The Wikipedia page for the Intel 8080 processor describes pin 11 as

The −5 V power supply. This must be the first power source connected and the last disconnected, otherwise the processor will be damaged.

How did the Altair (or other 8080-based computers such as the IMSAI) ensure that this was the first supply voltage to stabilize?

  • Err, is this about card insertion under power (connected) or power up sequence?
    – Raffzahn
    Mar 24, 2021 at 11:22
  • @Raffzahn: The power going to the CPU. Nothing about cards.
    – DrSheldon
    Mar 24, 2021 at 13:24

2 Answers 2


Never used nor saw an Altair so I might be completely wrong in here. However this doc for some Altair 8800 contains the circuits (at the end)...

The power supply is divided to 2 zones. one with transformers and rectifiers and second with voltage regulators.

My EE skills are telling me the start race of -16V wins due to input RC filtration filter has smallest T time to charge between all the voltages. That means either R (the first in series) is smaller or C is smaller or current is bigger...

The shut down time is related to the last R after the zener diode and or filtration caps and the device load itself. The smaller the discharge R is the quicker it shuts down ...

So by carefully selecting the input RC and output discharge resistor values among all voltages with respect to the Voltage and Current you can "program" the order of power on and shut down independently.

Here the circuit:

power supply

the input is without resistor (as for high current it would be big heat source and dissipated a lot of power) so the Voltage and Current of the transformer and sum of filtration Capacitors determine the order of power on.

This is the reason why you should recap power supply capacitors before first powering of old computer after long therm storage ... as old Caps have lost their original capacitance and the order of Voltages could be much different then designed.

  • Except that it's not the bus' job to ensure a handling specific to a certain CPU. That's must be done on the card.
    – Raffzahn
    Mar 24, 2021 at 11:22
  • @Raffzahn I do not follow ... did you mean that the voltage order is done by additional circuitry on board where the CPU is? Or this comment should be elswhere?
    – Spektre
    Mar 24, 2021 at 11:59
  • 1
    On the Altair everything about voltages, order and regulation is up to each card. Yes, sounds less than great, but that's how they cranked it out.
    – Raffzahn
    Mar 24, 2021 at 12:02
  • 1
    True, I agree, back in the days I never understood why it was such a wasteful design. it took time to understand that it was kind of an engineering compromise to keep entry level price low. Doing a capable regulation that would allow to grow from a few to many cards would have been quite expensive. But packing just the transformers, rectifiers and large caps into the box but leaving all expensive regulation to the cards saved a great deal for starters ... but adding up over time.
    – Raffzahn
    Mar 24, 2021 at 12:11
  • 1

It looks like the Altair 8800 didn’t have any provisions to ensure -5V was available first (but see @Spektre’s answer and the note below).

I found a schematic of the Altair 8800 CPU board power supply in this document, page 20.

This shows a very primitive -5V supply (just a resistor going to the unregulated -16V supply, a 5.1V Zener and a some capacitors for stabilization). The +5V supply is regulated by a 7805.

Altair 8800 CPU board -5V supply


As @Spektre noted, the -16V unregulated supply has lower capacitances than the positive supplies. That would have the -16V side arrive at its operating voltage faster at powerup. Because of the simpler (Zener) -5V “stabilized” power supply, it would probably ensure this was the first to arrive at its operating voltage as well.

It would be fun if someone with an original Altair 8800 could confirm this ;-)

  • Yes it did ... you look at wrong circuit you need to see the one just before this where the rectifiers and transformers are
    – Spektre
    Mar 24, 2021 at 7:50
  • 1
    I see what you mean and it might have been enough to make sure -16V was available first, and with it (because of the simplicity of the regulator) the -5V supply.
    – StarCat
    Mar 24, 2021 at 7:56

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