I am using the circuit below to switch a bi-latching relay once a wire is disconnected, i.e. to not switch as long as a voltage is present on the wire.
After assembling on a PCB, on which there are multiple instances of the circuit, about half appeared to work properly once, but after cycling the relays on/off by disconnecting the wire and resetting on the other side, all of them would stop working.
Specifically, the Q2 gate voltage appears to be internally pulled up to between 2V and 10V, and even when connected directly to GND it becomes active and brings Q1's gate to GND. Q1 appears to still function as expected.
In simulations it works as expected, and I find it strange that Q2 is the one that goes given how relatively shielded it is from the relay current and potential flyback voltage spike.
Potential solutions I see here are to use bigger MOSFETs or switch Q2 to a BJT, but I'd like to understand what could be causing this to avoid the same problems in future.
Best Answer
Seems like you've damaged the gate of Q2 by ESD or similar effects. It's a bad idea to expose an unprotected MOSFET gate to external connections, particularly a small MOSFET such as a 2N7002.
A series resistor (eg. 1K) and something like a 5.1 or 6.2V zener from gate to source will protect the MOSFET from most reasonable transients.
simulate this circuit – Schematic created using CircuitLab
You could switch to a BJT (with a series base resistor) but it would be a good idea to have a diode from base to emitter to prevent ESD-induced reverse base-emitter junction breakdown, so the component count does not look much different.