Hello all, press line here, CompactLogix with a 1769 OB16 sourcing output card. Every few months one output on it dies. Always one of the ten driving the interface relays, never the six going to the stack light and the beacon. Third card in 14 months.
New card each time, obviously. I measured the 24VDC rail and got 24.3 steady, then rang out the relay wiring for shorts and found nothing. The supplier reckons a bad batch and has offered us a credit.
Relays are 24VDC ice cubes in sockets, ten of them, driving contactors and a couple of valve coils. Is it the supply doing something we can't see on a meter, or is the batch story real? Ten relay outputs gone and the six on the lamps never touched is the part that bothers me, does that narrow it down at all?
What's on the relay sockets? Any little module clipped across the coil terminals, a diode, an RC? And the coils are DC, not AC through a rectifier?
Coils are 24VDC. Sockets are plain, nothing clipped on them. There's an LED on the relay itself and that's all it has. Pretty sure nothing else has ever been near them. Out of ideas here.
Every time an output turns off, the field in that coil collapses and the coil throws a spike out the wrong way, a few hundred volts for a few microseconds, straight back into the transistor that switched it off. The transistor takes that a few thousand times and then one day it doesn't. The stack light is resistive, no spike, so those channels live forever. It isn't a batch and it isn't the supply.
Put a diode across every coil, cathode to +24, anode to 0V. A 1N4007 does it. Tidier again, the plug-in suppression modules for those sockets, diode type, they clip straight on. Relays with the diode built in for the next panel. Panel locked out before you're in the wiring.
So the diode direction matters. Put it in backwards and it's a dead short across the coil the moment the output comes on?
Yes. Backwards it kills the output faster than the coil did.
Fitted the clip-on suppressor modules on all ten sockets, diode type, band toward +24 on every one. New card in.
The drive vendor's guy was in for something else and put a scope across a coil for us, probe referenced to the panel 0V, before and after. 300 and something volt spike before, nothing after.
Ten DC coils with nothing across them and a transistor eating the spike every time one dropped out. Three cards in 14 months and it was never the cards.
The older press panel next to it has the same relays and hasn't lost an output in 12 years. Don't get that one at all. Thanks ron, ed.
The old panel's card is relay outputs, I'd bet. Look at the number.