Introduction: ATX Bench Power Supply

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In this instructable I'll show you how to turn a working ATX PSU into a bench power supply.

For this design I got a lot of inspiration from similar projects.

Note: before starting this project make sure the power supply is unplugged and turned off.

Supplies

For this project you will need the following parts and tools:

Parts:

  • ATX power supply x1
  • Female Banana Plugs x10
  • 5x20 Fuse panel mount x5
  • 5x20 Fuse (value depends on your power supply) x5
  • 5Ω 10W Resistor x1
  • Perfboard x1
  • Multi-turn Potmeter x1
  • Voltage meter x1
  • On/Off Switch x1
  • Step-down Buck Converter XL4005 5A x1
  • M3x5x5 3D-print inserts x8
  • M3 screws x8

Tools:

  • 3D printer
  • Soldering iron
  • Wire Strippers
  • Cutting pliers
  • Screwdriver
  • Super glue

Step 1: The Plan

An ATX power supply can deliver different voltages with high currents. The plan is to modify the power supply to make it easier to use it for your electronic projects.

The different colour wires coming out of the power supply have different functions:

  • Black = Ground
  • Orange = +3.3 VDC
  • Red = +5 VDC
  • Yellow = +12 VDC
  • Blue = -12 VDC
  • Grey = Power Ok
  • Purple = +5 VDC Standby Voltage
  • Green = Power Supply On
  • Brown = Voltage Sense

To turn on the power supply we need to connect the greenwire to ground to do this we'll use a standard on/off switch. When the power supply is turned on there will be a +5V output on the grey wire, you could use this to turn on a led as a power indicator if you want to.

We also need to attach a small load on the +5V rail to trick the power supply into thinking it is connected to a motherboard. For this we'll use a 5Ohm 10W resistor but anything under 10Ohms will be fine.

The brown wire verifies the 3.3V output is realy 3.3V to do this we need to connect the brown wire to the other orange wires.

To control the variable output we'll use a buck converter that steps down the 12V output to 0.8V to 12V. We'll also replace the trimpot with a multiturn potentiometer to make controlling the voltage easier.

In the image above you'll see the schematic we'll be using to connect all the wires.

Step 2: 3D Print the Files

Because there are high voltage capacitors inside the power supply we wont open it. To work around this I designed a case that will be mounted to the front of the power supply giving you a place to put all the wires and components. (In my original design the spacing for the mounting of the buck converter wasn't correct but this is fixed in attached version)

Using a soldering iron press the inserts into the mounting holes of the Case.

Attachments

Step 3: Cut & Sort the Cables

To prepare the powersupply cut off all the connectors and sort the cables by colour. Make sure to keep some of the cut off wires as we'll use them in the next step.

I used some heatshrink to keep the wires separate during the building process.

Step 4: Assemble the Front Panel

After printing the Front Panel you need to screw in the banana plugs, fuse holders and the potmeter. Also press the switch in the hole and glue the voltmeter in the hole using superglue.

When everything is mounted use some of the cut of wires to solder the sides of the fuseholder to the positive plugs. The wires coming from the power supply are 20AWG. Because the amp capacity for 20AWG wire is anywhere between 5 and 11 amps I put 3 wires parallel to increase the amp capacity.

Step 5: Add the Buck Converter

Desolder the trimpot from the buck converter and solder some new wires to it. Solder the other side of the wires to the potmeter on the Front Panel.

Also solder the yellow wire of the voltmeter to the buck converter output.

Solder the output of the buck converter to the back of the fuse holder and ground banana plug.

I suggest using a multi-turn potmeter for a more accurate control of the voltage.

Step 6: Solder the Ground Wires

Keep 4 ground wires separate and solder the rest of the ground wires to the ground banana plugs.

Use 2 of the 4 wires for the negative input of the buck converter.

Solder 1 of the remaining 2 wires to one pin of the power switch.

Step 7: Solder the Buck Converter

Solder 2 of the +12 VDC wires to the input of the buck converter. Solder the red and black wire of the voltmeter to the input of the buck converter.

After soldering the buck converter screw it into the case using two M3 screws.

Step 8: Solder the Power Switch

Solder the green wire to the other pin of the power switch. This way when the switch is turned on the green wire is connected to ground and the power supply will turn on.

Step 9: Add the Power Resistor

To make sure the power supply stays on we need a small load on the +5 VDC output. To do this we need a resistor of 10 Ohms or less with a power rating of atleast 10 Watt.

I placed a resistor into the perfboard and made 2 holes 60mm apart for mounting.

Solder the last ground wire to one side of the resistor and one of the +5 VDC wires to the other side.

Screw the perfboard into the case using two M3 screws.

Step 10: Solder the Output Wires

Solder the remaining of the output cabels to the back side of the fuse holders. Make sure you connect the right colours to the right fuseholders:

  • Orange = +3.3 VDC
  • Red = +5 VDC
  • Yellow = +12 VDC
  • Blue = -12 VDC

Step 11: Assemble the Case

Screw the Front Panel onto the case using 4 m3 screws.

Before you glue the case onto the power supply make sure the wires go into the top right corner to make sure the wires dont touch the power resistor because it will get hot.

Step 12: Add the Fuses

Screw the correct fuses into the fuseholder. For my power supply I used the following fuses:

  • +12 VDC = 20A
  • +5 VDC = 10A
  • +3.3 VDC = 10A
  • -12 VDC = 0.2A
  • VAR = 5A

Check your power supply specs to make sure you stay under the max rated current the supply can provide.

Step 13: Test + Finish

Using a multimeter test if the output voltages are right.

And you're finished!!!