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Old May 6th, 2011, 08:49 AM   #1
SteinCollector
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DC into AC PLC inputs

I know this can be solved by using an interposing relay, but I'm trying to reduce cost. We have an application that has several AC inputs and 1 DC input - 24VDC, and I would like to use an Eaton smart relay. Can the AC inputs "see" a 24VDC input, or maybe a higher VDC? I'm more of a programmer, so I really don't know the behavior of the I/O in this situation.

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Old May 6th, 2011, 09:01 AM   #2
Christoff84
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Think of it this way...$20 relay or blown/intermittent input on the smart relay.
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Old May 6th, 2011, 09:13 AM   #3
Jeff23spl
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as stated i would keep the interposing relais option but maybe finding a micro size modele as it does'nt need much output current anyway

You may look at your input card construction and maybe their is some made with rectifier diodes on the AC side to actually trigger the input with DC voltage meaning it would work with DC at same voltage level. But the usual one i use got optocoupling where i don't believe would work on DC
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Old May 6th, 2011, 10:29 AM   #4
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Whenever you try to cheat the system, it WILL come back to haunt you in the future. Do it right the first time.
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Old May 6th, 2011, 01:03 PM   #5
Hydro Power Guy
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I guess I don't understand the savings. How can a $20 relay be offset by the cost of the engineer/designer time.
I would expect that you already spent at least 1 hour looking into this so if you do save the $20 relay, it is more than offset in labor.
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Old May 6th, 2011, 02:33 PM   #6
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This is a case where hundreds of this unit would go out in a year, so over time the savings would be pretty substantial.

My thanks to Jeff23spl for at least answering the question.
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Old May 6th, 2011, 02:55 PM   #7
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the input will work as most AC inputs have a diode bridge
outputs are not compatible as the AC is a Triac and DC is a transistor
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Old May 6th, 2011, 05:26 PM   #8
Mickey
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I'm not a Eaton smart relay user so I will ask,
are the inputs isolated ( don't mix Ac with DC). Is there one neutral connection. If so how would the DC input find a return.

As other have said don't do this. $20 buck would be an expansive relay
you can get a base and relay for under $10.
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