Link 0V from different PSUs

[cha1n]

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

 

[matthewm1965]

The way that I would do this is to take the 24V PLC supply through clean contacts on the machines.

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

 

[cha1n]

Hi Mathew,

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

 

[ScottyUK]

DIN terminal style relays take up as much space as... well, as much space as a DIN terminal. 90 of them would fit in about 2' of rail. Check out Weidmuller and Phoenix Contact for some examples, I think Wieland and no doubt others also make them.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[cha1n]

It's not a space issue, the electromechanical relays i've used are 15mm (width), but they're £5.50 each so if i could not use them, it's a win/win situation (space and money)!

 

[matthewm1965]

In many systems that I have worked on, the machines 24V psu would have the 0V rail connected to earth. This is effectively commoning up all the 0Vs anyway.
Matt

 

[roydm]

I agree with Matt, Ground all the power supplies and have a common wire as well so you arn't using the ground as a path.
Roy

 

[vinnygalbo]

Did you look at Weidmuller's RIDERSERIES? It's fairly new. I think it came out late last year.

http://www.clrwtr.com/Weidmuller-Relays-Relay.htm

 

[itsmoked]

I am very surprised that an opto-isolated Digital IN module needs all the field to have the same physical return?! How lame...

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

 

[CharlieGill]

I would agree with Keith. All of the optically isolated DC inputs I have seen do not require a common common. Each set of input terminals connects to its respective photo diode in the optocoupler (plus some current limiting circuitry).
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

 

[cha1n]

There is only one screw connection per channel, with all channels sharing a common. Very frustrating because i always need interface relays to solve the problem. The units are Siemens SM321 (6ES7 321-1BH02-0AA0) and definately have a common ground. I just wanted to know what implications of connecting all of the commons together was (as said, i currently use interface relays). If anyone has any further reading on this subject, i'd be very greatful. Is it classed as earth loops or something like that?

Thanks

 

[roydm]

I'm not sure why you are reluctant to ground the power supplies, that would be the easy solution. In my experience ungrounded supplies cause more problems than grounded ones. The chances are half your power supplies are grounded already.
Another solution would be to bond all the negatives together and tie them to the negative input to the PLC card.
Roy

 

[LionelHutz]

This was posted a while ago now, but if the power supplies are not grounded at the machine then you can run the wiring connecting the commons together at the PLC input card. There's no reason this would cause a problem as there is not a ground loop because the supplies are not grounded. If there is a power supply ground connection in the machine then you could remove it there and ground at the PLC card instead. This is called star grounding.

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.