Smoke and flames from my IFR-1000S...

I'd not really intended to have two posts in a row about repairing service monitors, but fate/opportunity intervened...

Figure 1:
The gray, charred stump of the failed tantalum capacitor in the center
of the image, just above the potentiometer shaft.)  The original,
nylon extension shaft broke several years ago and here, it is
shown having been mended with two overlapping
layers of heat-shrink tubing.
Click on the image for a larger version. 

The other night I had a few minutes to spare and I decided to take a quick look at my old IFR-1000S service monitor:  I'd remembered that the last time I dragged it to a repeater site there was something about it that was flaky - but I couldn't remember what - the 'scope, I thought...

So, I plugged it in, turned it on and everything was fine until I flipped the switch to apply power to the 'scope.  At this point the front panel lights dimmed momentarily, following by a loud bang (even though it was muffled by the unit's metal case) and smoke billowed out from every gap in the front panel along with a bright, flickering yellow light from a fiercely burning flame within.

Of course, I turned it off and fortunately, the flame quickly died out!

Hmmm...

Undoing about a dozen screws I soon had the cover off and discovered the culprit:  A dipped tantalum capacitor (150uF, 15 volts) on the the high voltage power supply board for the oscilloscope had incinerated itself.  Fortunately, aside from leaving a sticky, smoky residue on the nearby components, adjacent chassis panels and the inside of the wrap-around case, there didn't appear to be any real damage.

I should say that once I saw what had "flamed out" I wasn't too surprised:  Dipped tantalum capacitors don't fail too often, but when they do, they usually fail spectacularly, often burning holes in the circuit board and destroying nearby components!

Figure 2:
Inside the wrap-around cover - evidence of smoke and flames!
Click on the image for a larger version.

Grabbing the service manual I quickly located the faulty capacitor on the schematic diagram and noted that it wasn't anything too critical - a bypass capacitor on the power supply to filter the ripple from the 'scopes high voltage power supply (essentially an oscillator) from the main 12 volt power bus - this, to keep "noise" from getting into other circuits.

Carefully unsoldering the remnants of the capacitor (now a small chunk of charred tantalum) I shook out the other pieces of the capacitor that had fallen inside the unit and the powered it up.

Everything looked good!

Now, to replace the capacitor.  The original was a dipped tantalum unit, this type chosen because of its low ESR (Equivalent Series Resistance) and its ability to effectively filter the high-frequency noise produced by the high voltage inverter.  For this task I wasn't going use an "ordinary", cheap capacitor since its filtering ability may be somewhat diminished at the frequencies involved - around 20 kHz.

Back when the unit was made the best capacitors for high-frequency filtering were tantalum units or specially-made low-ESR electrolytics, but the latter weren't extremely common.  These days, with the proliferation of switching power supplies it's quite common to find high-performance, low-ESR electrolytics designed for just this task so I rummaged around and found a 330uF, low-ESR 105C (high temperature) capacitor that appeared to be well-suited for the task.

Figure 3:
The new (blue) CDE 330uF low-ESR electrolytic.
Click on the image for a larger version.

While it would have been ideal to have completely pulled the circuit board to install the replacement capacitor, I knew this to be a chore - having done it several times before - so I was able to do a careful "top soldering" job, heating the component's through-hole vias from the component side of the board.  Not having pulled the board out of the unit also meant that some of the sticky, smoky residue remained in some of the inner recesses and on some of the adjacent components, but I was content to clean off what I could reach using denatured alcohol.

The upshot?

The unit is now working again and the new capacitor seems to be doing its job.  If I have a reason to do so in the future, I'll pull the scope module go through it to remove the last traces of the smoky residue and, perhaps, preemptively replacing the other tantalum, but for now...

I still don't remember for certain the problem for which I was checking out the service monitor!

Additional random comments:

A few months later (8/13) I noticed that sometimes the IFR-1000S would hum when it was powered up - but not always.  Clearly 120 Hz ripple, it was pervasive enough that it would register as 200-300 Hz of deviation on an otherwise unmodulated carrier, appearing as a "dirty" waveform on the output signal as well as being visible on the scope and audible via the speaker.

Taking the cover off, the hum stopped, but I checked the filter capacitors in the power supply (some of which I'd replaced a few years ago) and found them to be good.  After having used it a few more times hum-free, the problem appeared again and this time I happened to notice, as I was picking it up while it was powered on, that the hum changed.  Pushing on the case and wiggling things I discovered that the hum changed radically when I wiggled them main AC power connector - "Jones" plug.

Upon disassembling the unit I saw that the solder joints on the connector were just fine, but that the Battery - lead from this connector (which can be used to operate the unit from DC power) shared a heavy black lead that came from the main power supply, bonding it to the chassis.

Hmmm...

Grabbing a screwdriver, I immediately noticed that this screw was a bit loose.  As it turned out it was this connection that was getting flaky, developing a slight amount of resistance.  Since it came from the power supply this caused the regulation (and consequently, ripple rejection) to suffer.  I put a drop of anti oxidant grease on the connections and properly tightened the screw, thus fixing the problem!

Resurrecting a CE-50A service monitor

Several weeks ago a friend of mine gave me his old Cushman CE-50A service monitor - a "Swiss army knife" piece of test equipment used for testing and evaluating 2-way radios, receivers and transmitters over the range 0.1 MHz to just under 1 GHz with the capability of testing receiver sensitivity, transmitter modulation and transmit power - all of this in addition to being a general-purpose audio and RF signal generator and low-end, general-purpose oscilloscope.  He'd had this unit for about a decade and had bought it in a non-working condition, having gotten it functional and had used it many times for radio maintenance.  His needs had changed and he no longer had the time and equipment to repair it so he figured that if he needed a piece of test equipment with the necessary functionality, he knew plenty of people (such as me!) from whom he could borrow the necessary gear.

Figure 1:
The now-working CE-50A with it's scope showing an "O'clock" - an
oscilloscope-based chronometer - just for fun!
Its display is slightly distorted due to my failure to compensate the
scope leads!
Click on the image for a larger version. 

Some time in the past year or so it quit working properly.  For a long time the front panel BNC connector from which the signal generator output - and the wattmeter was input - was flaky.  While a nuisance, it was still workable - at least until the built-in wattmeter quit working due to the internal RF relay not being triggered by RF any more.

Fixing the scope:

So, it fell into my hands along with the service manual.  Upon turning it on one of the first things that I noticed was that while the oscilloscope CRT was fairly dim, all displayed traces - from any source - had some "fuzz" on them at a frequency that was in the 20 kHz or so range.  My first inclination was that I should "shotgun" (e.g. replace) all of the electrolytic capacitors in the main power supply.

I should have gone with that first inclination.

I then thought that, perhaps, the scope's power supply board had lost some capacitors since not only did it supply the high voltage for the CRT, but it also generated other voltages (e.g. 90 volts for deflection and a negative voltage for other scope-related circuits) so I replaced all of the electrolytics on that board.

No change.

Poking around I then noticed that the high voltage was around -950 volts instead of, as indicated by the manual, -1400 volts.  Pulling the high voltage converter board again I realized that it didn't match the one in the book, being a different part number and further scrutiny revealed that about the only difference between the board and the one depicted in the book was that it was supposed to have a 3-stage high-voltage multiplier rather than just a 2-stage.  Rummaging around, I found some high-voltage 0.01uF disk ceramic capacitors and some 6000 volt, low-current diodes and constructed the extra stage, bringing up the high voltage to more-or-less what it should be.

This made the scope trace significantly brighter - as well as narrower and shorter.  Expectedly, the higher cathode voltage on the CRT increased the velocity of the electrons which meant that they were more difficult to deflect and this required that I re-tweak the vertical and horizontal amplifiers to get it back into calibration.  While doing this I couldn't help but notice that the service manual was obviously incomplete on some points, namely leaving out the descriptions and adjustment procedures for entire circuits within the vertical and horizontal deflection requiring a bit of on-the-spot decipherment of the apparent intent of the designers.  After a bit of tweaking, I got the scope back into calibration.  Even after all of this, the "fuzz" on the scope was still there - although slightly diminished - probably due to the increase in acceleration voltage.

RF power/signal generator transfer relay:

Setting aside the fuzz on the scope for the time being I attacked the problem with the internal relay.  Normally, the "Signal Output" jack is connected to the antenna input of a receiver under test and is used to apply a variable signal level used to test the radio's performance and as a signal source to aid adjustment.  When one transmitted into this same port an internal relay was supposed to switch the signal path from the output of the signal generator and connect it to the internal power meter, allowing one to measure transmitter power from less than 1 watt to 100 watts.

