Tuesday, October 17, 2017

A 10 MHz OCXO (Oven-controlled Crystal Oscillator)

Figure 1:
The 10 MHz OCXO (lower right) in use with my homebrew
24 GHz transverter.  At 24 GHz, the oven provides excellent frequency
stability, suitable for SSB or even digital modes, while providing a
frequency uncertainty of a few hundred Hz at most.
Click on the image for a larger version.
Why a frequency reference?

When operating on the microwave amateur radio bands, narrowband modes (such as SSB or CW) are often used to maximize the link margin - that is, to be able to talk when signals are weak - and when we use microwave frequencies and narrowband modes such as SSB or CW one must maintain pretty good frequency stability and accuracy:

  • Stability is important as a drift of even a few hundred Hz at the operating frequency (in the GHz range!) can affect intelligibility of voice - or, if CW is being used for weak-signal work, such drifting can move the received signal outside the receiver's passband filter!  Having to "chase" the frequency around is not only distracting, but it complicates being able to communicate in the first place.
  • Accuracy is also important because it is important that both parties be confident that their operating frequencies are reasonably close.  If a contact is arranged beforehand it is vital that both parties be able to find each other simply by knowing the intended frequency of communication and as long as the two parties are within several hundred Hz of each other it is likely that they will be able to find each other if the path "works" in the first place.  If the error was on the order of several kHz, "hunting" would be required to find the signal and if those signals are weak, they may be missed entirely.
Because achieving such stability and accuracy requires some effort, it is more convenient if our gear is constructed such that it can use a common, external frequency reference and lock to it.  In that way, we need only have one "master" reference rather than several individual references.

Figure 2:
The 10 MHz Isotemp 134-10 OCXO - one of many similar units that
often show up on EvilBay.  A 200uF, 16 volt capacitor is soldered
directly to the supply terminals of the OCXO to provide low-impedance
filtering of any noise that might appear on it - any value from 2000 and
up (to several thousand uF) would be just fine.  The green device is a 10-turn
trimmer potentiometer soldered directly to the OCXO's pins.  This
potentiometer is used to adjust the tuning voltage to precisely set the
frequency and locating it at the OCXO practically eliminates the possibility
of external noise pick-up on the tuning lines and the possibility of the I*R
drop on the wires causing a slight tuning shift as the oven power changes.
The OCXO is mounted in the case using rubber/metal shock mounts with "blobs"
of RTV (silicone) on the sides that prevent it from hitting the inside of the box
should the unit be accidentally dropped.
The corners/edges of the OCXO could be mounted in some stiff foam,
instead - but it should not be thermally insulated by this foam unless you have
demonstrated to yourself that doing so will not reduce the oven's stability.
Click on the image for a larger version.
Having one common frequency reference can also be convenient if one is operating portable using battery power since it can mean that one doesn't need to keep all of those individual pieces of gear "warmed up" all of the time to maintain stability.  If a particular piece of gear can accept an external 10 MHz input, this would allow one to turn on that gear (and drain battery power) only when it is needed.

At this point I might mention that Rubidium frequency references (such as one described here) are also readily available in the surplus market as well that provide at least an order or magnitude greater accuracy and stability and warm up in less time than the crystal reference, so why not always use a Rubidium reference instead of a crystal-based one?  The crystal-based unit is cheaper, easier to package and consume significantly less power than a Rubidium reference, and the stability/accuracy of a good-quality crystal-based reference is more than "good enough" through at least 24 GHz.  When I go out in the field to do portable microwave work I'll often power up the OCXO after putting it in the car knowing that by the time that I get to my destination and set up, it will be warm and on-frequency.  (To be sure, I bring the Rubidium as a "backup" reference!)

About this frequency reference:

The oscillator:

The goal for this project was to have a "reasonably stable and accurate" reference:  Based on an Isotemp OCXO 134-10 this particular unit has a rated stability of about +/-1.0x10-8 or better after it has warmed up for a while with short term variations approaching +/-1.0x10-10. In-field observations appear to confirm this stability, having shown that this unit seems to be able to hold the 24 GHz local oscillator to within 500 Hz or better once it has had 15-20 minutes or so to warm up -  and it seems to be fairly stable across a range ambient temperatures from "hot" to "below freezing."  The Isotemp unit - and others like it - are readily available on both the new and surplus markets, available via EvilBay and similar.

