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Modification of PC AT power supply

Modify feedback signal to DBL494 chip based AT computer switching mode power supply unit to enable variable voltage regulated output.

Check out the See also section.

Theory

Most voltage regulating circuits utilize a feedback signal to monitor output voltage and adjust the energy transfer from primary side to secondary. If the output voltage raises, the energy transfer is restricted until it drops. And if it drops, energy transfr is boosted. This way the output voltage remains constant (idealy).

Computer power supplies are switching mode. That means electricity is transfeted in pulses through transformer at high frequencies. All the regulation is done by PWM of the primary side switching transistors.

I suggest reading a bit about PWM and SMPS. While you are at Wikipedia, check out Voltage divider and Ohm's law. Here study the basic composition of PC PSUs. Soldering skills are required. Multimerer is handy.

For this mod you will need one potentiometer, a collection of resistors or some old equipment redy to disassemble and a handful of wires.

Safety

The input capacitors do carry enough energy to waporize metal. Please always disconnect the power when working on the circuit AND dicharge the input capacitors with resistor (or screwdriver, becouse life is too short).

Dont forget to check voltage rating on output capacitors. There is usualy only single volt headroom an this things can explode.

Whatever you do to the feedback, do not break the path from positive to pin1, again, output capacitors can explode.

Chips

Half of PSUs in the 200W range are based on TL494 chip and two power switching transistors. The other half is based on UC3843 chip and single MOSFET on the primary side. I do not want to talk about UC3843, it is too sad.

DBL494 is the same chip as TL494, the prefix letters can differ. KA7500 is pin-to-pin compatible and almost the same. And I am sure there are other. Please mail me your chip numbers. This chip contains all the logic of the power supply except short circuit protection.

Pin number 1 is the feedback imput of TL494. Pin 1 is one directly the to the left of an half moon shaped depression. This feedback input has a 2.5V reference. If we connect this pin direvtly to output we would get 2.5V at the output. To get different voltage, a voltage divider is used.

I don't want to repeate what have been written by others. You can find better and more technical descriptions on thoose referenced siter.

Notice how how the first three including mine are of CZ/SK origin :)

Remove other

Computer PSUs have variety of outputs at different voltages and max current. Voltage on unloaded outputs can climb to dangerous levels, especially while we mess with the feedback circuit, therefore I advice you to pick one so you can disable all other. Pick the one with the highest current rating. You can increase the output voltage only by a few volts, 3.3V is probably not suitable for electrolysis. My PSU did not have 3.3, so I've choosen 5V.

Do not disable the voltage which powers the TL494 chip. This chip is usualy powered from the negative outputs (-5V or -12V or both). Negative outputs are low power rated and are usualy safe to leave enabled.

First identify ground on the PCB. It is the biggest chunk of solder and connected to black wires. Don't do anythig to ground.

To disable an output easiest is to disconnect it at the secondary side filter inductor and disconnect any voltage sensign (usually resistor). The inductor is ferrite ring with multiple leads and different wires wrapped around and is usually yellow. To find which lead to disconnect you need to trace the printed circuit board.

Check the filter capacitors on the wanted output for voltage limits and replace them with high rated ones if the headroom is too narrow. You can get some caps from the disabled outputs.

Feedback circuit

Now trace the pin1 of TL494 chip. One or two resistors should lead to ground, one to +5V, one to +12V and maybe to other parts of the circuit. Disconnect all of them. If there is a capacitor, leave it.

Now design your own feedback voltage divider. You can use series and parallel resistors and you potentiometer.

Calculate the output voltage U using this formula, where Rp is resistance to positive and Rg is resistance to the ground (2.5V is reference voltage of TL494 chip):

U = 2.5 * (Rp+Rg) / Rg

Make sure your circuit provides valid feedback even when the potentiometer has bad contact or is turned to zero resistance. Do not allow setting of output voltages out of output filter capacitors limit. Remember, if the feedback breaks, expect blown capacitors.

Protection circuitry

In some units the overvoltage/undervoltage/overcurrent protection can kick in and prevent any voltage adjustment. This circuitry is usually built from many discrete components and is hard to trace.

I did not disable this protection. But if you have to, look into TL494 datasheet for "Dead Time Controll" pins. Also look for any silicon controlled rectifiers (they look like transistors), maybe desoldering thoose would help. Also prepare for sparks and smoke if something goes wrong.

Fan

If you have ATX PSU you still connect the green wire to the black wire to turn your unit on. Standby power supply is unaffected by this modification. You can connect the fan to +5VSB. Or if you have AT, connect it where it would spin.

Few photos

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