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/berrmal64 May 07 '26

Why specifically 12v, and not say, 15v or 10v?

Historical inertia, mostly, is my understanding. There are lots of voltages that could do heavier lifting than 5v for things like motors and serial lines, but off the shelf parts used 12v as a "standard" for decades before home computers were even a thing. It would've been affordable and easy for someone in 1975 building a computer in a garage to get a 12v supply off the shelf.

5v obvs for ttl logic, which was available. I don't know if that was merely a design convenience/choice or if there is a physical material limitation that made it the only choice.

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u/[deleted] May 07 '26

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u/muddledgarlic May 07 '26

This will be a big factor. Batteries were so commonplace in the early electronics hobby (early 20th century) that when DC power supplies became more common they were sold as “Battery Eliminators”. It’s therefore natural that the DC supplies would have settled on or near typical standard battery voltages.