Will using a modern ATX power supply on a retro PC cause the 5V rail to be overloaded? \ VOGONS

Will using a modern ATX power supply on a retro PC cause the 5V rail to be overloaded?

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First post, by zwrr

Posted on 2026-02-08, 11:23

zwrr Offline

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My retro PC system is built around an SBC motherboard and includes the following components:

1. Intel 855 SBC motherboard + Pentium M 1.7G
2. GeForce 6200 PCI
3. Voodoo 2 SLI
4. SB Live

The power supply I'm using is a FLEX power supply with a 5V current rating of 15A and a 12V current rating of 28A.

The system works normally, and runs 3DMark 2001 tests for extended periods without problems, but I noticed the power supply gets very hot. I suspect this is because the system is drawing more than 15A on the 5V rail, causing the power supply to overheat ?

SBC1: Cyrix 5x86-120, HS-5x86HVGA, 16MB EDO, GD54M30, SB Pro II, HardMPU-wt
SBC2: VIA C3-800, PCISA-C800, 128MB SDRAM, Savage4 Pro, SB AWE64 Gold
SBC3: Tualatin-S 1.4G, PCI-6872, 256MB SDRAM, FX5200 PCI, Voodoo2 SLI, SB Live

Reply 1 of 2, by dionb

Posted on 2026-02-08, 13:26

dionb Offline

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To answer this question we need to know exactly which SBC you have there - vendor and model. If you don't know, post some pics so we can help look.

Does it draw power exclusively through the PICMG slot? Or does it also have an ATX12V connector? Given So479 is normally ATX2.2 in terms of power delivery, I'd expect an ATX12V connector on the SBC and for the CPU at least to draw its power from it. I have a similar board here, an Axiomtek SBC81872 and it definitely has an ATX12V connector and refuses to boot without it.

Also, what sort of "FLEX" powersupply are you using? Is it ATX or AT? Here again, brand+model are useful information - particularly as some brands live up to their published specs and others are notorious for not doing so...

Reply 2 of 2, by Halofiber86

Posted on 2026-02-08, 17:03

Halofiber86 Offline

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If this could be of any help here, I have to report the following on a 5V rail when using a modern PSU on older motherboards: when using the 24pin to 20 pin adapter, I have noticed the 5V decreased from ~5.0 to ~4.73 or something. It so happened that I've also added a ~20cm 20 pin extension cable on top of that, and the juice available to the motherboard decreased to ~4.30V. This was too low even for the robust Tualatin motherboard, and it started to complain. Now I'm using the same modern power supply unit on the more modern LGA775 motherboard without any adapters and/or extensions, and it gives me solid 5V there when plugged directly into 24-pin socket.

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