Back current in a heating pad destroying a power supply?

For some biology experiments, I designed a heating pad (10W 5V) that is powered by a power supply (5V 1A linear) going through a MOSFET (TO-220 40V 195A). The MOSFET is controlled by a PID.

I've noticed that occasionally the power supply stops working entirely. I think the problem may be due to the temperature probe getting disconnected from the heating pad and causing a prolonged spike in heating pad temperature which generates a back-current? Is this plausible?

I am planning on using a 1N4001 diode on the power side of the heating pad. Would this be enough to solve my problem, or do you think there is something else going on?

-Ethan

If your heating pad is 10 Watts, 5V, that's 2 Amps. It's no surprise that your 1A power supply is shutting down.

Do you think a 1N4001 can prevent this?

You aren't going to get 2A out of a 1A supply. You'll need a power supply that can supply at least 2 Amps or more. 3 or 4 Amps would be better to give some margin.

This is not outside the range of a simple wall wart switcher like this one:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2082222_-1

The 'back-current' (more commonly called 'back EMF) you're thinking of is what results from switching an inductive load. Your heating pad is almost certainly a largely resistive load. It's doubtful that it's inductance is enough to cause a problem. Even if there were back EMF, the problem it would cause would be to blow out your FET, not cause the power supply to shut down.

That said, however, it wouldn't be a bad idea to have a 'catch diode' (aka 'flyback diode') on the output of the FET anyway. If you don't know the full characteristics of your load, it's good insurance to protect the FET. The 1N4001, placed on the FET output so that it is reverse biased, should work fine.

Thanks!

That's very helpful information