The oven module itself is rated to operate from 13 volts, +/- 2 volts, implying a minimum of 11.0 volts.  Even though testing indicated that it seemed to be "happy" with a supply voltage as low as 9.8 volts or so, it was decided to adhere to the published specifications and in looking around I noticed that most readily-available low-dropout regulators (and those that I had onhand) were not specified to handle the maximum "cold" current of this oven - about 800 mA or so - so I had to "roll my own" 11 volt "zero-dropout" regulator.  More on alternative regulators, below.
Figure 3:
The inside of the enclosure containing the OCXO, regulator and driver.
On the left is the shock-mounted OCXO while the circuit on the perfboard
is the "zero drop-out" regulator and the 10 MHz distribution amplifier.
The P-channel FET pass transistor can be seen along the top edge of
the die-cast enclosure, bolted to it to dissipate any heat while along
the right edge, inside the enclosure is a piece of glass-epoxy circuit
board material to provide a solid, solderable ground plane for the
distribution outputs and the DC input filtering.

A "zero-dropout" regulator:

Why regulate?  I noted in testing that slight variations of supply voltage (a few hundred millivolts) would cause measurable disturbances in the oscillator frequency due to the changes of the power applied to the heater, taking several minutes to again reach (thermal?) equilibrium.  Since battery operation was anticipated it is expected that the supply voltage would change frequently - between periods of transmit and receive - as well as due to normal battery discharge.

Referring to the schematic, U101 - a standard 5 volt regulator (the lower-power 78L05 is a good choice) provides a stable voltage reference for U103, a 741 op amp, which is used as an error amplifier.  A 7805 was chosen as it is readily-available but a Zener diode and resistor could have been chosen:  If a Zener is used, a 5.6-6.2 volt unit is recommended with 2-5 milliamps of bias as this voltage range offers good temperature stability.

If the output voltage is too low, the voltage on pin 3 drops, along with the pin 6, the op amp's output which turns on Q103, a P-Channel power MOSFET, which increases the voltage and once the voltage on the wiper of R119 reaches 5 volts - that of the reference - the circuit comes to equilibrium.  A P-Channel FET (a slightly less-common device than an N-channel) was used because it takes 3-5 volts of drain-gate voltage to turn on a FET and it would have been necessary to have at least 16 volt supply to bias the gate if an N-Channel FET were used.  Furthermore, with the use of a P-Channel power MOSFET the dropout voltage of the regulator is essentially limited to the channel resistance of the that FET.  In theory a PNP (possibly a complimentary pair arrangement) could be used instead if one can tolerate closer to a volt of dropout, but the FET was chosen to minimize the dropout voltage.

In testing, once the oven was warm (a condition in which the OCXO was drawing approximately 250 mA at normal "room temperature") the dropout of the regulator was approximately 50 millivolts - a voltage drop that is likely to be comparable that of the resistance of the wires used to power the unit.  This rather simple regulator seems to work quite well, holding the output voltage steady to within a few millivolts over the input voltage range of 11.1 to 17 volts with good transient response.
Figure 4:
The end panel of the OCXO module.  The power feedthrough/capacitor
is on the left, obscured by the red/white power cable with the yellow-ish
"ready" light to the right of it.  The three BNC connectors are the 10 MHz
outputs, allowing multiple devices to be connected while in use and/or while
its calibration is being checked.
Click on the image for a larger version.

"Faster warmup" feature:

This OCXO has a "status" output that, when "cold", outputs about 0 volts and in this state, Q101 is turned off, allowing R112 and R113/D102 to pull its collector high - turning on Q102 - which pulls the gate of Q103 low through R118, turning it fully "on."  In this state the voltage applied to the oven is nearly that of the battery supply and this higher voltage increases the power applied to the oven, allowing it to heat more quickly.  Once the oven's "status" line goes high, Q101 is turned on, illuminating the LED and turning off Q102, allowing the regulator to operate normally.

Note:  When the unit is warming up, the OCXO's voltage is unregulated which means that the supply should be kept below 15.0 volts to stay within the "safe zone" of the ratings of the oscillator itself.

