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Electronic resistor - variable dummy load for power supplies

     If you are testing various power supplies (switching or traditional), surely you will need a test load, whose current consumption can be regulated. Such a dummy load can help you to find out exactly the current threshold, where the ripple starts to occur, the current limit kicks in, the output voltage start to fall or the electronic fuse turns off.
     My dummy load is very simple. A MOSFET-N transistor is used here to turn the input power into heat. Current consumption is varied by changing the gate voltage using a potentiometer. MOSFET resistance is dependent on the gate voltage. The voltage for the potentiometer is regulated using Zener diode. In order to test a power supply with a low output voltage, you should use the logic-MOSFET (a MOSFET designed to be driven with logic level). It has a low gate threshold voltage and allows to test power supplies with voltages down to about 4V. For logic MOSFETs is suitable 4.3V Zener diode, for traditional MOSFETs about 7.5V Zener. MOSFET must be mounted on a large heatsink. For short duration tests, a smaller heatsink can be sufficient. The maximum power dissipation depends on the type of the transistor and the heatsink. My electronic dummy load operates in the input voltage range of about 4 - 28V. Logic MOSFETs have usually the max D-S voltage 30V. Never exceed the maximum voltage rating of the transistor. When using an appropriate MOSFET, this dummy load can work with higher voltages. In this case, increase the value of R1 resistor. Warning - a dummy load may get very hot! Risk of fire! Never operate it unattended. You may add a current and thermal fuse for extra safety.

Schematic of a simple dummy load for power supply testing.

PCB pattern and soldering plan, Temsi made according to my schematic (thank you). The files for download: PCB pattern in PDF for printer a soldering plan in Eagle.

Primitive construction of my variable dummy load without any PCB board :).

Completed dummy load for power supplies.