I'm building a USB solar charger for my tablet, which has so far worked well.

I'm using a 5 Volt, 5 watt solar panel (same as this one), which outputs about 4.5V and 0.6A in real use when charging my device, together with a 5.1V, 5w Zener diode (1N5338B) to regulate / limit the voltage, as it goes up to ~7V when not under load, which I presume could damage my tablet.

Now, I want to add a second identical solar panel (in paralell), to get more current to charge my tablet faster.

I need an appropriate voltage regulator, as this 5 watt zener diode would no longer work with (up to) 10w of power.

So far, I've looked into:

  • 10w Zener diode, but I have not been able to find one.

  • LM7805, But they require a higher input voltage than my panel produces, and have a ~1v voltage drop.

  • A DC-DC voltage converter, such as this one. Would this work ? Does it have a voltage drop ?

Any other possibilities ? What do commercially available USB solar chargers use ?

Essentially, I'm looking for an appropriate component to prevent the voltage from the solar panel going over 5V and damaging my device.

Any recommendations ?

  • \$\begingroup\$ Also check the USB charge resistors and what is the max current the device will try to draw. \$\endgroup\$ – Damien Sep 4 '15 at 19:05
  • \$\begingroup\$ I measured the max current, and it uses about 1.2 amps, so 2x 0.6a panels should be perfect. \$\endgroup\$ – JonasCz - Reinstate Monica Sep 4 '15 at 19:16

My first reaction is to put the two panels in series, then use a off the shelf buck regulator chip. If the two panels can't even put out 6 V or so in series (3 V per panel), then there is so little power available that it doesn't matter if the output is just shut off.

  • \$\begingroup\$ In full sun, these would produce about 9v in series. If I use a buck converter, how much efficiency could I expect ? Ie, if the panels produce 600ma at 9v, about how much current would I get at 5v with a good one ? \$\endgroup\$ – JonasCz - Reinstate Monica Sep 4 '15 at 20:09
  • \$\begingroup\$ @Jonas: You find these answers in the datasheets of candidate buck regulator chips. \$\endgroup\$ – Olin Lathrop Sep 4 '15 at 20:13
  • \$\begingroup\$ This seems like a good solution, will give it a try. I found a cool buck converter board with an lcd voltmeter, so will try that. \$\endgroup\$ – JonasCz - Reinstate Monica Sep 7 '15 at 16:40

I would look for a DC/DC converter with Maximum Power Point Tracking (MPPT, I suggest reading up on that).

Put your panels in series if required to meet the converter's minimum input voltage. Add bypass diodes as required, and set the MPPT regulation point to 2x the optimal panel voltage.

  • \$\begingroup\$ Will look into it - looks promising. \$\endgroup\$ – JonasCz - Reinstate Monica Sep 4 '15 at 20:02
  • \$\begingroup\$ If you search for 'MPPT' on the merchant sites you linked to, you will find a few items. No idea what they are worth, but worth a try. \$\endgroup\$ – Damien Sep 4 '15 at 20:10

Try using a Low-Dropout Regulator. Many vendors have tools where you can provide your device parameters and they'll suggest parts for your application.

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    \$\begingroup\$ With 3 V VDO, the '317 is hardly an LDO. In fact it's worse than the '7805 that OP already said is not good enough. \$\endgroup\$ – The Photon Sep 4 '15 at 19:03
  • \$\begingroup\$ Thanks for that. I've updated the answer to remove the reference to the LM317; hopefully it's not too vague of an answer now. \$\endgroup\$ – Adam Sep 4 '15 at 19:10
  • \$\begingroup\$ I've also looked into LDO's, but they still have too much voltage drop, considering the panel produces 4.5 volts under load which is the bare minimum for USB charging. Thanks anyways ! \$\endgroup\$ – JonasCz - Reinstate Monica Sep 4 '15 at 19:14
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    \$\begingroup\$ There are LDO's with Ultra-Low Dropout such as LP3855 which has 240mV drop when Io = 1.5A. \$\endgroup\$ – Bence Kaulics Sep 4 '15 at 19:51
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    \$\begingroup\$ This question looks a bit like what you need: building an LDO with a low-voltage shutdown: electronics.stackexchange.com/questions/9848/… \$\endgroup\$ – Adam Sep 4 '15 at 20:23

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