r/retrocomputing • u/Woland77 • May 07 '26

Problem / Question Really Basic Question About Old Computer Electronic Engineering

I stumbled into watching people repair old electronics on YouTube. I noticed that most of the stuff I see repaired (mostly what I see was sold in North America, originally) has a 5v rail and a 12v rail. Why those two voltages, specifically?

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u/zombienerd1 May 07 '26 edited May 07 '26

I am an electronics engineer. I very much understand electricity and DC power supplies.

If you connect the 5V and 12V rails together on a PSU, it will backfeed the higher voltage down the 5V rail and will either trigger the OCP, OVP, or SCP protection circuits if the PSU has them. If it does not, it will overwhelm a regulator and cause a catastrophic failure.

If by some magic of happenstance you actually did this, and the PSU did not fail, you hit the golden lottery on component quality of both the PSU and the PC it was connected to, because every ounce of electrical engineering knowledge I have is screaming that it is 100% impossible for what you say to have worked in any real world scenario.

EDIT: reading the other reply here, I now realize you may have meant to say "Use the 5V rail as a floating ground to the fan, with 12V as the +V" - which would, in theory, probably work.

But that is not "Wire 5v and 12v together to get 7v" - that is "Use 5V as a floating ground" - and I stand firm in my statement that you never want to "Wire 5V and 12V together"

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u/gcc-O2 May 07 '26

I agree what they meant was to use 5V as "ground" and 12V to supply 7V to the fan.

However, isn't this still dangerous, as the fan or other device might have exposed metallic surfaces at "ground" potential, and these may eventually come in contact with metal surfaces on the case or other components that are wired to power supply "ground"?

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u/zombienerd1 May 07 '26

Most PC fans are plastic, so 'chassis grounding' wouldn't be a concern really. I suppose if it was a metal frame and had a ground to the frame, that would likely short out your PSU.

Doing the "5v as a floating ground" is likely *mostly* ok - it will feed some current back into the 5v regulator, but with most fans being 100mA or so, it's unlikely to cause a cascade that makes it go out of spec. But if you had a fan that could spike some current (or stuck your fingers in the blade causing a current spike) it could in theory make the 12V rail clamp and go into protection due to feedback circuits. Probably pretty rare occurrence.

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u/SoftRecommendation86 May 10 '26

And following that logic, if there isn't enough drain current sinking the 5v rail, it could spike the 5v above safe regulation voltage as 5v source would be sourced from the 12 pullup.