I want to confirm this circuit is going to work and ask how to calculate the zener voltage. The load is going to be driven by 30 volts, and the gate voltage is the same 30v, through an optocoupler.

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  • \$\begingroup\$ The zener is there as a TVS diode, so you should size it just like any other TVS circuit. \$\endgroup\$ – Hearth Feb 16 at 20:08
  • \$\begingroup\$ Check the datasheet of the mosfet. Most mosfets do not allow for more than 20v on their gates \$\endgroup\$ – Huisman Feb 16 at 20:08
  • \$\begingroup\$ What do you want the circuit to do if it "works"? Please describe the load, specifying the maximum current it will draw. Also specify which transistor you want to use and give us a link to the datasheet. \$\endgroup\$ – Elliot Alderson Feb 16 at 20:54


simulate this circuit – Schematic created using CircuitLab

On your schematic the 10k resistor (R1) is not placed correctly. It must be in series with the zener cathode. Then there is another resistor to ground (R2) to make sure the gate is at 0V when the input to gate is off. The value can be between 100k, sometimes even less, and 1M.

The zener must be rated 15V (or somewhere between 10 and 15V) to reduce the voltage to under 20V. The 10k resistor prevent too much current flowing through the zener and just enough for the mosfet gate.

  • \$\begingroup\$ Depending on usage, R1 may well need to be smaller. A difference of 15 volts across a 15 volt zener is only 1.5 mA, and the gate charge current will be less than that, which will produce a rather slow turn-on. For things like lamp activation, that's OK, but PWM at kHz rates is not likely to work well. Granted, the fact that the gate drive is an optocoupler suggests low drive current, but you never know. \$\endgroup\$ – WhatRoughBeast Feb 17 at 4:16
  • \$\begingroup\$ Well, the switching frequency is about 1kHz, so I think R1 will prevent it from working. Anyways thanks for the answer. \$\endgroup\$ – user6740407 Feb 17 at 8:23
  • \$\begingroup\$ Yes, 10k is just an indication. Adapt the value according to the rest of the circuit and requirement. But you do need a resistor in any case, and if you need a small resistance value, be careful of the zener power and power dissipation. With 10k the power dissipation is 0,0225W, with 1k it will be 0,225W. With 0,47k, 0,45W. So you need a zener of more than 1/2W if you want to use a smaller resistor. The power of the resistor must be the same as that of the zener. \$\endgroup\$ – Fredled Feb 17 at 18:25

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