I am charging my capacitor so it can act as a battery for a short period of time. I already did the calculations for the battery discharging and charging.

My problem is now that I have a load, I want to prevent current to flow to the load until the capacitor is fully charged (or to my required voltage threshold) so I can get maximum output power/energy. When the capacitor is fully charged, the charge controller stops its current flow and stops charging.

What kind of switch or relay do I need that can read, detect or sense my capacitor is full and then connect the capacitor to the load until capacitor drops below a certain voltage?

My load is a wifi module which I don't want current to flow to it and cause problems before I get the correct power or energy levels that are needed.

My Capacitor will have a around 56 Joules of capacity. My Load needs 1.65 Joules for 1 second, then only 0.1 Joules/sec This will get me around 6-15 Seconds of discharging til I hit my minimum voltage of the capacitor which is 0.9V.

I want to prevent any energy to go to my load until the capacitor is full.

  • 1
    \$\begingroup\$ As requested in your previous related question, please tell us what you're really trying to do. What problem is this circuit supposed to solve? Your solution may have a better alternative. \$\endgroup\$
    – Transistor
    Commented Jul 7, 2020 at 18:10
  • \$\begingroup\$ @Transistor I added some more information. \$\endgroup\$
    – Sherman
    Commented Jul 7, 2020 at 18:25
  • \$\begingroup\$ This is an energy harvesting application, right? (making use of little bits of energy) You might want to add the energy-harvesting tag \$\endgroup\$ Commented Jul 7, 2020 at 19:08
  • \$\begingroup\$ What is the minimum expected interval between 1 second burst loads ? Can you define the min:max time to charge 1.65 J from your source? or give datasheets and more details. Then give the Vmin for the Wifi. I suspect your source is insufficient voltage \$\endgroup\$ Commented Jul 7, 2020 at 20:37
  • \$\begingroup\$ Even if you could get 2V on the cap after a very long time as the D1 drops to 0, the power of 1.65J in 1s means 1.65W @ 2V or 825mA which means your Cap ESR (R1) of 500 mOhm will drop 412 mV and reduce Vcap to 1.58V.. Try again with better specs e.g. a Maxwell Cap 25mOhm \$\endgroup\$ Commented Jul 7, 2020 at 20:41

1 Answer 1


A comparator with hysteresis can enable a switch device (power MOSFET, relay, etc.) when the cap voltage exceeds one trip point, then disable the switch device when the cap voltage falls below a second (lower) trip point. The switch will remain off until the cap voltage exceeds the first (higher) trip point, and the cycle repeats.

The comparator device can be a comparator IC or an opamp. It is important that all signals stay within the comparator's input and output voltage ranges. This forum and others have threads covering comparators with hysteresis.

  • \$\begingroup\$ I think this is the kind of switch I was looking for. I will look more into those forums! I need the cap voltage to be higher than 3.2V and enable the switch. when the cap voltage falls below 1.2V to open so it can be charged. \$\endgroup\$
    – Sherman
    Commented Jul 7, 2020 at 21:05
  • \$\begingroup\$ Linear Tech and TI have newer comparator chips (some CMOS) that will run on and work with those low voltages. What is the power source for the comparator circuit? \$\endgroup\$
    – AnalogKid
    Commented Jul 7, 2020 at 23:58
  • \$\begingroup\$ I have a solar panel, but using a mppt to charge a capacitor as a battery. \$\endgroup\$
    – Sherman
    Commented Jul 8, 2020 at 14:14

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