9

PC power supply breakout board

I bought a useful PC power supply breakout board, fused. It has the following outputs:

  • -12V
  • +12V
  • +5V
  • +3.3

What is -12V and how would I use it? The -12V has its own negative.

JohnnyVegas
  • 441
  • 4
  • 14
  • 2
    The -12V has its own negative. Normally, on an ATX PSU all outputs share the same GND/return. Is -12V output's negative isolated from the rest? – Rohat Kılıç Jan 16 '24 at 10:38
  • @RohatKılıç thank you - that reminded me of the old 7V fan hack, which lead to another answer – Criggie Jan 18 '24 at 06:19

4 Answers4

22

-12V is a negative 12V output. Used for something that needs a negative supply in addition to positive supply, for example a dual supply op-amp powered by +12V and -12V.

You can use it for anything you want or not if you don't need it. It will roughly provide 1A max anyway.

And no the -12V does not have a separate ground. All supplies are referenced to the only ground, the black wires on generic ATX supply.

What you have on your adapter and how you interpret what it meand can of course be different.

Justme
  • 147,557
  • 4
  • 113
  • 291
14

You can use it for things like analog circuits that use +/-12V. You can regulate it down to something lower such as -5V with a linear regulator. It's very useful for analog circuits. Not so much for digital- it can be used for old-fashioned RS-232 drivers that don't generate their own negative voltage.

The grounds will be common in any case, regardless of how the breakout board looks, because they are connected together within the power supply itself.

Spehro Pefhany
  • 397,265
  • 22
  • 337
  • 893
  • 3
    In fact I can't cite from memory any use of -12V in a PC other than powering old-fashioned "RS232" drivers, e.g. 1488, the typical interface from modems – fgrieu Jan 17 '24 at 15:08
  • 3
    @fgrieu Some of the early memory chips (DRAM and EPROM) used negative rails (-5V typically). The ISA bus has both -5 and -12, as well as +5 and +12. I don't think the original IBM '84 PC main board used the negative rails at all (the EPROM and DRAM were both +5V only), they just distributed the negative rails from the input power connector to the ISA connectors, but I could be wrong on that point. – Spehro Pefhany Jan 17 '24 at 15:42
9

If you use op-amps, or other analogue circuitry (for audio, perhaps), having balanced ±12V supplies can be very useful.

No (normal) op-amp can produce an output potential beyond its own power supply potentials. For example, if you connect your op-amp's supply pins to ground and +12V, that op-amp's output is constrained to be within 0V and +12V. Equally inconveniently, its inputs must not fall outside that range either. It's not just op-amps; with a few exceptions, such as DC-DC converters, most circuits will have similar constraints.

Often, when dealing with analogue signals, for convenience we would like to have all of our signals fluctuating above and below ground (0V), but that won't be easy when using supply potentials of 0V and +12V, for the above reasons. In such cases you would have to design your analogue elements to operate "in the middle" of the available range, having them oscillate and fluctuate about some average center potential between the extremes available, say, +6V instead of 0V. This can be quite challenging, and usually involves additional components at the input and output, and even between stages. Centering signals around a different average potential is called "biasing".

Things are usually much easier if we can provide our analogue elements with both positive and negative supply potentials, so that they can take positive and negative inputs, and produce positive and negative outputs. Working with signals centered around 0V (ground potential) renders biasing unnecessary, or at least greatly simplified, but will require a negative supply.

Simon Fitch
  • 34,939
  • 2
  • 17
  • 105
2

PRESUMING the -12V "ground" or 0V connection is common with the others as per the comment from Rohat Kılıç then you have a lot more voltage options.

Assuming that you only require low currents below 1A, the -12V line gives you:

  • ~15V when combined with the +3.3V line and the 0V/negative terminal is not used)
  • 17V when combined with the +5V line
  • 24V when combined with the +12V line (handy for dealing with 24V diesel car lights and components)

Likewise you can get other voltage differences with combinations:

  • ~1.5V between +3.3V and +5V (good for bench-powering a single alkaline battery device)
  • 7V between +5V and +12V (this one was popular for running a 12V fan on slow speed)
  • ~8V between +3.3V and +12V

The fancier a PSU is, the more likely it will object to this abuse and shut down. So in some respects a cheaper PSU is better.

Also do be aware of the maximum current on each rail. The +12V line can produce a lot more than others, so the lowest current rating is your cap.

Criggie
  • 869
  • 8
  • 22
  • 2
    I am not sure about your latter three options. They would only work if your PSU is able to sink current into the 3.3 V and 5 V rails. I doubt ATX standard specifies current sinking capability, so it would depend on the topology of the individual PSU. – tobalt Jan 18 '24 at 06:35
  • @tobalt yep that's partially why an older ATX PSU might be a better choice than a more recent design. A robust 200W PSU pulled from an old P3 would be more than enough for a bench supply. – Criggie Jan 18 '24 at 11:38
  • 2
    I used a PC power supply on the "bench" for years... the +12 would only reach 12 V if the +5 was loaded with 47 ohms or less. And all of them were noisy and rough as anything. – tomnexus Jan 18 '24 at 14:56
  • @tomnexus if OP's product is what I think it is, then there will be a load added somewhere to make the PSU happier. Could be a pair of simple 12V incandescent indicator lamps on the 5 and 12V rail, or somethign fancier like a power resistor. – Criggie Jan 18 '24 at 18:33