Does the "boosted" voltage actually help the oven warm up faster?  Probably only a little bit, but it took only 4 additional components to add this feature!

Status indicator:

It should be noted that this status line doesn't indicate that the oven has fully warmed up, but that it's still warming:  At "room temperature" it takes at least another 5 minutes before the frequency will be stable enough for use and another 5 minutes or so after that until it's "pretty close" to the intended frequency and it can be used at microwave frequencies without having to chase people around.

Why have the indicator light if it doesn't indicate that the unit is actually  "ready"?   While this indication isn't perfect if the light isn't on, you can be sure that the frequency output won't be valid for one reason or another.

Because the OCXO itself is somewhat load-sensitive (about +/-1.0x10-9 - perhaps a few 10s of Hz at 24 GHz) U102 - an LM7171 - is used as a distribution amplifier to both isolate the oven from its loads and to provide fan-out to allow multiple outputs to be driven simultaneously.  The LM7171, a high-output, high-speed op amp, is configured for a gain of 2, providing about 2 volts peak-to-peak output with the drive provided by the OCXO.

Mounting the oven:

Because this unit is intended to be used "in the field" it was decided to mount the OCXO module itself to prevent mechanical shock from affecting the reliability, frequency stability and accuracy and this was done using some rubberized mounting pillars from scrapped satellite equipment while some "blobs" of silicone were placed on the wall of the die-cast enclosure to prevent the OCXO housing itself from directly impacting it should the unit be accidentally dropped.

Figure 5:
Schematic of the OCXO-based unit, including the zero-dropout regulator and 10 MHz distribution amplifier.  It is important that the connection of the "ground" side of the 10 turn calibration potentiometer be made at the OCXO and not elsewhere, this to minimize possible frequency shifts due to I*R losses as the oven's heater power changes.
Click on the image for a larger version.
A few bits of stiff foam could also be used to provide some shock mounting in the corners of the OCXO but be aware that some oven-based oscillators have been known to become less accurate and stable if they are over-insulated and can't radiate at least some of their heat, so don't go overboard.


Like any crystal oscillator, it is somewhat "position sensitive" in that a frequency shift of a hundred Hz or so (at 24 GHz) can be observed if the unit is placed on its side, upside-down, etc. due to the effect of gravity on the quartz crystal itself.  While this effect is very minor, it's worth noting when it's being set to frequency and in operation.

In other words, when you calibrate it (see below) do so in the same physical orientation that it will be when it is in use.

DC input protection and filtering:

The input supply is RF-bypassed using a feedthrough capacitor to prevent the ingress or egress of extraneous RF along the power lead.   For power-supply short-circuit and reverse-polarity protection, R101, a 1.1 amp, self-resetting PTC fuse is used in conjunction with D101, a 3-amp diode.

Why not use a forward-biased diode for reverse-polarity protection?  If you recall, we are going through the trouble of minimizing voltage drop-out with our "special" voltage regulator and we could diminish this if we inserted something that caused a voltage drop - even the 0.3-ish volts of a Shottky diode would undermine this effort.

By using the "reverse-biased diode" and the self-resetting PTC fuse we get:
  • A means of current limiting should something to wrong:  If we accidentally short something out, the fuse resets itself when the fault is cleared - and no need to worry about not having a spare fuse when one is out in the hinterland trying to operate!
  • If the polarity is somehow connected backwards, the diode will conduct and the PTC fuse will "open" - no harm done, once the fault is rectified.
  • There is minimal voltage drop related to the fuse as its resistance is a fraction of an Ohm under normal conditions which means that we won't compromise the voltage "headroom" of a 12-volt lead-acid battery.

The best way to calibrate this device is to use a GPS disciplined oscillator or a known-good rubidium frequency reference.  If you have access to one of these, connect the output of the OCXO to one channel of a dual-trace oscilloscope and the frequency reference to the other, triggering on one of two signals - it really doesn't matter which one.

Note:  If you have an analog dual-trace oscilloscope with sufficient bandwidth you can use the "X/Y" mode to produce a Lissajous pattern (obligatory Wikipedia reference here) - but this doesn't always work well on modern, digital scopes when high frequencies are involved due to sample aliasing.

