Why Repurpose an ATX Power Supply from a Computer?
Converting a power supply from a computer (specifically an ATX form factor unit) into a benchtop DC supply is one of the highest-value projects in a maker’s workshop. You are essentially salvaging a highly engineered, multi-rail switching regulator that would cost upwards of $150 to $300 if purchased as a dedicated laboratory unit. Modern ATX units readily deliver 20A to 40A on the 12V rail and 15A to 25A on the 5V rail, making them ideal for driving high-current loads like LED arrays, stepper motors, or automotive accessories.
However, before tearing into the 24-pin connector, you must understand the topology you are working with and how it compares to traditional linear bench supplies. An ATX unit is a high-frequency switching power supply, which dictates its efficiency, thermal profile, and noise floor.
Topology Comparison: ATX Switching vs. Linear Bench Supplies
When deciding whether to use a repurposed ATX unit or build/buy a linear supply, you are trading noise performance for raw power density. According to Analog Devices' analysis on regulator topologies, switching converters excel in efficiency but introduce high-frequency ripple, while linear regulators offer ultra-low noise at the cost of massive heat dissipation.
| Criteria | ATX Switching Supply | Linear Bench Supply (Transformer + LDO) |
|---|---|---|
| Efficiency | 85% – 94% (80 Plus Gold/Platinum) | 40% – 60% |
| Heat Dissipation | Low (fan-cooled, minimal heatsinks) | Extreme (massive finned heatsinks required) |
| Output Noise/Ripple | 50mV – 120mV peak-to-peak | 1mV – 5mV peak-to-peak |
| Cost per Watt | ~$0.05/W (salvaged) to $0.15/W (new) | ~$0.50/W to $1.20/W |
| Transient Response | Moderate (depends on output capacitance) | Excellent (wide bandwidth error amp) |
Design Example: 12V/5V/3.3V Bench Supply Conversion
To build a reliable bench supply from an ATX unit, you need to manage the 24-pin motherboard connector, implement a dummy load for cross-regulation stability, and add user-facing protection and control.
Input Range and Built-In Protection
Most modern ATX power supplies feature a universal AC input range of 90VAC to 264VAC, utilizing an active Power Factor Correction (PFC) front end. This means you can plug it into 120V or 230V mains without flipping a voltage selector switch. Protection is handled by a dedicated supervisor IC (such as the Weltrend WT7527 or IN1S313IAG), which monitors the rails and provides:
- OVP (Over Voltage Protection): Triggers if the 12V rail exceeds ~13.4V or the 5V rail exceeds ~5.7V.
- OCP (Over Current Protection): Typically set at 120% to 150% of the rated maximum current per rail.
- SCP (Short Circuit Protection): Latches off the PWM controller within microseconds if a rail shorts to ground.
- OTP (Over Temperature Protection): A thermistor on the secondary heatsink shuts down the unit if internal ambient exceeds ~105°C.
Wiring the 24-Pin Connector and Dummy Load
The ATX standard requires the Power Supply On (PS_ON, Pin 16, Green wire) to be pulled to Ground (Black wire) to activate the main switching stages. Pin 9 (Purple, 5VSB) provides 5V Standby, which remains active whenever the unit is plugged into mains, useful for powering an indicator LED.
The Dummy Load Requirement: Older “group-regulated” ATX supplies use a single magnetic amplifier to regulate both the 5V and 12V rails. If the 5V rail has zero load, the 12V rail can drift dangerously high, triggering OVP. To prevent this, solder a 10Ω 10W ceramic power resistor between the 5V (Red) and Ground (Black) wires. This draws a constant 0.5A (dissipating 2.5W), stabilizing the cross-regulation. Modern “DC-DC” ATX units (where 5V and 3.3V are bucked directly from the 12V rail) do not strictly require this, but adding a 22Ω 5W resistor on the 5V rail is still good practice for minimum-load stability.
Adding a Variable Linear Post-Regulator
If your project requires a precise, low-noise variable voltage (e.g., 1.25V to 9V for op-amp testing), you can feed the ATX’s clean 12V rail into a linear post-regulator like the TI LM317.
Dropout and Headroom Math: The LM317 requires a minimum dropout voltage (input-to-output differential) of about 2.5V at 1A. If your ATX 12V rail measures exactly 12.1V, your maximum reliable output is 12.1V - 2.5V = 9.6V. Designing for a 9V maximum output leaves a safe 0.6V margin.