Except that it didn't, and that was the problem.

Of course, the module with this relay was the most deeply-buried circuit within the entire unit.

Getting access to this module required the un-mounting and disconnecting of several modules before its nearly two-dozen screws could be accessed and the cover removed.  Having just enough wires still connected to do so, I was able to transmit some RF power into the unit and saw that an 8 volt supply that fed nothing but that power detect circuit and its relay was going from its normal 8 volts down to 3 or so under load.  Referring to the manual I then noticed - with some annoyance - that this same 8 volt supply was now buried under the modules that I had to flip open to access this circuit to take the measurement, so I had to put everything back together.

Finding the 8 volt power source - a simple 7808 3-terminal regulator bolted to the chassis near the rear of the unit - I was immediately struck by the fact that its input voltage was varying between 60 and 70 volts.  Looking at the schematic I could see that its power source was either the main 12 volt bus from the power supply, or from the battery input - the choice between the two selected automatically with a pair of diodes in "diode-OR" configuration.  Putting the voltmeter on the source voltage I determined that yes, the 12 volt supply was correct, but the battery supply - which should have been at about 14 volts for charging the not-installed battery, was in the 18-20 volt area.

This last point was definitely wrong, but where was the 60-70 volts coming from?

Grabbing an oscilloscope I noticed, in looking at the battery charge line, that it was very "dirty" with 70-80 volt spikes on it which were then being rectified and filtered by the input diode and bypass capacitor on the 7808 which was apparently shutting down under even a very light load.

It was now that I finally "shotgunned" the capacitors in the power supply and in so-doing, I found that it was in the charging circuit for the battery that a capacitor or two had failed and because because of this, the circuit had gone into oscillation and was the source of the spikes.

I really should have replaced all of the capacitors in the power supply when I started!

Finding and replacing every electrolytic capacitor on this board (nearly all of them were found to be well out of spec!) I re-installed it and observed that the input of the 7808 was now in the 13-15 volt range and that the RF relay now operated normally - and also that the "fuzz" in the scope was completely gone!  Interestingly - but not too surprisingly - the scope now appeared to be even  "brighter" than it had been before owing to the fact that without the "fuzz" to fatten all of the lines, they were now fairly narrow and crisp, giving the illusion of additional brightness.

Using a handie-talkie I fed power into the jack and observed that I was now getting a wattmeter reading - but something was still wrong:  It read very low.  Further investigation showed that the meter reading seemed to change slightly every time it was activated, indicating a mechanical problem somewhere and even more revealing, I was getting a much higher and "less incorrect" reading when transmitting at 440 MHz than I was at 145 MHz indicating an "air gap" somewhere in the signal path with capacitive coupling across it.

At this point I connected the handie-talkie to the input side of the 20 dB, 100 watt attenuator inside the unit - a point "after" the relay - and observed that  the readings were closer to being correct and consistent both from one reading to another and over frequency, being fairly consistent between VHF and UHF andpositively indicating that the problem was likely inside the module that I'd previously taken apart - and mostly likely the RF relay.

This meant tearing the unit completely apart... again... and possibly replacing the RF relay.  Fortunately, Cushman had chosen a rather common component for this - a small Switchcraft RF relay that had been used in land-mobile gear in the 60's and 70's and a type of which I had some spares that had been pulled from scrapped gear.  While the relay itself was identical, the coil was different but inspection showed that it should be possible to drill out the rivets - leaving the posts - and then epoxy the old coil onto the "new" relay were it necessary to do so.

I carefully unsoldered and removed the relay and took off its cover to inspect its insides.  This relay is fairly simple - see figure 2 - an armature inside a milled-out channel (for RF impedance matching) with a pair of contacts at the far end with a plastic button transferring the motion from the armature on the coil.  What I noticed was that the armature was out of alignment, touching the other contact with only a "glancing" blow and thus explaining why it wasn't working properly.

Figure 2:
The guts of the same type relay used in the CE-50 and a potentialreplacement!  The problem with the original relay was
where it emerged from the coax and into the body of the relay (on the right) and connected to the armature:  It seems that
during the original installation the polyethylene insulation melted and allowed the armature and contacts to move out of
alignment, eventually causing the relay to become unreliable.
Click on the image for a larger version.

These types of relays were originally made with short length of coaxial cable crimped onto their ends and apparently the manufacturer of the service monitor (Cushman) had simply cut off the polyethylene dielectric coax, leaving a short portion of the center conductor to be soldered into the circuit.  What had apparently happened was that upon installation, a bit too much heat had been applied while soldering and the plastic dielectric had melted, causing the armature had moved out of position.

Using a pair of needle nose and carefully applying heat to re-soften the plastic I carefully repositioned the armature into proper alignment and then, to make sure that his wouldn't happen again, encapsulated the end of the armature (the far-right end in figure 2 where it would connect to the coaxial cable) in a small amount of clear epoxy to provide a more rigid substrate than the polyethylene had provided.  After the epoxy cured I took this opportunity to inspect the relay contacts more closely - which appeared to be nearly pristine - and then burnished them with a piece of scrap paper to remove any surface oxide that might have formed.  I then put a drop of "Stabilant 22" - a synthetic contact enhancer and anti-oxidant - on the relay contacts to assure continued operation.  Finally, I very carefully bent the armature itself so that its "springiness" would be modified to more positively and forcefully make contact.  After all of this, I reassembled the relay, installed it into the circuit and tested it.

It worked!

Putting everything back together I went about recalibrating the wattmeter and found that as with the section that described the 'scope, the service manual was woefully incomplete requiring that I reverse-engineer their original intent, invent, and perform the calibration procedure - and then note it in the manual for future reference!

Why doesn't the PFM-Generate mode work?

At this point everything seemed to be working so I went about checking and recalibrating the various sub-instruments as required - until I came across the need to check and calibrate the "PFM Generate" function.  As it turns out, in addition to AM and FM, there's a "PFM" (presumably meaning "Pulse FM") mode that isn't well described in the manual.  When set to its equivalent in the "monitor" (receive) mode, this seems to insert a low-pass filter into the demodulator path to remove high-frequency noise, but when set to "generate" mode, the PFM setting seemed to do nothing at all except generate a CW (dead) carrier.

Again, I tore into the unit and with the aid of the manual I started tracing the signal path of the front panel selector switch and found that it was getting everywhere it should have.  I finally got to the audio board where these signals were routed and noticed that when in PFM, the audio path went through a separate adjustment and audio switch just for the PFM mode - and it seemed to be working.  Moving to the next circuit earlier in the audio path I found an audio gate transistor that seemed to be disabled in PFM mode with a diode.  At this point I went back and reviewed the manual's circuit description and interestingly, it described in some detail the audio paths for all modes - including PFM - but then, in a separate paragraph it mentioned in passing that this particular diode was there to disable the audio in the PFM mode!  Why, then, was there extra circuity for the PFM mode if it was ultimately disabled?  At least this answered the question and told me that "PFM-Generate" was supposed to do nothing!  While I could easily remove the diode and make this mode functional, I decided to leave it alone for now.

Comment:

Further testing and "reading between the lines" of the manual it would appear that the "PFM" mode is intended only for modulation applied through the "External Modulation" input jack - a point that the manual doesn't make clear.  Since external modulation is, in fact, possible in the other modes, it is unknown why that would have yet another switch position to accomplish this!

Hummm...

While chasing out the PFM-Generate mode I noticed a small amount of hum coming from the speaker.  In checking the 12 volt supply I could see that there was about 35 millivolts of ripple on it, so apart came the power supply again.  This time, I traced the 120 Hz ripple to the switcher board and then noticed that a previous modification had been done to it where an output filter capacitor had been installed with a series resistor - presumably to reduce inrush current - replacing a smaller-value capacitor that the diagram had showed as being located directly across the switcher supply's output.  Putting a scope there showed that there was a few hundred millivolts of ripple at the switching frequency (25 kHz or so) but that this was being filtered out by a later power supply stage - the one on which I'd shotgunned all the capacitors.  Using a low-ESR capacitor specifically designed for switching supplies, I reinstalled the device that had been missing and not only did the switching frequency ripple decrease significantly, but the 120 Hz ripple on the 12 volt supply went down to the 10-12 millivolt area (which was quite acceptable) - but the hum in the speaker, being much lower, was still audible.

