Relay Coil Spike Suppression

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daneloctober

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Hello again!

This is very elementary but I need more insights since I'm not a power electronics guy...

The problem is, I have this circuit here:



Just the encircled part, don't mind the rest...

The mosfet (2N7000) keeps breaking. The relay (NY24WK) coil is rated 24V, at 120mW... So that's about 5mA of coil current, right? The 2N7000 is rated at about 115mA max. I'm really not sure what happens, but after a few switches (SW2+ and SW2- opening and shorting), the mosfet gets busted. I keep reading that the diode should be enough but apparently, in this case, it's not.

What more can I add to protect the mosfet? Thanks very much!
 

Make sure the current path for the flyback current formed by D4 and the bypass capacitors has low parasitic inductance. If you do that then overshoot shouldn't be a problem. But looking at your circuit I feel your problem isn't overshoot, but rather slow switching times of the FET, especially during turn-off.
 

mtwieg said:
But looking at your circuit I feel your problem isn't overshoot, but rather slow switching times of the FET, especially during turn-off.
Click to expand...

That's what I think, too. Looking at the datasheets, the 1N4001 has a reverse recovery time of 30us, while the 2N7000 has turn off time of 10ns (well it depends on the conditions... but what the hey).

Should I change the diode as IanP mentioned?...
 

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