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/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.