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Electronic resistor - Dummy load for power supplies

     If you are testing various power supplies (switching and classical), surely you will need a load, whose consumption can be regulated. Such load can help you to find out exactly the current threshold, where the ripple starts to occur, output voltage start to decrease or the electronic fuse turns off.
     My dummy load is very simple. As a disssipator (component, which converts energy into heat) MOSFET-N is used here. Current consumption is regulated by changing the Gate voltage using a potentiometer. MOSFET resistance is dependent on the gate voltage. The voltage at potentiometer input isstabilized using zener diode. In order to test the source with a small output voltage, you should use the logic-MOSFET (MOSFET designed to be driven with logic level). It has a lower Gate threshold voltage and allows to test the power supplies with voltages down to cca 4V. For logic MOSFETs is suitable 5V zener diode, for the classical MOSFET about 9V. MOSFET must be placed on large heatsink. Short term, it is possible to overload the MOSFET, the TO220 housing can handle power up to 100W for short moment. Permanently it can operate with a loss up to about 50W when the heatsink is sufficient. My electronic ballast operates in the input voltage range of 4 - 25V. Logical MOSFETs have usually the max D-S voltage 30V. When using an appropriate MOSFET the dummy load can work with higher voltage. In this case, increase the series resistor.

Schematic of simple dummy load.

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 without PCB :).

Completed dummy load.