I have

  1. a solar panel that outputs maximum 1.2W (7V, 0.17A).
  2. a 5V 1A USB power supply
  3. a 3.5-9V, 1-3W water pump.

I'd like to have the water pump running all the time using power from (1) and (2), preferably not drawing unnecessary power from the USB power supply... So let's say, if the solar panel gives out > 1W of power then don't draw from USB. Wondering what kind of circuit I'd need to achieve this.

Background: I have very basic knowledge of electronic circuits and can connect components following the diagram... that's about it.

Thanks so much!


A diode, preferably one with a low forward voltage, probably followed by a large capacitor:


simulate this circuit – Schematic created using CircuitLab

Whichever power source is able to supply the higher voltage drives the motor of your pump. If the pump is off and the sun is shining, no current will flow, and C1 will be charged to your solar cell's full voltage of 7V.

However, as soon as you start drawing current, the voltage supplied by the solar cell will drop. For a long time, this will still mean the right side of D2 has a higher potential than the left side, so no current flows from the USB power bank.

However, as soon as you draw so much current that the solar cell's voltage sags below 5V, D2 starts to conduct and you start "helping" the solar cell from the USB supply.

This is not really a great arrangement: Much more elegantly, you'd build a solar charger for Lithium-Ion cells like in your USB power bank, and use the sun to charge that, and also supply to the motor. ICs to do that exist, it's just way more work.

  • \$\begingroup\$ Thanks for your suggestion! Quick question: why is the battery charging circuit a better arrangement? It would involve extra battery, which might have limited charging cycle. \$\endgroup\$
    – Art
    Jun 5 '19 at 16:11
  • \$\begingroup\$ no, I meant replacing your USB battery bank with a battery that you can directly charge from the solar cell. That's not one more battery - that's still the same number of batteries :) \$\endgroup\$ Jun 5 '19 at 16:18
  • \$\begingroup\$ Oh I'm using an AC adapter to USB, not a USB battery. \$\endgroup\$
    – Art
    Jun 5 '19 at 17:09
  • \$\begingroup\$ ah, ok. then, never mind the last paragraph. \$\endgroup\$ Jun 5 '19 at 21:25
  • \$\begingroup\$ Will try it out this weekend:) \$\endgroup\$
    – Art
    Jun 6 '19 at 1:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.