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The power supply of my 386DX computer (an unknown brand M320 motherboard) is not trustworthy anymore, so I'm planning to use an ATX power supply, plus an ATX to AT adapter for P8-P9 connectors.

The thing is that AT power suuplies provide +5V, +12V, -5V and -12V. ATX power supplies, until not much time ago, supplied the same outputs, plus 3.3V, but it seems that the -5V output has phased out, and some ATX supplies mark that pin as NC (not connected).

So when it comes to chosing the right ATX power supply (otoh, one that does not have any fans in the upper side, but in the back side, because ATX supplies are mounted upside down in an AT case and the fan faces the upper side of the case which is most ofently closed) it's hard to tell if the ATX connector includes the white cable that should carry -5V.

So I would like to know if AT-class motherboards need that supply for its internal things (chipset, CPU, SuperIO, etc). I can say that the very usual UM82C206L chip, used to implement most of the AT system chips (DMA, RTC, PIC, Timer, etc) does use only +5V.

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  • -5V on ATX supplies was phased out almost 20 years ago now.
    – user722
    Commented Aug 21, 2019 at 21:11
  • 20 years? Boy! Time flies! I thought it was about 10 years or so.
    – mcleod_ideafix
    Commented Aug 22, 2019 at 7:12
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    @mcleod_ideafix -5V was removed from the ATX requirement in 2002 and, indeed, it was only used to service the -5V requirement for the ISA bus. See : Related. The "custom applications" they refer to are mostly niche industrial motherboards (we have some such 2nd and 3rd generation Core i7/i5 systems, for example, on motherboards that also include legacy ISA and PCI slots, if you can believe it.)
    – J...
    Commented Aug 22, 2019 at 13:21
  • @J... the only thing that surprises me is that the number of legacy ISA controlled industrial/medical/scientific/etc machines has finally dwindled low enough that the specialty board makers haven't released a newer ISA slot board since gen 3.
    – Dan Is Fiddling By Firelight
    Commented Aug 22, 2019 at 14:50
  • @DanNeely They may exist - haven't shopped for an ISA board in a few years.
    – J...
    Commented Aug 22, 2019 at 15:28

4 Answers 4

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Anecdotal answer, because I don't feel like googling for lots of schematics:

  • Most AT mainboards only use +5V.
  • In some cases, +12V is used for miscelleanous stuff, I remember an auto-voltage mainboard where the +5V/+3.45V switch for the 80486 processor used the voltage detection pin, and controlled the +5V Vcc connection using a FET that got its gate drive from +12V (via a resistor; pulled down if a 3.45V cpu is installed).
  • If a board has onboard serial ports, the 1488/1489 level shifters also use +/-12V.
  • You should nearly always be fine with missing -5V.
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Looking at the three ATX PSUs I have to hand, all three of them have a bottom-mounted fan, and only one (the oldest, least efficient and noisiest) provides a -5V line. That's a Nexus NX-3500.

One of the others is actually an SFX-L form factor, which mounts to an ATX case using an adapter plate which leaves a significant gap on all sides relative to a full-size ATX PSU. This may expand your options somewhat, especially as the SFX-L mounting bolts are symmetric, allowing you to mount it with the fan inwards in free air.

enter image description here

To fit an ATX PSU to an AT m/board, you'll need to insert an adapter between the plug and socket on the low-voltage side. You could take advantage of this to derive the -5V supply from the -12V one, as not much current is carried on that line to begin with. A 7905-series linear regulator should do the trick; just add a small heatsink on top and a bypass capacitor on each side.

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  • Oh! Just to clarify: when I said "one that does not have any fans in the upper side" I meant the upper side of an AT case, as ATX supplies have to be mounted upside down so the normally bottom faced fan is left facing up. Most ATX supplies I've seen, including the one I have as spare, have a bottom-mounted fan.
    – mcleod_ideafix
    Commented Aug 21, 2019 at 19:06
  • I think SFX-L still solves that part of your problem. I've expanded the answer to explain why.
    – Chromatix
    Commented Aug 21, 2019 at 19:08
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    As fot he 7905, yes, I considered that. Once I needed to service an ATX supply with a not standard form factor, so finding a replacement was hard and I wanted to give the old one a try. As I opened it, I quickly noticed the footprint of a 7905 regulator in the silk screen, obviosuly to add the -5V which this particular power supply didn't have (but it had the white wire).
    – mcleod_ideafix
    Commented Aug 21, 2019 at 19:10
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Engineering is Driven by Specifications

In general, it is always dicey to not implement the full breadth of a spec.

