I am a CSE student working on a project.

We are using a solar powered microcontroller. The solar panel's power goes through a fuse and an RC circuit with a varistor to the charging system.

What does this circuit do and is it possible for the capacitor to be replaced with a smaller value one without compromising its function?

6V solar Panel Varistor with 5.8V working voltage and 18V shunt voltage

Circuit Schematic

  • 1
    \$\begingroup\$ I dont think you normally would use a varistor when using a solar panel as a supply.. You use a varistor to shunt unwanted voltage spikes that could damage your system. A solar panel has a given maximum open circuit voltage and you design your circuit after that. It would be a waste of power to just dissipate it as heat. \$\endgroup\$
    – Linkyyy
    Nov 29 '18 at 22:09
  • \$\begingroup\$ As for the cap C1, if you are just going to power a small MCU this could definitely be smaller, unless you are using it as somekind of battery/energy storage. \$\endgroup\$
    – Linkyyy
    Nov 29 '18 at 22:11
  • \$\begingroup\$ To me a 1mF capacitor is likely a (rather small) EDLC/supercap, so they can run (or at least stay alive/asleep) during periods of no USB power and insufficient light. An electrolytic capacitor is sub-optimal because of leakage (at least for general purpose types). Also there should be some kind of automatic transfer switch between the USB and PV power sources if the PV can get above Vbus levels. Varistors are not meant to be used as a continuous shunt regulation device (eventual failure). \$\endgroup\$
    – isdi
    Nov 29 '18 at 23:17

I don't see the "R" in your RC circuit.

I see a capacitor and a varistor doing separate jobs, right next to one another.

C1 is there to provide current for a little while when there's a shadow on the solar cell.

It also provides current when your load needs more current than the solar cell can provide - short bursts, of course. It can't provide high current for long, but often times you just need a little burst to get something going - starting a motor or switching a relay, for example.

The varistor is there to protect your five volt load from over voltage from the solar cell.

As the voltage rises above 5V, the varistor will start to conduct. Somewhere around 5.6V it will turn into a short circuit. Depending on how much current the solar cell can deliver, it will either prevent the voltage from getting above 5.6V or it will blow the fuse.

Either outcome protects your 5V load from damage.

Without knowing what else is on the "USB" line, there's no way to tell whether or not you could use a smaller capacitor for C1.

There might be something there that needs a lot of current once in a while. It might also be there just because one of the other folks in your group has a bad tendency to stand in the wrong spot and block the light from getting to the solar cell.

That is a strange way to do things. It would be better to have a buck/boost switching regulator in there to maintain a proper 5V supply as long as the solar panel is providing enough power.


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