Xjyuk

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.

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.

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.

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.

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.

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.