APPS: APPS: MicroLogix1400 / Solid-State Relay Application Question

Tue Sep 29 07:02:02 EDT 2009

==> Automation List post by curt wuollet...
To view or reply to this thread: http://www.control.com/thread/1254145201#1254193414

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Re: APPS: APPS:  MicroLogix1400 / Solid-State Relay Application Question
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I'm right with you on the driving relays with relays issue. But I've seen it done in many places. The key here, I think, is the very low switching rate. If cost is no object, the transistor/triac outputs with replaceable relays might be the way to go, with suppressors on the outputs. That would be the gold standard way. But with only very infrequent switching, it would be hard to justify the cost, and the relay outs, used within ratings, should be fine. One thing that may make the decision easier is that you often need isolated contacts when replacing relay logic. The MLs I've seen had bussed relay contacts, usually broken up in groups. If that will work, I'd probably use the PLC relay outputs.

If you can't have the common contacts, then I'd get the solid state outs and use relays. In either case use suppression or you don't have to worry about which will be more reliable because neither will be. This is neglected a lot these days and then the relays are called unreliable. If you can see a relay arcing from the other side of the plant, it's not going to be reliable.

Regards,
cww
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