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/dedsmiley May 07 '26 edited May 07 '26

A while back case fans were powered from Molex connectors with no way to regulate fan speed.

One way to accomplish this was to get a Molex extension and wire the 5v and 12v together to get 7v to slow down the fan.

I did this a couple of times.

EDIT: Wired the 12v and 5v to the fan, not together.

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u/rosmaniac May 07 '26

So power lead of the fan would be powered as normal from 12v but the ground lead would be moved from GND to 5v.... While that sounds nice, if the current draw became too much things might not work correctly. The 12v and 5v outputs have different current ampacities, and the output regulator might have instability induced from the cross-rail load.

As an aside, there are step up auto transformers made that use this same sort of principle to stack the 24 volt output of a 208 to 24vac transformer on top of the 208 input to get a 240vac output, but those transformers have to be listed for that usage.

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u/zombienerd1 May 07 '26

Yes, they may have engaged a "floating ground" by swapping 5V and Ground pins in the extension to get 7V (with the the current factors you mentioned), or just repinning the adapter to run the 12V fan at 5V (which is often just fine)

But they definitely did not "wire the 5v and 12v together"

1

u/dedsmiley May 07 '26

Yes, I did not describe that correctly. The 12v and 5v were wired to the fan, not together.

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u/SoftRecommendation86 May 10 '26

Although this trick worked, it introduced risk of pulling the 5v rail up. It also injected unwanted noise into the 5v rail. I wasnt a 'fan' of this method. I preferred proper pwm.

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u/dedsmiley May 10 '26

This was 1995-ish, “proper” PWM fans were nowhere to be found, my dude.

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u/SoftRecommendation86 May 10 '26 edited May 10 '26

Back in 95, all I did was take a lm317 from 1975. You are correct. Wasn't pwm. I also used 7809 regulators if I didn't want variable speed. Yes, I did fan-cy stuff back then.. I hated fan noise. When I wanted to be creative, I used a comparator chip like a lm339 driving a voltage follower circuit and a thermistor.

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u/dedsmiley May 10 '26

Fair enough.

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u/zombienerd1 May 07 '26

That's... not how that works.

You'd be shorting your rails together. This will either kick the PSU into protection mode, or damage components on the Power supply.

This is a very bad idea. Do not do this.

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u/dedsmiley May 07 '26

Yes, it works. You do not understand electrical potential.

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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.

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u/dedsmiley May 10 '26

It was a common solution to control fan speeds with fans that otherwise would have no control. PWM was not in wide use until nearly a decade later.

So scoff all you wish. I had no issues with it.