Diodes on IO when used as an dry contact?

shr00m-dew

So I've got a job with three cable boxes. No discrete's so I ran the switched outlet to a 110 relay and ran the NO side of that relay to three IO's on a NI4100.

Two of those IO's are now dead. Luckily we had two IO's open and the setup is again working. I know about using diodes when driving a relay with the IO's, but do I need to put something on them when using them as a dry contact?

Or should something be put on the 110v coil? I'm trying to prevent any downtime if any more IO's fail.

Thoughts?

Kevin D.

John Gonzales

Normally the diode is placed on the coil because when the magnetic field collapses in the coil, there's a large voltage spike. The Voltage spike is bad for whatever is driving the coil which is the cable box in this case. Could be something arcing on the relay causing the problem.

Instead of relaying the 110, can you use a small wall wart 12vdc transformer in the switched outlet of the cable box and use a 12vdc relay instead? Maybe less current and voltage will be better.

--John

shr00m-dew

That was one thought. I've got a stack of the Altronix relays with the diodes at the shop. I'll bring back three of them and three wall-warts.


Thanks,

Kevin D.

Thomas Hayes

The CEMF from 110V relay coil can be very high. I'd use a small electronic relay if you can to switch the bigger one.