help converting ATX into varible power supply - KA7500B chip

[eevblogfan]

hey

I have got few of that's boards , they consistent of the KA7500B's chips and I know I should be able to convert then into CC and CV power supply ,

nothing fancy I know it will have lousy ripple noise etc , that supply is not ment to power sensitive stufff, I need something like two of these but since I have got couple of them laying around I'll do something like 3 or even four

each power supply will have two panel meter's , one for CV and one for current displaying . I do like to do that "CCadj" button and see what is my CC set to , but don't be bothered if too hard ,

now I think of re-wind the transformer , what exactly I shall do ?

please I really need one to guide me throo that.

thank you in advance !

[Alana]

If you want CC/CV supply then build some linear PSU from scratch, eventually converting ATX PSU to replace mains transformer.
But if you insist on converting ATX one then go for simple CV solution - something like this: http://danyk.cz/at_atx_en.html

[eevblogfan]

hey

I can't maneg to acompish that :

http://www.chirio.com/switching_power_supply_atx.htm

please guide me here !
thank you in advance !

[Alana]

Show us what you already did and what does not work.

[darko31]

I was also playing a bit with ATX PSUs to serve as battery charger, and had successfully upped the voltage by modifying resistor values.
But you want CC and CV, and you cannot achieve that without some more circuitry.

I think that best thing to do is up the voltage to whatever you need and then use a LM317 for variable voltage and another one for constant current.

If you aren't sure how to change the output voltage this schematic in attachment will help you out.

Look at the resistor connected as voltage dividers to the pin 1 on KA7500 (R33, R34, R76).

Note that aren't all PSUs the same, so values may be different.

Add for example a 100k variable resistor (pot with only 2 pins hooked up) in series with one of resistors (R34) and hook up a voltmeter to the 12V output (also 5V, and 3.3V will increase) and turn the pot, and you'll notice how the voltage increases.

At some point everything will turn off (don't worry, you didn't burn anything) it's the over voltage protection doing its thing.

Look at the A2.1 op amp, that its job, to send signal to KA7500 when to turn off to pin 13.

You will have to disable or adjust the that op-amp (easiest thing to do is to desolder pin 13 and isolate it, cut it or whatever if you aren't going to use it).

And that's about it. If you need help, ask.
 

 

[Alana]

Provided that i replace 16V caps to 25/35V whats the safe margin for such conversion?

[darko31]

I've completely forgotten about caps, thanks for the reminder Alana, got to order some.

So KA7500 can take from 10V to 40V as the unregulated input voltage. So basically we are just limited by the transformer turns ratio. I have no idea what's the transformer secondary voltage. Probably something like 20V - ish.

Besides filter caps i think that there's nothing else that would blow if the voltage is increased.

[gnif]

I performed a similar mod to a PSU for the same reasons. The PSU I used already has CC once you disable the protection circuit. I am able to get 5v - 16V pot adjustable (have not upgraded the caps yet).

The output of the device needs to be modified, in all designs I have seen, the feedback voltage from the 5V and 12V are tied together via some resistors before being fed into the feedback circut. This means, if you pull too much current from the 12V rail, the voltage will drop. You need to disconnect the 5v feedback and then just deal with the 12V side.

As for adjustable CC, the switching chip on the hot side handles it, it is fixed by some resistors, lookup the databasheet and you should be able to replace these with a pot. I am getting adjustable 1A to 20A CC.

[eevblogfan]

hey

not that it's a big deal but . is that CC or CL mode ?

thank you i advance an when I'll have enough time I''ll play with the ATX and update as well

thank you in advance

[LukeW]

I'll give you a hint that should prove valuable... the KA7500 is a cheap almost-exactly-equivalent knockoff of the common TL494 SMPS controller chip.

You should be able to search for lots of examples and reference designs using the TL494, and use that knowledge with the KA7500 chips (or trace out the circuits in your existing boards) once you know they're the same thing.

[eevblogfan]

hey

just updeating :

I have 3 UPS transformers , one I've rewound and two are the geniuen windings .

I have decided to build from them 3 CH varible power supply

CH1) 0-12V ,0-4.5A / 0-6V , 0-9A ( 54W)

CH2) 0-13V , 0-5A / 0-6.5V , 0-10A ( 65W )

CH3) 0-20V , 0-3A ( 60W)

all linear and all based on e3610's design  

so the SMPS hacking will have to be pushed aside and will be futural side project if will ,

thank you any way to all of you

[crazyfrog]

new poster, I'm in Bangkok Thailand.
try KA7500B modification on my old PSU, and it works !!    ;-)
I could vary Voltage.

but, question
a) how to adjust Current ?