Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system

Figure 1:
Renogy 200 watt folding panel, in the sun
Click on the image for a larger version.

Update - 10/24: 

I recent spent most of a week in a national park where I extensively used the solar panel and charger - along with some LiFePO4 batteries - as my main power source.  The panel was set up with 3-6 feet (1-2 meters) of my portable HF antenna.  I did not notice any interference from this system at all.  I did notice a bit of QRM from my fridge/freezer cooler to which I have since added its own RF filtering.

A year or so ago I got a 200 watt foldable solar panel system.  This unit - made by Renogy - consists of two glass panels in metal frames equipped with a sort of "kickstand" assembly to allow it to be angled more favorably with the sun to improve its output.  I use this panel when "car camping" to charge the batteries to run the sorts of things that one might bring:  Lights, refrigerator, amateur radio transceivers and who knows what else.  

On that last point, I've done some "in the field" operating on the HF amateur bands while the battery is being charged and noticed that the charge controller (and not the panel itself!) produced a bit of "hash" on the radio - mostly in the form of frequent "birdies" that swished around in frequency as the solar insolation and temperature varied - as well as a general low-level noise at some frequencies.   

This problem is not specific to the Renogy panel's charge controller, but common to almost any panel+controller combination that you will find.

Nearly all "portable" and RV solar power systems cause QRM:

You will find similar systems built into RVs and campers and these are also well known (notorious, even!) for generating RFI.  The techniques described here to quiet interference from these devices applies equally to those as well - but note that one may have to "scale up" the inductors/capacitors to accommodate higher voltages and currents that may be found in those systems.

By placing the solar panel with charge controller and the battery being charged some distance away from the antenna, this interference could be reduced, but that fact that it was even there in the first place annoyed me, so I did what I have done many times before (see the links to other blog entries at the end of this article) and mitigated it by making fairly easy, reversible modifications to the panel's controller.

Portable solar panels and RFI In my travels, I've been around other users of portable solar panels of various brands and I have yet to find any commercially-available portable panel+controller combination that does NOT produce noticeable RFI on HF/VHF among the half-dozen or so brands that I have checked.  In comparison with most of the others that I've been around with radios, the Renogy is comparatively quiet - producing less overall QRM with fairly long wires between the panel/controller and battery - than the others - but I decided that I could make it even quieter!

Where does the QRM come from?

It is NOT the solar panel itself that produces the radio frequency noise, but rather the charge controller attached to it.

Modern charge controllers electronically convert the (usually higher) voltage from the solar panels down to something closer to the battery voltage and this is typically done using PWM (Pulse Width Modulation) which means that these devices contain high-power oscillators:  This is true for simple "PWM" types of charge controllers as well as those using "MPPT" techniques.  It's this oscillation / switching action that produces a myriad of harmonics that can extend through the HF spectrum - and even into VHF/UHF!

The "Antenna" in this case consists of two parts of the PV system as depicted in the drawing below:

Figure 2:
A typical solar charging system showing the separation of the two major components that can radiate interference:  The panel itself, connected to the input of the charge controller and the wires and load connected on the output side.
If there is even a slight amount of differential between the two at radio frequencies, the system will radiate.
Click on the image for a larger version.

 

In other words:

• The wires connecting to the load.  Typically a battery being charged - which can be connected to other things (e.g. vehicle, inverter, etc.)  The wires connecting the panel to these other things - and those devices themselves - act as part of the "antenna" that potentially radiates noise.
  

• The solar panel itself.  The solar panel consists of large plates of metal - not only the silicon of the panel, but any metal frame and wiring:  This large area of conductive material offers a suitably large aperture to permit radiation of HF RF.
  

The "load" and solar panel constitute two different parts of the charge controller's system when it comes to radiation of RF:  The panel is connected to the INPUT of the PWM circuitry while the wiring is connected to the OUTPUT of the PWM circuitry, effectively forming a dipole antenna.  To a degree, the electrical lengths of these two conductors - which can include power cords or even a vehicle - overall can broadly resonate, affecting certain frequency ranges more than others.