This is engineering, not magic or wish.

In the absence of other information, you must assume that all voltages in the spec are required.

So to explicitly answer your question, the only sound answer is "yes", although there may be many exceptions.

Your Board May Be an Exception

For a particular AT motherboard, one could examine the schematic or trace the -5 V pin to be sure it doesn't connect with anything.

Seeming to Work Does Not Equal Actually Working

The -5 V rail may not be needed at all, or it may only be needed for functions you aren't currently using. Any particular motherboard may or may not require the negative voltage. Add-on cards may or may not require the negative voltage.

At Least One Did Require -5 V

At an earlier company, we designed a 386 motherboard. As I remember, it used -5 V, to power the on-board RS-232 drivers, probably with a capacitive voltage doubler. I think we also had an OP-amp using negative voltage. I would need a schematic to verify those statements.

For early motherboards, it is possible that PMOS parts were used, some of which used a -5 V supply.

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    While all statements in your answer are true it doesn't answer the question. The question was about the actual necessarity of -5V for the main board. Mcload excluded the need of additional cards. You may want to improve your answer regarding this point.
    – harper
    Commented Aug 22, 2019 at 3:38
  • I thought I had answered it clearly enough, but I now added an specific statement, and personal recollection.
    – cmm
    Commented Aug 22, 2019 at 4:03
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    I know that -5V were needed at first for a reason: ancient DRAM memories needed it. That's why I wanted to narrow the question to more "modern" motherboards: those that use SIMM modules, with +5V only DRAM chips. My guess (not expressed in the original question) is that the -5V supply rail was there mostly because of those DRAM chips.
    – mcleod_ideafix
    Commented Aug 22, 2019 at 7:20
  • Yes, a negative voltage was sometimes required. I think for NMOS it was sometimes for substrate bias before on-chip charge pumps were included. For PMOS, it was, in effect, the VCC.
    – cmm
    Commented Aug 26, 2019 at 16:29
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I would say: test it. If everything you need works fine without -5V then it would not matter even if your motherboard used it for something.

If you find out it does not work properly without -5V (or if you want to be extra certain nothing is missing), why not just add a 7905 regulator to generate -5V from -12V. Those cost roughly a dollar a pop.

They have 3 pins, Input (connect -12V), GND (connect GND) and Output (connect to motherboards -5V)

79xx series regulators are made by many manufacturers and are generally negative voltage linear regulators. Linear regulator are easier to use than switching regulators, but has the downside of burning excess wattage to heat. So if your motherboard would draw 100mA of -5V, 7905 would dissipate 7V*100mA = 700mW of heat.

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  • There is a middle-ground between efficient-but-sensetive DC-DC converters and simple-but-inefficient linear regulators: A inductor-less charge-pump DC-DC converter. There are a lot of charge-pump controller chips on the market that allow you to "flip" +5V to -5V, with nothing but a few capacitors, simple to use, and efficiency is typically greater than 70%. The only disadvantage is their low power capabitilites: 100-200 mA only. For example, LT1054 or LM2663.
    – 比尔盖子
    Commented Aug 22, 2019 at 12:48
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    While I agree that there are more alternatives than I listed, my main aim was to offer inexpensive and simple one part solution. Ineffectiveness is not a big problem if load is very small. If there is need for higher loads, then I would consider prebuilt switching DC-DC modules. Even though charge-pump controller chips only require a few extra components, they can still be overwhelming for anyone who is not into electronics. For once off job prebuilt module would probably be more cost effective solution anyway.
    – Ville
    Commented Aug 23, 2019 at 13:40
  • Back in the 1980s, I used a card extender with a 7905 on it to operate a Zoom brand 1200-baud modem from a Tandy 1000, whose card slots didn't have a -5 supply wired to them.
    – supercat
    Commented Apr 10, 2020 at 18:20

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