PLC Relay Outputs Going Bad.

[EE93PSU]

I have an A-B Micrologix 1000 PLC (1761-L32AWA) with relay outputs which are rated for 2 amps. These outputs control MAC valves. The outputs fail frequently--I'm guessing due to the voltage transients caused by the solenoids, but would they really exceed 2 amps? What kind of surge suppression should I use here? (Manufacturer/cat.# would be very helpful.)

Thanks.

 

[Skogsgurra]

Voltage transients rarely exceed 2 amps .

But voltage transients do easily exceed hundred of volts. Especially if there are no free-wheeling diodes, snubbers or MOVs.

If your valves are DC, you need a free-wheeling diode or some other device. If they are AC, you need a snubber or varistor, aka MOV.

If you cannot tolerate the dela caused by diodes, use varistors or a series resistor with the diode.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Skogs has posted some good tips above.

Interposing relays are cheap. So are fuses. If these valves are mains voltage I'd isolate them from the PLC through an interposing relay, regardless of the PLC relay's capability. PLC cards are too expensive to blow up, especially at Rockwell prices!


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

I toasted a relay running a solenoid. I had a setup that chattered. A blue light came out of the opaque relay housing. The contacts were completely gone from the relay. There were just charred copper arms left.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

http://literature.rockwellautomation.com/idc/groups/literature/documents/sg/1761-sg001_-en-p.pdf

page 44.

Looks like 2 A is the continuous rating. Make/break rating is 28 VA.

 

[SteveBailey]

If these are dual solenoid valves, check your program logic to make sure that you're not energizing both coils simultaneously. That's a sure-fire PLC output killer.