How to Convert an ATX Power Supply into a Multi-Voltage Bench Supply

Introduction

A dedicated bench power supply is one of the most-used tools on any electronics workbench, but quality units cost $100 or more. An old ATX computer power supply sitting in your closet can be converted into a multi-voltage bench supply with 3.3V, 5V, 12V, and -12V outputs for less than $20 in parts. ATX supplies are robust, current-capable (20+ amps on 5V and 12V rails), and often free from discarded desktops. This guide walks through the safe conversion process from start to finish.

What You Need

• ATX power supply (any wattage, 250W or higher recommended)
• Power resistor 10 ohm 10W (ceramic sandbar type)
• 5 binding posts (banana jack type): red, black, yellow, orange, blue
• Power switch (rocker or toggle, rated for mains voltage)
• Fuse holder with 5A fuse (for the 12V rail protection)
• LED with 1k ohm resistor (power indicator)
• Project enclosure or ATX supply case
• Wire (18 AWG minimum, 16 AWG preferred for high current rails)
• Heat shrink tubing assortment
• Multimeter for verification
• Drill and step bit for mounting holes

Safety first: ATX supplies contain lethal mains voltage on the primary side even when unplugged. The large filter capacitors can hold a charge for minutes. Do not open the supply until you have safely discharged it.

Step 1: Verify the ATX Supply Works

1. Before modifying, verify the supply powers on.
2. Plug it in. Do not connect to a motherboard.
3. Use the paperclip test: short the green wire (PS_ON) to any black wire (ground) with a paperclip or jumper.
4. The fan should spin. If it does not, the supply may be faulty or require a minimum load.
5. Check output voltages with a multimeter: yellow = +12V, red = +5V, orange = +3.3V, blue = -12V, white = -5V (on older supplies).

Step 2: Discharge and Open the Supply

1. Unplug the supply from the wall. Wait 5 minutes.
2. Discharge the filter capacitors:

• Use a 10k ohm 5W resistor with insulated leads. Touch the resistor leads across the large cylindrical capacitors (200V+, 470uF or larger) for 10 seconds each.
• Alternatively, a 60W incandescent light bulb with test leads works well and provides visual confirmation of discharge.

1. Remove all screws and open the metal case.
2. Cut the existing cable bundle from the PCB, leaving about 10 cm of wire length for reuse.

Step 3: Understand the ATX Wiring Colors

Standard ATX wire color coding:

• Orange: +3.3V
• Red: +5V
• Yellow: +12V
• Blue: -12V
• White: -5V (on older ATX 1.x supplies, may be absent on newer)
• Black: Ground (common return for all rails)
• Green: PS_ON (pull to ground to enable output)
• Purple: +5V standby (always on when supply is plugged in)
• Gray: PWR_OK (status signal, not needed for bench supply)
• Brown: 3.3V sense (connect to orange wire near connector)

Step 4: Wire the Minimum Load Resistor

ATX supplies require a minimum load on the +5V rail to regulate properly. Without it, the 12V rail may drift high or the supply may shut down.

1. Take one red (+5V) wire and one black (ground) wire.
2. Connect the 10 ohm 10W resistor between them.
3. Mount the resistor inside the case where it has airflow. It will get warm during operation (P = V^2/R = 25/10 = 2.5W continuous).

Alternative: Some modern supplies work fine with a 22 ohm resistor (lower heat) or even a 12V automotive bulb as a load. Test your specific supply.

Step 5: Wire the Power Switch

Option A: Momentary Switch (latching behavior)

Wire the green (PS_ON) wire and a black wire to a momentary pushbutton. Pressing it toggles the supply on and off.

Option B: Hardwired Always-On (recommended for bench supply)

1. Simply twist the green wire together with a black wire.
2. Cover with heat shrink. The supply turns on when plugged in and the rear rocker is on.
3. Add a front panel switch in the mains AC line for safety.