The reason for the generation of the interference is due to the fact that the PWM circuitry (which is operating at a frequency of 10s or 100s of kHz) uses square waves, rich in harmonics.  As the voltage input (from the panel) and the output (to the battery/load) are different parts of the PWM circuit, they necessarily have different waveforms on them.

Figure 3:
Charge controller with additional filtering showing added
bifilar-wound chokes/caps on both the input and output leads.
Click on the image for a larger version.

While this device does have some filtering to provide a degree of input impedance reduction (fairly high capacitance) and smoothing of the PWM waveform of the output (more capacitors and likely some inductance) the extent to which this filtering is implemented is suitable for the purpose of providing clean DC power to the load and maximize power conversion efficiency.  This filtering - and likely the controller's circuit board itself - was likely not intended to provide the high degree of RF suppression needed to make it quiet enough to avoid the conduction of RF energy onto its conductors which is then picked up by a nearby receiver.

Containing the RF energy

 

As the controller itself is potted with a silicone material, it's not practical to modify it directly to make it RF-quiet - and there is no need to do so:  Instead, we must take steps to eliminate any differential RF currents that may exist between DC Input and DC output terminals.

Ferrite alone is NOT the answer!

One may presume that the answer to this problem is the implementation of RF device such as snap-on or toroidal ferrite devices - and you would be partially correct.  Any practical inductor - such as that formed by the introduction of a ferrite device onto an existing wire - will have rather limited efficacy in quashing RF currents.

Snap-on devices (e.g. those through which a wire passes) have very limited usefulness at HF frequencies (<30 MHz) - especially on the lower bands - as they simply cannot impart a significant amount of reactance in the conductor onto which they are installed.  At higher frequencies (VHF, UHF) they can have a greater effect - but their efficacy will usually be disappointing at HF.

Using a device that can accommodate multiple turns through its center such as a toroid (or even a larger snap-on device) it may be possible to get up to a few hundred ohms of reactance on a conductor across a fairly wide frequency range - but even this will be capable of reducing the amount of RF by 10-20 dB (2-3 "S" units) at most:  Depending on the intensity of the RFI from the solar controller, this may not be enough to quash the interfering energy to inaudibility - particularly in a remote and otherwise "RF Quiet" location.

To be sure, it's worth trying just the ferrite devices by themselves to see if - in your situation - it reduces the RF interference from the controller to your satisfaction, but remember that the location where you are likely to be using this panel is probably far quieter (RF-wise) than your home QTH:  A "quiet" Solar charging system may seem quiet enough at your noisy home QTH, but could still be noisy in the middle of nowhere.

The addition of capacitors to the circuit can improve the efficacy over ferrite alone by orders of magnitude.  Consider the diagram below:

Figure 4:
Diagram, including additional filtering.  L1 and L2 are the bifilar chokes seen in Figure 3, above while the capacitors (C1a, C1b, C1c and C2a, C2b and C2c) and their implementation are described below.
Click on the image for a larger version.

 

Ferrite devices L1 and L2 are comprised of bifilar-wound inductors on the DC input/output lines, respectively.  These inductors will suppress common-mode RF energy that may appear - but these alone are not likely to be quite enough.

 

In order to force the RF energy to common mode to maximize L1/L2's effectiveness, capacitors C1a, C1b do so for the "external" connections (e.g. those connected to large devices, long wires) while C2a and C2b do so for any RFI emanating from the controller itself.

 

C2c - which is placed between the DC input and output of the charge controller - effectively shunt RF energy differences between the in/out terminals to minimize the differential currents.  Figure 3 shows C2a placed between the two positive terminals, but it could have been placed in any combination (+ to -, - to -, etc.) and been just as effective since the capacitors C2a and C2b effectively short the + and - terminals together at RF frequencies.  If your OCD bothers you, could could add additional capacitor combinations, but the three shown above for C1 and C2 proved to be adequate.

