How to convert a PC PSU into a variable power supply?

[nrodie]

convert a pc power supply to a variable

Hi,

Im worse than a novice... im a novice who knows a little bit!, anyways....

As a little project I would like to convert my PC PSU into a variable power supply.
Is it possible for me to just plonk a variable resistor into the circuit somewhere to adjust the output voltage. Ideally I would like to step it up to the max voltage I can get, which I would imagine would be somewhere around 16VDC ( I make this assumption based on the caps which are 16V rated ).

I would like you to assume that the transformer can supply the voltage

To give you some idea of what level im at I can read schematics, identify components, follow paths on a circuit board, and follow instructions, essentially I know just enough not to hurt myself.

So is this possible? even if a little rough, any ideas would be appreciated, im doing this for love not money so please dont suggest alternatives, just tell me it cant be done if it cant be done. Thanks

 

[IanP]

Re: Convert PC PSU

Some of PC PSUs can be modified reasonably easy, some not at all. Specially the older units will be worth lookig at.
Obviously there is no general recepy on how to do it; and the voltage range will also vary.

Some of these unit use LM431 ( **broken link removed** ) which is a shunt zener regulator.
It usually has an input voltage divider and drives an optocoupler to the primary side. The Vref input has to have exactly 2.5V from that voltage divider, so all what you need is to change the ratio there, and this is how you can do it with a PSU that uses 431 circuit in its voltage feedback path ..

Regards,
IanP

 

[throwaway18]

Re: Convert PC PSU

Switch mode power supplies contain hazardous voltages, eough to hurt you or kill you. They contain large capacitors that can hold a charge for some time after they are switched off. If operated without the lid parts inside can explode with enough force to cause eye injury.

PC power supplys can produce enough current to make a home project run from one a fire hazard.

Elektor published a short article a few years ago describing how to modify PC PSU's for different output voltages.

 

[nrodie]

Convert PC PSU

thanks, i will have a look and see if i can figure it out

 

[ahmad_abdulghany]

Re: Convert PC PSU

I think you can use a variable zener regulator with suitable voltage and current requirements to adjust the PS output level...

 

[Code Warrior]

Convert PC PSU

I dont know weather PC power supply can converted to variable power supply but you can use it for fixed voltage PS. Its output is +5V, -5V, +12V and -12V. These voltages are good for electronic experiment and testing.

 

[niket_304]

Re: Convert PC PSU

thats risky as far the security of the pc is concerned

 

[thushi]

Re: Convert PC PSU

I have enough experience on that. You may find TL494 PWM IC on the SMPS. find the feed back circuit from the output and remove the resistor there and put a variable resistor on to that. I did it several times this type of job. you can convert the PSU to vary the voltage upto 24V, provided that you have to change the caps, resistors and windings on the transformer. good luck