Adjusting the 'scope to see one of the waveforms, one should see a stationary wave (the one on which the 'scope is triggered) while the other will be "sliding" past the first.  Adjust the OCXO's frequency (after the OCXO has warmed up for at least 30 minutes - preferably more) while it is sitting in the same physical orientation in which it will be used as this can (slightly) affect frequency.  It is suggested that the cover of the enclosure containing this circuitry be left on except during the brief periods to access the 10-turn potentiometer unless provisions are made to access it (via a hole) from outside the box.

The OCXO's frequency is then adjusted to minimize the rate at which the two sine waves are moving with respect to each other:  It's sometimes easier to make this adjustment if the 'scope is adjusted to that the two waves are atop each other and about the same size.  With careful adjustment it should be possible to get two waveforms that take more than 10 seconds to "slide" past each other - maybe longer.  The Isotemp OCXO noted should, in theory, be able to hold to that "10 second" slide rate over a wide variety of temperature conditions.

If you don't happen to have access to a rubidium reference or a GPS Disciplined oscillator, you can do "reasonably" well by zero-beating the 10 MHz output with the signal from WWV or WWVH, be note that Doppler shifts can cause their apparent frequencies to shift by 1 Hz or more.  I'll leave the explanation of methods of successfully zero-beating an off-air signal to others on the GoogleWeb.

The best time to attempt this is when you are hearing only one of these two stations (assuming that you can ever hear them both) and when it's signal is the most "solid" - that is, it's fading in and out is at minimum.  Often, the worst time to make this sort of measurement is when any part of the radio path between you and WWV (or WWVH) is within a hour or two of sunrise or sunset as this is when the ionospheric layers are in a state of flux.  If you are hearing both WWV and WWVH, don't try this as the two frequencies and signal strength will not likely be consistent and the results will probably be confusing.

If you don't happen to live in an area where you have a reasonable signal from WWV or WWVH then I'd suggest you ask around to find someone who has appropriate gear to help with this task.

Comments about alternative schemes for low-dropout regulation for the OCXO:

There are a number of "low-dropout" adjustable regulator ICs on the market that may be suitable for your this project - but there are a few caveats.

For example, there is the Linear Technologies LT1086-Adj which is rated for up to 1.5 amps of current.  While much lower dropout than a conventional adjustable regulator such as an LM317, it does have approximately 1 volt of dropout which means that if you set the OCXO's supply voltage to 11.0 volts - the minimum recommended in the OCXO's specification - your battery voltage must be at least 12.0 volts:  While this represents a lead-acid battery that mostly depleted it is likely that a small, but healthy, lead acid could drop to such a voltage under transmit load - particularly if the resistance of power leads is taken into account.  This 3-terminal regulator is used in a manner very similar to the LM317 - except that you really must have some good quality, low-ESR capacitors (probably tantalum) very close to the regulator itself - see the data sheet.

Also made by Linear Technologies is the LT1528 that is rated for up to 3 amps that has a (nominal) 0.6 volts of dropout - more typically in the 0.3 to 0.5 volt area for the amount of current consumed by the OCXO, particularly once it has warmed up:  This extra margin would keep one in the "safe" region of the OCXO's operating voltage range down to around 11.5 volts allowing both "deeper" battery discharge and more voltage drop on connecting wires.  This part is somewhat more complicated to use than the LT1086, above, but it is, overall, simpler than the op-amp based regulator described earlier in this page.

If the "fast warmup" were to be implemented on either of the above regulators it would take a different form than the above - likely using several resistors and a transistor or two to "switch" the resistor-programmed voltage setting to something higher than the normal voltage.

There are a number of other, similar, low-dropout regulators that are made by different manufacturers, but very few have as low dropout (e.g. about 50 millivolts) as the simple FET/Op-amp circuit described on this page.

Additional comments:
  • It is recommended that one not use a switching regulator to power the OCXO unless it has been extremely well filtered and bypassed.  Unless such a regulator is a buck-boost type, it will probably have a higher drop-out voltage than even a standard low-dropout linear regulator.  Because of the rather low overhead voltage involved, there is not much loss in the linear regulator, anyway - only 10-15% or so with a 12.5 volt supply.
  • If you are interested in an example of this project being built with an etched PC board with surface-mount parts, visit VK4ABC's 10 MHz OCXO Web Page.

