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AT power supply handled +5 and +12 volts, +5 for ISA and processors, +12 for motors in disk drives. In 1995 486DX2 was introduced with 3.3V core, so it derived that from a simple but inefficient linear regulator connected to the +5V rail. In 1999 Athlon was introduced, which consumed 60W of power through +5V line, with total current up to 30A. Power supply cords started to burn under high loads. In 2000 Pentium4 was introduced, which had processor core voltage derived from +12V line, not from +5V.

I'm asking, what could be better if linear regulators to power 3.3V processor cores in 1995 where fed from +12V line, not from +5V?

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    I’m voting to close this question because “what if” questions are explicitly off-topic: “avoid asking subjective questions where you are asking an open-ended, hypothetical question: What if ______ happened?”
    – Stephen Kitt
    Mar 17 at 7:54

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Keep in mind that linear regulators work by converting the voltage difference (multiplied by current to make power) to heat. If the linear regulators were fed 12 volts instead of 5 volts, the power dissipation, as heat, would be over 5 times as much (dropping 8.7 volts vs 1.7 volts).

So feeding 12 volts into the linear regulators would not only not be better but would be five times worse!

You need to replace the linear regulators with switching regulators to take advantages of higher input voltages.

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  • so, basically, the reason is cost?
    – Ivan Borsuk
    Mar 17 at 8:26
  • @IvanBorsuk what do you do with all that extra heat? Move it out, of course. That requires radiator fins and fans; that adds not only cost (passed on to you the consumer, bur adds noise and yet another thing that can break.
    – RonJohn
    Mar 17 at 8:58
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    It also means more power consumed from the power supply and therefore from the wall.
    – DoxyLover
    Mar 17 at 12:25

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