24 Vdc

VigneshGunasekaran

Member
V
Join Date
Nov 2009
Location
India
Posts
116
Hi

I have PLC IO modules which has its own 24VDC power supply power supply from PLC panel but field devices having its own 24VDC.

1. Is the field input is connected to DI module will it work (note the field device and module supply are from different source)

2. what happens if i short/connect 24VDC negative from two different sources to make common reference.

3. what happens if i short/connect 24VDC positve from two different sources to make common source.
 
If both pwr source are isolated from ground it may work connecting both negative toghether and using only pwr from field device...
but is field device is cheap of far from the plc....anything happening to it's negative pwr can burn all your plc...(Bad)

If you parrallele both pwr (+ with + and - with -) voltage difference while be shorted somewhere....(Bad)

You should use an isolator connected to field pwr and isolated contact to your plc....if slow a small relays would make it if not an opto or a small SSR
 
Jeff23spl said:
If both pwr source are isolated from ground it may work connecting both negative toghether and using only pwr from field device...
but is field device is cheap of far from the plc....anything happening to it's negative pwr can burn all your plc...(Bad)

If you parrallele both pwr (+ with + and - with -) voltage difference while be shorted somewhere....(Bad)

You should use an isolator connected to field pwr and isolated contact to your plc....if slow a small relays would make it if not an opto or a small SSR
Click to expand...


I feel if anything happens to field negative, my module will not sense any voltage because it donot have any reference right?
 
VigneshGunasekaran said:
Hi

I have PLC IO modules which has its own 24VDC power supply power supply from PLC panel but field devices having its own 24VDC.

2. what happens if i short/connect 24VDC negative from two different sources to make common reference.
Click to expand...

I have never had any trouble with option two. Tie the commons together. If that connection is broken AND you exceed the isolation rating of the PLC input card it may be damaged, but that is unlikely to happen with most hardware. Refer to the specifications of your hardware to determine those ratings.

This is assuming that your PLC input is looking for +24vdc, and that the negative side of the power supply is considered common.
 
OkiePC said:
I have never had any trouble with option two. Tie the commons together. If that connection is broken AND you exceed the isolation rating of the PLC input card it may be damaged, but that is unlikely to happen with most hardware. Refer to the specifications of your hardware to determine those ratings.

This is assuming that your PLC input is looking for +24vdc, and that the negative side of the power supply is considered common.
Click to expand...

What you mean if the connection is broken.....

If the connection is broken there will not any potential at all, so how it spoil my hardware....can you pls state clearly
 
All aplications that I see is similar #2 option. I think you have no any problem with this kind of connection.
 
To me, the right way to do it is with relays to separate the power.

That being said if you common the 0vdc of both power supplies and connect both to ground, you should be fine. It's not the right way to do it but it will work OK.

Technically you don't even have to connect the 0vdc to ground, but I would. I notice I am the only one that said to connect 0vdc to ground so I am the lone wolf so far.
 
Last edited:
As Paul B said, tie both 0V together and to ground. Without doing this the voltage could 'float' and you could damage something.

Rev
 

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