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two DC power supplies in parallel with different voltages

two DC power supplies in parallel with different voltages

(OP)
Hi,

This is a basic question I guess but would greatly appreciate anyone's input. I have an existing DC power supply feeding the existing control system PLC and the loop powered instruments in the field. The PLC requires a change and will now have a higher DC voltage as power supply. Assuming that the field instruments is flexible to handle the higher dc voltage, can we connect both the DC power supply in parallel and then remove the lower/older power supply later. The idea is to have a seamless transition when changing the power supply. Before migrating the PLC they would like to change the power supply first without any downtime. The existing PLC is able to handle the higher DC power supply. Question is:
1. If we have two DC power supply in parallel with different voltage what would the resultant voltage be? Say power supplies are 12Vdc and 24Vdc.
2. What are the precautions needed with this configuration?

Thank you in advance.

RE: two DC power supplies in parallel with different voltages

IR gave the "Better safe than sorry" answer.

But there are a few variations. If both power supplies are well protected with current limiting but with no crow-bar and with components that can handle the higher voltage, which they usually can for the very short time that both are connected together, then there could be a happy ending.

Get the specifications and schematics for both of the supplies to see how they are built. Or get the 12 V spare out and do a test with both supplies connected together.

I would also consider letting the new (also 12 V) PSU stand on the shoulders of the original one and feed the system that need the higher voltage from the resulting higher voltage. Both systems sharing the same zero potential, of course.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: two DC power supplies in parallel with different voltages

I don't think your scheme to parallel the outputs of a 12Vdc and a 24Vdc PS will work (ie you might well end up with an unintended shutdown).
Why not install a temporary adjustable power supply. Start off with the new PS paralleled to the existing PS (& set at 12Vdc). Disconnect the existing PS; this leaves only the new PS in play. Increase the output voltage, of the new PS, up to 24Vdc. Connect in your permanent 24Vdc PS and remove the adjustable PS. Voila.
Piece of cake. All you need to do is to locate an adjustable power supply with the appropriate current rating. BTW, how many amps are we talking about here? Perhaps a benchtop type PS might work?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: two DC power supplies in parallel with different voltages

(OP)
Thank you gentlemen! I'm glad I'm not the only one hesitant about this idea. The PSU vendor mentioned that this should not be a problem but I'm not convinced. I just want to understand more and the theory behind it. GroovyGuy I like your idea about the adjustable power supply! That seems to be the logical solution. We are still on the planning portion of this design hence there are lots of ideas floating around.

For discussion purposes only, what would be the resultant voltage if the 24Vdc and 12Vdc are connected in parallel? Let's assume the PSU's have diodes or fancy electronics that would avoid one PSU burning the other. What voltage would the load receive?

RE: two DC power supplies in parallel with different voltages

I would look at the range of voltage supply that all those instruments can handle.
Low to high range on the spec sheets for all instruments, see if all covers this 12-24vdc range.

I would think you would want the DC commons tied together with the PLC input card. This would cause issues if the commons are not tied together.

you also have differences in wire voltage drops between a 12vdc and 24vdc power supply.
Ask this question: Is the wire that is pulled for the power supply daisy chained to the instruments or pulled separately (home runs from plc to field)?

I would not recommend putting 12v and 24v dc on same wire, i think you would probably cause the 12v to burn out or the 24v might see a short depending on the typical "short circuitry" designed inside the power supply.

RE: two DC power supplies in parallel with different voltages

I haven't thought this through but you may be able to add load resistors (or resistor network) on the output of the supplies and then hook them in parallel. This will give each supply something to regulate too and shouldn't cause any issues if you make sure the resistor is big enough - probably need to parallel some resistors so you don't have a huge wattage resistor to source. Hooking them directly up to one another will probably burn them up. At the very least it will be erratic voltage.

I am also confused as to what is causing the higher voltage you need that can be replaced while operating? Must just be an I/O card or something.

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