 

The real work for our filtering magic is actually done by C1c.  As seen from the diagram it's shunting RF currents that might appear on the "external" sides of L1 and L2 - which will have been significantly reduced in amplitude by L1 and L2 anyway:  The low impedance of the C1c at RF (a few ohms) coupled with the high RF impedance of the conductors through L1 and L2 work together to make sure that differential RF currents that might exist between the input and output of the charge controller are minuscule, and thus there is effectively no RF energy that can be radiated.

 

Figure 5:
Three 0.1uF monolithic capacitors placed across the
controller's terminals (C2a, C2b, C2c).
Click on the image for a larger version.

Implementation

A glimpse of what was done may be seen in Figure 3.  Some 14 AWG paired copper wire (red/black) was wound on two FT140-43 ferrite toroids - about 6 bifilar turns in this case:  Individual wires could have been used other than "zip" cord - just be sure that the two parallel conductors are laid in parallel to maximize the effectiveness of the bifilar configuration.  Two of these wire/bifilar devices were constructed - one for the DC from the panel and the other for the output to the battery/load.  "Spade" lugs were installed on one end of the red/black wires - two lugs per wire/bifilar assembly.  (FT240-43 or FT240-31 toroids could also have been used, but the FT140-43 is a fraction of the cost, half the diameter, and perfectly suitable for this application.  The FT240 size may be more appropriate if such a filter network is constructed for a higher-current system with larger-gauge wire.)

On the solar controller itself, small 0.1uF, 50 volt monolithic capacitors were installed (C2a, C2b, C2c) to form part of the filter circuitry:  Minimal lead length is important for maximum effectiveness.  While monolithic ceramic capacitors are preferred because they are small (and will fit more easily in tight spaces) and have very low ESR (Effective Series Resistance) one could use disk ceramic capacitors instead.  Film/plastic capacitors are less effective at higher frequencies.

Figure 6:
Terminal strip with capacitors C1a, C1b and C1c.
As described, these capacitors do much of the "bypassing"
of RF differential currents between the input and output.
Click on the image for a larger version.

As can be seen from this picture and Figure 5, the terminals are the "clamp" type and are connected in the same manner as the lugs on the cable on the bifilar toroid assembly. - and also note that this "modification" is completely reversible as nothing at all was changed on the controller itself.

The other end of the red/black wires were soldered to a four-position screw terminal strip as seen in Figure 6 - similar to the one on the back of the charge controller.  As with the terminal strip on the controller, three 0.1uF 50 volt capacitors were soldered (C1a, C1b, C1c) on the back for RF bypassing.  It is possible to have connected the capacitors under the clamps as was done on the controller, but soldering them to the back means that they would not be prone to falling out or being lost if the cables were changed.

 

With these connections made, the wire on the toroids and the connections to the added terminal strip were covered with "Shoe Goo" - a robust rubber adhesive (which may be used to fix shoes, as the name suggests) both as mean of strain relief and to provide electrical insulation.

 

The reader may have noted that we have physically brought together the input/output cables again at this terminal strip - and this was intentional.  By keeping the leads with the bifilar inductors as short as possible and then bringing them back together, we can use the shortest-possible leads on our capacitors to effectively "short" the input and output cables together at radio frequencies, making it impossible for the wires to radiate effectively at HF.  With this, the RF energy is contained within the area of the charge controller itself and the terminal strip/cables and since this is a very small aperture at HF, it can't radiate effectively and additional metallic shielding is unneeded.

 

At VHF/UHF frequencies - where the physical size of the controller+bifilar chokes is a larger proportion of the size of the wavelength (plus the fact that the components used won't work as well at these frequencies) means that some RF energy could radiate, but testing shows that the amount of VHF/UHF RF energy conveyed by the panel and cables was reduced below the point of detection more than a few feet (a meter) or so away from the system.

