# Solar panel reverse voltage protection

Hi

my project is to charge a USB power bank using a 12v 250mA Solar panel. To do that I have connected the solar panel to a USB buck converter (which takes 6-20v and turns it into 5v) which will be used to charge my USB powerbank.

My question is should I add a blocking diode between solar panel and buck converter to prevent reverse voltage when sun is down or will the buck converter stop the reverse voltage without a diode?

Hi, you will have to find and post a schematic of the buck convertor for anyone to give a definitive answer on that. Could you simply test your theory with a multimeter? Wire it up between the panel and the buck convertor/battery in sunlight and verify that you see current flowing in the right direction. Then cover the panel completely with something thick to block out all light. Do you then see a negative current on the multimeter?

Paul

As well as the diode question, you have other questions to find answers for. Such as how much current does the usb power pack draw when charging. Can the pannel provide enough current at 12V for charging at 5V? In low light, even if no reverse current leaks through the panel, does reverse current leak through the buck convertor.

I think you need to experiment in different conditions, monitoring the input & output voltages and currents flowing between the panel, the convertor and the usb power pack.

I also think you should be more truthful in your posts. You can't connect up something you haven't even purchased yet.

It's possible that your solar panel already has a built-in blocking diode, try testing it with a multimeter. If not, a Schottky diode would be a good idea since they have lower forward voltages than normal silicon diodes so they waste less power.

PaulRB:
As well as the diode question, you have other questions to find answers for. Such as how much current does the usb power pack draw when charging. Can the pannel provide enough current at 12V for charging at 5V?

A quick calculation as someone who has no practical experience with buck converters or solar panels: 12V at 250mA = 3W, 3W at 5V = 600mA, assuming the buck converter is ~90% efficient as they often are that should be enough (the pack is probably designed to work at around 500mA minimum), but it'll be close and slooooow. Personally I'd probably go up to a 5W or greater panel but assuming Noobian lives in a sunny country it should just about do.

PaulRB:
I also think you should be more truthful in your posts. You can't connect up something you haven't even purchased yet.

That's an oddly accusatory way to say "Please be more specific when asking questions".

To do that I have connected the solar panel to a USB buck converter

vs

well I havent bought the buck converter yet

That's what i was referring to!

neema_t:
It's possible that your solar panel already has a built-in blocking diode, try testing it with a multimeter. If not, a Schottky diode would be a good idea since they have lower forward voltages than normal silicon diodes so they waste less power.

A quick calculation as someone who has no practical experience with buck converters or solar panels: 12V at 250mA = 3W, 3W at 5V = 600mA, assuming the buck converter is ~90% efficient as they often are that should be enough (the pack is probably designed to work at around 500mA minimum), but it'll be close and slooooow. Personally I'd probably go up to a 5W or greater panel but assuming Noobian lives in a sunny country it should just about do.

That's an oddly accusatory way to say "Please be more specific when asking questions".

Nope, I don't think my solar panel has a blocking diode. Its just a flat panel with epoxy coating and 2 terminals.

Will it be bad if I use schottky diode along with buck converter? just to be safe

The USB powerbank I have uses AA Ni-MH batteries, so even if the solar panel is of high wattage I believe the batteries will still charge very slowly since its not supporting li-ion fast charge.
Also time is not of very important concern because I intend to use it during emergency situation like a flood or something, that's also why I chose a smaller panel which is easier to carry around.

Yes, Noobian lives in a very sunny tropical place. Always sunny and usually always above 30 C.

@PaulRB - Sorry, my english is not very good. I meant in my project which I plan to make, virtual project before I actually connect the real parts.

Noobian:
Nope, I don't think my solar panel has a blocking diode. Its just a flat panel with epoxy coating and 2 terminals.

That doesn't mean there isn't a diode, though, it could be built into the actual substrate.

The only problem with using an external diode is a loss in efficiency, say the forward voltage is 400mV and your current is 250mA, you'd be burning 100mW for no reason. Not really an issue if you have plenty of sun, but an unnecessary waste (about 3.33% of your panel's 3W) if your panel does already have an internal diode. Maybe you could consult the datasheet or try a diode test on the panel, but as I said I have no practical experience with them so I don't know if the test would actually work as expected.