Computer Lab PSU hack

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This is a great way to get a cheap lab power supply. My very first successful electronics project also. Most computer psu’s give 5v 12v and even 24v, which is useful for most beginner projects. These PSU’s can also deliver a 100 – 400 W of power, and cost only a few bucks if bought used. These PSUs won’t function outside of a computer without some hacking first though.

 

 

First you have to know how much power each wire will supply. Typical wiring configurations for AT and ATX PSUs are shown below.

 

AT Power Supply

Pin

Name

 

Color

Description

1

PG

 

Orange

Power Good, +5 VDC when all voltages has stabilized.

2

+5V

 

Red

+5 VDC (or n/c)

3

+12V

 

Yellow

+12 VDC

4

-12V

 

Blue

-12 VDC

5

GND

 

Black

Ground

6

GND

 

Black

Ground

 

ATX Power Supply

Pin

Name

 

Color

Description

1

3.3V

 

Orange

+3.3 VDC

2

3.3V

 

Orange

+3.3 VDC

3

COM

 

Black

Ground

4

5V

 

Red

+5 VDC

5

COM

 

Black

Ground

6

5V

 

Red

+5 VDC

7

COM

 

Black

Ground

8

PWR_OK

 

Gray

Power Ok (+5V & +3.3V is ok)

9

5VSB

 

Purple

+5 VDC Standby Voltage (max 10mA)

10

12V

 

Yellow

+12 VDC

11

3.3V

 

Orange

+3.3 VDC

12

-12V

 

Blue

-12 VDC

13

COM

 

Black

Ground

14

/PS_ON

 

Green

Power Supply On (active low)

15

COM

 

Black

Ground

16

COM

 

Black

Ground

17

COM

 

Black

Ground

18

-5V

 

White

-5 VDC

19

5V

 

Red

+5 VDC

20

5V

 

Red

+5 VDC

 

To run outside of a computer PSUs require a load. A fair sized resistor between +5v and ground will do fine. A 10ohm 10W resistor will work with most PSUs, but if you are running a very small or very large PSU, different resistor sizes may be required. A rule of thumb is 10-20 % of advertised wattage.

 

Also the Power Good or Power OK must be shorted to ground to make the PSU run in some cases. Putting a LED between them will indicate when the PSU is on.

 

Your PSU should now be up and running! If all is good you should be getting 12v on the yellow wires and 5v on the red wires. Depending on how many watts it can handle the amperage will be between 10 and 40 amps on the 5v rail. 

 

Variable Voltage?

Converting an ATX supply for variable voltage is surprisingly simple, and only takes one potentiometer. The PSUs I've hacked used a KA7500B for PWM, but the TL494, HA17339, KIA494, KA7500, IR3M02, and MB3759 are pin equivalent, so if your PSU uses one of these you are in luck.

To regulate the voltage down:

Find the resistor which goes from pin 14 on the PWM IC to pin 2. Now simply place a 10K potentiometer in series with this resistor and pin 14. What this does is mess with the regulating reference. Pin 14 is the internal regulated +5V from the IC. The resistor we put a pot in series with is part of a voltage divider, which supplies around 2.5V originally. Turning the pot decreases this voltage and the PWM IC thinks the output voltage is increasing, so it compensates by reducing the duty cycle. Regulating the voltage down can be done without modding other components.

To regulate the voltage up:

If you want the PSU to live long you’ll need to replace the rectifiers and capacitors for the voltage. Otherwise the overvolted components will die, killing your PSU.

 

 24.3 volts from the 12v rail.

 

First find pin 1 on the PWM IC. Then trace back until you find 2 or 3 resistors connected to the output lines. If shorting these shorts the PSU you most likely have the right ones. Now disconnect these resistors a wire a potentiometer is series with them and pin 1, and wire a pot in series with it. The potentiometer should be around 10-20K. Turn the pot to 0 ohms before you turn on the PSU. Now turn it on and slowly turn the pot up until the PSU shorts out. If you just measured a higher voltage on the 12v rail you have the right resistors, if not solder them back in place and keep looking.

This modification works much like the previous one. The resistors we put a potentiometer in series with form a voltage divider, which feeds 2.5V to pin 1. Their voltage source is the output voltage, when this changes the voltage on pin 1 changes and the duty cycle is altered. Turning the pot reduces the voltage at pin 1 and the PSU thinks the output voltage has dropped, and increases the duty cycle.

If you need a circuit diagram to follow things better, this one is similar to almost every PSU I've come across:

http://www.pavouk.comp.cz/hw/en_atxps.html

Each of these hacks trick the PWM chip into thinking the PSU is running at a lower voltage, and therefore it compensates by increasing the duty cycle and thus the overall voltage.

 

Disabling overvoltage shutdown:

Now you might have noticed the PSU shorted at a mere 14v or so. This is because the overvoltage protection kicks in and kills it. Overvoltage protection works by setting pin 4 high, or pin 13 low. A transistor is often used to change the signal level, and disabling it will prevent the overvoltage/short-circuit protection from kicking in. So find pin 4, and trace back to the first transistor you find. To see if it’s the right one short the collector and emitter. If it shorts the PSU cut it's collector off.

Congratulations! Now your PSU hack is finished! Drill some holes and put the potentiometers in place!

50.8 volts obtained by combining -12 and +12 volt rails, which after the hack can supply 24 volts each.


Safety

Always make sure you've unplugged the PSU, before poking around inside. I've been reckless enough to operate on a PSU while it was still plugged into the wall, and I've learned my lesson. The shock was just across my fingers but still enough to trip the breaker in the house. Also its a good idea to be careful around the capacitors, as they may still hold charge.

 

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