Spectrum analysis plots - updated 12/24

In the original version of this post I "loosely coupled" a spectrum analyzer (the "Tiny SA Ultra") to the leads to/from the controller by wrapping them around - but this only tells part of the story, showing the worst of the interference.  In this update, I connected  to the DC leads "directly" - using a 0.0022uF capacitor as a DC block - before and after the filtering to show the difference.  (Ignore the markers in the following plots - I forgot to shut them off)

First, the bad (no filtering):

 

Figure 7:
Un-filtered 0-30 MHz spectrum from the controller, directly coupled.

Figure 7 shows what the spectrum looks like from 0 to 30 MHz - each horizontal division representing 5 MHz - covering the extent of the HF spectrum but as expected, the worst of it is found in the first several MHz as shown in the next plot:

 

Figure 8:
Un-filtered 0-10 MHz spectrum from the controller, directly coupled.

Figure 8 shows from 0 to 10 MHz - with each horizontal division representing 1 MHz - and we can see that the peak energy at around 5 MHz is stronger than -40dBm - a level which correlates with an S-meter reading of about "30 over S-9".  This plot along with that of Figure 7 show that even as high as 20 meters (14 MHz) the noise can exceed "10 over S-9".

Remember that we are connecting the analyzer directly to the terminals of the equipment - something that you would not normally do, but these plots give you an idea as to how much energy is available to be radiated on the wires that go between the panel and the battery:  Even if the signals at 5 MHz were down by 30dB (1000-fold) due to coupling, they would still be about "S-9" in strength (e.g. pretty strong).

Below are the same plots - this time with the filtering depicted in figures 4-6:

Figure 9:
0-30 MHz, directly coupled to the controller - after filtering.

 

This shows a dramatic decrease in the amount of RF present from 0-30 MHz and as can be seen above, the noise floor above 5 MHz is VERY much reduced.  Let's look at the 0-10 MHz range, below:

Figure 10:
0-10 MHz, directly coupled to the controller - after filtering.

 

As expected, the filtering is less effective at frequencies below 1.5 MHz (more inductance and larger capacitors could help with that) but even at 2 MHz we see that the "grunge" is attenuated by about 20dB (100-fold, around 3 S-units worth), and is further reduced to be down by roughly 40dB (10000-fold, or about 7 S-units) around 80 meters (3.5-4.0 MHz) and higher, approaching the noise floor of the spectrum analyzer (approx. -100dBm with an RBW of 100 or 300 kHz) around 7 MHz, rising slightly in the 13-16 MHz range.  As can be seen in Figure 9, the worst-case noise output anywhere above 7 MHz are only around -85 dBm (S-7) and this would be with the receiver connected directly to the solar panel leads:  Again, no-one would ever do that!

 

The above measurements show how much RF energy is inputted into the wires connecting the panel and the battery bank - both of which will act as halves of a dipole antenna - and these measurements show that we can very significantly reduce this energy, likely reducing it to the point of inaudibility. 

Figure 11:
The same filtering - this time, applied to a Renogy
Rover 20 amp MPPT solar controller.
Click on the image for a larger version.

As noted in the update at the top of this posting, I've operated the solar panel and controller right next to my HF antenna and could not hear any interference from it!

 

Figure 11 shows the techniques depicted in figures 4, 5 and 6 applied to a different Renogy controller, also using FT140-43 toroids and 0.1uF monolithic capacitors.  In this case, the only capacitors used were those attached directly to the back side of the four-screw terminal strip (C1a, C1b, C1c) which works because this particular controller already has the other set of capacitors (C2a, C2b, C2c) on its circuit board.

 

Conclusion:

 

Prior to the modification, getting within several feet/meters of the solar panel with a portable shortwave receiver equipped with SSB revealed drifting "birdies" from the controller's normal operation and holding the antenna against either the panel or the output cable made this orders of magnitude worse.