Option C: Combined Mains + Enable Switch (best)

1. Keep the rear mains switch functional.
2. Add a front panel rocker switch that breaks the green wire connection. This lets you turn the DC outputs on and off while keeping the supply plugged in.

Step 6: Add Binding Posts for Each Rail

1. Drill holes in the front panel for 5 binding posts.
2. Mount posts from largest to smallest voltage:

• Red post: +12V (yellow wires)
• Yellow post: +5V (red wires) — use yellow post to avoid confusion with +12V
• Orange post: +3.3V (orange wires)
• Blue post: -12V (blue wires)
• Black post: Ground (black wires)

1. Group all wires of the same color together and solder to the binding post terminal:

• +12V: Bundle all yellow wires together, solder to the red binding post
• +5V: Bundle all red wires together, solder to the yellow binding post
• +3.3V: Bundle all orange wires together, solder to the orange binding post
• -12V: Connect the blue wire to the blue binding post
• Ground: Bundle all black wires together, solder to the black binding post

1. Use 16 AWG wire for the high-current rails (+12V and +5V). Twist wire bundles tightly and solder thoroughly.
2. Cover all connections with heat shrink tubing.

Step 7: Add the Power LED

1. Take the purple (+5V standby) wire and a black wire.
2. Connect the LED in series with the 1k ohm resistor.
3. Mount the LED on the front panel as a power indicator.
4. The LED lights whenever the supply is plugged in, even if the DC outputs are off. This reminds you the unit is live.

Step 8: Add Fuse Protection

1. Install a fuse holder in the +12V line between the yellow wires and the binding post.
2. Use a 5A fuse. This protects your projects from overcurrent without the supply shutting down entirely.
3. Optional: Add a 10A fuse to the +5V line if you plan to draw significant current.

Step 9: Reassemble and Test

1. Double-check all connections. Verify no bare wires touch the metal case.
2. Bundle internal wires with zip ties to prevent movement.
3. Close the case and install screws.
4. Plug in and turn on.
5. Verify all output voltages with a multimeter:

• +12V rail: 11.8-12.2V under no load
• +5V rail: 4.9-5.1V
• +3.3V rail: 3.2-3.4V
• -12V rail: -11.5 to -12.5V

1. Test under load: Connect a 12V automotive bulb or power resistor. Verify voltage does not sag more than 5%.

Step 10: Add Optional Features

Variable Voltage Output (LM317 or Buck Converter)

For adjustable voltage (1.25V to 12V or beyond):

1. Add a buck converter module or LM317 linear regulator to the +12V rail.
2. Wire the input to the +12V binding post, output to a new pair of binding posts.
3. Add a potentiometer for voltage adjustment.
4. Include a voltmeter display for readout.

Current Limiting

Simple current limiting with a LM317 and sense resistor:

• Sense resistor R = 1.25V / I_limit
• For 1A limit: R = 1.25 ohms, 2W minimum

Voltage and Current Displays

Add cheap digital voltmeter/ammeter modules to each rail for real-time monitoring. Wire the meter power to the +5V standby (purple wire) so displays are always on.

Safety Notes

• Never open the supply while plugged in. Mains voltage is present on the primary side.
• Discharge capacitors before working inside. The large filter caps hold lethal voltage.
• Ground the case. The ATX case should already be grounded through the mains cord. Verify continuity with a multimeter.
• Use appropriate wire gauge. 18 AWG is rated for 10A. Use 16 AWG or heavier for the +12V and +5V rails.
• Fuse protection is mandatory. A shorted +12V rail can deliver 20+ amps instantly, creating fire and burn hazards.
• Do not exceed rated current. Check the label on the supply for maximum amperage per rail.

Conclusion

A converted ATX bench supply gives you four regulated voltage rails at currents that would cost hundreds of dollars in dedicated power supplies. With proper wiring, fuse protection, and a clean enclosure, it is as safe and functional as a commercial unit. The project takes an afternoon, teaches you about switchmode power supplies, and clears out that old desktop tower gathering dust in the closet.