Convert Computer Power Supply into Multi-output

Bench Supply

A computer SMPS (switched mode power supply) can easily be converted into a versatile general purpose multi-output bench supply for workshop use. A large number of articles are available on the internet to do this. But to make a true regulated bench power supply, voltage & current metering is a must. Also, besides having +12V, -12V, +5V & +3.3V output voltages, one often needs a variable supply voltage source (say 0-10V) to make it a complete power supply for hobby use.

This article describes the construction of a simple good quality multi-output power supply with voltage & current metering and having a variable dc voltage supply of 0 to 10V.

Fig. 2 shows the picture of a common SMPS/PSU supply salvaged from a junked desktop computer. Encased in a shielded enclosure with AC mains socket, cooling fan and a large number of output wires with connectors, it provides +12V, -12V, +5V, -5V & 3.3V voltages with different current ratings.

Fig. 2

  It has a large main 20-Pin (sometimes 24-pin) connector with several other smaller connectors carrying different power supplies. The main 20-pin connector also carries SMPS control ( PS-ON for switching on the supply), power supply health monitoring signal (PS-OK) & stand-by 5V supply.

Fig.3 shows the details of 20-pin connector – pin number, their function, colour code of wires & description.

 Fig. 3

Note the following control/status pins:

Pin    Name          Colour                   Description

8        Pwr_Ok      Grey Power         Ok (+5 VDC when power is Ok)

9        +5VSB        Purple                  +5 VDC Standby Voltage

14      Pwr_ON      Green                  Power Supply On (active low)

The wires on the main 20 pin (or 24 pin) connectors are colour coded. These are the same for all ATX power supplies:

3.3V wires are orange; +5V wires are red; -5V wires are white; +12V wires are yellow; -12V wires are blue; all ground wires are black.

The green wire is the "power on" sensor. This wire is internally connected to 5V with a pull-up resistor. If you connect this wire to ground (any black wire) the power supply will get switched on.

The purple wire is the +5 "stand by" power. This outputs a 5V signal even if the rest of the power supply has not yet turned on, allowing you to power any circuit that might control the ON/OFF signal. We are not using stand-by supply in this project.

The gray "power good" indicator is at 5V if each of the output wires is operating at the correct voltages.

Besides 20pin (or 24pin) connector, there are a number of other small connectors. Wires on these connector follow the same colour code. For example all Yellow wires in other connectors are also for 12V.

So cut off all the connectors. Join all the similar colour coded wires together. This will result in higher amperage output for all the supplies voltages.

As an additional feature, an adjustable voltage regulated power supply of 0 to 10V is also included. You could use LM317 adjustable voltage regulator or a simple basic voltage regulator circuit using 2N3055 transistor. The 12V supply serves as the source for making this variable regulated supply.

Fig.4  shows the circuit diagram of the completed bench top power supply.

(To ZOOM-IN, press the button on the lower corner on the right-hand side)

The output voltages of 5V, +12V, -12V and 3.3V directly go to output terminals. Their ground return wires carry current sense resistors R1 through R4 of 0.1 ohms (you could use 0.01 ohm resistor in case you work more with higher currents).

Notice R11, a 22ohm / 10W power resistance across the +5V power supply output. This provides a minimum load to the SMPS to function properly. You could also use any other output supply volatage to provide the minimum load requirement for this PSU.

For monitoring the output voltages and current, two 3-1/2 digits LED DPMs have been used. These DPMs operate from an independent +5V power supply. This is because these DPMs require a floating ground (note that DPMs ground is not connected to ground wires of other supplies). A common 5V mains voltage adopter has been used for this purpose.

A 2-pole 4-way band switch is used for selection of one of the four output voltages.  The voltage monitoring DPM has a resistive divider of 1/100 using R7 & R8. The current monitoring DPM has a resistive divider of 1/10 using R5 & R6.

Decimal point is selected by shorting the middle pad of the three decimal selection pads to the ground.

 Fig. 5

 Fig. 5 shows the inside view of the power supply cabinet. The overall performance of this supply is good enough for all hobby work. We can not put such a supply in the 'Laboratory Grade'  power supply category, but it is certainly a versetile  multi-output regulated general purpose bench top supply. Also a great fun project using an otherwise junked stuff!!