Build a Simple Analog Active Load for Testing Constant Current

It’s always good to have an active load on hand for testing power supplies to ensure that their output voltage and current are within specs over a range of load conditions, as faulty power supplies can damage electrical equipment and hardware. There are plenty of load testers on the market that offer multiple modes: constant voltage, constant current, constant power, and constant resistance, at constant high prices.

While it’s possible to utilize a heat element or some large resistors to test power supplies, an active load brings more control to the table. Sure, those pricey units will get the job done, but what if you only need one mode for testing or are on a fixed budget? Those are questions electronics enthusiast Charles Ouweland asked himself, so he decided to build his active load for testing constant current (only) using very little hardware.

Ouweland designed his active load using an IRFP250 MOSFET, LM358 opamp, 100nF capacitor, and a 20k trimmer resistor (10 turns lin or log). He also used a 0.1 Ω 10W shunt resistor, and 1kΩ resistor, all packed onto a 10cm x 11cm PCB outfitted with an aluminum heatsink and 12V fan, along with a 2-pin (2X) and 4-pin header. Ouweland states his active load is current settable between 0A and 10A, can handle a maximum power of 120W, a maximum voltage of 200V, and can dissipate up to 190W.

Ouweland has uploaded a detailed walkthrough of his active load on his project page, complete with schematics, and link to the necessary files on GitHub for those interested in recreating his build.