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New PSU on old hardware - is the -5V rail important?

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First post, by aurasdoom

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I have bought an old Pentium III Sony VAIO PC which came with a 120V PSU.

I replaced the PSU with a more modern one which doesn't have the -5V rail anymore. The computer complains about that at boot time. More specifically it thinks there's -12v on the -5v pin, but I checked and that pin is not present.

Did no computer ever use it and that's why it was removed?

Should I ignore it?

Reply 1 of 10, by Matth79

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-12 being detected on the -5 is alarming, but that is presumably the voltage sense somehow floating - and if it's floating, then there is no load.
Some soundcards need -5 for the audio stages, and if you can spare an ISA slot, there is the "voltage blaster" https://www.philscomputerlab.com/voltage-blaster--5v.html.
In the AT era, it was needed a lot more

Reply 2 of 10, by aurasdoom

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Thanks for the reply. There is no ISA slot in this pc. Was it only used for those slots?

Reply 3 of 10, by mkarcher

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aurasdoom wrote on 2025-08-22, 16:04:

Thanks for the reply. There is no ISA slot in this pc. Was it only used for those slots?

Since the 4116 memory chips (which required -5V if I remember correctly) were obsolete in 1983, Mainboards No longer required -5V, and the only remaining use was to forward that voltage to the slots.

Reply 4 of 10, by aurasdoom

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The computer works fine. I really hate that it stops the boot process every time and I have to press F1.

Reply 5 of 10, by DaveDDS

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The current requirement on the -5v rail is/was very low.

IIRC many PSUs implementer it with a simple analog regulator on the -12v rail.

You can do the same, with something as simple as a 7905.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Reply 7 of 10, by aurasdoom

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@red-ray do you have a clearer picture? Or the do you still know the values of the resistors?

Reply 8 of 10, by red-ray

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aurasdoom wrote on 2025-09-08, 13:23:

@red-ray do you have a clearer picture? Or the do you still know the values of the resistors?

It may depend on the board, I used -12 ... 1K ... -5 ... 680R ... earth, measure the actual -5 voltage and adjust the 680R as needed.

Doing the sums -12 x 680 / ( 1000 + 680 ) = -4.86 volts

Reply 9 of 10, by jh80

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aurasdoom wrote on 2025-09-08, 07:36:

The computer works fine. I really hate that it stops the boot process every time and I have to press F1.

Sometimes there is a way to turn off the -5V alarm in the BIOS. I'd check there first.

Reply 10 of 10, by aurasdoom

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jh80 wrote on 2025-09-08, 22:40:
aurasdoom wrote on 2025-09-08, 07:36:

The computer works fine. I really hate that it stops the boot process every time and I have to press F1.

Sometimes there is a way to turn off the -5V alarm in the BIOS. I'd check there first.

I've set the BIOS to ignore the voltage on the -5V rail but it still gives the warning at boot and requires me to press F1

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