Building a benchtop power supply from a surplus ATX computer PSU is a staple DIY project, but most guides stop at wiring up binding posts to the raw 12V, 5V, and 3.3V rails. If you are testing sensitive analog circuits, op-amps, or high-resolution ADCs, the raw switching noise from an ATX supply will wreck your measurements. The direct answer for clean DC is a hybrid topology: use the ATX PSU for high-current bulk conversion, then add a linear post-regulator to scrub the ripple. This guide walks through the exact topology trade-offs, the headroom math required for linear regulation, and the protection circuits you need to keep your bench safe.
Topology Comparison: Raw ATX Switching vs. Linear Post-Regulated
When deciding between linear vs switching for a benchtop load, you are trading efficiency for noise performance. A raw ATX PSU is a forward or half-bridge switching converter. It is incredibly efficient but generates high-frequency ripple (typically 50kHz to 200kHz). A pure linear supply is silent and clean but wastes massive amounts of energy as heat when dropping high voltages to low voltages.
The optimal approach for a benchtop power supply from a PSU is a hybrid cascade. The ATX switching stage steps the 120V/240V AC mains down to a rough 12V DC bus with high efficiency. A secondary linear regulator then drops that 12V to your exact target voltage, filtering out the switching noise.
| Topology | Efficiency | Heat Dissipation | Output Noise / Ripple | Cost per Amp |
|---|---|---|---|---|
| Raw ATX Switching | 85% - 92% | Low (Fan cooled) | 50mV - 100mV p-p | ~$0.50 / A |
| Pure Linear (Transformer) | 30% - 50% | Very High (Massive sinks) | < 1mV p-p | ~$3.00 / A |
| Hybrid (ATX + Linear Post) | 70% - 82% | Medium (Managed via sink) | < 2mV p-p | ~$0.80 / A |
For digital logic (Arduino, ESP32, 74-series ICs), the raw ATX 5V rail is perfectly adequate. For RF mixing, audio pre-amplification, or sensor bridging, you must use the hybrid linear post-regulation stage described below.
Design Example: 1.2V–9V Variable Regulator with Headroom Math
Let us design a variable 1.25V to 9V output capable of delivering 5A continuous current, fed from the ATX PSU's 12V yellow wire rail. We will use the Texas Instruments LM338 5A adjustable linear regulator.
Dropout Voltage and Headroom Math
Linear regulators require a minimum voltage difference between input and output to regulate properly, known as the dropout voltage. The LM338 has a maximum dropout voltage of 2.5V at a 5A load.
An ATX PSU's 12V rail is nominally 12.0V, but under a heavy load, it can sag to 11.8V. Therefore, your maximum achievable output voltage is:
V_out(max) = V_in(loaded) - V_dropout = 11.8V - 2.5V = 9.3V
If you need a clean 10V output, the 12V rail will not work; you would need to cascade the 12V and 5V rails or use a boost converter first.
Thermal Dissipation and Heatsink Sizing
Linear regulators burn excess voltage as heat. If you set the output to 5.0V and draw 5A from the 11.8V input, the power dissipated by the LM338 is:
P_diss = (V_in - V_out) * I_load = (11.8V - 5.0V) * 5A = 34 Watts
The LM338 junction-to-case thermal resistance is roughly 1°C/W. To keep the junction temperature below 100°C in a 25°C room, your external heatsink must have a thermal resistance no greater than 1.5°C/W. A standard TO-3 finned aluminum extrusion (like the Aavid Thermalloy 6118) is mandatory here.
Component Spec Sheet
| Component | Value / Part Number | Purpose |
|---|---|---|
| U1 (Regulator) | LM338K (TO-3 package) | 5A variable linear regulation |
| R1 (Set Resistor) | 120 Ω, 1% metal film | Sets minimum 1.25V reference current |
| R2 (Variable) | 5 kΩ, 10-turn wirewound pot | Voltage adjustment (1.25V to 9.3V) |
| C_in (Input Cap) | 10 µF, 35V Tantalum | High-frequency bypass, prevents oscillation |
| C_out (Output Cap) | 1 µF, 25V Ceramic (X7R) | Transient response improvement |
| D1 (Protection) | 1N5408 (3A, 1000V) | Reverse current protection if output is shorted |
Input Protection, Thermal Derating, and Ripple Expectations
When adapting a PC power supply for the bench, you are bypassing the motherboard's careful power sequencing. You must implement your own protection and understand the physical limits of the ATX topology.
