Thread: *sigh*, Another ATX PSU question

1. *sigh*, Another ATX PSU question

I have an ATX PSU with dual 12v rails.

I have read that it could be possible to connect both rails (floating ground) and it would act like a daisy chained power supply.

If this is correct, how does this exactly work?

Any details would be great. Like what wire(s) to trace back to what part(s) and what wire(s) to connect to each other.

2. If you connect a load across the +12 and -12 rails you have a voltage potential of 24 volts. (Likewise, if you connect between, say, +12 and -5 the potential is 17 volts.)

The ground rail is just the PSU’s ground state, e.g. its lowest potential. So when you see wires labeled +12, -12, +5, -5 or whatever, that is always in reference to ground.

Thus, you will need to make sure all equipment in that circuit (PSU, motors, drivers, etc.) all share common ground. If two portions of your system have different ground potentials you’ll probably experience problems.

3. I dont want a 24v low amp line.

I was talking about using the 2nd rail as if it were a separate PSU.

4. Greg,

"Daisy chained" would imply you wanted the supplies in series giving you 24V. As neutronics stated, "So when you see wires labeled +12, -12, +5, -5 or whatever, that is always in reference to ground." That means also that the ground leads are all common to one another. You could get a +12 V from the -12 volt rail by using the ground as the positive and the -12 as the negative, however.... the positive side of that supply will be common to ground of all the others.

Are you trying to get higher current +12 volts?

Steve

5. Originally Posted by vger
Greg,
"So when you see wires labeled +12, -12, +5, -5 or whatever, that is always in reference to ground."
Steve
I think either you guys are missing what I am saying, or its my ignorance on this.

The power supply is a "Dual Rail" PSU. It has two +12v rails. +12V1 +12V2....etc

So, like setting up a daily chained PSU, connecting +12v of the second PSU to the Ground of the first, and cutting the Ground of the second to create a floating ground.

I am asking if its possible to do the same thing within the single PSU.

If im not explaining this correct, or it just isnt possible, thats fine... I really have no idea due to lack of knowledge, which is why I am asking.

6. I think that you are asking your question in an understandable manner - at least I understand what you are trying to get at.

You have an ATX power supply with two independent, high-current 12-volt supplies, and you want to know whether you can daisy chain them to get 24 volts at high current.

Without seeing a schematic, I wouldn't speculate either way.

However, I would caution you that in the past I have read some in-depth PC power supply reviews (unfortunately I don't remember where - it might have been on the Tom's Hardware site or anandtech, but it might have been somewhere else as well), where they actually opened up some power supplies that advertised having multiple independent 12-volt buses, only to discover that inside the supplies the 12-volt leads were all wired to a single common bus. So, I wouldn't trust any power supply to have independent output rails unless I were to open it up and verify it.

7. Greg,

Here is a link to a schematic of an ATX supply....

http://electronicsrepairguide.files....9/09/atxps.png

If you look on the far right at the +12 Volt output you will see a line running down and back to the left. Following that line it comes to a junction, take it to the left to another junction and down to R25, a 27K resistor, and on to the left... labeled as "FEEDBACK" and on down to the left to another pair of resistors and to pin 1 of the switching regulator control chip TL494. You will notice that pair of resistors terminate to ground placing pin 1 of the TL494 at the center of a voltage divider comprised of R25/26 and and R30/31. This feedback voltage is used to control the output voltage of the supply.

At the center tap of T3 you will see it is connected to GND, with the upper end of it connected to the +12 V output section. In a supply with dual +12 V rails, you may have that end of the transformer connected to 2 +12 V output sections. They will both be referanced to GND by the feedback line, and by the center tap of the transformer.

If you can find the schematic of your supply, and post it here, I will be able to tell you quickly if you can do what you are asking.

Steve