Turning my attention to the audio amplifier I noticed that the speaker had been coupled to the output in an odd way.  This amplifier was a fairly simple, transistor-based circuit using a pair complimentary transistors in the output to the capacitively-coupled speaker.  Typically, the speaker coupling capacitor is connected between the output of the "totem-pole" transistors and the speaker, but in this case, the DC blocking/coupling caps were on the "low" side of the speaker - and there were two of them:  One between the speaker "low" side and ground and another between the +12 volt line and the speaker "low" side - and it was this latter capacitor that appeared to be coupling the power supply hum into the speaker.  I'm not sure why they did it this way, but my guess is that it prevents a loud "pop" in the speaker when the power is turned on and off, so I left it this way, deciding that the hum wasn't that bad anyway...

Front-panel connector:

The final item was to address the problem with the front-panel RF connector.  As often happens with BNC and N-type female connectors, the "leaves" on the springy center connector weaken and break off - and that had happened to the previous owner.  Unfortunately, a rather specialized chassis-mount BNC connector had been used on the end of small-diameter PTFE hardline coaxial cable.  For trouble-shooting purposes, the previous owner temporarily connected a male BNC connector via a length of cable, but now that I'd gotten the unit back together and fully functional I wanted to make a permanent fix.

Figure 3:
In Tracking Generator mode, testing a 10.7 MHz ceramic filter.  Because the
filter was fed/sourced with 50 ohms instead of 330 ohms, there's extra
passband ripple!
Click on the image for a larger version.

In rummaging around the junk box I found plenty of crimp-type BNC chassis-mount male connectors that would fit in a 1/2" diameter hole - but the one on the unit was a 3/8" hole with a flat spot and I wanted to avoid - if possible - drilling it out.  What's more, I didn't want to use a standard chassis-mount solder-on BNC connector as this RF connection had to be both RF tight and fairly impedance "flat" from essentially DC to 1 GHz - difficult to do with a solder-on connector.  Finally, I found one 3/8" O.D. chassis-mount female BNC connector with a coaxial crimp fitting it to RG-174 sized PTFE flexible coax so I carefully disassembled it and managed to successfully attach it - with a bit of soldering - to the small-diameter hardline inside the unit.  A bit of testing showed that it worked nicely over the entire frequency range, so the unit was reassembled and the project was considered to be complete!

Comment:  I later noted that the exact replacement connector was available via Pasternack Enterprises for somewhere around $50 - something to keep in mind should this repair fail at some point.

Conclusion:

Overall, the repair was a fun project, taking me several places that I didn't anticipate going and reminding me, again, of that old adage: 

"When in doubt, check the power supply!"

Additional comments:

More recently (8/13) I had a strange problem occur:  When in the spectrum analyzer mode the synthesizer lock would come and go while the frequency offset meter would slowly drift up and down.  Apparently, something was slightly unstable, preventing the main PLL from locking.

In trying to troubleshoot this I looked at some of the plug-in boards and noticed that a lot of the 100uF, 16 volt capacitors were starting to leak - and there are a couple dozen of these scattered throughout the unit on most (if not all) plug-in boards that are used as power supply bypass/filter capacitors.  While no damage seemed to have been done other than a slight amount of surface corrosion that was easily removed, I did replace pretty much all of them and at some point the problem with the synthesizer's locking went away - although I don't know for certain on which board the "fix" occurred - or if it somehow fixed itself with my re-seating the boards.

Anyway, it would be a very good idea to take a close look at all of the 100uF capacitors (and other electrolytics, as well) on the various boards and replace them as they are probably starting to leak on your CE-50, too!

Throwing one's voice 95 miles on a lightbeam

For more information about long-distance optical communications, go to the modulatedlight.org web page  (link)

This past weekend (September 15-16, 2012) was the 2nd part of the annual ARRL 10 GHz and up contest and we decided to use one of the highest-available amateur bands - the one known in the FCC rules as "275 GHz and up."  Actually, this covers a lot of territory including submillimeter radio frequencies and far infrared wavelengths, but the part that we are more interested in is that for which most of us are equipped to detect directly - light.

We've done this before, managing to have spanned 107 miles (173 km) on several occasions and even 173 miles (278 km) (read about those efforts here - link) so we weren't going to break any of our own DX (distance) records, but it's fun to do this, anyway - and it gave us an excuse (as if we really needed one...) to go out and test some new gear that had not yet been tested over anything but relatively short (20 km or so) paths.

The two locations for the stations were about 96 miles (154 km) apart with Ron and Elaine Jones (K7RJ and N7BDZ) being at the far end at an elevation of about 5600 ft (1700 m) ASL near Park Valley, Utah in the extreme northwestern corner of Utah, a few miles from where the U.S. Transcontinental Railroad was joined for the first time in 1869 and only a few hundred meters away from the historic stagecoach route that paralleled part of that later railroad.  Along with friends Gordon (K7HFV) and Gary (AB1IP), I  was closer to home at about 9300 feet (2830 m) near a minor protuberance known as "Bountiful Peak" about 10 miles (16 km) north of Salt Lake City.  As it turns out, the path is a grazing one and were it not for the slight refraction of the Earth's atmosphere, it may not even quite be "line of sight."

We'd tried this same path during the first weekend of the 10 GHz and up contest but the thick veil of smoke from wild fires elsewhere in the western U.S. prevented a successful contact - although our light beam was occasionally just visible to the binocular-aided eye in Park Valley.  This time, however, the air was reasonably clear, only somewhat hazy from the still-burning fires:  Since we "almost" made contact a month ago we were confident that this time we would have no problems.

Figure 1:
The high-power red LED shining to the north-northwest.
The lights of Layton, Utah and surrounding communities
may be seen way below, in the background! The dot at the terminus
of the red shaft of light is the light from Ron's end of the path.
Click on the image for a larger version.

Soon after we arrived on site Ron shone a 500,000 candlepower halogen spotlight in our direction and immediately we noted a lone, flickering, yellow-red dot in the blackness "above" the last ribbon of visible lights from the populated areas of Layton and Ogden about a mile (1600 meters) in elevation below us.  Using this as a visual reference I swung my high-power LED in his direction, using the Rayleigh-scattered shaft of red light as a guide, and immediately Ron reported that it was easily the brightest light visible:  Considering that there were only a small handful of lights visible from his dark, rural location, anyway, this wasn't saying much, but if anyone where to have dropped by and looked in that direction they would have seen the bright, red light and asked, unprompted, "What's that?!?"

Using our light as a guide Ron immediately fine-tuned his pointing and soon, a very obvious red light appeared in the darkness.  Initially starting out with the lower power 3-watt LED he soon switched to the much higher-powered 20-30-ish watt LED and the red dot in the distance was even more striking than before.  The dot at the end of the red shaft of light in the above picture was from Ron's LED.

Soon after we brought our transmitters up to full power we reduced them again to 1/4-1/15th as each other's signals were strong enough that there was noticeable distortion in the received audio - and it also allowed us to run full-duplex (e.g. both sides being able to send and receive simultaneously) without intercepting as much of our own, scattered transmit light and causing acoustic feedback between our speaker and microphone.

This was the first actual "long distance" test of the Phlatlight-based optical transmitter - these using CBT-54 LEDs and permitting a 20dB improvement on the audio received at the far end.  This also was the first test of some APD (Avalanche PhotoDiode) based optical receivers that I'd built some time ago (see the link at the bottom of the page) so we set about reduce each other's LED currents to do a sort of "limbo" dance - that's to say we wanted to answer the question "How low can we go?"

It immediately became apparent that even though we could read the Phlatlight modulators' current with a resolution of 0.1 amp, this was still too coarse when we got down to the lowest readable current and were still able to hear each other, so Ron switched to the older 3 watt Luxeon on which the LED current could be measured and adjusted down to the single digits of milliamps.  As it turned out, speech was copyable - with some difficulty - down to the 40-50 milliamp range with the old receivers but the APD receivers extended this down to around 20 milliamps - an approximately 6-10dB improvement, a number that agreed reasonably well with what had been calculated using similar measurements done at home on my "Photon Range" using a very dim LED and test receivers.

Practically speaking this meant that at full power with the Phlatlight LEDs we had about 50dB of  excess signal at the output of the receivers as compared to the minimum possible signal level using baseband speech and the "naked" ear.  Switching to MCW (tone-modulated Morse code) we could extend this by another 6-10dB and the the use of narrowband digital signalling techniques (such as WSPR or QRSS CW - very slow Morse) could have extended this by even another 20 dB or so.  The implication of this is that, in theory, we could communicate over that distance with only a milliamp or two of LED current!

Figure 2:
This time, a high-power green LED!
Click on the image for a larger version.

