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Modification of PC supply AT / ATX to an adjustable 3-15V supply
Halfbridge topology supply mod:

    Everyone sometimes needs a regulated power supply. Laboratory power supplies are expensive and therefore we usually use what is available. As a small voltage supply with a large current we the most frequently use AT or ATX supplies from computers. Their disadvantage is not truly stabilized voltage and often need to load both the two main outputs (5 and 12V) of approximately the same power. Therefore, I present a simple modification of the PC power supply of type AT and ATX to a regulated source of 3 - 15V with the true stabilization and the current corresponding to the original 12V output. Feedback stabilization is connected to 1 outlet circuit TL494 (Or its equivalent, KA7500, KIA494, DBL494 - the same circuit is used in 90% of common supplies). The reference voltage is 2.5 V (ie circuit regulates the output voltage so that voltage divider created just 2.5 V). Originally stabilization is linked to outputs 5 and 12V and works well only when you equally load both outputs. After this modification the feedback is associated with the 12V output only. Potentiometer regulates the voltage in the range of 3 - 15V. Potentiometer can be replaced by resistor to set constant voltage if needed. The ATX supply fan can be connected to the 5VSB so that it does not affect regulation. During modification you can find usefull the AT and ATX PC computer supplies schematics .

AT or ATX PC power supply outputs
Yellow ... 12V
Red ... 5V
Black ... 0V (GND or COM)
Green ... Standby power. This is only in case of ATX. Connect it to black (0V) tu turn the supply on.


     WARNING !!!
There's a dangerous lethal line voltage inside the power supply. Capacitors can remain charged even after shutdown. Inappropriately modified source can be dangerous. You do everything on your own risk.


Schematic of the modification of PC AT or ATX supply to a adjustable source of stabilized 3-15V
Schematic of the modification of PC AT or ATX supply to a adjustable source of stabilized 3-15V

Pin 1 of xx494
Pin 1 of xx494


Forward converter supply (single MOSFET) mod

Sometimes I come in contact with ATX, which instead of NPN transistors halfbridge uses so-called single switch forward topology with a MOSFET with Uds = 800-900V. In these supplies is no circuit TL494. Theres UC3843 circuit and it is situated on the primary side. Feedback is introduced via optocouplers. Voltage is sensed by a circuit TL431 (GL431, AZ431 - letters may vary). This circuit also has a reference 2.5V, so the principle is similar. From the reference pin (R) resistors leads again to the ground, +5 V, +12 V Moreover, there is a link between the RC reference and cathode, it is necessary to leave. Adjustment resistor divider is seen in schematic below. If the TL431 has the anode resistor (in picture it is 22R), short it. Furthermore, it is necessary to enable the main power source and eliminate the protection against undervoltage and overvoltage at output (it does not allow regulation). This is done by shorting the emitter and collector of one of optocouplers (there are a total of 3-4). This is the optocouplers, which provides the standby function. Connecting the green wire (PS ON) to the ground is no longer necessary. After this modification the source stops to shut downwhen in case of short circuit - it enters into current limiting mode. Short-circuit current is probably too large, therefore you should set the current limit a bit lower. It is done by replacing the current sensing resistor (shunt) to greater value. This resistance is connected between Source of main MOSFET and the negative pole on the primary side. I suppose nobody will have problem to find this resistor. In my case there was 0R15 (2W), it should be replaced with around 0R27 to 0R51. This resistance can also set the output current in case of modificating the supply into the battery charger (eg for car battery 12V voltage is set to about 15V and the current is set corresponding to the battery). Fan is connected to the auxiliary source 5VSB (so it is not affected by the voltage adjust).

Schematic of modification of the single MOSFET forward converter ATX PC-supply into an adjustable stabilised power supply of 3-15V
Schematic of modification of the single MOSFET forward converter ATX PC-supply into an adjustable stabilised power supply of 3-15V

Optocoupler, which turn on the supply from the standby mode. Short the emitter and collector together (they are on primary side).
Optocoupler, which turn on the supply from the standby mode. Short the emitter and collector together (they are on primary side).

TL431 integrated circuit (in a TO92 packages).
TL431 integrated circuit (in a TO92 packages).

The board under TL431. Removed resistor to 5V, connected wire to regulation.
The board under TL431. Removed resistor to 5V, connected wire to regulation.

Modified supply
Modified supply

Warning for dudes
If you open the feedback, the output voltage can reach 30V or even more (maybe 60V). The electrolytes are rated 16V, so they will explode. The modification to voltage of more than 15V will be much more complicated, you will have to rewind the transformer and replace electrolytes, and so on.



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