Driving 40 mA relays with Mesa 7i76, over-current warning.

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05 Jan 2018 18:29 #104054 by Patrik T
Hi
I read in the 7i76 manual that the outputs could source 350 mA.
I designed the system with a 24 V, 600 Ohm coil (40 mA), relay to be driven directly from a 7i76 output.
On first try I received an over-current warning after ca 10 seconds and I could smell hot electronics.
I disconnected the output wire and measured the current to 40 mA.

Am I missing something? When searching for Mesa + relays I find no indication that driving 40 mA coils should be impossible.

Thanks
Patrik

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05 Jan 2018 18:39 #104055 by PCW
7I76 outputs are tested driving a 68 Ohm load with a 30V Vfield (~400 mA)

If you get an over current warning this suggests a shorted load or maybe a
backwards flyback diode. A overcurrent alarm should instantly disable the
outputs so if something is overheating on the 7I76 that's a bad sign

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05 Jan 2018 18:42 #104056 by Patrik T
Thanks
Then I know it should work.
I'll try again and look for any mistake I might have done.

Thanks for super fast support.

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05 Jan 2018 20:24 #104057 by Patrik T
Wrong polarity over internal flywheel diode. Smell came from diode.
No Nobel prize for me today :)

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