Never open the ATX PSU casing to modify the primary side. The bulk capacitors hold lethal charges long after unplugging. Furthermore, do not use this raw benchtop supply to directly charge lithium-ion or LiFePO4 cells. ATX supplies operate in Constant Voltage (CV) mode only; lithium charging requires a dedicated Constant Current/Constant Voltage (CC/CV) BMS or charge controller to prevent thermal runaway.
Input Range and Fusing
The ATX PSU handles the 90V–264V AC mains input range and provides internal Over Current Protection (OCP) and Over Voltage Protection (OVP). However, the OCP trip points on the 12V rail are often set dangerously high for bench wiring (e.g., 30A+). You must add a fast-acting 7A or 10A glass fuse on the 12V yellow wire immediately after it exits the ATX casing. This protects your 18 AWG bench wiring from melting if your linear regulator fails short-circuit.
Thermal Derating Notes
ATX power supplies are typically rated for their full continuous wattage at an ambient temperature of 40°C to 50°C, assuming high-velocity airflow from their internal 120mm fan. If you mount your linear post-regulator heatsink inside a poorly ventilated enclosure and the local ambient rises to 35°C, you must derate the ATX PSU's 12V rail capacity by roughly 20%. If the ATX is rated for 18A on the 12V rail, limit your bench draw to 14A to prevent the ATX's internal thermal overload from tripping mid-test.
Ripple and Noise Expectations
A standard 80 Plus Bronze ATX PSU will exhibit 40mV to 80mV peak-to-peak ripple on the 12V rail, dominated by the switching frequency and its harmonics. According to practical measurement guides from All About Circuits, measuring this requires an oscilloscope with a tip-and-barrel probe to avoid ground-loop antenna effects.
By passing this 12V rail through the LM338 linear regulator, the low-frequency ripple is rejected by the regulator's Power Supply Rejection Ratio (PSRR), typically 60dB to 80dB. To kill the high-frequency switching spikes that bypass the linear regulator, add an LC pi-filter on the LM338 output: a 10 µH ferrite choke followed by a 100 µF low-ESR polymer capacitor. This drops the final output noise to under 2mV p-p, making it indistinguishable from a $300 laboratory linear supply for most hobbyist and prosumer applications.
Frequently Asked Questions
Can I use a benchtop power supply from a PSU to charge lithium batteries?
No, not directly. ATX power supplies are strictly Constant Voltage (CV) sources. If you connect a deeply discharged lithium cell to a 4.2V CV source, it will draw massive, uncontrolled current, likely tripping the ATX OCP or causing the cell to vent and catch fire. You must place a dedicated CC/CV buck converter module (like an XL4015-based CC/CV board) or a dedicated BMS charge controller between the ATX supply and the battery pack.
How do I turn on an ATX PSU without a motherboard connected?
ATX supplies require the PS_ON signal to be pulled low to enable the output rails. Locate the 24-pin ATX connector and identify the green wire (PS_ON) and any black wire (COM/Ground). Bridge these two pins together using a simple SPST toggle switch or a jumper wire. For added safety, place a 1kΩ resistor in series with the switch to limit the control current, though a direct dead-short is generally tolerated by the ATX's internal optocoupler circuitry.
Why does my converted PSU whine or click under light loads?
This audible noise is caused by magnetostriction in the switching transformer. Under very light loads (e.g., drawing less than 1A total across all rails), modern ATX PSUs enter Discontinuous Conduction Mode (DCM) or 'burst mode' to maintain efficiency at light loads. The erratic switching pulses fall into the audible frequency range. The fix is to add a permanent 'dummy load' to the 5V rail. Soldering a 10-ohm, 10W power resistor between the 5V (red) and Ground (black) wires draws 0.5A, forcing the PSU back into Continuous Conduction Mode (CCM) and eliminating the whine.
Do I need to connect the ATX PG (Power Good) pin to my linear regulator?
No. The gray Power Good wire simply signals the motherboard that the ATX internal rails have stabilized (usually holding +5V when the 12V/5V/3.3V rails are within regulation). For a DIY benchtop supply, you can safely tape off and ignore the PG wire. Your linear post-regulator will naturally ignore the ATX output until it rises above the dropout voltage, inherently handling the soft-start sequence without needing the PG signal.
