Simple dual constant-current load tests low-current power supplies

Texas Instruments LM358

Today’s small electronic appliances, such as washers, dryers, and stoves, use switched-mode power supplies to replace bulky, heavy, linear-power supplies. The engineer testing these power supplies, which range in current from 50 mA to 1 A, typically uses resistors or standard off-the-shelf electronic loads. An engineer would employ a variety of high-wattage resistors to verify multiple loading conditions to satisfy a proper design. Most off-the-shelf electronic loads target an average of 300 W. When measuring 50 to 300 mA, a display is inaccurate; most of them display 0.1 A, but accuracy is questionable at that low range. You can alternatively use the simple dual constant-current-load design in Figure 1, which you can build with inexpensive, common parts.

China PCB Prototype and Fabrication Manufacturer

This dual constant-current load can measure the performance of dual power supplies supplying 0 to 1.25 A per channel.
Figure 1. This dual constant-current load can measure the performance of dual power supplies supplying 0 to 1.25 A per channel.

The load current passes through a MOSFET and a 1%, 1 Ω sense resistor, R6. Pin 2 of IC1A compares the voltage drop in the resistor to a reference voltage. IC1, an LM358 op amp, compares the two inputs and adjusts its output accordingly. The reference voltage at Pin 3 of IC1A comes from a voltage-divider potentiometer, R2 or R3, which derives from a TS431 1.25 V 1% reference. Because the maximum voltage can be 1.25 V and the sense resistor’s value is 1 Ω, the maximum current per channel can reach 1.25 A.

R2 and R3 are 15-turn, 1-kΩ potentiometers, which you can finely adjust to the desired load. One can set a minimum current, and the other can set a maximum current. Switch S1 can then switch between minimum load, no load in the middle position, and maximum load. Furthermore, by attaching a standard DMM (digital multimeter) across R6, you can directly read the current and adjust it to the proper level.

Input-voltage change does not affect the DMM’s reading because it monitors the constant current through sense resistor R6. The second channel is a duplicate of the first. Each channel can control 0 to 1.25 A and can handle a voltage of 3 to 50 V. The capacitor input and the MOSFET set the upper limit. The two inputs can be in parallel to a load of 2.5 A. For a two-output power supply, you can set the minimum and maximum current by precisely reading the level on a multimeter and then quickly testing a matrix of no load, minimum load, and maximum load. A 9 V battery powers the unit.

Materials on the topic

  1. Datasheet Vishay IRFP450
  2. Datasheet Texas Instruments LM358
  3. Datasheet STMicroelectronics TS431

EDN

You may have to register before you can post comments and get full access to forum.
User Name