AFIK -5 is only used as a bias in some designs, and I've never seen more than a few ma drawn from it.

Looking over the supplies I have on the shelf, even the ones with high current +5 and +12 outputs, all list 0.5A for -5 (and I think that's generous)

The few times I've had to do it, I've not bothered with a DC-DC converter ... I've just used an old-school linear -5v regulator(7905) on the -12 line.

The original use was old RAM chips in the original IBM PCs and XTs, maybe if you have one of those and a up to 1MB RAM card entirely populated by 1x64kb then it might get strained a little. Though some sound cards seem to make use of it for amplifiers. Known examples have it noted in their page on Retro Web.

Anyway, it is frequent that PSUs only supply 0.3 A on that line and never myself having to dig into -5V issues over the years, I assume that is generally quite adequate. Some motherboards whine about not having it though. If it's only whining, then sometimes, later boards, you can turn off monitoring of that line, older boards it might be more of a PITA and actually need to see -5V to POST, which is the motherboard being overzealous in looking out for you rather than using it for anything.

DaveDDS wrote on 2026-05-23, 10:20:

The few times I've had to do it, I've not bothered with a DC-DC converter ... I've just used an old-school linear -5v regulator(7905) on the -12 line.

That's a good idea... but, Im guessing with what ATX power supplies are like today, the -12V rail won't be impaired by anything used along with the -5V, is this true? Also did you use a heat sink, I would make a PCB for the regulator and/or heat sink but with a heat sink it would make the adapter a little bulky looking and conductive to parts, my old adapter had a big piece of heat shrink over the PCB

Take a look at this old Intel ATX Power Supply Design Guide 0.9 from 1998:

Page 13, 3.3 Typical Power Distribution, Table 7: Typical Power Distribution for a 300 W Configuration

Output Min. Current (amps) Max. Current (amps)

[...]

-5VDC 0.0 0.3

There are multiple 'typical' examples like this. All have 0.3A max on -5V line, which explains why this is frequent as BitWrangler already said.

FWIW, 0.8A is specified for the -12V line.

-12 is used more than -5 ... but I've never seen significant current drawn from either - it can't hurt to measure what is actually drawn just in case for your configuration it's getting "close".

Since -5 allows 300/500ma (depending on who's specs you read), you technically should use a heatsink on a 7905 - especially since it will be dropping 7 volts
... but practically you might not need one ... I'd check the actual current drawn to determine what actual wattage (current x7) will be dissipated (if you're making a PCB, I'd leave room for a small heatsink ... if you're more like me and just hang the regulator somewhere on "flying leads", it should be easy to position it somewhere that would accommodate a heatsink if needed).

Most PSUs I have looked into have a 7905 without a heatsink on the -12V line to make the -5V rail, sometimes inside heatshrink to prevent shorting into chassis etc., so one can definitely get away without any thermal considerations...

See also this project as another example.
https://www.philscomputerlab.com/voltage-blaster--5v.html

Tiido wrote on 2026-05-26, 17:07:

Most PSUs I have looked into have a 7905 without a heatsink on the -12V line to make the -5V rail, sometimes inside heatshrink to prevent shorting into chassis etc. ...

That does seem odd to me - not that it would work (I'm sure it would), but many supplies rate -5 at 500ma ... and I don't see how it could safely meet that spec without a heatsink.

These little 3-terminal regulators are typically rated at 1a ... and I just happen to have a good "stash" of 14v center-tapped transformers, so I make little PSUs to power my handbuilt projects all the time. Using a full-wave bridge rectifier, I get about +/-7<>8v from the ends of the bridge to the center tap (under load - it goes higher when unloaded)... perfect for RS-232, and using a 7805 on the +8 side gives regulated 5v to power the electronics.

But ... I find that drawing 0.5a (fairly typical) from it can get uncomfortably warn without a heatsink ...
8-5=3x0.5 = 1.5watts - This would probably be OK (but does get warm)

12-5=7x0.5 = 3.5watts - which I think would get too hot without something to dissipate it.

Granted - nothing's going to actually draw 0.5a - but that's what it's speced for!

DaveDDS wrote on 2026-05-26, 22:29:

That does seem odd to me - not that it would work (I'm sure it would), but many supplies rate -5 at 500ma ... and I don't see ho […]
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Tiido wrote on 2026-05-26, 17:07:

Most PSUs I have looked into have a 7905 without a heatsink on the -12V line to make the -5V rail, sometimes inside heatshrink to prevent shorting into chassis etc. ...

That does seem odd to me - not that it would work (I'm sure it would), but many supplies rate -5 at 500ma ... and I don't see how it could safely meet that spec without a heatsink.

These little 3-terminal regulators are typically rated at 1a ... and I just happen to have a good "stash" of 14v center-tapped transformers, so I make little PSUs to power my handbuilt projects all the time. Using a full-wave bridge rectifier, I get about +/-7<>8v from the ends of the bridge to the center tap (under load - it goes higher when unloaded)... perfect for RS-232, and using a 7805 on the +8 side gives regulated 5v to power the electronics.

But ... I find that drawing 0.5a (fairly typical) from it can get uncomfortably warn without a heatsink ...
8-5=3x0.5 = 1.5watts - This would probably be OK (but does get warm)

12-5=7x0.5 = 3.5watts - which I think would get too hot without something to dissipate it.

Granted - nothing's going to actually draw 0.5a - but that's what it's speced for!

Don't forget it's in a closed box with forced airflow. That could get you into workable territory. Admitted, most power supply manufacturers knew that there's virtually no load on the negative rails.

tehsiggi wrote on 2026-05-27, 05:40:

Don't forget it's in a closed box with forced airflow. That could get you into workable territory. Admitted, most power supply manufacturers knew that there's virtually no load on the negative rails.

Fair enough, but IME airflow doesn't help much if you are relying on just the little tab on the regulator itself. Even a small heat sink can makes a huge difference.

And... I'm pretty sure "in heatshrink" doesn't help airflow/heat-transfer either.

We all know that nothing is going to draw significant current on these lines - and there shouldn't be problems!

It's more about not-meeting published specifications (and we know that NEVER happens in the clone/knock-off computer "industry" :- )

DaveDDS wrote on 2026-05-27, 13:17:

Fair enough, but IME airflow doesn't help much if you are relying on just the little tab on the regulator itself. Even a small h […]
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tehsiggi wrote on 2026-05-27, 05:40:

Don't forget it's in a closed box with forced airflow. That could get you into workable territory. Admitted, most power supply manufacturers knew that there's virtually no load on the negative rails.

Fair enough, but IME airflow doesn't help much if you are relying on just the little tab on the regulator itself. Even a small heat sink can makes a huge difference.

And... I'm pretty sure "in heatshrink" doesn't help airflow/heat-transfer either.

We all know that nothing is going to draw significant current on these lines - and there shouldn't be problems!

It's more about not-meeting published specifications (and we know that NEVER happens in the clone/knock-off computer "industry" :- )

It's another case of "nobody cares about specs" 😉

If it was actually loaded with 500mA then yeah, that heatshrink will probably shrink a little more 🤣 (and the regulator perhaps even enters self preservation mode, they are actually very hard to kill if you try to overload or overheat them)

Tens of mA is most one will see on that rail, at least with sound cards that need the -5V. -12V can see couple hundred mA, depending on how big the analog sections are on each card. Couple tens with one card, but can be many more with several cards...