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# Using 2 ATX power supplies to get 24v

#### feetsdr

##### New Member
Trying to get 24v from 2 ATX power supplies - call them A & B

I bound the yellows of A together, the blacks of A together, the yellows of B together, the blacks of B together.

I get 12v across the black & yellow of A & B separately,. but ALSO from yellow of A to black of B & vice versa. The cases aren't touching. They ARE plugged into a common 3 prong wall outlet

Because they have a common ground, right?

So I opened 1 of them. there was a wire from ground connector to case. Cut that off. There were caps from ground to hot and another from ground to neutral. Cut them out.

Still have 12 v between the 2 power supplies.

You need to connect them in series to get 24V.
So connect the blacks of one to the yellows of the other.

You have connected A and B in series, but to get higher voltage you need to connect in parallel.

Question is will the ATX supplies series and still regulate ?

Regards, Dana.

Opening 1 of the ATX, I saw the little board with cicruit breaker / LED is screwed to the case. I isolated that and the circuit board. and now I get 24 v. from the black of A to yellow of B, with the yellow of a connected to black of B,

Anything I need to be aware of?

You have connected A and B in series, but to get higher voltage you need to connect in parallel.
Uh, I don't know too much, but that's wrong? Parralel - more current, series, more voltage? at least for batteries? take 2 AA batteries, stack them (makes them in series) and measure voltage on each end... 3v, right?

You have connected A and B in series, but to get higher voltage you need to connect in parallel.

Ummm. No. Connecting the two supplies is series would double the voltage....if they were isolated.

Uh, I don't know too much, but that's wrong? Parralel - more current, series, more voltage? at least for batteries? take 2 AA batteries, stack them (makes them in series) and measure voltage on each end... 3v, right?
Oh forgive me, I just woke up.

Anything I need to be aware of?

Some ATX PSUs are not fully regulated & they may rely on load on the 5V output to provide enough power to maintain voltage on the 12V outputs.

Just keep an eye on the voltage & see if it drops with a moderate load, rather than holding up to its full rated output.

Some ATX PSUs are not fully regulated & they may rely on load on the 5V output to provide enough power to maintain voltage on the 12V outputs.

Just keep an eye on the voltage & see if it drops with a moderate load, rather than holding up to its full rated output.
GOSH!!! thanks for the tip!

This is a nice try but bad idea. PC PSU's use AC gnd= DC gnd,
They use CM filters to suppress CM noise via the AC Gnd. Now anything you use on tha 24V will also get CM noise, which may work for some things not not if sensors or controls are involved.

rjenkinsgb is correct about regulation. I'd say, they are all this way. It's called a Forward converted with high mutual coupling between all secondaries so that they all track the 5V regulated output. Any error is called cross-load regulation error. This means if you loaded the 5V by 10% of rated current the energy is increased on the primary but the other outputs might only increase 0.2% if made with good transformers.

If you are just using the 24V for LEDs no problem to pull the AC ground on the higher 12V supply, but now that chassis might be unsafe (hot) during a lightning storm. Be careful with DC motors and measure the DCR to understand the surge currents. The onboard error detection on the PCB might be more sensitive to transients and shutdown easier. Because there is capacitance coupling in the transformer, switched RF noise passes thru, so it is not isolated at RF frequencies. The AC ground only diverts/attenuates the RF with the X caps and does not affect the isolation at RF. (it also helps divert lightning noise) to AC gnd. So expect common mode RF noise to interfere with any communication parts.

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All of the ATX power supplies I have come across have their 0V connected to earth (the casing) so this will effectively short out one of the 12V supplies.

All of the ATX power supplies I have come across have their 0V connected to earth (the casing) so this will effectively short out one of the 12V supplies.
He cut the earth to one supply and isolated the cases.

This is a nice try but bad idea. PC PSU's use AC gnd= DC gnd,
They use CM filters to suppress CM noise via the AC Gnd.
Well, at here - IT net (from power company) is very common - i.e. much of the ac outlets are not grounded. Not sure though if this cause a negative impact regarding cm noise.

Well, at here - IT net (from power company) is very common - i.e. much of the ac outlets are not grounded. Not sure though if this cause a negative impact regarding cm noise.
I've no idea what you mean by 'IT net' (and googling doesn't help), but earthed sockets is more a question of your building wiring than the power company.

Ok, sorry that was not the correct English term (i.e bad translation), it was Isolated ground I meant to say.

Ok, sorry that was not the correct English term (i.e bad translation), it was Isolated ground I meant to say.

According to that it's usually only used on special equipment?.

According to that it's usually only used on special equipment?.
Yes, I know. I live in a kind of odd country when it comes to the most used solution of power companies.

From what I've understand, it's a preferred system use in hospitals because when designed properly it gives a very low ground fault current, and thus when a ground fault does occur, it doesn't cause a circuit beaker to trip immediately (because power cut is considered more dangerous than an enduring ground fault).

Medical power supplies must be very low RF ground leakage currents with tighter standards with more ferrite absorption filtering and less shunt current (Y cap) to ground as patients with electrodes are more sensitive. There are universal stds. for these.

Trying to get 24v from 2 ATX power supplies - call them A & B

I bound the yellows of A together, the blacks of A together, the yellows of B together, the blacks of B together.

I get 12v across the black & yellow of A & B separately,. but ALSO from yellow of A to black of B & vice versa. The cases aren't touching. They ARE plugged into a common 3 prong wall outlet

Because they have a common ground, right?

So I opened 1 of them. there was a wire from ground connector to case. Cut that off. There were caps from ground to hot and another from ground to neutral. Cut them out.

Still have 12 v between the 2 power supplies.
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