Building an Adjustable Constant Current Load

Published:2012/11/29 0:22:00 Author:muriel | Keyword: Adjustable, Constant Current Load | From:SeekIC

Constant current dummy load designs built around the venerable LM324 quad OPAMP have become quite popular in the hobbyist community ever since David Jones at the eevblog popularized the concept in one of his excellent videos. The basic concept has been around for a long time though, and it's essentially an OPAMP based current source design. The current source portion of thecircuit below is very similar to Dave's design. It always bothered me though that the original design didn't use the four OPAMPs in the LM324, and that there was limited protection in the circuit. Therefore, I decided to use one of the two spare OPAMPs for thermal overload protection and another to activate a cooling fan. This approach should result in a more reliable design which is an important consideration for test equipment. Figure 1 shows the circuit schematic. The Current Source portion should be familiar to anyone that has seen similar designs on the web. The U2D OPAMP is a voltage follower that ensures the voltage at the 0.1 Ohm resistor follows the voltage applied to its non-inverting input. Therefore the current at the output through the 0.1 Oh resistor is I = Vi / (0.1) = 10 x Vi. I decided to use a 0.1 Ohm resistor (instead of the common 1 Ohm design)to allow larger currents. The smaller resistor value reduces the power dissipated in the resistor for a given output current. Furthermore, the source-to-ground voltage is also reduced thus ensuring that VGS is kept well above the MOSFET's threshold voltage for all practical operation conditions. I used two 50N06 N-channel MOSFETs in parallel to reduce the total Ron and improve reliability. In this design, the maximum current should be about 7A and it is limited by the 5W resistor I used; not by the MOSFETs. Larger currents can be achieved with a resistor capable of 10 or 20 W dissipation (which I didn't have handy). The input voltage should not exceed 60V (maximum VDS for these MOSFETs). As an added protection measure, I added a power MOV from the input to ground to protect the MOSFETs against high-voltage transients.



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