After the modification these "birdies" were inaudible on the cables:  It took holding the portable receiver's antenna within a few inches/cm of the charge controller to hear its operation.  By the addition of these nine components (two bifilar inductors, six capacitors and the terminal strip) the RF energy is confined to the (small!) physical space of the controller itself and is no longer being introduced differentially to the panel and output cable, causing it to be unable to radiate effectively at HF, making it very quiet and "Radio Friendly".

While the supplied charge controller for the Renogy panel was a simple PWM type rather than an MPPT (Maximum Power Point Tracking) and is thus somewhat less effective at extracting all-possible energy from it, there is no reason why this sort of filtering could not be applied to either types.

 

This shows how a typical portable solar panel+charge controller can be made to be RF-quiet and "POTA" or "SOTA" compatible.  This (reversible!) modification has rendered this panel completely quiet across the HF spectrum and inaudible on VHF/UHF frequencies as well at distances of more than a few feet (a meter or so) as well.

* * * * *

Related articles:

• The December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link 
• See also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link
• The article "Minimizing VHF and HF RFI from electronic ballasts and fluorescent tubes - link - from December 17, 2015 may also be useful in quashing interference from these sources.
• The December 1, 2017 article "Containing RF noise from a sine wave UPS" - link - also details how interference from an RF-noisy device may be suppressed.
• Quieting an insanely noisy LED floodlight - link.  This describes how a constant-current LED supply that produced enough interference to quash HF reception was quieted down to the point of undetectability. 
• Quieting a 150 watt Samlex sine wave inverter - link.  This page talks about making an otherwise RF-noisy sine wave inverter quiet at radio frequencies. 
• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave. 
  

This page stolen from ka7oei.blogspot.com

 

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Teardown and analysis of a solar Powered USB charger

This is a "teardown" and analysis - and then a bit of modification of a 1 watt solar charger with an integrated 2.7 amp-hour 3.6 volt Lithium-Ion cell sold by Harbor Freight as Item Number 68691.

Figure 1:
1 watt solar charger with built-in
2.7 amp-hour Lithium-Ion
cell.
Click on image for a larger version.

The instructions supplied with this unit are rather sparse and only appear on the packaging material - not on any printed matter included with the unit - and, unfortunately, these instructions do not really match the solar charger itself too well.

For starters, the instructions imply that one presses and holds the button for a couple of seconds to enable/disable the USB charger, this being indicated by the illumination of a blue LED along with a beep.  In fact, this doesn't happen:  Pressing the only button on the device simply turns on/off a three-LED flashlight - and that's only if the internal cell isn't fully-discharged.  There is an LED on the side that glows red when charging, changing to green when complete, but that's not really well-documented, either.

Aside for the miserably inaccurate documentation, what about the unit itself?

As it turns out, it does work, but not in the way that is documented.

What's inside:

Popping it open, I did a bit of quick reverse-engineering of the device (See Figure 4, below):

• One section of a 4013 dual CMOS flip-flop is wired to function as the push-on/push-off control device, its output turning on a FET switch for the three white LEDs comprising the flashlight.  It should be noted that as wired, the 4013's state is completely random should power be applied.  What this means is that if the internal Li-Ion cell had been run down so far that its internal protection circuit disconnected it, the flashlight could turn itself on when charge was applied either by solar or external means!
• A small surface-mount switching controller U1, a XC6368, drives a FET switch to function as a voltage up-converter to boost the 2.7-4.2 volts from the LiIon cell to the nominal 5 volts for the USB. connection.  A self-resetting thermal fuse ultimately limits the maximum current that can be drawn by the device being charged.
• U3 is the on-board is a LiIon charge controller chip, a TP4056.  Its job is to disconnect the cell to prevent overcharging (above 4.2 volts) and to "trickle charge" the LiIon cell while its voltage is below about 2.9 volts - both important factors in terms of safety and cell longevity.  Among other things, it limits the maximum charge current - which could come from either the solar panel or the coaxial charging jack - to 0.5 amps as set by R9.
• There is an undocumented coaxial power connector next to the on/off pushbutton switch for the flashlight.  It's difficult to be certain of its precise size, but it appears that a connector with an inside diameter of 1.35mm and an outside diameter of 3.5-3.6 mm will fit.  An IEC type "C" connector is probably the closest "standard" size and the tip (inside) is positive.



