Atx modify help me

[Aplicant]

Hi to all
I want to modify atx power supply to battery charger which gives 14.4 v 1-7A
How to do this
I have done this but there is a problem :
I rewound transformer windings to 5 turns
Original turns were=4 for 12v
Then i check output voltage is 18v with no voltage control
When i load a 12v fan output drops 17v ,the lower resistance the more drop voltage but max drop 15v inspite of much lower resistance
My aim is 14.4 v constant voltage
Pwm:tl494
How to do 14.4 v contant voltage

 

[(*steve*)]

You need to find a power supply where the 12V rail is regulated and modify the feedback loop to set it to the required voltage.

 

[Aplicant]

I modified 12v of PS but its max output 13.5 with no load ,with load 12.7
Steve
Let's imagine
We have 17v power supply with no feedback control and we need modify it from 17v to 14.4 v
Have we got some way to do it
Feedback control or other ways
Thanks in advance Steve

 

[davenn]

I modified 12v of PS but its max output 13.5 with no load ,with load 12.7
Steve
Let's imagine
We have 17v power supply with no feedback control and we need modify it from 17v to 14.4 v
Have we got some way to do it
Feedback control or other ways
Thanks in advance Steve

show us the circuit you are working with so better suggestions can be offered

 

[AnalogEL]

Not sure if this is worth anything, but here's some more data. I recently modified an ATX supply to do something similar to your second comment above. I just added a single resistor to the regulation feedback circuit, and left the transformer turns ratio unchanged. I only increased Vout from 12V to about 13.8V (that was my goal value), but it kept Vout constant for both large and small load currents.

It sounds like your supply is running open loop, like the feedback function is somehow broken in both of your experiments. With your output set to 12.7V or thereabouts I would expect tight regulation.

I think that these supplies need a minimum load current, so I hooked a small incandescent light bulb designed for 12V to the 5V output (that was a convenient power resistor in a small package). These supplies tend to pay more attention to the 5V output than the 12V, maybe having a constant 5V load present makes a difference?

My supply uses a different regulator IC than yours (.pdf attached, see page 2 for my mod).

 

[Aplicant]

AnalogEL thanks for information
I regulated my voltage from 17v to 14.4
And it works fine
But i have another problem:
I need current limit
I designed current limit circuit

Voltage 14.4
Current limit :5a
When i load lower load than 5amps
It works fine
But when i load higher load than 5amps
It sounds noise: chrrrrrrrrrrr