Modifying a PC Power Supply.

On a sunny day (Mon, 7 Apr 2008 12:12:13 -0700 (PDT)) it happened Arete wrote in :

Most will do 12V > 5A. Depending on what you want to use it for, pehap you could consider this:

3V 5 V load between 12 V and 5 V = 7 V load between 12 V and 3 V = 9 V 12 V This gives you 5 steps, perhaps enough for your application?

Modifying the control to the 12V goes all he way down to 3 V may not be so simple. Could even be easier to add a second switcher on the + 12V.

Use with LM338K (pricey, ~$8 @ Mouser/Digikey) for up to 5A, or use an LM317 (< $1) for up to 1.5A

Michael

I once modified an ATX supply (i.e +5V standby supply drives the main PWM controller) to go from 0 to 5V @ 25A. It used TL494 and I just modified the error amplifier section. I found TL494 based PC power supplies are less common these days, with the ever increasing power/volume density. Modifying for 0 to %100 should be possible with any ATX power supply because the you have the +5V standby. You need to somehow inject a positive signal into the voltage control loop and therefore adjust its output voltage range %100 to %0.

Primary side controllers (e.g UC3842 forward converters) will use a TL431 on secondary as the voltage error amplifier. Secondary side controllers (e.g TL494) use the PWM controller's internal amplifier.

I was gona publish a simple How-To web page until I realised there was no real common ATX power supply design to make my modification useful to people.

Adam