# 120 ac or 24 vdc plc cards?



## JRaef (Mar 23, 2009)

24VDC I/O cards will be less expensive and higher density in your PLC cards. But the two main drawbacks are voltage drop over distance and large loads like contactor coils. So if these are just for status feedback of your starters and are not going to deal with the actual coil power, I would say you are fine using 24VDC. If you want to run the coil with this circuit, use the interposing relays and 120VAC controls.


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## NC Plc (Mar 24, 2014)

When using power from PLC cards is it common practice to use a fuse to protect the card?

My Directlogic cards recommend it.


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## splatz (May 23, 2015)

NC Plc said:


> When using power from PLC cards is it common practice to use a fuse to protect the card?
> 
> My Directlogic cards recommend it.


There's a very good discussion of this here: 

http://www.electriciantalk.com/f28/proper-way-wire-plc-7327/

There is no consensus on the best way to wire PLCs, but fuses seem like a good idea if the output doesn't go to relay inputs.


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## NC Plc (Mar 24, 2014)

I saved that link for another day. Thank you.


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## garfield (Jul 30, 2009)

JRaef said:


> 24VDC I/O cards will be less expensive and higher density in your PLC cards. But the two main drawbacks are voltage drop over distance and large loads like contactor coils. So if these are just for status feedback of your starters and are not going to deal with the actual coil power, I would say you are fine using 24VDC. If you want to run the coil with this circuit, use the interposing relays and 120VAC controls.


In another thread there was discussion of the contacts on a relay having to little power going through them and then they quit conducting electricity. Is this relevant when discussing ac versus dc?


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## splatz (May 23, 2015)

garfield said:


> In another thread there was discussion of the contacts on a relay having to little power going through them and then they quit conducting electricity. Is this relevant when discussing ac versus dc?


If I remember right, that's more of a problem with the contact surface on the contact side glazing if there's not enough juice going through there to zap it off. That would be an argument for driving small loads directly through the PLC, or using a solid state relay.


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## triden (Jun 13, 2012)

NC Plc said:


> When using power from PLC cards is it common practice to use a fuse to protect the card?
> 
> My Directlogic cards recommend it.


Yes, always put overload protection on your outputs. I personally like to use thermomagnetic breakers, but fuses will work (just a pain to replace if none are handy).


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## oliquir (Jan 13, 2011)

i tend to use 24vdc everywhere if possible ( no risk of electrocution) but some of my old school clients don't like it :blink: so i still use 120v cards for them


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## Cow (Jan 16, 2008)

I like 120v, because there are no issues if you have to mix power and control in a conduit.


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