You can turn a standard ATX computer power supply into a useful fixed-voltage bench-power adapter without opening or modifying the supply itself. Expose its +3.3 V, +5 V, +12 V, and −12 V rails through an enclosure-mounted connector and banana-style terminals, then use the ATX PS_ON# control wire to switch the supply on.
What you are building
You can turn a standard ATX computer power supply into a useful fixed-voltage bench-power adapter without opening or modifying the supply itself. Expose the supply’s +3.3 V, +5 V, +12 V, and −12 V rails through an enclosure-mounted connector and banana-style terminals, then use the ATX PS_ON# control wire to switch the supply on.
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The result is useful for hobby electronics, repair work, testing, and prototyping. It is not a full laboratory bench supply: the voltages are fixed, there is no inherent adjustable current limit, and you should not assume it includes a display, precision regulation, ripple measurement, or laboratory-grade protection.
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Before you start: check the power supply
Use a conventional ATX power supply with a documented 20-pin or 24-pin motherboard connector. Read the label on the individual supply before designing the outputs. Its wattage and rail-current ratings determine what the adapter can actually deliver.
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Do not add every amperage printed on the label and treat the total as simultaneously available. ATX supplies commonly impose combined limits on the +3.3 V and +5 V rails, and available current can also depend on cross-loading, temperature, protection circuitry, wiring, and connector ratings. Use the PSU’s own label and documentation as the authority.
The standard ATX colors below are a useful starting point for conventional supplies, but inspect and verify the particular unit before connecting anything:
| Wire color | Typical ATX function | Use in this project |
|---|---|---|
| Orange | +3.3 V | Fixed +3.3 V output |
| Red | +5 V | Fixed +5 V output |
| Yellow | +12 V | Fixed +12 V output |
| Blue | −12 V | Fixed −12 V output relative to COM |
| Black | COM/common | Ground or return terminal |
| Green | PS_ON# | Active-low supply-enable control |
| Purple | +5 VSB | Standby voltage; normally not used as a switched output |
| Gray | PWR_OK | Power-good status signal; normally not used as an output |
On a standard ATX supply, PS_ON# is pulled high internally and turns the main outputs on when connected to COM. That is why a switch between the green wire and a black wire can control startup.
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Core parts
- A standard ATX computer power supply with a documented 20-pin or 24-pin connector
- A compatible female ATX connector, salvaged connector, or purpose-built ATX power supply breakout board
- A plastic electronics project enclosure large enough for the connector, terminals, switch, wiring, and ventilation
- Eight insulated banana binding posts or equivalent protected output terminals
- Wire suitable for the expected current, plus jumper wire for control wiring
- Crimp spade terminals or another mechanically secure termination method
- Heat-shrink tubing
- A latching switch suitable for the PS_ON# control circuit
- Labels for every voltage, polarity, and common connection
A breakout board is the simplest non-destructive alternative to salvaging and wiring a motherboard connector. When comparing one, check that it is designed for the correct 20/24-pin ATX connector, clearly maps the pins, identifies each rail, provides suitable terminal or fuse provisions, and states the ratings of its traces and connectors. Do not assume that every board advertised as an ATX tester or adapter is suitable for carrying your intended load.
For component sourcing, an electronics component supplier can be useful when you need a connector, binding post, resistor, wire, heat-shrink, or test lead with a specified rating rather than an unverified generic part.
Conditional and optional parts
- 10-ohm, 10-watt power resistor, only if the particular PSU needs a minimum load
- Small fan or ventilated enclosure if the dummy load or operating conditions generate significant heat
- Inline fuses or protected output modules where the application and wiring justify them
- 12 V DC barrel outlet
- USB outlet or regulated USB power module
- Insulated, appropriately rated banana-plug test leads
You will also need ordinary enclosure-making and assembly tools: a drill, a way to cut the connector opening, a soldering iron, solder, wire cutters and strippers, a crimp tool, a multimeter, and suitable fasteners or adhesive for mounting the connector.
Plan the front panel
The original arrangement uses four pairs of banana-style terminals: one pair for +3.3 V, one for +5 V, one for +12 V, and one for −12 V. In each pair, one terminal connects to the selected rail and the other connects to black COM.