Satisfied with our tests I switched to a green CBT-54.  Interestingly - but not too surprisingly - Ron reported that subjectively the green LED wasn't really any brighter than the red had been.  On previous tests at much shorter distances (a few 10's of miles/km) the green far outshone the red owing to the fact that the human eye is at least 5 times as sensitive to green than the wavelength of red LED that we were using.  For these distances the atmospheric attenuation was sapping the vast majority of our light since the shorter (green) wavelengths are attenuated at a far higher rate than the longer ones, a fact that explains red sunsets and that we observed, at the beginning of our testing, that his white, halogen spotlight appeared to us as distinctly yellow/red in color.

The silicon photodetector didn't fare any better since it had far less sensitivity at green than red, the two factors (atmospheric and the Si sensitivity) adding up to between 20 and 30dB in degradation - assuming that the subjective measurement of "equal" brightness between red and green was correct.  As it turns out the degradation was probably far greater than that as the APD-based receiver could hardly detect voice at all, but this may have been also due, in part, to the fact that the gain of a standard APD drops off precipitously with shorter wavelengths and that it was likely not focused properly for green light due to chromatic aberration of the Fresnel lens!  In retrospect we should have switched to a receiver with a larger, "non-APD" detector - and thus less sensitive to misfocusing due to chromatic aberration.

In addition to using high-power LEDs, we also exchanged 2-way communications using plain, ordinary, cheap low-power red LED laser pointers.  The signals were far weaker - mostly owing to the lower optical power of the laser pointer - but each other's lasers were visible to the naked eye over the distance.  Because of the combination of the laser's (relatively) coherent light and its small exit aperture (small beam diameter) the scintillation (fading) on the laser-based link was terrible while on the LED-based link it was only just noticeable.  Some of the methods and techniques to communicate using laser pointers may be found in the September 5th entry of this blog.

After several hours of standing around in the dark on the mountain, we decided that it was getting early (approaching 2 AM!) and packed things up and made our way down the mountain.

Overall, it was a fun little jaunt giving us a healthy dose of nerdiness... enough to last for a few weeks, anyway!

Afterward:

While we run these tests, we'll often play something from portable audio players so that we have a continuous source of sound.  In this case, one of the audio sources that I used was from a Soldersmoke podcast.

For the heck of it, I emailed Bill, N2CQR, who produces this podcast and he put it on his blog page (link) as well as commenting on it in his next Soldersmoke podcast (link)!  This may have had something to do with this post appearing on Hack-A-Day (link)!

Links from the "Modulated Light" (link) web site:

• Using Laser Pointers for voice communications -  This page describes in more detail the methods by which one may successfully use inexpensive laser pointers to cast voice through many miles of the ether!
• A Highly-Sensitive Optical Receiver Optimized for Speech Bandwidth - This is one of the most sensitive speech-range optical receivers yet devised that uses standard photodiodes and is much more sensitive than the simple receiver depicted in figure 1, above.
• Receiver for low-bandwidth optical (through the air) communications using an Avalanche Photo Diode (APD) - This optical receiver is an enhancement of the one above, achieving another 6-10dB of ultimate sensitivity using an APD.  This receiver is about as sensitive as you can get without resorting to an exotic red-sensitive photomultiplier tube!
• A "Cheap" Optical Transceiver lens assembly - If you really want to improve your receive sensitivity, the best way to do this is with a large lens.  This article describes how one could use inexpensive foam-core poster board to make an assembly that will focus the distant light on the detector diode of an optical receiver.
• A "Mini" full-featured Pulse Width Modulator for high-power LEDs and laser diodes -This describes a simple, computer-based PWM modulator that will not only transmit audio, but generate test tones as well.

Be sure to check out the "ModulatedLight.org" web site's other pages as well!

[End]

This page stolen from ka7oei.blogspot.com

Two repeaters, One frequency (Part 3)

For a follow-on article in this series, see Part Four (link)  for a discussion of how the voting receiver system works.

In parts One and Two the general overview of a "synchronous" (or "simulcasting") and voting repeater system was discussed.  In a nutshell:

• Both repeaters operate on the same frequency saving spectrum and simplifying the system's use since the user doesn't have to remember which particular frequency of a "normal" linked system covers a certain area best.
• The coverage of the two repeaters overlaps to a degree.
• Because of precise frequency control, the two transmitters don't really clobber each other in overlap areas, particularly in a moving vehicle.
• Because of voting receivers and multiple transmitters, the users can seamlessly move between coverage areas with no intervention on their part.
• The total coverage is greater than the sum of the parts owing to the increased likelihood of one or another site hearing the user and/or being heard - particularly if in an area where coverage is spotty to one or both sites.

 Originally (back in the late 90's) the idea was to frequency-convert the received signals from the 2-meter frequency to a subcarrier-baseband and send them to the main site where they could be voted upon and then a master modulator would then ship back (via a microwave link) a subcarrier which was then up-converted to the transmitter frequency.

The details were worked out and some of the equipment was actually built and tested - and it worked!  However, the magnitude of the task bogged things down and one thing led to another and the project languished - until 2009.

By then I'd already put together 2 voting systems and one multi-transmitter synchronous system (using GPS frequency references) and had other ideas on how to do things a bit more simply which translated to "being more likely to get completed!" The project got underway in earnest in mid-July of 2009 where the plans were re-draw and tasks divided as appropriate.

Instead of building the transmit and receive gear from the ground up it was, instead, decided to modify off-the-shelf GE MastrII radio gear to fit the bill.  This equipment is readily available on the surplus market and the individual pieces could be used with little or no modification - which meant that spares of those same pieces (receiver, transmitter, power amplifier, etc.) could be kept on hand as spares!  What's more, for the most part these units used common, off-the-shelf parts (resistors, capacitors, transistors) and were thus field-repairable now and for the foreseeable future.  Finally, a lot of information is available on these radios on the web so if, in the future some trouble shooting is required, there's plenty of advice to be had online.

What modifications were required to the radios were fairly simple:

• Instead of a standard crystal module (called an "ICOM" by GE) a simple, plug-in module (using a "gutted" ICOM) was plugged into the exciter instead.  This was connected via coax to an external module that provided the low frequency (at 1/12th of the transmit frequency) at the precise frequency.
• Transmit audio was fed into the subaudible tone input port.  This was done because it did not have the highpass and lowpass filters that the normal microphone inputs had:  We would do the high/low pass filtering externally!
• The receiver modification (for the Scott's Hill site) simply involved obtaining discriminator audio.

There were some additional modifications done to provide interfacing to the rest of the system - namely an outboard de-emphasis, a low-pass filter and a switchable notch filter (for a "quirk" we later discovered) but these were mounted on the backplane - a more-or-less passive board that would likely never require replacement!  Pretty much everything else was "stock" and could be tuned up and adjusted according to the original manuals!

Transmit frequency control:

The most important aspect of a multi-transmitter (simulcasting) repeater system is that the transmitters be where they are supposed to be, frequency-wise!  While there are several ways of doing this, we took a somewhat unique approach.

A standard transmit crystal (at 1/12th of the VHF transmit frequency) was ordered and placed into an "EC" type ICOM.  This is, in effect, a self-contained oscillator module that has provisions to be frequency-controlled with an external voltage.  This module is completely standard and off-the-shelf and it could be plugged into any GE MastrII VHF transmitter and work normally.

In our case, however, this "EC" module is plugged an external module - called a "Disciplined Oscillator" - that takes the crystal frequency (which is 12.2183333 MHz for a 146.620 MHz transmit frequency) and locks it to a reference based on a 10.0 MHz oven-controlled crystal oscillator.  This is done by synthesizing an audio frequency, using a PIC microcontroller clocked to the 10 MHz oscillator, that has a resolution of a few parts per billion and with a bit of dividing, mixing and comparison has the result of locking the 12.2183333 MHz oscillator to the 10 MHz reference to within a tiny fraction of a Hz.  Essentially, the frequency accuracy is that of the 10 MHz oscillator!

The 10 MHz oscillator is an oven-controlled crystal oscillator (OCXO) pulled from scrapped satellite gear and is well-aged (made in about 1990) and has a stability of about 10E-8 - within 1 Hz or so at the 2 meter transmit frequency.  This OCXO also has an external voltage control tuning line that is under control of the PIC microcontroller and with it, the 10 MHz frequency (and thus the transmit frequency) can be tweaked to set each transmitter on the desired frequency - which also means that the frequency difference between the two transmitters may be precisely controlled.  In the nearly 3 years since the system was made operational we've observed that the transmitters have stayed within about 1 Hz of their intended frequencies relative to each other over the course of the temperature excursions during the year!

