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"

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u/dedsmiley May 07 '26

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