Controllable electronic load

The controllable power supply range of the electronic load is 2V~12V, 200mA, and the variable power supply is generated by the voltage regulator LM317.

The electronic load circuit uses a pair of parallel power transistors T3 and T4 (both 2N3773). T3 and T4 are mounted on a large area heat sink and can be used to test a DC power supply with a voltage range of 2V to 50V and a maximum current of 10A. If two 2N3773 power transistors are connected in parallel, the maximum current can be expanded to 20A.

Electronic load circuits have two major advantages: 1. No need to use high-wattage resistors as loads; 2. The load current is well controllable.

In the figure, T1 and T2 are connected to form a Darlington pair. It is connected to a variable 12V power supply by the voltage regulator chip LM317. R2 provides base current for the Darlington pair. The size of the base current can be controlled by the 5kΩ, 10-turn potentiometer VR1 connected to the ① pin of IC1. The output of the Darlington pair is taken from the terminal voltage of R4. It is sent to T3 and T4 through the current limiting resistor R3.

The power supply under test is connected between the collector and emitter of the power transistor, as shown in the attached figure. The load current is detected by the 0.2Ω, 10W resistor R7 connected in series with the load, and the voltage drop across it is read using a digital voltmeter. Therefore, when the load current is 1A, the voltage drop should be 200mV. Conversely, a voltmeter reading of 1mv corresponds to a load current of 5mA.

Note: Although the rated power consumption of 2N3773 transistor is 150W, the power consumption of each 2N3773 should not exceed 120W when in use.