Computer PSU +12V fixed output

[bugmike25]

Hi, I want to use a computer PSU as a suply for my design.  The +5V output is pretty much constant however the +12V ouput decreases to approximately 10.8V when the load is low.  Does anyone have an external circuit that i could build to keep the voltage at 12V?

[Ret12_12]

I think when the voltage is drop down then that means the power supply can't give all the current requested by the load so drop the voltage for keep the current.

Or the power supply it is old or used for many many hours, so in this case or you replace or change the capacitor.

Check also the what current it is on the power supply is it the same or more or less from the load?

[bugmike25]

Well the strange thing is that when there is no load the voltage drops.  With a high load the voltage increases.

[meirmark]

Hi.
A PC power supply is a switch mode power supply that needs a minimum load of ~ 0.5A so put a 24 ohm 7W resistor on the 12V.
 

[Walkura]

Pc powersupply's are usualy regulated on the 5 Volt .
Afterall much more then make the hdmotor run isn't needed from it .
You can include the 12 volt line into the voltage regulation loop by adding a resistor from the 12 volt .
Under ebooks you can find Marty brown's powersupply cookbook & switch mode powersupply's by Pressman .
In those books they give a simple and sure way to calculatoe your extra resistor for your voltage regulation .

Goodluck and be carefull

[mmemory]

When you buy an expencive powersupplay , 5 V and 12V supply may be designed separately and works independent

[Walkura]

Although i never seen it that can ofcourse be true .
All pc powersupply's i seen so far were with 1 transformer With pwm usualy based on the tl494 or clones .
The feedback loop goes with a resistor from the 5 volt line to the tl431 .
Usualy the current through the feedback loop is about 1 mA which would make a 2k5 resistor to the Vref of the tl431 .
(Ofcourse thats a matter of choice of the designer so always check the value of the resistor so you can calculate the true amount of mA)
For 12 Volt that would mean you need to add a resistor of 12 Volt - 2.5 Volt / 1 mA = 9.5 Kohm .
By doing it this way you include the 12 volt into your feedback loop and it will now regulate both lines .
Just look up that resistor between the 5 Volt line and the Vref connection of the tl431 further its relative simple math

[amacolp]

Hi! All

Can any one of u give me the circuit diagram for Old PC power supply :
Name of the power Supply is COMPAQ - Model is PS-7201-2C
Having a 24 pin connector. Thanks.
amacolp

[tAhm1D]

Hi,
I don't have exactly the circuit you wanted but I have something similar.

Thanks.
Tahmid.

[Jehan]

If you are conversant with SMPS, the easiest way is change the main feedback from 5 volt to the 12 volt output, but there is a problem, the 5 volt output becomes unregulated, yuo can also give a double feedback with the prominent part from 12 volt and a marginal part from 5 volt section, this scheme will give you a regulation of around 3 %

WARNING: ANY OPEN LOOP FEEDBACK CONDITION WILL DESTROY THE POWER SUPPLY