Is Back EMF From Heating Pad Damaging 5V 1A Linear Power Supply? Diode Protection?

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#1 21670457 22 Nov 2013 14:58

Ethan Hathaway Ethan Hathaway

Anonymous

Post #1
21670457 22 Nov 2013 14:58

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
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#2 21670458 22 Nov 2013 16:08

Gary Crowell Gary Crowell

Anonymous

Post #2
21670458 22 Nov 2013 16:08

If your heating pad is 10 Watts, 5V, that's 2 Amps. It's no surprise that your 1A power supply is shutting down.
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#3 21670459 22 Nov 2013 16:14

Ethan Hathaway Ethan Hathaway

Anonymous

Post #3
21670459 22 Nov 2013 16:14

Do you think a 1N4001 can prevent this?
#4 21670460 22 Nov 2013 16:21

Gary Crowell Gary Crowell

Anonymous

Post #4
21670460 22 Nov 2013 16:21

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.
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#5 21670461 22 Nov 2013 16:26

Gary Crowell Gary Crowell

Anonymous

Post #5
21670461 22 Nov 2013 16:26

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
#6 21670462 22 Nov 2013 16:42

Gary Crowell Gary Crowell

Anonymous

Post #6
21670462 22 Nov 2013 16:42

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.
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#7 21670463 22 Nov 2013 17:14

Ethan Hathaway Ethan Hathaway

Anonymous

Post #7
21670463 22 Nov 2013 17:14

Thanks!

That's very helpful information
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Topic summary

A 10W 5V heating pad powered by a 5V 1A linear power supply through a TO-220 MOSFET controlled by a PID occasionally causes the power supply to fail. The heating pad draws approximately 2A, exceeding the 1A rating of the power supply, which likely causes the shutdown. The concern about back EMF damaging the power supply is unlikely since the heating pad is a resistive load with minimal inductance; back EMF typically arises from inductive loads and would more likely damage the MOSFET rather than the power supply. Adding a flyback diode such as a 1N4001 diode reverse biased on the MOSFET output is recommended as protection for the MOSFET but will not address the current overload issue. A power supply capable of delivering at least 2A, preferably 3-4A for margin, is advised. A simple wall wart switch-mode power supply is suggested as a suitable replacement.
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Is Back EMF From Heating Pad Damaging 5V 1A Linear Power Supply? Diode Protection?

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Topic summary