XT Power Supply Question

Can anybody direct me to the function of the –12 VDC power on the old XT/AT power supplies for PCs?

The reason I ask is the older XT power supplies are not easily found these days for replacement. Today I took a newer power supply that had absolutely no –12 VDC output and wired on the old XT/AT connectors. This was to replace a bad power supply AND it all worked with a bit of cheating! So just what did the –12 VDC do?

 

ALL modern ATX PSUs have a -12V line. It is a power line for the motherboard.

 

Hi Sno,

The serial ports use +/- 12 volts for the output signals. That is about the only use that was ever made of it.

Luck, Jim

 

Somehow the new power supply I picked up did not have –12V.

Thanks Jim, there was something in my human RAM that brought up the original RS-232 specs as having +/- 12V signals. While in reality just about any level shift works. I guess now days with so many computers coming without serial ports they no longer build -12 VDC into all power supplies.

 

According to the ATX standard, the -12VDC line is pin #14, which is the only blue wire on the coding table.


References: http://en.wikipedia.org/wiki/Power_supply_unit_(computer)#Wiring_diagrams

 

Are you sure it wasn't the -5volt rail (white)?
a lot of modern ATX PSU's don't have that rail any more but I don't think they ditched the -12 yet.
The minus 5 rail was part of the ISA slot standard and no longer used.

 

Opps, I believe you are correct in correcting my short-term memory. After I went over "collinsl"'s last post link with my info I was afraid I made that mistake between the -5V & -12V wires. Fortunately it was only a memory mistake and not a physical mistake. Unfortunately I will not be back into the machine to confirm as it is at another location being used as a Voice Mail computer. 24/7/365 use stuff so it does not come out of service often.

Good to know about the ISA slot info. Interesting because the cards in this computer are all ISA cards. Must be that none of them require or use the -5V power.

Thanks all.

 

Hi again Plumm,

If I remember correctly the spec is 5v to 15v on each supply. I have designed many devices for my own use with only a swing from 0 to +5 and gotten away with it.

Luck, Jim

 

"Interesting because the cards in this computer are all ISA cards. Must be that none of them require or use the -5V power."

This is a good point you have bought up, I have changed out more PSU's than I care to remember and on rare occasions I come across an old atx machine with an ISA slot and card in it that needs a new PSU.
Fortunately there has never been an issue so obviously those cards didn't need it but it's certainly something to keep in mind when selecting a new PSU for an older machine with ISA.

There would be a work around for this by feeding a negative 5volt regulator from the -12 rail with a capacitor either side if it really needed it or get a PSU with a -5 rail, so thanks for bringing this up

 

Hi All,

Just a thought. Can't really remember that far back. The original PC may have used some Geranium Transistors or devices. Those guys were so leaky that they required an added 'Shut Off Bios'. My guys use -10. May the PC guys used -5. Power supplies where pretty well copied for years so all voltages remained the same.

Jim