Basic wiring PLC

zeddie · Sep 11, 2012

Hi!
I discussed with my friend today about wiring a switch to a digital input. The discussion was about the zero in the case in the picture. Is this a proper wiring or do I have to wire the zero to the input-card? We both know there is more simple solutions but we couldn't agree about this example.

James Mcquade · Sep 11, 2012

using your picture.
from the 230 volts, you go into the 24 volt power supply.
from the power supply, you goto your device which powers your input. there should be a common on the input card to connect the (-) of the power supply to complete the circuit and make the input work.
regards,
james

Lancie1 · Sep 11, 2012

Yes, a circuit must be a loop with two wires. One wire cannot supply current. Some ground all DC power supplies and allow the ground to be the second wire. This is a big no-no. Ground should never be used for a current-carrying conductor.

zeddie · Sep 11, 2012

Think I forgot to say that the common (-) at the input card is connected to the left 24 VDC power supply common wire. Is'nt that a proper loop then?

KalleOlsen · Sep 11, 2012

No. It's the same situation as when you jump start a car from another. Needs both + and - cables.

Kalle

Liam Moran · Sep 11, 2012

The 0 volt reference at the input must be the same potential as the 0 volt from the power supply supplying the sensors. If both power supplies have a floating output then it is not possible to reference between them. As Kalle points out it is the same as 2 batteries.

Liam Moran · Sep 11, 2012

By the way, Kalle, I am in Norway at the moment.
What a breathtakingly beautiful country.
Friendly people too. I love it.

KalleOlsen · Sep 11, 2012

Liam Moran said:
By the way, Kalle, I am in Norway at the moment.
What a breathtakingly beautiful country.
Friendly people too. I love it.

Nice words, thank you!

I recon you haven't bought a beer yet?

Btw where appx are you? Let me guess; if the people are nice, it's likely up North.

Kalle

zeddie · Sep 11, 2012

I don't get the diffrence between my first picture and this one from ABB, see ch1. It's still the same main supply, same (+)-feeder and same (-)-feeder..?

Liam Moran · Sep 11, 2012

KalleOlsen said:
Nice words, thank you!

I recon you haven't bought a beer yet?

Btw where appx are you? Let me guess; if the people are nice, it's likely up North.

Kalle

Yes I bought a beer. Called my wife to sell the house to pay for it

I am in a small town south of Oslo called Drobak.

I am working near to Oslo on the south side.

I am removing a machine which is being shipped to Ireland.

Liam Moran · Sep 11, 2012

zeddie said:
I don't get the diffrence between my first picture and this one from ABB, see ch1. It's still the same main supply, same (+)-feeder and same (-)-feeder..?

Your first drawing shows 2 power supplies.
The remote power supply is powering the switch.
The (-) must also come from this power supply as a reference otherwise there cannot be a potential difference to activate the input.

brendan.buchan · Sep 11, 2012

Your first diagram would only work if the 0VDC output of both power supplies were connected together, or bonded to earth.

Without this they are essential 2 electrically isolated 24VDC floating supplies. There is no current path or complete circuit from 24VDC of one supply to 0VDC of the other supply. You can test this with a multimeter.

zeddie · Sep 12, 2012

So if I connect both minus (-) at 24vdc- power supplys to earth it would work right? Is it ok way to do that with 24vdc? Normally the (-) is connected to ground at 24vdc ps i think, because of PELV.

Lancie1 · Sep 12, 2012

So if I connect both minus (-) at 24vdc- power supplys to earth it would work right?

Yes it will work. People do it all the time. Here in the US it is illegal to use a ground or earth conductor as a current-carrying conductor. A 0-potential earth ground is just that and should stay that. Using it as the -24 volt side of your power supply is techinally not allowed, although many ignorant people get away with it. They even label the -24 volt leg as "0 volts". Of course it is not 0 volts relative to the +24 leg, but can only labeled that because that leg is also grounded. It then measures 0 volts to ground, but it is still your -24 volt power leg of the power supply.

Is it ok way to do that with 24vdc?

Sure, go ahead and join the crowd. If enough people do it, maybe they will change the rules.

Lancie1 · Sep 12, 2012

To test your power supplies to see if they are wired correctly, temporarily remove the ground wire. If you lose power somewhere in the system, then your wiring was incorrect. You were using the grounding or earthing wire as a current-carrying conductor. Minus 24 volts should be labeled "-24 VDC" and never confused with the true grounding wire. You should never be able to measure any current flowing through the ground wire at any point.

Basic wiring PLC

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