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Label the terminals by voltage, not merely by color. The −12 V terminal deserves especially clear marking: it is negative 12 volts relative to COM. If you use a red binding post for consistency with the other rail outputs, the red plastic does not mean that the −12 V terminal is positive. A user measuring between COM and −12 V should see approximately −12 V with the meter’s red probe on the −12 V terminal and black probe on COM.
Keep the control switch separate from the output terminals, and leave enough room inside the box for strain relief, insulated joints, and airflow. Do not pack a heat-producing resistor against a plastic wall or place it where someone can touch it through the enclosure.
Build the adapter
1. Identify the connector wires
With the PSU unplugged, identify the 20-pin or 24-pin connector and sort the wires by function. Several wires may carry the same rail; using multiple wires in parallel can reduce the current through an individual conductor and connector contact. Preserve the connector’s original contact arrangement and do not rely on a color chart alone.
If you are using an ATX breakout board, follow its documented pin labels. If you are wiring a loose female connector, keep a pinout diagram beside the work and inspect every connection before power is applied.
2. Mount the ATX connector
Mark and cut the opening in the plastic enclosure. Deburr the edges so they cannot damage insulation. Secure the connector with its intended fasteners where possible; otherwise use a suitable adhesive in addition to a mechanical strain-relief method. The connector should not move when a cable is inserted or removed.
3. Mount the terminals and switch
Drill the mounting holes for the eight insulated output terminals and the PS_ON# switch. Arrange the terminals so adjacent voltages cannot easily be bridged by a loose lead. Fit washers and nuts as appropriate, and confirm that no terminal body or solder lug can contact another terminal or the enclosure hardware.
4. Wire the output rails
Connect the orange conductors to the +3.3 V terminal, red conductors to +5 V, yellow conductors to +12 V, and blue conductors to the −12 V terminal. Connect black COM conductors to the common terminals for each output pair.
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Use a mechanically secure connection before soldering or crimping. Cover soldered joints with heat-shrink tubing, route the wires so they cannot be pulled directly from the connector, and avoid making a high-current connection through a thin jumper or undersized terminal.
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5. Wire the power switch
Connect the green PS_ON# lead to one side of the switch and connect the other side of the switch to a black COM lead. Insulate both soldered joints with heat-shrink and make sure the control wires cannot touch any output terminal.
A latching switch gives the adapter a simple on/off function. The switch carries the PS_ON# control connection, not the main output current, but it still needs suitable insulation and construction for the enclosure and environment.
6. Close and label the enclosure
Label every terminal with its voltage and relationship to COM. Include a prominent warning that the outputs are fixed and that the unit is not an adjustable, current-limited laboratory supply. Add strain relief where the ATX cable enters the enclosure and ensure no exposed conductor remains accessible during normal use.
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Test it before connecting electronics
- Unplug the ATX supply and inspect the wiring for shorts, stray strands, reversed connections, and uninsulated joints.
- Check continuity with a multimeter while the supply is disconnected. In particular, verify that each rail is connected to the intended terminal and that adjacent rails are not accidentally shorted.
- Connect the ATX supply, keep the output leads disconnected, and turn on the adapter switch.
- Measure each rail from its output terminal to COM. Confirm the polarity as well as the approximate nominal voltage.
- Measure the −12 V rail with the meter’s red probe on −12 V and black probe on COM; the reading should be negative relative to COM.
- Switch the unit off and confirm that the main rails shut down. Remember that +5 VSB may remain present while the AC supply is connected.
- Only after these checks should you attach a project, and begin with a low-risk load when practical.
Use insulated, appropriately rated leads and jacks. A multimeter confirms nominal voltage and polarity; it does not by itself establish ripple, transient response, maximum safe current, or laboratory-grade regulation.
If the supply starts and then shuts down
Some ATX supplies expect a minimum load or behave poorly under an unusual load distribution. Start by testing whether the unit remains on with no external load. If it shuts down, do not immediately assume the wiring is wrong: first recheck PS_ON#, COM, rail polarity, and accidental shorts, then consult the PSU’s documentation.
