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schematic

simulate this circuit – Schematic created using CircuitLab

I try to solar charge a supercapacitor C1. When it's fully charged, I use a Zener diode D2 to the gate of an logic MOSFET M1, which then opens the circuit to power an ESP32/Arduino (using an MCP 1700-3302E converter from 3-5 V to constant 3.3 V. I think I need another part to use the full 5V of the capacitor). Once the MOSFET turned on, however, it says on. I can't use a pull down resistor, because I need the capacitors energy down to 0.5 V.

So. Is there a way to pull the gate to gnd, only once the voltage is below any threshold?

I use the zener, because I don't want to power any electronics unless the voltage of 4.7V is reached. But I need to find a way to pull the M1 mosfet back to GND, once the voltage drops below say 0.9V.

Basically I want a MOSFET that turns on at 4.7 V and turns off below 0.5 V.

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    \$\begingroup\$ there is a schematic editor built into this website ... edit your post, click the button that looks like a circuit ... draw your schematic and then click save and insert \$\endgroup\$
    – jsotola
    Oct 30, 2023 at 3:45
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    \$\begingroup\$ It sounds like this question could attract useful answers, if people can understand exactly what is going on... Right now, this is a bit hard. Please insert a schematic \$\endgroup\$
    – tobalt
    Oct 30, 2023 at 4:34
  • \$\begingroup\$ Welcome! "step up converter from 0.5-5 V to constant 3.3 V" You need a buck-boost to make that happen. A step up (boost) can only step up, so it can't produce 3.3 V output from 5 V input. \$\endgroup\$
    – winny
    Oct 30, 2023 at 7:10
  • \$\begingroup\$ As @winny said or maybe even look into a LDO. Less noise and smaller BOM. \$\endgroup\$
    – S_G
    Oct 30, 2023 at 7:16
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    \$\begingroup\$ Why a super cap if the only function is to push the MOSFET into conducting? If you want to use it as a energy storage you need to connect it the right way. \$\endgroup\$
    – MiNiMe
    Oct 30, 2023 at 9:00

3 Answers 3

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It could work but not this way, the schematic is not correct. The negative side of the supercap should be connected to GND. Otherwise the current will never reach the arduino.

Then, before the LDO, instead of the mosfet you should use a voltage supervisor, like this one: MCP100-315 or something similar. It monitors the line and switchs on the arduino only when there is enougth voltage.

Maybe it could work without any supervisor, just with the LDO, but I think that the arduino will start to ripple when it's around 2.8V.

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You may want to look into some integrated solution to accomplish the desired function. What about something like MAX6778?

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schematic

simulate this circuit – Schematic created using CircuitLab

The Mosfet should be placed between the arduino and ground, on the low side, if you use a N-MOSFET. You can use a P-MOSFET on the supply pin but it's easier to control a N-MOSFET in this case.

R4 is protecting the Zener diode Z1 against excessive current. The S in the triangle is a Schmitt trigger. R3 and R2 can be used as a voltage divider to adjust the voltage at the Schmitt trigger input. At the same time, R3 protects this input and R2 pull it to zero volt in absence of current. This way, the Schmitt trigger will trigger the MOSFET at the desired voltage. R1 protects the MOSFET gate. R5 pulls the gate at zero volt in absence of enough current in the circuit.

MCP1700 or other voltage regulator should be able to supports any voltage from the solar panel system. (This schematic doesn't show the components used to protect and properly supply the MCP1700.)

As said previously, the capacitor (C1) should be connected to GND on one end, and to the supply on the other end. (In your schematic the voltage of the MOSFET gate will always be zero.)

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  • \$\begingroup\$ The use of a supervisor IC may be more appropriate, However, if you wants to starts the Arduino at a very specific voltage, you can use this circuit. I think there are supervisors which allow you to program a specific voltage but I'not sure. \$\endgroup\$
    – Fredled
    Oct 30, 2023 at 19:55
  • \$\begingroup\$ Thank you for this very detailed answer. However, if the capacitor is not fully charged, the Schmitt trigger already draws current. My ESP32 will need like all of the capacitor's energy. So I tried to start with the zener to open the mosfet. And only after that, all other components should be powered. The ESP32 can send a "stop" signal after the work is done. But I fear, that if it did not finish the work, it might get stuck in a zombie state. So, yes, a Schmitt trigger might be the right thing. But it should only power on, once the capacitor is fully charged (the zener breaks through). \$\endgroup\$
    – KungPhoo
    Oct 31, 2023 at 4:52
  • \$\begingroup\$ The Schmitt trigger will draw very little current for functioning. It should not affect filling the capacitor. When the capacitor is fully charged, the voltage at the Schmitt trigger input will reach the trigger voltage and actuate the Mosfet. The trigger voltage is 3V, so in order to trigger at 4.7V, you have to reduce the voltage before the Schmitt trigger input with R3 and R2. \$\endgroup\$
    – Fredled
    Oct 31, 2023 at 8:35

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