Current handling capability of a power supply system often needed to be checked. Finding high power rated variable resistor to provide variable load testing might be difficult. There is wire wound type but it’s not always available at your local store, and the price and performance might be disappointing. This variable high power “resistor” uses 2N3055 power transistor to provide the required function, see the circuit’s schematic diagram in the figure below.
This circuit can be used to replace the load resistor in power supply testing, and you can adjust the current by setting the potentiometer. The maximum voltage applicable for this variable resistor is 60 Volt. Although 2N3055 is specified for maximum collector current for 15A, but practically the current is limited by maximum power dissipation of the transistor, remember a 2.5A current at 50V is 125 Watts!!! Put the circuit in a box with sufficient heat sink. The current consumption of the battery is maximum when this “resistor” is not connected to a power supply output since there is no voltage reaction coming from collector current at R2 to counter the base current flow for the whole transistor, but this current would never exceed 1.9 mA. The current consumption when the circuit is in use to test a power supply system is around 0.9 mA, and it’s good idea to switch off the witch SW1 if the circuit is not used. [Circuit's schematic diagram source: freecircuitdiagram.com]
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