This "Disciplined Oscillator" module also has another function that, since it was computer-based, was easy to implement, and that's as a simple dual cross-band repeater.  On Scott's (the remote site) it simply cross-bands the 2 meter receiver to the 70cm link transmitter - taking care of thinks like proper IDing, timeout timers, etc. and it also takes the 70cm link receiver and controls the 2 meter transmitter coming back the other direction:  Both operate independently of each other...

Squelch control and voting:

It also does one more thing:  COS (Carrier Operated Squelch) signalling.  The 146.620 repeater is one of the few repeaters in the area that does not have a subaudible tone requirement, this being because it's an "open" repeater and that extreme care is taken at all receiver sites to keep the receive frequency as clean as possible - a task that is arguably easier since the demise of analog television in the U.S.!

Since the Scott's Hill transmissions are relay to/from the master site via a UHF link there would be an extra squelch tail (the "ker" in "ker-chunk") if the loss of a signal at Scott's were signalled simply by its UHF transmitter being keyed/unkeyed.  Instead, the loss of squelch is signalled by the appearance of a strong, 3.2 kHz tone sent over the link which performs two functions:

• It signals to a decoder at the master site that the squelch as closed at the other end.
• It signals to the voting controller at the master site that the signal being received is "bad" and should NOT be used.

(This 3.2 kHz tone is "notched" out and its brief appearances in the system audio are not heard by the users.)

So, what happens if a user's signal into Scott's is dropping rapidly in and out?  As the squelch opens and closes, the tone is turned off and on (tone on = squelch closed/signal dropout.)  When the tone is turned on the voter disqualifies this tone, but if that same user is getting into the other receiver (at the master site) then this tone will guarantee that the signal from that receiver will be used, instead.

There is a short "hang time" on the link transmitter which means that when an input signal disappears from the 2 meter receiver at Scott's, the tone will turn on instantly, making the master site ignore the input, and then the UHF link transmitter signal will drop and in this way, the extra squelch tail from the UHF link transmitter dropping is never heard by the user.

As it happens, the signals coming the other way (from the master site to Scott's over the UHF link to be retransmitted on 2 meters) also use this 3.2 kHz tone - this time, to control the Scott's VHF transmitter.  In this case, however, the activation of the tone starts the "Unkey" sequence at Scott's allowing time for the disciplined oscillator to put an extra "beep" on the transmitter (so that users know which transmitter they are hearing!) and then unkey the VHF transmitter.

Since the 3.2 kHz tone being sent to Scott's occurs just before the master site's 2 meter transmitter unkeys, it's possible to set the timing so that both sites unkey at precisely the same time:  If both site's didn't unkey simultaneously, many users would be annoyed by the presence of an extra squelch tail if they could, in fact, hear the "other" transmitter hanging in there for a short time!

At the master site:

As it turns out, the master site's interfacing a was bit easier... sort of...  This repeater's master site is actually split, with the receiver and antenna being several hundred feet away from the transmitter, this being done to put it farther away from the megawatt of RF being emitted from all of the TV and Radio transmitters on the main site!  It is connected via transformer-coupled cables (for lightning protection) and has operated with minimum maintenance since the early 1980's.

Since we already had on-hand local COS (squelch) and audio from the receiver, there was no need for the tone signalling schemes of the remote site but, instead, the audio and COS lines could be input to the voter.  There was a problem:  The "local" receive audio was "too" good!

The way the voter works is that it analyzes mostly the audio above 2.5 kHz and of the receivers being compared, it is the receiver with the MOST audio above 2.5 kHz that is considered as being the one with the worst signal.  The reason for this is pretty simple:  As an FM signal gets weaker, it gets noisier, so it stands to reason that given two otherwise identical signals, the one that is also noisier will have a total signal level that is higher - particularly at higher audio frequencies.

The problem was that the audio from Scott's had already passed through a radio link which tended to scrape off the audio above about 3.5 kHz or so while the "local" audio, being coupled via wire, had no such low-pass filtering, so we had to add some.  What was happening is that the "local" audio - with its additional "highs" (as compared to the audio from Scott's) was being considered to be "bad".  By removing those extra high-frequency components and making the two audio signals pretty much equal we were able to make the more-or-less directly comparable.

Next time -Part Four:  A bit more about the voting controller and some of the remote control/monitoring capabilities.

[End]

This page stolen from ka7oei.blogspot.com

Voice on a laser beam...

Sending voice over light is nothing new.  The first wireless voice communications system - using light - was the PhotoPhone, demonstrated in 1878 by Alexander Graham Bell - a full 25 years before Fessenden demonstrated the same feat using radio waves.  To be sure, optical communications has certain practical limitations, namely the blinding presence of the sun and the occasional opacity of the atmosphere due to weather, but it's still a fascinating and fun topic of discussion.

I'm one of those people who find wireless communications of any sort to be interesting and I have a particularly keen fascination with optical wireless communications - that is, using "radio waves" that I can see with my own eyes.

For short-range experimentation it's hard to beat a cheap laser pointer - and here is a bit of info on how one might go about this.

Modulating the laser pointer:

The laser pointer consists of a laser diode and like any diode, it has a maximum current rating that should be observed with more caution than its voltage.  What this means is that you cannot connect a laser diode to any sort of battery and expect it to work properly:  Too little voltage and it won't lase while too much voltage, it will never lase again!  What is needed is a simple circuit that limits the amount of current fed into the laser diode to a safe level and fortunately, cheap laser pointers always have something that does this.

Increasingly, cheap laser pointers simply rely on a combination of a simple circuit (or even a single resistor!) and the internal resistance of the battery powering it to keep the laser current at a safe level and since a laser pointer already has the necessary parts, why not use them?

In my opinion, one mistake that I often see on web pages that describe the modulation of a laser pointer is to attempt to modulate by varying the voltage/current of operation - typically using a transformer in series with the power source.  There are several things wrong with this:

• It's not certain how far down in current one can go before the laser drops out of its "laser" mode or how high one can go before it gets "blowed up."

• Laser current versus output isn't terribly linear which means that distortion can occur.

• With the min/max current uncertainty, one can't fully modulate the laser's output safely which means that the audio on the beam will be somewhat "quiet" - something that reduces the efficacy of the link!

The better way to modulate a laser is to simply turn it on and off using Pulse With Modulation (PWM)  and taking advantage of the circuit already present to safely operate the laser from its intended power source - say, a pair of AAA cells (or 3.0 volts.)  While more complicated than simply putting the laser's power supply in series with a transformer, it's pretty much bulletproof and can sound pretty darn good!
 

Figure 1: Laser transmitter/receiver by K7RJ.
For a diagram of this unit, see Figure 4 at the bottom of this page.
Click on the image for a larger version.

A simple circuit to do this may be found in the diagram in figure 4 at the bottom of the page..

I won't take credit for this circuit which was thrown together by my friend Ron, K7RJ.  When built, this circuit was intended to be quick and easy and high performance was NOT in mind - just enough effort was put into it to make it work for demonstration purposes.

Contained within the diagram is enough information to connect your cheap, 3-volt laser pointer - just be sure to pay close attention to its positive and negative battery connections when you take it apart!

Also contained in this diagram is a very simple, low-performance receiver intended solely for across-the-room (or across-the-parking lot) testing of the transmitter to make sure that it works.  It should be emphasized that this receiver is not at all intended for longer-distance use - say, more than a few hundred meters at most, and its performance can be spectacularly enhanced with the careful installation of a small magnifying glass lens with the phototransistor at its focus.  Even when enhanced thusly, other optical receiver circuits will still run circles around it!  A link to a web page describing a far more sensitive circuit may be found at the bottom of this page.

At this point I'll make a few comments about laser safety and legality:

• Make certain that your "laser range" is end-stopped - that is, when the beam goes beyond the receiver it does not cross a road or have any likelihood of being intercepted by aircraft in flight or landing/taking off where they can dazzle and distract!  In other words, the receive end should be against the side of a building or hill.

• While cheap, red laser pointers are probably too weak to cause permanent eye damage, it's best not to stare into it or point it directly at people!  A standard, cheap red laser pointer will, at its worst, probably just dazzle and maybe cause a brief headache or eye pain as well as a temporary loss of night vision.  The farther you are away from it, the less dangerous it will be.

• In some states and areas laser pointers are highly regulated or even illegal - including some U.S. and Australian states/localities - check your local listings!

• It is NOT recommended that any but cheap, red laser pointers be used for this purpose.  Why?  First, they are the cheapest and secondly, they are fairly safe and low power.  It's also worth considering that typical electronic detectors respond far better (e.g. are more sensitive) to red light than green or blue - not to mention there being less atmospheric attenuation at "red" wavelengths!  Some of these "fancier" laser pointers of other colors have electronic circuits in them that can prevent them from being modulated effectively.

