Common 0V only on 2 x 24VDC power supplies

Hi guys,

I have 2 separate power supplies (240VAC mains to 24VDC - both 10A @24VDC). The fan draws 8A hence its own power supply and the other is for the PLC and its I/O.
The fan has 3 contacts, 0VDC, 24VDC (this is where the first power supply connects to.
The 3rd contact on the fan is control input (24VDC on this contact to start the fan) - The PLC switches this and the PLC and control is fed from the 2nd power supply.

All simple but the fan will not start as the voltage between the fan control input and 0VDC on the fan is not 24VDC as separate power supplies at different potential. I don't want to use a relay as space is limited but my question is can I just common up the power supplies 0VDC only so this fan will run as 0VDC on both PSU at the same potential?

I cant see any reason why not as I am not paralleling up the power supplies so no diodes etc required and the fan can only draw power from the fan PSU and the PLC/control from the control PSU?

if its the same source for both supplies there wont be any problem connecting the 0V into eachother.

I do this with both power supplies and VSDs so you wont have to use relays.

Hi ojz0r,

thanks for the quick reply.

For workshop testing, the PSU will be connected from the same source (240VAC mains)

But where this project gets delivered, the PSU will be provided/installed by the client.

But say the PSU were off different sources (ie different gen-sets, would this still cause any problems as the 0VDC outputs from the PSUs are commoned up only?

Thanks
Matt

I would hope that you tie your OV down to earth anyway. Having said that 0VDC is 0VDC. Tie away I say.

qwsoftware said:

Hi ojz0r,

thanks for the quick reply.

For workshop testing, the PSU will be connected from the same source (240VAC mains)

But where this project gets delivered, the PSU will be provided/installed by the client.

But say the PSU were off different sources (ie different gen-sets, would this still cause any problems as the 0VDC outputs from the PSUs are commoned up only?

Thanks
Matt

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If you got a floating 0VAC (not equal to ground) on one of the power supplies and a grounded 0VAC on the other you could run into problems.
Like HJTRBO says, if your countrys electrical regulation allows it, you should ground the 0VAC.

I cant see any reason why not as I am not paralleling up the power supplies so no diodes etc required and the fan can only draw power from the fan PSU and the PLC/control from the control PSU?

Click to expand...

Unless you know how the 24Vd.c. control signal is used inside the fan you cannot say that you won't be paralleling the supplies. You will have two 24V d.c. supplies going in to one device. Use a relay, the DIN rail interface relays are the same thickness as standard DIN terminals and so take up hardly any extra room.

On the subject of grounding the 0V on power supplies outputs, you will find various threads here. The answer as far as I can understand is that all 24V d.c. supplies that are used anywhere near higher voltage sources must be grounded for safety reasons. Because if there is contact between a high voltage and an ungrounded (floating) low voltage, the low voltage will 'float' to the high voltage.

Bryan

HJTRBO said:

I would hope that you tie your OV down to earth anyway. Having said that 0VDC is 0VDC. Tie away I say.

Click to expand...

I agree, we as a common pratice on our 24volt systems always earth ground our 0volts.

mvaughan said:

I agree, we as a common pratice on our 24volt systems always earth ground our 0volts.

Click to expand...

Yes but still run a negative wire between the two systems, you shouldn't rely on circuit through ground to make it work.
Try to ground the two power supplies at the same point if they are close together otherwise you may get a ground loop.

Roy