Hello,

I'm using a basic NPN 2N7002 series mosfet in a low side switch configuration as part of a larger circuit. I have a 100k resistor to ground on the gate and have been cycling the circuit from low temperature to room temperature (-10F to 60F) and have noticed that the switch will close while warming back up to room temperature. What could be causing this? Any help is very much appreciated.
Thanks

 

Best to post your circuit so we can see what is happening, sounds like thermal runaway, or hf ringing.

 

Here is the circuit. The probe to ground initiates the cirucit, while the 5V out of the regulator latches the power on once directed by the uC. Could you explain thermal runaway, and any way to prevent it?

 

I suspect that it could be condensation on the device providing a path from gate to drain.

Les.

 

I take it Q26 is at fault, and self latches at warm up from minus temperatures, if that's the case sounds like dampness on the gate to drain, reducing the 100K gate resistor to 4K7 may improve it, or surrounding the mosfet in silicon sealant will improve it.

Thermal runaway
The problem with increasing temperature causing increasing collector current is that more current increase the power dissipated by the transistor which, in turn, increases its temperature. This self-reinforcing cycle is known as thermal run away, which may destroy the transistor. Again, the solution is a bias scheme with some form of negative feedback to stabilize the bias point.