Figure 2:
Charger with adapters for various types of devices
Click in image for a larger version.

• There is a mini USB female connector on the charger that mates with a short cable with a coaxial power connector on it (of different size than the one on the unit itself) to which a variety different-sized supplied adapters fit, including a mini-USB, a two different types of micro-USB, and a number of other proprietary telephone connectors.  (No "iPhone" adapter, however.)

In poking around on the board I became curious if/how the voltage booster was disabled if nothing was being charged, especially considering that the device didn't behave as the instructions indicated when it came to starting/stopping charging.  It was soon apparent that there was, in fact, no on/off switch for the 5-volt switching converter and neither did it have any means of "auto-sensing" when something was to be charged, so how much current did it pull from the battery "all of the time"?

The answer to this question is "about 1.5-1.8 milliamps" - and a significant amount of that current is solely from the voltage divider consisting of R1 and R2!

What this means is that at all times, there is a constant drain of around 1.8 milliamps being pulled from the internal cell.  Actually, while a bit annoying, this isn't really too bad since:

• At 1.8 milliamps, it would take over 60 days to run down the 2700 milliamp-hour cell.
• The above would be true only if it was always kept in complete darkness.

Over a 24 hour period, this continuously-running inverter would pull less than 50 milliamp-hours from the internal cell, an amount easily made up if the unit were stored anywhere that it received even indirect sunlight for a couple of hours per day:  Perhaps even standard room lighting would suffice to break even at this level of discharge.

Charging from an external source:

What about charging this thing from an external source rather than from the solar panel?  In reverse-engineering the circuit - and also using the TP4056 data sheet - that one could apply a 5 volt supply (say, from a USB device such as a computer or a charger) to more-quickly charge the unit's internal power cell.

There is one important "gotcha", however:

You cannot apply voltage to the mini-USB connector built into the device to which the charging cable attaches as doing so may damage the device's voltage converter!

In my opinion, the choice of these two connectors is a bit idiotic as the two connectors (the coaxial and USB) on the charger itself are backwards from what they should be as without instructions to the contrary, it would be natural for someone to plug a voltage source from a charger or computer into the mini-USB connector!

I've not timed how long it actually takes to fully charge the internal cell from a depleted state, but my calculations indicate that it could be anywhere between 4 and 12 hours, depending on the current capability of the device that is doing the charging:  Toward the "long" side of this charging time from a computer USB port and on the short side of this for a dedicated, high-current USB wall-charger.  When the LED on the unit turns green, charging is complete and it should be disconnected.

A bit more about the circuits:

Figure 3:
Diagram of the solar charger showing the charge controller and the voltage up-converter circuit.
Click on the image for a larger version.

 

I decided to draw the diagram of the circuitry while I was at it - a simple enough task since this thing is more or less a collection of rudimentary circuits lifted from the various manufacturer's data sheets!  As can be seen in Figure 4 there are a number of surface-mount components and identifying these components can be a bit tricky at times.  U3, the charge controller, is comparatively large so there is room for the part number to be stamped on it.  The two transistors, T1 and T2, are smaller and there's room for only a "marking code", and the same was true of U1.  Fortunately, I could tell by the lack of a series resistor on the outputs of the switching regulator and U2, the flip-flop that they couldn't be bipolar transistors and had to be FETs and that information along with the marking code put into a web search rapidly revealed their true identity!

