A project log for Reboot-o-matic

Automatic power-cycling made safer

Nick SayerNick Sayer 11/15/2019 at 05:440 Comments

It's occurred to me that if you power the circuit at 12 volts, Vgs for the P MOSFETs will be the full 12 volts when they're turned on. This is problematic as the MOSFETs I've designed with have an absolute maximum Vgs of ±8 volts. The fix is simple - each of the P MOSFETs needs a zener diode to limit Vgs. For the output MOSFET it's as simple as placing a zener diode from the gate to the positive rail. There's no need for a pull-up because the gate is normally pulled-down and the switching is across the zener. For the switching P MOSFET, we need to add the same zener to the positive rail, but also must add a pull-up across the zener because the switch pulls the gate momentarily down. There also needs to be a current limiting resistor added in series with the switch to limit the zener current. When the supply voltage is less than the zener voltage, then you can imagine that they're just not installed. When the supply voltage is higher, then you can sort of assume that the voltage on the anode is lower than the cathode by the zener voltage amount.