* * *

This is a revised version of one of my web pages, the original being found at http://www.ka7oei.com/10gig/10meg_oven_1.html


This page stolen from ka7oei.blogspot.com

 Note:  This post is partially an attempt to test means of reducing the "scraping" of content of this blog by sites such as "rssing", who seem to "swipe" content and "load" search engines' result with unwary readers NOT ending up at my page.     xe2XV6SJ9914C50H08S8  QY2IU7TU0C11c57804Q8

xe2XV6SJ9914C50H08S8 QY2IU7TU0C11c57804Q8

Thursday, October 5, 2017

Fixing my failed "Kill-A-Watt" meter - and a bit about capacitive dropper power supplies.

A few days ago I had the need to measure the load of an appliance, so I dug out my "Kill-A-Watt" power meter.  The purpose of this device is to measure not only the load of the appliance in watts, but it will also measure the line voltage, frequency, and provide a running total of consumed power over time in kilowatt-hours (kWh).  Usefully, this device will also measure things like power factor and volt-amps (Vars) - both things that can be useful in determining how much actual load something may be putting on your generator.  (For more info on power factor and volt-amps, read the Wikipedia article linked here.)
Figure 1:
147 volts on the power mains?  I don't think so!
Click on the image for a larger version.

I was both surprised and annoyed when I plugged in the unit and it informed me that the line voltage was about 147 volts so I grabbed a voltmeter and found it to be a more reasonable 123 volts - a typical voltage in the U.S. for the circuits powering lower-power household devices:  Something was wrong!  As I looked at the Kill-A-Watt's display I noticed something else:  It seemed to be flickering slightly - something that I'd not noticed it doing before, but it was also a clue as to what might be wrong.

This was clearly due to a power supply problem within the unit - but since power supplies in these sorts of devices are often very simple I figured that it would be pretty easy to fix and upon opening it up, I immediately recognized it as a typical "capacitive dropper".

A "capacitive dropper" power supply:

One of the simplest and cheapest ways to get a low current supply from mains voltage is to put a capacitor in series with it - and this is both smaller, lighter and less expensive than using a power transformer.  If you aren't familiar with the use of capacitor droppers, it may seem strange - but it can be quite effective and safe if done properly.  While a simple series resistance may seem more intuitive, it has the problem that it can generate quite a bit of heat.

Let's take a simple example.
Capacitor dropper supplies have, by their very nature, a very poor "power factor" - that is, the waveform of the voltage is not in phase with the current through them.
What this means is although our example may be pulling about (235 volts * 0.01 amps =) 2.35 watts,  because the voltage and current aren't following the sine wave at the same time, if you were to put a Kill-A-Watt on such a circuit, it would not read 2.35 watts - in fact, it may not read anything at all!

Why?  Remember that watts is "volts * amps" - but if the current and voltage are out of phase far enough, the voltage may be zero while the current is at maximum - and later in the sine wave's cycle, the voltage may be at maximum but the current may be zero - and in either case, the math tells us that that would be zero watts.

The Kill-A-Watt can also read "volt-amps" - which will correctly indicate the "volts * amps" - even when they don't happen at the same time.  If the "power factor" is 1.0, watts and volt-amps will be the same, but if it is something other than 1.0, the volt-amps will be higher than the watts.

Why do we care?  A generator or inverter can deliver only so many amps - and the "watts" rating that they have always assumes that the power factor is perfect.  If your generator load has a terrible power factor - say 0.5, that means that the "watts" reading is about half of the "volt-amps" reading - but since the amps is the same in either case it may appear that the generator cannot supply the power.  In other words, if your generator is trying to power, say, computers that have a terrible power factor, you may find that it will trip out at a lot lower wattage than you might expect!

Suppose that we need 10 milliamps at 5 volts for a hypothetical oven clock, ignoring the "converting to DC" part for the moment.  Doing the math we see that we need to drop from a nominal 230 volt mains (230 - 5 =) 225 volts - such as what might be found in the power supply for an electric oven.  With 225 volts to drop, Ohm's law tells us that we need (225 volts / 0.01 amps = ) 22500 ohms - so let's pick the closest standard value of 22000 ohms (a.k.a. 22k).

