I want to make a benchtop power supply from a 20 pin ATX power supply from the last millennium. I have seen posts, but still need help.
So far, I have shorted PWR_ON to ground and have connected an LED to PWR_OK. I connected a 12V fan to a 5 pin peripheral connection. When I connect a CD drive to to 5V, the LED lights up and the fan spins. I measured 5.05V between the CD's supply and ground and 0.55A across it. I took this to mean that I should use a 9 - 10 ohm power resistor (2.7775 watt?!) for a preload.
After removing the CD drive, I connected a 50 ohm 25 watt potentiometer to 5V. PWR_OK comes up (repeatably) only when the resistance drops to below 1.2 ohm. I measure 4.5V across the potentiometer and 3.2A going through it (it gets on hot side of warm). When I try to slightly change the resistance up or down the PWR_OK LED goes out.
What accounts for the apparent difference in behavior between attaching the CD drive and this external resistance? Is this getting distorted by looking at the low end of the potentiometer and my meter?
Despite my abuse, the ATX psu seems to work. Would I have better luck building a benchtop supply using a more recent ATX?