Figure 2:
Cheap laser pointer on a tripod
Click on the image for a larger version.

One thing that you'll immediately notice about laser pointers is that despite their name, they can be fiendishly difficult to aim them - particularly as the distance increases!  For this reason it's best to contrive a means by which a camera tripod can be used to hold a laser pointer - but even this can be tricky since even a fairly expensive tripod is quite "touchy"!  To the left you can see Ron's laser pointer mount with the pointer module itself being contained within a cheap project box from Radio Shack and connected by a short cable to the rest of the circuitry.

This brings up another point as well:  Do not put both the modulator electronics and your laser pointer in the same box.  By connecting them with a cable you will be able to make adjustments and turn the thing on and off without touching the tripod and possibly disturbing your carefully-aimed beam!

Another example of a laser pointer modified for such use may be seen on the right.  When I got this pointer I couldn't see how I could remove the laser module without the possibility of damaging it so I simply used it as-is:  A wooden dowel, the same diameter as AAA cells was used and at the inside end of the dowel was a small screw to which the minus (-) side connection was made.  The connection to this screw was made via a wire laid in a shallow groove along the length of the dowel and the positive (+) side was connected to the case of the pointer itself by using some copper foil wrapped around the end of the dowel opposite the screw.  The dowel was tack-glued into place, pushing against the internal battery spring and the laser's "on" button was simply taped down.  The entire pointer was then "hot-glued" to a cheap project box that itself has inside it a 1/4"-20 bolt glued into place to allow attaching to a tripod mount while electrically insulating the laser pointer's positively-connected case from the tripod.

Figure 3:
Minimally-modified pointer on a tripod mount.  This just happens to
be mounted atop an 8" astronomical telescope (a Celestron C8)
  with a equatorial mount which allows
precise aiming - and it also includes a telescope!
Click on the image for a larger version.

A lot has been glossed over in this brief article - namely techniques about how to accomplish a laser communication over longer distances including links to descriptions of higher-performance gear and methods of precisely aiming - and if you are really interested, you can take a look at my page:

 "Using Laser Pointers for Voice Communications" (see the link below) for a lot more detail than can be covered here.

How far can a lowly laser pointer go?

Under clear-air conditions on a line-of-sight path and using the very same lasers pictured above I've had a 2-way laser pointer to laser pointer communications on a 107 mile (173 km) path with fairly good signals.  This was, of course, using high-performance receivers with orders of magnitudes better sensitivity than the one shown it Figures 1 and 4 on this page!  In the "Using Laser Pointers..." link just above one can even find additional links to actual "off the air" recordings made via long-distance laser-pointer communications systems.

There are other problems with using lasers over distance, however, namely that of scintillation - the rapid fading or "twinkling" caused by the irregularities in the atmosphere.  While this affects all types of light sources the combination of the coherent laser light and the small diameter of the beam as it exits the laser greatly exacerbates the problem - but that's a topic for another article!

Links from the "Modulated Light" (link) web site:

• Using Laser Pointers for voice communications -  This page describes in more detail the methods by which one may successfully use inexpensive laser pointers to cast voice through many miles of the ether!
• A Highly-Sensitive Optical Receiver Optimized for Speech Bandwidth - This is one of the most sensitive speech-range optical receivers yet devised that uses standard photodiodes and is much more sensitive than the simple receiver depicted in figure 1, above.
• A "Cheap" Optical Transceiver lens assembly - If you really want to improve your receive sensitivity, the best way to do this is with a large lens.  This article describes how one would use foam-core poster board to make an assembly that would focus the distant light on the detector diode of an optical receiver.
• A "Mini" full-featured Pulse Width Modulator for high-power LEDs and laser diodes -This describes a simple, computer-based PWM modulator that will not only transmit audio, but generate test tones as well.

Figure 4:
Schematic diagrams of a simple (but deaf) receiver for testing and a simple PWM laser/LED transmitter, described
in the text above.  This unit was designed by Ron, K7RJ and is shown in Figure 1, above.
Click on the image for a larger version.

[End]

This page stolen from ka7oei.blogspot.com

Problems with Lithium Iron Phosphate (LiFePO4) Batteries

Update:

For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - link

In 2010, About 2 5 years ago over the period of several months, I got three 13 volt, 6+ amp-hour Lithium Iron Phosphate (LiFePO4) packs from Batteryspace.com for about $95 each.  These packs seemed to be a reasonable alternative to my old standby of portable battery power - the ubiquitous 12 volt, 7 amp-hour sealed lead-acid (SLA) battery, often (mistakenly) called "Gel Cells."

Why switch from SLAs?

The three LiFePO4 battery packs in question.

LiFePO4 packs seemed to be attractive for the following reasons:

• LiFePO4's were lighter than the same-capacity SLAs - roughly 1/2-2/3 as much weight.
• Claimed 1000-2000 charge durability for LiFePO4's versus 100-200 or so for SLAs.
• Claimed 10 year lifetime for the LiFePO4's versus 3-5 years for SLAs and conventional Lithium-Ion packs (even the polymer types.)
• With all of the above, the relatively high initial cost ($95) of the LiFePO4 batteries that would last 10 years seemed to be reasonably comparable to $15-$30 (when new) on a "per-year" basis with typical 7 amp-hour Lead-Acid packs - and the lighter weight was a plus!

As it turns out the LiFePO4 packs aren't quite as "energy dense" as  "normal" Lithium Ion cells - that is, when cylindrical LiFePO4 cells are assembled in a battery pack it takes about the same amount of space as a lead acid battery of the same capacity - but they weigh much less.  It's also worth remembering that conventional LiIon packs will typically last 3-5 years from the date of manufacture and thus didn't have much longevity advantage in that respect over SLAs.

So, over the period of several months, I ordered three of these 6.2 amp-hour LiFePO4 packs that put out about 13-ish volts over their discharge cycle - slightly higher than SLAs, but still well within the realm of what typical "12 volt" gear will accommodate.

As I typically do with newly-acquired batteries I checked the amp-hour capacity of each of the three battery packs shortly after arrival using my West Mountain Radio Computerized Battery Analyzer at 700 milliamps and found that they were reasonably close to the advertised capacity - that is, around 5.8 amp hours:  Typically such batteries are rated at the "20 hour" rate which would have been about 310 milliamps and the higher rate that I used would reduce the measurement by 10-20% so I was pleased with the results.

At about the same time I acquired some 2 year-old 12 volt, 7 amp-hour lead-acid batteries that had been pulled from UPS service on a routine basis and these were found to have about 6.2-6.5 amp-hour capacity at the same 700 milliamp rate.

In the intervening years I used these batteries (both LiFePO4 and SLA) about equally, running radio equipment and the like and earlier this year I suddenly realized that something was amiss:  The LiFePO4 packs were dropping out far earlier than they should have.

A bit of explanation here:

All rechargeable lithium-ion packs (should!) have built-in circuitry to protect against excess over-discharge, the reason being that if you run a lithium battery down too far an irreversible chemical change occurs and they cannot be safely recharged ever again.  For this reason when a lithium pack runs down too far it will suddenly drop off, the internal circuit disconnecting the battery to protect it.

Lead Acid packs, on the other hand, do not do this:  Their voltage slowly drops down and their effective internal resistance goes up and one eventually realizes that the equipment being powered is no longer working correctly.  (Note:  This ignores longer-term permanent damage from sulfation that will occur if a lead-acid cell remains discharged for a long time.)

As it turns out both Lithium-Ion and Lead-Acid packs are charged in similar ways.  One simply connects a power supply of voltage appropriate for the type of battery pack and let it charge.  Both types of batteries, when discharged, will pull more charge current but this will gradually drop off as the battery approaches full charge and for this reason it's typical for these power supplies to be current-limited as well as be fixed voltage.

A major difference between how one treats Lithium-Ion (including LiFePO4) and Lead-Acid (SLA) batteries appears at the point of full charge:

• For SLAs one obtains the best lifetime by continuously maintaining them at a constant voltage - typically 13.5-13.8 volts for a "12 volt" lead acid battery

• Lithium types should not be maintained at the "full charge" voltage after full charge has been achieved.

What happens with Lithium-Ion batteries (including LiFePO4) is that if you maintain the "full charge" voltage its internal chemistry degrades much more rapidly than if you were to fully-charge the battery and then immediately disconnect the source, allowing the voltage to sink down a bit on its own.