While we could use an 22k resistor for this, knowing the voltage drop (225 volts) and current (0.01 amps) we can see that we would be dissipating (225 volts * 0.01 amps = ) 2.25 watts.  While this doesn't sound like much power, containing this much heat in a very small box would cause it to get a bit warm - and it would heat up everything else as well, likely shortening the life of other components.

Alternatively, we can take into account the fact that our mains power will be a sine wave - 60 Hz in the U.S., 50 Hz in most other places - and use this to provide reactive current limiting - and by using the reactance of the capacitor, we can get the same voltage drop, but without any heat!  This lack of heat has to do with the fact that unlike a pure resistance, a pure reactance - like that of an ideal capacitor (or an ideal inductor) is theoretically loss-less.

An "inductive dropper" power supply is also possible - and in many instances, it would be preferable - but it is far easier and cheaper to make small, low-loss capacitors than low-loss inductors, so it is done in only very special circumstances.

Remembering that capacitors will block DC it makes sense that as the frequency increases, more current can flow through a given capacitance and this is calculated using a simple formula:

Z = 1 / (2 * Pi * Frequency * capacitance)

Z = reactance in ohms
Frequency is in Hertz
capacitance is in Farads
For our purposes we can simply consider the "reactance" to be equivalent to resistance - the clue being that like resistance, it's value is expressed in Ohms.

We can see from this formula that with frequency and capacitance in the denominator (bottom) of the fraction that if we increase either one, the equivalent resistance goes down in proportion.  Because from the math above we already know that we need 22k of equivalent resistance, we can rearrange the formula, swapping the locations of capacitance and "Z" to solve for capacitance, as in:

capacitance = 1 / (2 * Pi * Frequency * Z)

So, for 60 Hz:

1 / ( 2 * 3.14 * 60 Hz * 22000 ohm) = 0.00000012 Farads = 0.12 uF (microFarads)

And for 50 Hz:

1 / ( 2 * 3.14 * 50 Hz * 22000 ohm) = 0.00000014 Farads = 0.14 uF

Recalling that the lower the frequency, the higher the effective resistance and the lower the current so if we want to make this work for both 50 and 60 Hz systems, we'll pick the closest higher standard capacitor value for the lower frequency, 50 Hz, or 0.15 uF (a.k.a. 150nF) to make sure that we can get the minimum current that we need (0.01 amps) in either case.

At this point it's worth mentioning that neither the resistor or capacitor will actually reduce the voltage - it only limits the current:  If you want to reduce the voltage to something useful, some sort of regulator circuit is required - typically one that clamps it at or below the desired value.

A practical circuit for doing this is depicted in Figure 2, below:

Figure 2:
A typical "capacitive dropper" power supply.
This image is from the Wikipedia article
"Capacitive Power Supply" - link
This supply was designed for use with 220-240 volt mains as
described in the text.

Click on the image for a larger version.


While this may seem a bit complicated at first, it's easy to break down.

On the left we can see "C1" - the "dropping" capacitor that we calculated.  Across this capacitor is R2, a 470k ohm resistor and the purpose of this high-value resistor is to bleed off any charge that might be across the capacitor if it happened to be disconnected at the instant the the sine wave of the AC mains was at its peak, preventing it from shocking the user with that stored charge.  The resistor R1 is used to provide a bit of series current limiting:  If the power switch were closed at the instant that the sine wave of the mains was at its peak, there would be a sudden surge through capacitor C1 and R1 limits this amount to prevent damage to other components while its resistive value - 100 ohms - is quite low compared to the reactance of C1 so it produces little heat.
These capacitive dropper circuits, while simple, have a drawback:  They cannot and must not ever be used on circuits that can come in contact with anything that might be referenced to ground such as a body.  Because it is directly connected to the mains without the benefit of a transformer's isolation it is possible that if a person or animal touches any part of the circuit - or even touches something powered by it - it could result in a dangerous or fatal electrical shock!

The other caveat is that these circuits rely on the fact that a mains supply produces a pretty good sine wave.  If someone plugs this type of circuit into an AC power supply that is not a nice, clean sine wave - such as an inexpensive 12 volt power inverter - it may be fed with a square-type wave - sometimes called a "modified sine wave" - which, unlike a true sine wave, has a lot of energy at higher frequencies.  What this means is that the reactance at these higher frequencies will be lower and too much current will flow through the capactive dropper and the device that it is powering such as a night light - even a Kill-A-Watt - will probably be damaged/destroyed!

