Adjustable / Variable Voltage from ATX supply.

[fault]

I have seen projects online for converting an ATX PSU into a vairable bench supply. Most bolt an adjustable regulator onto the available switched outputs. But I do not see anyone adjusting the switched output directly from the feeback circuit.

Questions:
1: Can a 12V rail be adjusted for a range (eg 4V-12V) by modifying the switcher feeback sense?

2: If I can, and I do, what kind of trouble might I be getting into? (eg fires, fried equipment))

Backgound:
I do not know a lot about SMPS circiuts. And I know I am not going to get a lab quality bench supply out of an ATX PSU. But it seems to me that modifying the feedback sense to regulate Vout would be more effective and use less parts than a linear adjustable addition.

[japlytic]

Re: Adjustable / Variable Voltage from ATX supply.

Welcome to the forums!

The easiest supplies to convert have a TL494 or KA7500 PWM controller (or its equivalent) on the secondary side. I also managed to do the ones with a optocoupler and a TL431 (or equivalent) for the feedback.
You would need a large and fairly accurate test load, because the current sensing circuit may need to be modified in most cases, because some power supplies check for tolerance on the -5V and -12V rails.
When I modify a computer power supply, I make a circuit of the PWM controller and its associated components, to make conversion easy.
To turn on an ATX power supply, ground PS_ON, which is ATX connector Pin 14 (usually Green).
Use a series lamp limiter when performing tests under light load e.g. 50W 12V halogen lamp, and a clamp meter which can perform DC current measurements for larger loads.

I've converted a few computer power supplies for bench use (e.g. amateur radio, providing the supply is well filtered), and I am planning to do a 580W unit bought from a local junk shop for $3.

Also, I plan to have a webpage on PC power supplies I have modified for various applications.

[Wizard]

Re: Adjustable / Variable Voltage from ATX supply.

I have modified my PSU for 15V for the toshiba DTR and added ATX 12V connector as a pigtail through the power input hole. Still needs good 15" LCD because DTR was beat up.

It is a 2.8GHz P4 HT so it is my lucky find.

Cheers, Wizard