Posing a bit of challenge was the identity of U1, the switching voltage controller, but this soon fell into place due to its somewhat unusual pinout, that there are only half a dozen manufacturers of 5-pin switching regulators in that particular type of SMD package, and the fact that it was really a 2.7 volt regulator reconfigured for 5 volts using R1 and R2 as a divider:  Why they didn't use the version that was pre-set internally for 5 volts is anyone's guess - maybe they just had a lot of the 2.7 volt versions around!

U1, which is apparently a Torex XC6368A271MR, is a fairly good device with low internal power consumption and designed specifically to drive an outboard switching transistor, including a built-in "soft start" circuit and operating in the 70-100 kHz frequency range. To reduce power consumption slightly, the "Vdd" pin of the regulator is connected to the voltage input rather than the output which would be at a higher voltage (about 5 volts):  Normally, such a circuit would allow the DC current to flow through the inductor and diode to "bootstrap" the operation of the circuit since U1 itself is capable of starting at below 1 volt.

Interestingly, L1's inductance value is 250 uH - a strikingly high inductance value for such a voltage up-converter.  Ideally, the inductance would be much lower for the typical POL (Point-Of-Load) voltage converter - maybe 1/3 or 1/6 of this value (perhaps 47uH) in order to maximize conversion efficiency at higher output currents, but the choice was likely made here to minimize the quiescent current of the voltage converter.  I've not measured the efficiency of this converter, but I have little doubt that it could be made to be significantly better by using a heavier-duty FET (or several FETs in parallel) and more optimal value of an inductor - not to mention an inductor with a higher current rating.  If this were done, the quiescent current would be higher, however!  As it is, the device did a fairly reasonable job of increasing the charge on my Razor phone by over 30% over the period of an hour or two:  Not terribly fast, but it did work.

It would appear that if one wanted to shave nearly 1 milliamp from the converter's quiescent current, R1 and R2 could be rescaled upwards, keeping about the same proportion and, possibly, adding a small capacitor (47pF-100pF) across R1 as compensation to keep it stable:  Why didn't they do that in the first place?

Figure 4:
Inside the solar charger - including the added on/off switch.
Click in the image for a larger version.

To the solar charger, I added an on/off circuit (and indicator) shown in the diagram above, inserted at point "X" consisting of an SPST switch, "Rz", a 10k resistor and "Dz", a high-brightness blue LED.  With the value of Rz calculated to set the LED's current at only a few hundred microamps, its added current load is rather minimal while providing an obvious indication that the inverter is powered up.

The upshot:

How well does it work overall?  It's "OK", but not really great.  There are a number of fine points throughout that incrementally reduce overall usefulness - no doubt to save a bit of cost.  One problem with some phones is that they may attempt to pull too much current from the charger, cause the voltage from the converter to drop below the device's charging threshold and then it would stop charging - only to repeat the cycle over and over.  Unfortunately, when this happens many phones' displays will light up and eat up a large portion of the charging current, the result being that charging may never progress!  If this happens to you, there's probably nothing you can do about it unless there happens to be some way to keep the display from "waking up"!

For charging audio players and most phones, it does work - although a bit slowly.  The 1 watt capacity - which is only accurate for direct sunlight on a clear-sky afternoon - is likely not enough to maintain anything but rather light usage of a telephone over the course of a day, but it is more than enough for the making of the occasional telephone call or putting enough charge on a phone to make an emergency call.

A warning:

Considering its construction - mostly of ABS plastic - one should NOT place a device like this on the dashboard of a car to charge it:  Not only is the temperature in a closed-up car going to be high enough to melt/warp its plastic case, but it will also damage/reduce the life of this - or any - Lithium-Ion cell!

(It's also worth noting that the natural tinting of almost all car windshields is going to be enough to significantly reduce the amount of light reaching the panel and thus slow its charging!)

Anyway, there you are - in case you were wondering!

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This page stolen from ka7oei.blogspot.com