This test load is used to test power supplies (switching and linear) and chargers.
It allows you to determine the behavior of the PSU at different levels of current consumption,
the current at which the voltage starts to ripple, the output
voltage decreases or the electronic fuse trips.
A Darlington power transistor is used to convert the consumed power into heat.
At first, I used a BDX33C, then I replaced it with a more powerful TIP142.
The consumed current is regulated using a potentiometer. This determines the reference voltage for the MC33171 operational amplifier (OpAmp),
which is applied to its non-inverting input. The voltage drop of a 50mΩ shunt is applied to the inverting input, this is how the consumed current is sensed.
The output of the OpAmp controls the base of the transistor so that the selected current flows through it, thus achieving voltage equalization between the two inputs.
To ensure that the current regulation does not depend on the input voltage, the voltage of the potentiometer is stabilized by the LM317 circuit.
Without a resistor divider, it provides its lowest selectable voltage of 1.25V.
To be able to test even power supplies with a low output voltage (down to 4V), use a so-called logic-MOSFET, which has a low gate threshold voltage.
The type of transistor and its heat sink must of course be apropriate to the maximum power dissipation (P = V · I). For short-term testing, a smaller heat sink is sufficient.
The potentiometer value is not critical, anything from approximately 1k to 47k can be used.
This electronic load can be used with input voltages from 4V to 30V and currents up to about 4.25A.
Caution - this artificial load can get very hot! Risk of fire! Do not leave it in operation unattended.
For greater safety, it is possible to add a current and thermal fuse.
