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# What voltages i can obtain from ATX by combine the outputs? Answered

I have not opened an atx, to measure the outputs to understand. I google it and i find like -12 with +12 you get 24V, -5v and +5V you get 10V but no one say all the outputs. What happens if i connect +12V with -3,3V, or others.

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All the atx voltages on the outputs share a common ground/earth, so nothing can be combined to increase or decrease existing voltages, what you see is what you get.

Sorry, I disagree.

You may be confusing the Common 0V pole with Ground/Earth, which is different here.

The stated voltages are potential to the 0V common.

Actual Ground/Earth is not a factor in this at all, except for grounding the case for safety.

Cheers!

On my atx psu the ground leg on the AC input side is the same as the 0V black leg on the DC outputs as well as the case.

Measured 1 ohm on my DMM, so for all practical reasons is the return path.

So then why when you connect -12V with +12V you get 24V? This question is from the perspective of making a power Supply!! This is what i talk about: http://i41.tinypic.com/14b6p77.jpg , but are more other combination to obtain other voltages and i want to know them before i begin to disassembly the ATX .

What you're measuring is a potential difference between -12 and +12v and there's a difference between theory and practice, theoretically there is 24v according to your meter, but practically you wont actually have a load working at the combined 24v for any practical length of time.

You wrote: "...but practically you wont actually have a load working at the combined 24v for any practical length of time"

IMO the atx circuits are designed to have a load on the V outputs, eg +12V , with the return path being the chassis ground.

You in essense are hooking up 1 neg load path and trying to use another pos load path as the return, instead of the chassi/ground, which is how it was designed to operate.

Because you will STILL have +12 and -12 with respect to 0V, and 24 V across the ends. You WON'T have 24V with respect to ground, and you'll only be able to take current at the maximum rating of the lower rated supply - so if you have +12@10A and -12@1A you will be able to have a 24V @ 1A output.

Your confusing your terminology - If you measure between + 12 and -12 you will measure 24 volts.

You don't get different voltages by combining the different voltage outputs. You'll blow up your power supply. -5v means it's a ground for a 5v output, combining it with a 5v output will just route the 5v directly to ground. Combining different voltages will burn out the lower voltage capacitors on the lower voltage outputs as you will be feeding a higher voltage to them from the higher voltage outputs. This is also why you cannot just take a cable and plug it from one computers usb port to another.

Sorry, Darryl, (perhaps I'm not reading you right, but) not only are you wrong about what -5V means, you are making assumptions about USB connections that are irrelevant to your statement about the voltages.

In other words, USB to USB is not readily compatible for different reasons than those that you think are responsible for output combinations being incompatible.

In fact, special transfer cables exist to transfer data from USB to USB... not relevant anymore, but they do exist.

As for the voltages, yes they work. There are current limitations, of course, but it works just fine -- Been doing it for years. The output capacitors don't need to be capable of handling the higher voltage completely... Only the potential difference between the rails used.

Ex: Between +12V and +5V there is 7V. The capacitors on the 5V rail need not handle the full 12 volts. They only see the 7 volt difference.

ATX PSUs are a lot tougher than you may think, AND they have protection.

This is the front panel from my design... enlarge it to see a list of potential voltages available by combining rails.

I have modified an ATX PSU for bench use and have found that many usable voltages are available by combining output rails.

Since all voltages use the same 0V reference then they are potential to each other as well. So, the potential between any two wires will give you the difference between their values (in reference to 0V). Visualize it as a line from -12V to +12V with the 0V in the middle.

(Color coded wires in order: Blue -12, White -5, Black 0, Orange +3.3, Red +5, Yellow +12)

-12...........-5............0....+3.3.....+5........+12

|<---7v--->|

|<--------------17v------------------>|

|<------8.7v------>|

(Sorry, if the scale doesn't line up properly see text instead.)

Between the +5V wire and the +12V wire is 7V... its polarity is determined by which wire you use as your positive. Between +12V and -5V you get 17V. Between +3.3V and +12V you get 8.7V... and so on. You can use any combination that will give you the voltage you need. Pick any two points on the scale above and use the one on the right as the positive.

The only real limitation is amperage. Since more than one rail is being used, the current from the weaker rail should always be seen as the most amperage available for these voltage combinations... Of course, if more than one combination results in the same potential voltage, then choose the one that will give the current you need.

Ex: There is a 7 volt potential between +5 and +12... But also between -12 and -5.

The difference is that the second choice may only deliver 1 amp or less.

As long as these limitations are heeded, there is no reason to think this will not work.

Cheers!

But if you have two ATX and do not let them touch frame to frame,
you can get 48VDC !!!

BTW I think the -12 is the lower current and
that is the current rating you are to expect as the output of 24 or 48VDC..
If you try to pull more current .... Expect something to fail

Some power supplies work fine when combining the voltage sources because they have isolation circuits integrated, If your power supply does not have isolation circuits built in it may cause capacitors to blow after a short period of time because they're not rated for what you might be pulling. Do note that combing 12 with 12 or 5 with 5 may work fine but combing 12 with 5 on a psu without isolation circuits may burn out the capacitors with a load due to the 2 sources being unbalanced.