Whenever I want to test some USB power supplies, drawing I-V characters of a power supply and working on a lab bench supplies based on SMPS and linear mode I face the problem of testing its full capabilities. Usually I want to see how much power I can get as an output from a unit to test the best method to connect a low value resistance between its output wires through a multimeter in ammeter mode. The value of resistance and voltage across the power supply is responsible for the current which is basically Ohm’s law. But how to decide the value of that resistance, if its value is higher than recommended one it will not draw max current and if it’s value is lower it shorts the output and turns on the protection feature. And if we are measuring higher current say 10A then a higher wattage low value resistance is required. All this is not possible at a time and took a lot of precision to accurately characterize a power supply unit.

That’s why electronic loads came into the picture, in which the resistance can be variable or we can say they can draw a constant current out of any device. I made this electronic load from scratch, which has a max capability to draw 200W. This is a huge power, the concept of electronic load is based on heat dissipation, they convert this power into heat with the help of big mosfets. The basic working of a constant current load is stated in the section below. I made a PCB combning all the features, Big thanks to PCBWay for sponsoring the PCBs for this project! Their high-quality manufacturing and quick turnaround made this build possible.

How an Electronic Load works:

Suppose you have a 3V battery and would like to discharge it with a constant current of 1 A. The operating point is the (voltage and current) setpoint where the battery output voltage intersects the programmed constant current load line of the electronic load. When the load operates in CC mode, it loads the output of an external voltage source (for example, a 3 V battery), with a variable resistor to reach the desired programmed current. Most electronic loads use power transistors, FET’-s or IGBT’-s that act as a variable resistor to regulate the current flowing into the load.

The transistors are typically arranged in a parallel array configuration to handle more power. The current flowing into the load is monitored via a shunt resistor (for example 1 Ω). The voltage drop proportional to I*Rshunt is fed to a current amplifier. The current amplifier compares the voltage drop on the current shunt against the reference programmed value (example 1 A * 1 Ω = 1 V). The amplifier output signal regulates the FET resistance and electronic load’s input current. This feedback configuration allows the load to dynamically change the resistance and maintain the programmed current independent of the voltage change of your sourcing device.

The ability to sink high currents at exceptionally low voltages is challenging and a highly required feature for electronic loads. Sinking at low voltages is mandatory when testing fuel cells, power management ICs, or other devices operating at low voltages and high currents.

Circuit Diagram:

I have taken the reference from a lot of sources and finally came up with this approach of keeping two MOSFETs in parallel to each other. The schematic is divided into 5 main sections.

First section is the input section, which is a basic input section and this load needs a 12V external power supply which is different from the test supply to work. Because of onboard electronics and operational amplifiers.

Second section is a basic voltage set section, it is basically a voltage divider network which feeds the non inverting supply of the operational amplifier. This section has two potentiometers, one to adjust coarse and other for fine adjustment. The voltage output from this section is made equivalent to the current drawn from the test supply. Which is based on the 3rd section operational amplifier...