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RELAY8: FET failure modes

Posted: Sat Jan 24, 2015 1:09 am
by JollyJohn
Hi All,
I suspect I may have damaged some of the 2N2007 FETs on a RELAY8 board, but the failure mode seems odd. The three channels that have failed will not turn off. At power on the red LED comes on, the attached device activates and functions as expected - but I cannot turn off the channels. Other channels behave as expected. I would expect the failure mode to be open, with the device permanently off, but I haven't blown up enough FETs to be sure. :D

Is this a failure of the FET, or something more bizarre?

Re: RELAY8: FET failure modes

Posted: Sat Jan 24, 2015 6:07 am
by andrew
Hard to say, could you please describe what you've got connected to the channels that aren't behaving?

Re: RELAY8: FET failure modes

Posted: Sat Jan 24, 2015 10:36 pm
by JollyJohn
Problem solved and it's my bad. The load is a solenoid. I did all my calculations based upon the AC ratings, but I was driving it with DC. I’ve checked the inrush and holding currents and on DC (hence no reactance) the load is much higher – and pop go the MOSFETs! I’m about to try a fit a bunch of relays into the case, which I was hoping to avoid.

In the meantime I now own the very first pair of a new product line – RELAY5.

Thanks for the follow-up…

Re: RELAY8: FET failure modes

Posted: Sat Jan 24, 2015 11:55 pm
by andrew
JollyJohn wrote:Problem solved and it's my bad. The load is a solenoid. I did all my calculations based upon the AC ratings, but I was driving it with DC. I’ve checked the inrush and holding currents and on DC (hence no reactance) the load is much higher – and pop go the MOSFETs! I’m about to try a fit a bunch of relays into the case, which I was hoping to avoid.

In the meantime I now own the very first pair of a new product line – RELAY5.

Thanks for the follow-up…
Ah there you go. Thank you for letting us know. We all learn by making and breaking :)