Thermal Math: At 9V output and 1A load, the LM317 must dissipate (12.1V - 9V) × 1A = 3.1W. A bare TO-220 package has a junction-to-ambient thermal resistance of ~65°C/W. Without a heatsink, the junction temperature would rise by 201°C, instantly triggering thermal shutdown. You must attach a heatsink rated for at least 10°C/W to keep the temperature rise under 35°C.
Thermal Management, Derating, and Ripple Expectations
Repurposing a power supply from a computer requires understanding its environmental limits. ATX units are designed for forced-air convection inside a PC chassis, not passive benchtop use.
Thermal Derating Curves
Most quality ATX power supplies are rated for their full continuous wattage at an ambient temperature of 40°C or 50°C. If you operate the supply in a hot garage or enclosure where ambient reaches 60°C, you must apply a derating factor. Typically, ATX units derate linearly by about 1% to 1.5% per degree Celsius above their rated ambient. At 60°C ambient, a 600W supply might only safely deliver 450W. Always ensure the ATX’s internal 120mm or 140mm fan has unobstructed intake and exhaust paths when mounting it in your bench enclosure.
Ripple and Noise Realities
Do not expect laboratory-grade ripple from an ATX unit. The Intel ATX12V Design Guide specifies that the 12V rail may exhibit up to 120mV peak-to-peak ripple, and the 5V/3.3V rails may show up to 50mV peak-to-peak. This noise is primarily generated by the 50kHz to 150kHz switching frequency of the primary MOSFETs and the secondary rectifier diodes.
For driving motors, charging batteries, or powering high-wattage LEDs, this ripple is irrelevant. However, if you are powering sensitive audio preamplifiers, RF transceivers, or high-resolution ADCs, this switching noise will couple into your signal path. To mitigate this, add an LC pi-filter on the bench output: a 10µH toroidal inductor in series, followed by a 470µF low-ESR electrolytic capacitor and a 100nF ceramic capacitor in parallel to ground. This will attenuate the high-frequency switching hash by 20dB to 40dB.
Frequently Asked Questions
Can I use a power supply from a computer to charge a 12V car battery?
Not directly, and doing so is risky. A standard ATX 12V rail outputs a fixed ~12.1V to 12.2V. A 12V lead-acid car battery requires a Constant Current / Constant Voltage (CC/CV) charging profile, peaking at 14.4V for bulk absorption and dropping to 13.6V for float. Connecting an ATX 12V rail directly to a deeply discharged battery will cause the battery to pull massive current, likely tripping the ATX’s Over Current Protection (OCP) or overheating the secondary rectifiers. If you must use an ATX for this, you need to interpose a dedicated DC-DC buck-boost battery charge controller (like a module based on the XL4015 or LM2596 with CC/CV potentiometers) between the ATX and the battery.
What is the minimum load required when building a bench power supply from a computer?
It depends on the internal topology. If you open the ATX and see two large toroidal inductors on the secondary side (a group-regulated design), the 5V rail typically requires a minimum load of 1A to 2A to keep the 12V rail from over-voltaging. If the ATX uses a DC-DC topology (indicated by small daughterboards with buck inductors near the output cables), the 12V rail is the primary regulated output, and the 5V/3.3V rails can often handle zero load without destabilizing the 12V rail. When in doubt, a 10Ω 10W dummy load on the 5V rail is a universal failsafe.
How do I safely discharge the capacitors inside a computer power supply?
Never short the primary bulk capacitors with a screwdriver; the instantaneous current spike can weld the tool, destroy the capacitor’s internal bond wires, and blow the input fuse. Instead, use a discharge tool made from a 100Ω to 220Ω, 5W power resistor mounted on an insulated stick with alligator clips. Clip it across the capacitor terminals for 30 to 60 seconds. Always verify the voltage is below 1V DC using a multimeter before touching any internal PCB traces.
Is an ATX switching supply too noisy for sensitive audio or RF circuits?
Out of the box, yes. The 120mV p-p switching noise on the 12V rail will manifest as an audible whine in high-gain audio circuits and cause spurious emissions in RF receivers. However, you can clean it up. By routing the ATX output through a secondary linear regulator (like the LM317 or LM7812) or adding a multi-stage LC low-pass filter tuned to the ATX’s switching frequency (usually around 100kHz), you can drop the ripple to <5mV, making it perfectly suitable for sensitive analog bench work.
