Wiring a plc

When wiring a plc to a circuit, does both the input and output need a voltage source? If so, How do you wire a circuit using a relay?

Yes, inputs and outputs need a voltage source, but they don't have to BOTH come from the SAME source.

Using Phil's example...



Think of the input to the PLC as the "coil" of the relay, and the output from the PLC as the "contact" of the relay

beerchug

-Eric

Whether an input or an output, you need a voltage source and a complete circuit. A PLC doesn't allow you to violate any of the rules you learned in Elementary Circuit Theory.

Whwn you ask about wiring a relay, are you asking about a relay output module in the PLC or wiring to an interposing relay?

In the former case, the PLC relay contact output is simply an electromechanical switch that allows you to complete the circuit under the control of the PLC.

In the latter case, the PLC output is wired to the coil of a relay. You need a current source to power the coil. A contact from the relay controlled by the PLC is then used to switch the ultimate device. You need a current source for that circuit as well. It could be the same source that powers the relay coil, or it could be separate.

As Terry would say...IT DEPENDS

BUT???

Does a LAMP need a power source?

Does the switch to turn on a LAMP need a power source?

The power requirements for Inputs is usually low, but output requirements can be diverse/high. This is an area/subject you have to learn.

Actually, Terry would say...

Ralph, you're asking us to make a few assumptions regarding your questions.

"When wiring a plc to a circuit, does both the input and output need a voltage source?..."

The answer to this question (as posed) is...

Of course they both need a voltage source! That is, if you plan on using them!

I think the real question is...

Does the PLC provide power for inputs and/or outputs? Or, must power be provided by some external means?

NOW, The answer to this question is... It depends!

INPUTS:
Some PLC's provide 12-30VDC for Inputs. (With only a limited amount of Power!- Be careful!)
Some don't.

OUTPUTS:
I don't know of any PLC's that actually provide the power for outputs in the same way that a 120VAC PLC might provide 12-30VDC for Inputs.

The PLC simply acts, as a switch, in one of two ways...

• A SOURCING SWITCH - The PLC Output is a "switch" between the HOT Source and the Field Device. (The other side of the Field Device is connected to N/COM.)
  
• A SINKING SWITCH - The PLC Output is a "switch" between the Field Device and Neutral/COM. (The other side of the Field Device is connected to the power source.)

...If so, How do you wire a circuit using a relay?"

This is an example of a question being too general.

Are you talking about an Input Circuit? ...Output Circuit? ...Power Circuit? ...Control Circuit? ...WHAT?

I think this might be a case of not being experienced enough to know what question to ask. Or, if you do know what question you want to ask, then it's a case of not knowing "how" to ask that question.

Ralph, I think most of the guys here would agree that, when it comes to designing, it is more important to have the "right questions" than it is to have answers.

A bucket-full of answers to the wrong questions are useless!