Blown PLC Outputs for bidirectional control valves
Blown PLC Outputs for bidirectional control valves
(OP)
Having numerous 24VDC (rated @ 2A continuous) output cards that are going bad over the past year (a point or two will remain ON/OFF.) Cards are 3 years old. 16 point in 2 groups of 8 with. The outputs are 24VDC Solenoid Control valves (rated @ 1.25A with built in surge suppression). They cycle ON once every 30 seconds and are on for about 15 and then turn OFF.
Have not been able to put a clamp current meter on it yet.
Anyone know what the make\break max amp. means? It is 1.2 Amps. Our load is 1.25A?
What if we have the valve energized on one side and de-energize it and energize the second side at the same time. Could that increase the arc to more than the SS diode could handle?
Just looking for ideas.
Thanks,
Funguy
Have not been able to put a clamp current meter on it yet.
Anyone know what the make\break max amp. means? It is 1.2 Amps. Our load is 1.25A?
What if we have the valve energized on one side and de-energize it and energize the second side at the same time. Could that increase the arc to more than the SS diode could handle?
Just looking for ideas.
Thanks,
Funguy





RE: Blown PLC Outputs for bidirectional control valves
RE: Blown PLC Outputs for bidirectional control valves
The 2 A rated current is most probable resistive load. For inductive loads it is usually a lot less. Your surge protection is probably an RC combination. That is not good enough for a DC solenoid. Try putting a free-wheeling diode there instead. That should help. But you are still operating close to what the contacts can take.
Gunnar Englund
www.gke.org
RE: Blown PLC Outputs for bidirectional control valves
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I don't suffer from insanity. I enjoy it...
RE: Blown PLC Outputs for bidirectional control valves
You should probably consider the solid-state solution or just plan to replace the modules once a year as a standard maintenance project.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Blown PLC Outputs for bidirectional control valves
With an inductive load the contacts are derated quite a bit right? So with a 1.25 A load on 2 A contacts derated
by X amount with this amount of cycles this could be why
we are seeing the failures? How many cycles are PLC relay contacts typically rated for? I heard as high as 20 million?
I really appreciate the help and am going to look at SSR cards.
Thanks,
Funguy
RE: Blown PLC Outputs for bidirectional control valves
RE: Blown PLC Outputs for bidirectional control valves
In that line of thought, I think I recall an optical relay of some sort? The I/O card switched the optical relay, which then turned the current on/off to the solenoid?
Or, I could be mistaken.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Blown PLC Outputs for bidirectional control valves
Dumb question: There are no gotchas when it comes to SSRs like derating them for inductive loads or anything right. I found some that are up to 3 A continuous and so I don't derate that do I?
Thanks in advance,
Funguy
RE: Blown PLC Outputs for bidirectional control valves
Gunnar Englund
www.gke.org
RE: Blown PLC Outputs for bidirectional control valves
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Blown PLC Outputs for bidirectional control valves
I can't quite picture what product you are describing - could almost be a standard SSR because most of them use an optocoupler for the front end.
funguy123,
You might save some money by using an open collector transistor output card to drive the SSR's rather than an SSR card in the PLC. The input of an SSR is a very friendly load and could be driven by almost any of the open collector cards available. They only draw a few mA at, say, 12V.
Pay heed to skogurra's comments about a diode in antiparallel with the relay (i.e. diode reverse biased in normal operation).
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I don't suffer from insanity. I enjoy it...
RE: Blown PLC Outputs for bidirectional control valves
Could be. I am going on recollection.
Thanks.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Blown PLC Outputs for bidirectional control valves