12V 8A Power Supply Using ATX supply componenets

[saeed_pk]

Dear All,
I want to design a Battery Charger 12V 6-8 A
I want to make a switching supply using 350W ATX Flyback transformers.
My problem is i can't use PC supply as it is please tell me how do i make power supply from PC Supply components.

 

[mister_rf]

Any ATX power supply might be converted to provide up to 14V-15V to charge batteries.
You'd first have to determine which switching regulator IC was in use, and then look up for the datasheet and later to reverse-engineer the whole supply to find the voltage feedback and to change the value of the resistors in order to get a different voltage on the 12V line.
(also need to change resistors for the over-voltage protection).

 

[Tahmid]

All you need to do is modify the voltage feedback and overvoltage protection crowbar circuit in the ATX power supply. Most ATX power supplies I've seen use TL494. See if yours uses TL494 or not. Then, just change the resistors in the feedback section so that the output is around +14.5V. 14.5V*8A = 116VA. So, your power supply should be able to take it. Use a 10 ohm resistor on the +5V line as a dummy load so that there's always some load at the output. Now, all you need is a current limiter circuit, so that the output current does not exceed 8A. This can be done with op-amps or microcontroller. Choose microcontroller if you're familiar with it as it's going to be easier and will require less parts and will give you more control. If you're not familiar with microcontroller, use opamps.

Don't get the output voltage too high since the output caps are rated (usually) at 16V. So, try to keep the output at <14.5V if possible.

Hope this helps.
Tahmid.

 

[saeed_pk]

Thanks for your response
I want to re-make a new board using same components the difference is only that i have to effectively control voltage using some MCU for intelligent charging application.
my flyback transformer gives (on secondary of course)the voltage waveform out but when i apply load it dies down
Is my Switch( BJT in this case) is not getting base current so i am not have output.
Please advice why i am not getting load -

 

[Tahmid]

What frequency are you driving it? Can you post your circuit?
Which transistor are you using?