What this means is that you will get much better longevity out of a Lithium pack if you do not keep a high-level float charge on it.  In fact, the best longevity of Lithium-type rechargeable batteries can be obtained if you store them in a half-discharged state - provided that you check once in a while to verify that their self-discharge hasn't caused their voltage to go so low that they become damaged from that!

* * *

That is how I treated the LiFePO4 battieries:  I would attach the pack to a 1-amp, regulated 14.2 volt 1.5 amp power supply for 12-18 hours and then disconnect it and then place it on the shelf, possibly topping it off briefly just before using it.  The Lead-Acid batteries, on the other hand, are left connected to a 13.6 volt power supply and allowed to sit there all of the time when not being used.

I was, therefore, chagrined when after just two years the now 4 year-old SLAs were outlasting my LiFePO4 packs.

This observation spawned some further testing, so I put the LiFePO4 packs back on my battery tester I was further distressed to note that those that had originally tested out as having 5.8-6 amp hour capacity were now, at the very most, in the 1.5-2 amp-hour range while the much older SLAs were still in the 5.0+ amp-hour range.

Comment:

 In the time since I did the testing for this entry, the LiFePO4 packs have continued to degrade at about the same, alarming rate while the old Lead-Acid cells are still holding in, degrading much more slowly.

Hmmm...

So, what's the deal?  Why are the 4+ year old SLAs still in better shape than the 2 year old LiFePO4 packs?

I really don't know.  I've attempted to correspond with the sellers of the LiFePO4 batteries (batteryspace.com) to find out their "take" on this observation, but I've not heard back from them - too bad since I've had reasonable luck with their customer service in the past...

Perhaps they got a batch of "bad" cells - but since the three LiFePO4 packs were actually purchased several months apart it would seem to me that it's more a problem with manufacture/chemistry of the cells themselves. 

What to do?

At the moment I'm sticking with the old, heavy SLAs since I'm now understandably "gun shy" when it comes to LiFePO4s since the former do seem to be fairly predictable in their longevity and performance - at least when treated properly!

Update:

For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - link

Update on battery longevity (June, 2016):

I recently re-tested the three batteries depicted above and found that their capacity ranged between 4.8 and 5.4 amps-hours - this for batteries that were at least six years old.  Based on their capacity when they were new, they have lost somewhere around 20% of their original capacity in that time.

While I'm a bit skeptical that they will make it to the 10 or 20 year mark, it is worth noting that practically any lead-acid battery of this same age would have since been relegated to the recycler!

[End]

This page stolen from ka7oei.blogspot.com

A more practical capacitor-Powered Flashlight

In an earlier post, "A mechanically-powered capacitor flashlight" I wrote about those cheap LED-based "shake-powered" flashlights that were seen on many an annoying commercial several years ago.

You might recall that their promise was that they would never need batteries and one simply shook them back-and-forth to generate all the power that was needed.  In that same post I also noted that many of these same flashlights actually did contain batteries and that while they still worked if those batteries were removed, it took several minutes of shaking to get any usable light and that it was quite an effort to maintain a useful light output!

At the end of this article, I mentioned a few things that might make such a light more practical and useful, including:

• A better capacitor.  The cheap flashlight had a rather small (0.22 Farad) capacitor for energy storage - not very much energy, really, approximately 6.6 Joules maximum or less than 1/1000th of what a single AA alkaline cell contains!  Being a standard "super cap" its internal resistance was quite high (10's of ohms) which meant that a large percentage of the energy dumped into it during charging and that extracted from it to run the LED was lost as heat - not much heat, but heat just the same.

• A switching converter to run the LED.  The LED didn't even begin to light until 2.7-3.0 volts or so appeared across the capacitor and it isn't usefully bright until there is 3.6-4.2 volts available which meant that a significant portion of the energy in the capacitor (all of that at voltages of 3-ish volts and below) was unusable.  A simple switching converter would allow both extraction of that additional energy as well as regulate the LED's current so that its brightness was more consistent over the entire charge range and, in theory, could also be adjusted upwards or downwards as necessary.  The efficacy of trying this with a capacitor of high internal resistance would probably be dubious...

One of the conclusions in this earlier article was that the back-and-forth shaking motion wasn't a very efficient means of generating electricity - both in terms of expended muscle energy (since you have to move and stop the entire mass of your arm!) and compared to a conventional crank-type generator - and it would necessarily be larger and heavier in order to be more efficient.  By using a conventional spinning generator and gearing up rotational speed, one can more-efficiently rotate a smaller magnet faster amongst a larger number of poles with a motion that requires less human effort.  What's more, a crank-type generator is quite "scalable" in its input:  You could crank it fairly gently for a long time or do so vigorously for a shorter time and get roughly comparable results in terms of total energy output - within reason, of course.

Figure 1:
The prototype capacitor-based flashlight using a Maxwell Energy
2600 Farad, 2.5 volt "Boostcap".
Click on the image for a larger version.

What is more likely in most situations is that one actually has a source of power somewhere (an already-charged battery, solar panels, a plug-in power supply, etc.) that can be used to charge the flashlight and that it's unnecessary to actually bring along the means of charging the battery with you.

Such devices are already available in the form of batteries, particularly rechargeables, so having a capacitor-powered rechargable flashlight is more of an intellectual exercise rather than one of practicality, but being practical has not always been much of a deterrent to the experimenting nerd!

Some time ago The Electronic Goldmine in Arizona had a large quantity of  Maxwell BCAP0010 BoostCaps ^tm* available. These were obtained for just $6 each had a rated capacity of 2600 Farads (yes, that's 2.6 kF or kiloFarads!) at 2.5 volts with a "surge voltage" of 2.8 volts - whatever that means...

Comment:  I noted that at other times they had models that were rated at around 3 kiloFarads at 2.7 volts, but these were sold for far more than $6 each.  Alas, as is the nature of surplus, the supply was limited and they sold out fairly quickly.  Sometimes these types of capacitors will show up elsewhere on the surplus market so if you want some, it would pay to look around!

Compared with the 0.22F capacitor in the original flashlight, these units have 10000+ times larger capacity (albeit lower voltage) and very low internal resistance - in the milliohm area - as their intended use was to provide a large burst of current for a short time, say, on an electric vehicle.

To demonstrate, I charged one of these capacitors to 2.5 volts and then I carefully shorted out the terminals with a length of #14 AWG bare copper wire, holding it in pliers.  Within a second or so the current from the capacitor had burned this wire open and in so-doing, it only lost about 0.1-0.15 volts!  For these particular capacitors the maximum rated current is on the order of 600 amps so I have no doubt that I could have repeated the same trick (not recommended!) with larger gauge wire!

What this means is that resistive losses of this type of capacitor (e.g. a "Boostcap") are negligible when it comes to its being charged by a power source and then being discharged by an LED.  As an example, let's assume that we need to draw 100 milliamps to run our hypothetical LED circuit from two different types of capacitors:

• A standard "supercap" with an internal resistance of 10 ohms - an nice, round value, typical of these types of capacitor.

• A "boostcap" power system with an internal resistance of 100 milliohms - that value being mostly that of thin wires connecting to the capacitor:  The capacitor itself would likely have an internal resistance a fraction of this!

If we take the formula:  P = I^2 R (that is, power equals the square of the current multiplied by the resistance) with the resistance values above and assuming an LED current of 100 milliamps - and ignoring other losses we get:

• A loss of 100 milliwatts from a standard super cap.

• A loss of 1 milliwatt from the "boostcap" and its connecting wires.

Now, if that LED were running from, say, 2 volts at 100 milliamps, the total LED power in each case would be 200 milliwatts - but you can see that the super cap would be losing 100 milliwatts of that in heat while the boost cap would be losing just 1 milliwatt - a considerable difference!  (This assumes that we are somehow ignoring the power loss of the resistance when we are running our LED...)

Clearly, the use of a boostcap offers superior efficiency when discharging, but it also works in reverse:  One could dump many amps into the capacitor (if you used thicker connecting wire) and charge it very quickly and efficiently.

We still have the problem of running the LED, however.  The boostcap capacitors that I obtained were designed to be charged to just 2.5 volts or so and this is too low to run a standard white LED, which needs 3.6-4.2 volts just to light up brightly, so an electronic boost circuit is required and this was accomplished using a variation of the ubiquitous "Joule Thief" circuit:

Figure 2:
Schematic of the flashlight.  This diagram includes a "blocking oscillator" (a.k.a. "Joule Thief") and a current sensing circuit.
See the text for recommendations on transistors to use for Q1.
Click on the image for a larger version.

