Link 0V from different PSUs
Link 0V from different PSUs
(OP)
Hi,
I'm in a situation where i have 45 machines, each with their own 24VDC power supply.
I'm taking a +24VDC output from a limit switch and a start button on the machine and feeding them into a digital input card for a siemens PLC.
The PLC card uses one common 0V for each +24VDC input to it, so can i link all of the machines 0Vs together even though they use different PSUs?
If i can't i know i can use a single pole changeover relay (or SPDT) and just switch +24VDC from a single PSU for use with all of the machine PSUs to interface with the PLC card, but i'd rather not buy 90 relays if i can help it!
Thanks,
Ross
I'm in a situation where i have 45 machines, each with their own 24VDC power supply.
I'm taking a +24VDC output from a limit switch and a start button on the machine and feeding them into a digital input card for a siemens PLC.
The PLC card uses one common 0V for each +24VDC input to it, so can i link all of the machines 0Vs together even though they use different PSUs?
If i can't i know i can use a single pole changeover relay (or SPDT) and just switch +24VDC from a single PSU for use with all of the machine PSUs to interface with the PLC card, but i'd rather not buy 90 relays if i can help it!
Thanks,
Ross





RE: Link 0V from different PSUs
Can you disconnect the machines internal supply from the start PB and limit sw?
It depends on the spec of the PLC input card. If they are individual isolated inputs, you could probably take the external machine supplies in, but if not, you could end up in a situation where you are paralleling the different PSUs, which would definately give you problems.
Also think of the safety aspect of having 45 different supplies coming into a PLC panel. How would you isolate for safe working?
Matt
RE: Link 0V from different PSUs
The PLC card inputs are optically isolated so i haven't got any circuit protection between field and PLC card.
As mentioned the circuit diagram for the PLC card seems to illustrate that every +24V input must have a common 0V.
I did request clean contacts, but that was not available. I might have to just bite the bullet this time and use relays to switch a seperate 24VDC supply within the control panel and hope nobody notices as the job is so urgent. However in future i'd definately like to know if i could link all the 0V together as these Electromechanical relays with bases are about £5.50 each!
Thanks for the reply.
Ross
RE: Link 0V from different PSUs
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If we learn from our mistakes I'm getting a great education!
RE: Link 0V from different PSUs
RE: Link 0V from different PSUs
Matt
RE: Link 0V from different PSUs
Roy
RE: Link 0V from different PSUs
http://www.clrwtr.com/Weidmuller-Relays-Relay.htm
RE: Link 0V from different PSUs
Does each DI have one screw or two? If it has two, ohm out the screws between a neighboring channel, if there is no continuity there should be no need for tying any returns together.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Link 0V from different PSUs
I would hesitate to arbitrarily earth all the supplies if they are not already. Particularly if you are powering any instrument loops.
Just connect the common and signal line from each machine. It should work.
Charlie
RE: Link 0V from different PSUs
Thanks
RE: Link 0V from different PSUs
Another solution would be to bond all the negatives together and tie them to the negative input to the PLC card.
Roy
RE: Link 0V from different PSUs
If the supplies are grounded at the machine and have to remain grounded at the machines then connecting the commons together can cause ground loops and I'd say keep the relays like you are using.