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I have looked, but haven't find an answer.

Are ATX computer PSUs capable of having a load connected between their positive rails, so that for example, current would flow from a 12v rail to a 5v rail?

Specifically, I need to know:

  1. Does the specification of the standard say anything about this?
  2. If not, what is the likelihood that it would be OK?
  3. What's the worst that could happen?

Thanks

Addition: Same question, but between the -12v and any of the positive rails (paricularly +12v and +5v). Similar question here says -12v to +12v would be OK.

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  • \$\begingroup\$ When you connect a load between two outputs the max current shouldn't be higher that the max current of the weaker output. Based on that if you use any of the 12v/5v power lines that can provide several Amps with the -12 line you will barely be able to use 1A (depending on the -12v specs) \$\endgroup\$ – alexan_e Jan 18 '14 at 19:15
  • \$\begingroup\$ It depends on the load. If you connect from +12V to +5V as the only load it's probably not OK because these power supplies typically have buck derived output stages so can only supply current from the positive outputs and take current on the negative outputs. If you have enough other load on the system to guarantee this you should be OK but if the 5V is otherwise unloaded the 12V will be trying to charge up the 5V rail and since these power supplies are typically designed to provide more 5V current than 12V. The 12V rail will probably collapse. \$\endgroup\$ – Warren Hill Jan 18 '14 at 19:15
  • \$\begingroup\$ @WarrenHill Does that mean that if you use only the 12v output with a load that draws high current (load between GND and 12v) with the 5v line unloaded (or just a small load in case the psu doesn't turn on) there will be a problem with the operation of the psu, or what you say only applies when connecting loads between positive supply lines (not GND) \$\endgroup\$ – alexan_e Jan 18 '14 at 19:20
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    \$\begingroup\$ The problem is that the positive outputs can't sink current so you should be OK if your load is between +12V and 0V. If you want to have a load between 12V and 5V add another load between 5V and 0V so the 5V is always supplying and never sinking current. \$\endgroup\$ – Warren Hill Jan 18 '14 at 21:33
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The answer, as to many engineering questions, is "it depends". The most cautious advice is that you can allow current to flow from any rail to any other rail provided that the specified minimum output load current (which may be zero) is drawn from any rail.

What is the minimum load, you ask? It's specified in the ATX specification. In no case should you allow a rail that normally sinks current to source current or vice versa. That's what "0" means.

enter image description here

Should you break that rule, the voltages present at the rails may go outside of tolerance and (worst case) damage the supply or something attached to the rail (one exception to the minimum load rule is that if you open all the outputs, damage should not occur to the supply, but the supply is not guaranteed to operate).

For example, the 5V rail could rise to 8V, exceeding the rating most 5V chips as well as the output filter capacitors. High quality supplies may have OVP (overvoltage protection) which will likely prevent damage to the supply or attached load.

On the other hand, there are ATX power supply designs that stack a 7V rail on top of the 5V rail to give +12V, such as this one. You can draw current from the +12V rail to the 5V rail on this one (but not the 3.3V rail).

enter image description here

There's no guarantee that you'll get a supply made this, so it's best to stay within the specifications that all ATX supplies must meet. There are multiple revisions of the specifications.

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Connecting a load between any positive rail to any negative rail should be fine provided that both rails can handle the current, and provided that the load can tolerate any common-mode noise that may be present.

If both rails are positive or both negative then there's an additional wrinkle which is that one will be sourcing current into a positive rail or sinking it from a negative rail. Provided that the amount of current sourced into the less-positive rail doesn't exceed the amount consumed by other loads, and likewise with current sunk from the less-negative rail, there should be no problem. If, however, one were to try to connect a 1.1A load between the +12 and +5 rails at a time when there was only 100mA of other loading, then the supply's +5 rail would have to sink 1A. In theory, a supply could harvest the energy fed into the +5 rail and feed it to other rails, but doing that would require extra circuitry which most supplies would have no reason to include.

If your load is going to be small relative to the minimum expected load on the +5 rail there shouldn't be any problem, but if your load could be higher than that you should ensure that current has a path to ground without having to go through the +5 rail.

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Bottom line is NO, the one suggested answer does not even show a switching power supply that is a normal bridge rectified, capacitor/choke regulated supply. Power rails are physically separate coils on your transformer(s) nor does this in any way sync up the switching regulators to work perfectly timed together. (remember switching psu's switch the regulator on and off very rapidly this is literally why they are called 'switching' PSU's) With a normal regulated (Always on) psu its possible to link up several regulators or transformers coils by using low ohm (< 1ohm) high wattage resistor to balance the amp draw.

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