hello , I've had this psu that I took from a PC , that I want to modify into a lab bench psu , I succeeded in altering the feedback and have it regulate on the 12v rail however I can only go from 10v to 14v any more or less than that the psu will shut down.

Also the psu shut down at any load that exceeds 1 amp on the 12v rail even while i'm loading the 5v rail output, I'm confused on how to disable the protection on this psu, anyone knows how to do this on this 2003 IC ?

here's a diagram for it

 

it looks like nobody knows about this chinese 2003 IC.

too bad ...

 

I have modified these before with this chip. You have to fool the Ovp sense on pins 3,4,6, by fixing them at 3V on pin 3, 5V on pin4, and 4 to 6v on pin 6, by using preset pots, i used two 4K7 pots across the 5V chip supply pins 1,12 and the wiper to pin3, and pin6, set them to 3v and 4v, and desolder pin4, and connect to pin 1, this disables the 5V ovp.

When all this is done remove all resistors on pin 14, and put a 10K variable resistor across the 12V output ,wiper to pin14, this will give you your variable supply. If sucessful change the output capacitors to 63V.


Pin 5 is the current sense pin, you can temporary link it to gnd for testing when you get it to give out 28V.

 

oh exactly the confirmation I've been looking for , I was so suspicious that pins 3, 4 and 6 are the voltage sensing pins but I was afraid of blowing this thing up before any confirmation I didn't do anything and there's no datasheet for that IC out there either.

one question tho , should I desolder the pins or leave them linked to the PCB , obviously lifting them up right ? and about pin 5, as long as I ground it i'll be able to draw it's maximum current ?

looking forward to your reply .. you ROCK dude, thanks .

 

Yes desolder the pins, or cut tracks and use link wires. As i remember pin5 needs approx 0.7v to go into over current sense.

 

Aright, I'll get going with this modification and I'll post results here

thanks alot

 

When all this is done remove all resistors on pin 14, and put a 10K variable resistor across the 12V output ,wiper to pin14, this will give you your variable supply. If sucessful change the output capacitors to 63V.

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All the resistors including the one between pin 13 and pin 14 and the one between pin 3 nd 14 ?? those one connected to those caps ?

 

No, just the 5 resistors on pin14.

 

did all you told me to do I got pin 3 at 3v , pin 4 at 5v and pin 6 at 4.6v , but I only R62 and R60 and put a 10k pot instead of R6 , now I can only go from 0 to 3v , and the the PSU shuts off.

I'm a bit confused , is it a must that I remove all the 5 resistors connected to pin 14 ?

 

I even removed R40, R39 nd R38 , and used R60 place to hook up my pot and wired the 3rd leg to GND and I'm still getting only 0 to 3v max

 

I think now this chip needs pin6 (12v sense) needs to be fooled by the preset resistor put across C15 supply and set it to 12V., this is how i did it, it's a awkward chip to work with,.

The easiest chip is the TL494.

 

I see, i'm having pin 6 at 4v just like u told me aright then, i'll change its value to 12v, but what do u mean across c15 supply?

 

Put your preset resistor across the capacitor C15, as it will have about 15V as i remember.

 

I did some changes , put back pin 3 and 6 to their places and left pin 4 connected to pin 1 and grounded pin 5, got the resistors back now i'm able to go from around 9v to 16v (cap's limit) but I keep hearing this buzzing sound , almost like a static and it gets louder as i get my hand closer to the PCB , some times the PSU even shuts down , could it be some sort of EMI ?

 

That is the pwm controller on the pulse transformer, ringing due to stray capacitance, you need to use screened cable for the variable resistor across the 12V output, and wiper to pin 14, this should give you a voltage range of 9 to 28V if you can fix the shutdown like i said previously.

This is the worst chip to work with.

 

I could alter it now I can go from 2.5v to 15v, i didnt risk going beyond cuz i didnt change the electrolytes, now i just want to get rid of the buzzing it seems to get less when i touch the output wires with my hand.
True this is the worst chip to work with

 

hello Dave , can you tell me how to disable the overvoltage and over current protection on a SDC 7500B IC ??

 

Sure can mate, its the same chip as the TL494, pin 4, is the deadtime cut off it needs 0.7V to shut it down,.

To control the voltage and current the chip uses its inbuilt error amps on pins 1,2 and 15,16. Ideally if you can draw out the circuit around pins 1,2, and 15,16, or take pictures of both sides of the pcb.

This is the easiest chip to mofify.

 

Well i just wired a pot to the 12v rail and GND and wiper goes to pin 1, now i'm able of varying the voltage from 2.5v to 13.6v any more and the psu shuts down, also it won't handle any load, before modification i was able to draw 8 amps on the 12v rail.

 

The psu probably has a Lm339 chip as an extra shutdown, this feeds pin 4, i usually short pin 4 to gnd (7), pot across the 12V rail, and put the wiper to pin 1, and i get 5 to 28V on the 12V rail, . The over current is sensed on pins 15,16, using a current transformer, again this can be altered with a preset..



Here is an example circuit with a TL494, pin 16 is overcurrent sense, and the reference is set by pin 15, note the Lm339 on pin 4.