Important Note:

• This discussion assumes that one is using an LED with a 3.6 volt threshold as is typical for most white and blue LEDs.  LEDs with lower voltages (e.g. typical red or yellow that operate in the 1.6-2.5 volt region) can't be used with this circuit because their operational voltage would be below that of the full-charge voltage of the capacitor and would be immediately destroyed by the current, from the capacitor, through T1.

While there are more efficient circuits out there, there are almost none that are simpler than the Joule Thief and adaptable to parts that might be found in scrounging around the junk box.

What I came up with is the circuit in the diagram.  At it's heart (Q1, T1, R1, LED1)  it is the Joule Thief circuit comprising a "Blocking Oscillator (link)" that, using inductive "kick" from T1, will produce a voltage higher than that of the power supply (our capacitor), sufficient to light the LED.

While the simplest version of the circuit using the aforementioned components did work, it was very bright at the higher capacitor voltage (above, say, 1.8 volts) but it got noticeably dimmer - but still useful - at lower voltages.  Since the intent was to provide a "useful" amount of light I decided that I didn't need "maximum brightness" at the higher voltages and that I'd be happy with a much dimmer - but consistent - brightness at a much wider range of capacitor voltages.  This also had the obvious and beneficial side-effect of allowing a much longer run-time since, overall, the power consumption was reduced to a fairly steady level over the entire voltage range.

To regulate the LED current a simple circuit was added consisting of T2, D1, R2, R3, C2 and Q2.  The way this circuit works is that the AC current through the LED goes through the primary of T2 and is then integrated by D1, R3 and C2 and if this resulting voltage is too high (correlating with higher average LED current) Q2 would conduct, "pinching" off the drive to Q1.

Originally, a circuit consisting simply of a series resistor along with a transistor like Q2 was tried in which the current through the resistor - if it exceeded the 0.6 volts required to turn on the transistor - would be used to turn off the oscillator and regulate it, but this added resistor required that a bit of the LED's current to be lost as heat through it - plus, it just didn't work very well!

Using a simple transformer arrangement to "transduce" the current into voltage reduced the efficiency losses that occurred with a series resistor while still being fairly simple.  Being simple also meant that there was still a fair amount (say, 25% or so) of LED brightness variation between the target 1.1-2.5 volt range, but that was considered to be acceptable for a simple circuit.  This circuit is also somewhat affected by temperature owing to the fact that not only do the various current gains of the transistors change, but so do the threshold voltage of the transistors and D1.

In this circuit there's really only one critical component and that's Q1, an NPN transistor that was specifically designed for use in photoflash inverters and as such it can switch several amps of current with low collector-emitter drop, this rating being several times that of the more ubiquitous 2N3904 or equivalent.  While a standard NPN like the '3904 will work, it will not work very as well and will be much less efficient.  The KSD5041 may be bought from Mouser Electronics, substituted with a 2SC695, an NTE11 or maybe even found on the flash board of a discarded disposable camera.

An even better alternative for Q1 was suggested by Brooke Clarke (a link to one of his web pages analyzing the Joule Thief may be found here) and that is the Zetex ZTX1048A, available through Mouser and Digi-Key for approximately $1 each in small quantities.  This device - like the KSD5041 and 2SC695 - offer increased efficiency by virtue of its very low collector-emitter saturation voltage - an important consideration when one has conflicting needs of both high current and low voltage in a circuit such as this and according to the specification sheets, the '1048 offers the possibility of even lower saturation voltage than the '5041!

Figure 3:
The capacitor flashlight's circuitry.
Click on the image for a larger version.

The two inductors were toroids salvaged from a defunct computer power supply - and even some of the original wire was salvaged!  In this particular power supply - and several others that I have seen - it's common to see several different-sized toroidal inductors and I happened to choose the larger one for T1.

The circuit itself was built "dead bug" - that is, components were hanging in free space, soldered to each other's leads with the entire assembly being "potted" in thermoset ("hot-melt") glue to stabilize the components to prevent shorting and lead breakage.  As can be seen from the pictures a small piece of PVC pipe was used to not only contain the circuit, but also to shield the positive terminal of the capacitor so that it was not possible to accidentally short it out - something that could conceivably start a fire!

The LED itself is a 3-watt Luxeon III Star that I had kicking around but it's not being run at anywhere near its maximum ratings so about any 1-3 watt white LED that you might find would suffice.  While it's not running a watt of power, the converter probably produces too much output for a single, epoxy-cased white LED, but 3-6 identical units in parallel would probably have be fine with the added benefit that they could be aimed so that their built-in lenses could be used to advantage to shape the resulting beam of light.

Originally, I considered putting a lens on the single LED to concentrate the light but I soon realized that without using a special lens designed specifically for this LED I'd end up with less light overall due to optical inefficiencies.  Even with the LED being "bare" its light output is more than enough to be useful, even walking along a mountain trail in the dark, and its beam is broadly cast so that one isn't as subject to the "spotlight effect" of some LED flashlights where you can see only that which is directly in the beam while the surroundings disappear!

To charge the flashlight I set a variable-voltage bench supply at exactly 2.60 volts and then applied it to the connector (not visible in the pictures) which is wired directly across the capacitor.  From a state of complete discharge (0 volts) it will take several hours for a 1 amp bench supply to fully-charge the capacitor!  Whatever you do, do not allow the capacitor's voltage to exceed its maximum ratings or else it may be damaged:  I have no idea what actually happens if you do this, but I wouldn't recommend trying!

It's worth mentioning at this point that my charging method is extremely inefficient since, when using a linear supply, most of the power input would be lost as heat!  A far more efficient (and somewhat more complex) method would be to use a switching converter to provide the capacitor charge current and have its maximum voltage set to 2.60 volts and this would be much preferred in a power-limited situation where one had only battery or solar as the energy source.

* * *

Update - As of the time of this posting (August, 2013) I've used this flashlight for more than a year, now (since August, 2011) - both around the house and at night while hiking in the mountains and in that time I have only charged it once - and it's still going strong.

Additional Update - As of this update (January, 2017) there is still enough remnant of the original charge on the capacitor to power the light to reasonable brightness.  For most of the years this device has been sitting on a shelf, having been used for a while during an extended power failure to find another flashlight.

* * *

While it may sound like this capacitor can store a reasonable amount of energy storage (and it can!) it's worth noting that the total amount of energy stored in one of these capacitors when it is fully-charged (approximately 8200 joules) is in the same ballpark as the amount of energy contained in a single fresh AA alkaline cell!  Anyone who has actually used a reasonably efficient AA-cell powered LED flashlight knows that it's perfectly capable of providing 10's of hours of useful light, so the duration of the single charge thusfar shouldn't be too surprising.  Just for fun, I dug up some typical numbers:

• For an AA Alkaline cell, given an average of about 1.25 volts and a usable capacity of 2.2 amp/hours at that voltage, this correlates with a energy storage capacity of 9000-9500 joules, depending on load, temperature, end-of-charge voltage, etc.

• These calculations ignore the fact that some of the energy being stored in the capacitor or battery at low voltage is not usable as the LED's converter circuit will not operate below approximately 0.9 volts and be able to extract energy.  This is arguably a greater factor with the capacitor because by the time an alkaline battery drops below 0.9 volt, it has almost no residual energy (only a few percent, at most) while 10-12% of the original energy remains in the capacitor.

These numbers are a bit misleading since not all of that energy is usable with equal efficiency in each case over the entire voltage/charge range, but it gives a general idea as to the magnitude.

So, does this flashlight actually work?  Yes, it does!

Is this flashlight really practical?  No, not really.

As it turns out the capacitor itself is not only fairly heavy - about 525g (1 pound) - but it is also quite large - 60mm (2-3/8") diameter and 172mm (6-3/4") long - not including the circuity or bolts:  I have fairly large hands and I find myself moving the flashlight from one to the other as I hike along owing to a bit of muscle fatigue from its diameter and weight.  Again, the capacitor itself was $6 from a surplus seller but that was just a fraction of its original cost (perhaps $150-$200) and one could buy an awful lot of AA cells for its original price!

The main advantage of the capacitor is that unlike a battery, it really doesn't have a fixed number of cycles that it will last before wearing out.  Another advantage is that by knowing its voltage, one can precisely gauge how much useful power remains - a tricky proposition with batteries, especially considering that over time, they lose capacity as they age to a degree that isn't easily determined ahead of time.

I suppose that as time goes on capacitor technology will improve and eventually the power/size/weight will approach (and even surpass!) that of conventional battery technology, but until then a flashlight such as this is a bit of a nerdy novelty!

* "BoostCap" is a trademark of Maxwell Technologies 

[End]

This page stolen from ka7oei.blogspot.com