24v power supply question

Timeismoney08 · Feb 17, 2014

I have a 120ac to 24vdc 40amp power supply that went bad.

Can I take two 120ac to 24vdc 20amps and put them in parallel?

I know this works fine with batteries, what about a power ac to dc power supply?

I only ask because we'll have to wait on a 40amp ps

rpoet · Feb 17, 2014

You can only do that if the power supplies have the explicit ability to be paralleled. If your 20A supplies don't have paralleling ability, you can get modules that will allow you to do it. The issue is that since normal supplies will never be at EXACTLY the same output voltage, they will fight each other, circulating currents will develop, and the power supplies will fault (if you're lucky). If your're not lucky, you'll damage them.

If you can split the loads roughly in half, you can always have your two 20A supplies share a common ground, and feed the positive sides to each load half.

-rpoet

Lemming · Feb 18, 2014

40 a is a big dc supply... What are you driving ?? If 20 a will cut it, any supply can be paralleled through a diode box... Also get the advantage of redundancy..

Timeismoney08 · Feb 18, 2014

Lemming said:
40 a is a big dc supply... What are you driving ?? If 20 a will cut it, any supply can be paralleled through a diode box... Also get the advantage of redundancy..

When you say diode box, what do you mean exactly? I was wondering if that was something I could do.
Could I just put a diode on the power supply if I bought some from radio shack?

Thanks for the help

mbartoli · Feb 18, 2014

Here is an off the shelf Phoenix Contact part that we use extensively for paralleling DC supplies; specifically for redundancy.

https://www.phoenixcontact.com/onli...mdetail:pid=2938963&library=usen&pcck=P&tab=1

keithkyll · Feb 18, 2014

Here is a diode bridge from Radio Shack. It says internet only.
Mount it on a heatsink or aluminum panel with heat sink compound.
Put a supplies on AC leads, and output on + terminal.
Use tinned wire, at least 10 gauge. Solder. QC connectors won't last long at 20 Amp.

JHarbin · Feb 18, 2014

keithkyll said:
Here is a diode bridge from Radio Shack. It says internet only.
Mount it on a heatsink or aluminum panel with heat sink compound.
Put a supplies on AC leads, and output on + terminal.
Use tinned wire, at least 10 gauge. Solder. QC connectors won't last long at 20 Amp.

This is not what you want to use for this application!

keithkyll · Feb 18, 2014

JHarbin · Feb 18, 2014

keithkyll said:
Why?

Because he it's not trying to rectify AC he is thinking about paralleling DC power supplies.

jrwb4gbm · Feb 18, 2014

Just one more alternative would be to use two 12 VDC 40 amp supplies in series.

danw · Feb 18, 2014

Here's a schematic of the two diodes used by the Phoenix Contact redundant (paralleled) power supplies decoupler.

Lemming · Feb 18, 2014

Timeismoney08 said:
When you say diode box, what do you mean exactly? I was wondering if that was something I could do.
Could I just put a diode on the power supply if I bought some from radio shack?

Thanks for the help

You could make your own, is just a suitably rated diode after each ps output... But the mean well / weidmuller options are relatively in expensive..

keithkyll · Feb 18, 2014

JHarbin said:
Because he it's not trying to rectify AC he is thinking about paralleling DC power supplies.

A full wave bridge is 4 diodes. Half of it will do the job.
Exactly the same as the Phoenix Contact solution. I like the Phoenix Contact, because it's Din rail mountable, and has terminal blocks.
The OP mentioned Radio Shack. I was answering yes to that. Sounds like he has a couple of 20 Amp supplies, and wants to get running now. A full wave bridge should be easy to get and cheap.

Vic · Feb 18, 2014

Do the 20 amp power supplies have adjustable outputs? A diode in series with each power supply output will prevent circulating currents between the power supplies but will not insure the power supplies will share the load evenly. You will have to set the output voltages of each supply to be EXACTLY the same. If you need 24 volts at the load you will need to set the output voltages to about 24.7 volts to compensate for the voltage drop across the diodes.

keithkyll · Feb 18, 2014

Single output supplies are adjustable. With the voltages mismatched, the higher voltage supply will take most of the load. Sensing is local. That means output voltage will drop a little as load increases. Soon, the other supply will start to deliver. I expect voltage drop to be about 450 mV.
Most 24 volt devices have a wide tolerance range, so it wouldn't be a big deal if both supplies were set at 24.0 volts.

24v power supply question

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