The rest of this circuit converts the AC to DC and limits the voltage to a reasonable value:  D1 is a full-wave DC rectifier and C2 filters the ripple to make it more pure.  Resistor R3 provides a bit of current limiting so that IC1, a programmable Zener diode (an ordinary Zener diode would have worked, too) that in combination with R4 and R5 can do the job of clamping the current to no more than 5 volts.

What was wrong with the Kill-A-Watt?

By now you have probably figured out where I was going with this discussion.

The display reading a high voltage indicated that it was possible that the computer within the Kill-A-Watt was getting too-low a voltage, causing its internal voltage reference to also be low which, in turn, caused its reading of the voltage to be too high.  The flickering display was also a clue indicating that there was AC mains ripple on its internal  power supply.  Carefully checking the voltage at the input of the onboard 5 volt regulator of the unit I saw that it was just 5.2 volts instead of the needed 7+ volts, with about 300 millivolts of ripple - far too low a voltage for the regulator to work properly and thus allowing ripple to get through, explaining why the display was flickering.

Figure 3:
The "bad" capacitor.  Although marked 0.47uF, it measured
0.31uF.  This is an "X1" type "safety capacitor, specifically
designed for this use.  Generally speaking, this could be
replaced with either an X1 or X2 type of appropriate voltage
Click on the image for a larger version.
Both of these pointed at the high likelihood of its "dropping capacitor" being bad, and upon opening it up I spotted the capacitor on the rear board.  Removing this capacitor, marked "0.47 uF" and checking it, I noted that it read 0.31uF - about 2/3rds of its original value.

On discovering this I knew that I would need another 0.47uF capacitor of the same voltage rating (e.g. 250 volts AC) and that it should be an "X1" or "X2" safety capacitor.  This last point is important as these types of capacitors are designed to fail open rather than short out - something that would certainly result in spectacular destruction of the device!  Its worth noting that for capacitive dropper power supplies, one should only use the appropriate safety-rated capacitors, such as an X1 or X2.  (Note, the original X1 capacitor was rated for somewhat higher peak voltage than the X2 with which it was replaced, but both are rated for this particular type of service at this voltage.)

How the capacitor failed:

This "safety" property is also a clue as to why its value reduced from 0.47 to 0.31uF.  One of the reasons why these capacitors can go bad is that their internal conductors, when subject to a fault - say a power surge, a significant spike, or just age - will fail open.

Often, this doesn't happen with the entire capacitor, but small portions of the thin, conductive metal film within the capacitor will degrade, the result being that the value of the capacitor will gradually drop.  In this case, as the capacitor failed, the Kill-A-Watt's circuitry was no longer able to get enough current, causing the voltage to be pulled below the threshold at which it would operate properly and resulting in erratic operation.
Figure 4:
The new capacitor on the board.  While the new capacitor was the
same height as the old - important for being able to fit in the case - it was
longer.  The circuit board has a series of holes on one side allowing
different-sized capacitors to be used.
Click on the image for a larger version.

I was able to rummage around in my box of film capacitors and find a 0.47uF film unit that was similarly rated and was a safety capacitor (with an "X2" rating).  Even though it was the same height as the old one (a "taller" capacitor would not have fit in the case) it was about 25% longer - but the designers of the circuit board had left a series of holes to accommodate several different sizes of capacitors.


With the new capacitor installed I plugged the unit in and it worked normally!

* * *

Many devices such as appliance timers, the electronic controls of stoves and ranges and nightlights uses these "capacitor dropper" supplies.  If you have one of these devices that has stopped working - or it has slowly "faded out" over time - there is a good chance that this is because the main dropper capacitor in its power supply has reduced in value.

If you do attempt a repair, extreme care must be taken if you test it as the entire circuit will be "lit up" by and not isolated from the power mains and posing a possible (fatal!) shock hazard.  If you do replace its capacitor it must not only be of the marked capacitance value of the original capacitor, but it must be an X1 or X2 type safety capacitor of equal or greater voltage rating!


This page stolen from ka7oei.blogspot.com