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A commonly used starting remedy is a 10-ohm, 10-watt resistor between +5 V and COM. At a nominal 5 V, the ideal calculation is:
I = V/R = 5/10 = 0.5 AP = V2/R = 25/10 = 2.5 W
The 10-watt rating provides thermal margin, but the resistor will still become hot. This value is not universal. Modern supplies differ, and some may require another load or may regulate poorly with an unusual cross-load. Add a dummy load only after determining that the particular PSU needs one and calculating the resistor’s operating dissipation.
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Optional 12 V and USB outputs
A 12 V DC outlet can be connected to the +12 V and COM rails if its connector, wiring, and protection are appropriate. Label the voltage and polarity directly beside the outlet.
For USB, a protected, purpose-built USB power module is preferable to treating a loose USB extension as a universal output. USB applications can involve negotiation, over-current protection, connector orientation, and voltage requirements that a bare connection to the ATX +5 V rail does not address. If you add one, identify its actual output voltage and protect it appropriately.
What this adapter can—and cannot—do
| Good use | Use a different tool when you need… |
|---|---|
| Fixed 3.3 V, 5 V, 12 V, or −12 V for hobby circuits | Continuously adjustable voltage |
| Powering a prototype or test fixture | Precise, independently adjustable current limiting |
| Testing fans, motors, LED projects, or digital boards within the PSU’s ratings | Known ripple, transient, or regulation performance |
| A low-cost source for several common DC rails | Laboratory measurement displays, remote sensing, or calibrated output accuracy |
Respect the PSU’s rail-specific limits, combined +3.3 V/+5 V restrictions, over-current protection, over-temperature protection, and connector ratings. Do not short the outputs and assume the internal protection will make every error harmless. Fuse an output when the load and wiring require it; choose the fuse from the expected load, conductor, connector, and PSU limits rather than copying a generic value.
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- Unplug the PSU before changing wiring or opening the adapter enclosure.
- Do not open the PSU housing. It contains hazardous mains circuitry and energy-storage components even after disconnection.
- Perform this project only on the low-voltage output side, leaving the mains-side electronics untouched.
- Verify voltage and polarity with a multimeter before connecting valuable electronics.
- Use insulated or shrouded terminals and leads, enclosed connections, and adequate strain relief.
- Keep the dummy-load resistor ventilated and inaccessible while operating.
- Do not use the adapter where a certified, adjustable, current-limited laboratory supply is required.
Electrical safety requirements can vary with location and intended use. This design is best treated as a hobbyist adapter for supervised bench work, not as a certified piece of commercial test equipment.
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Bottom line
A spare ATX supply makes a practical multi-rail DC source when you need fixed +3.3 V, +5 V, +12 V, and −12 V outputs. The safest version is an external enclosure using a compatible connector or breakout board, clearly labeled terminals, a PS_ON# switch, verified polarity, and adequate ventilation. Treat the PSU’s label as the limit, add a dummy load only when testing proves it necessary, and choose a real adjustable bench supply whenever current limiting or controlled voltage is important.
Frequently Asked Questions
Can an ATX power supply replace an adjustable bench power supply?
No. An ATX supply normally provides fixed nominal rails rather than continuously adjustable voltage. It also does not inherently provide the precision current limiting, displays, or measurement features of a laboratory bench supply.
Does an ATX power supply need a dummy load?
Not necessarily. Some ATX supplies shut down or regulate poorly without a minimum load. Test the specific unit first. If required, a common starting point is a 10-ohm, 10-watt resistor from +5 V to COM, mounted with ventilation; the value is not universal.
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Can I use any modular power-supply cable?
No. Modular PSU cables can use different PSU-side pinouts even when they fit physically. Use the original cable, verify the exact model’s pinout, or use a compatible breakout solution. Never rely on wire color alone.
How should the −12 V output be connected and labeled?
The standard ATX color is blue for −12 V, measured relative to COM. Label it explicitly as −12 V. A red terminal does not make that output positive if it is wired to the negative rail.
How much current can the converted supply provide?
Use the PSU’s own label and documentation. Do not add all printed rail amperages together: combined-rail limits, wiring, connector ratings, cross-load behavior, temperature, and protection circuitry affect the safe output.
The Bottom Line
An ATX-to-bench adapter is an inexpensive fixed-rail DC source, not a laboratory power supply. Build it externally, verify the exact PSU pinout, label the −12 V output clearly, ventilate any dummy-load resistor, and never open the PSU or rely on its headline wattage as an output guarantee.
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