The following current limiting circuit keep the voltage drop to a minimum until a certain current amount is reached. This circuit provides automatic current limiting up to 8.4A. Using a potentiometer, this current amount limit can be adjusted from 1.4A to 8.4A. The value of the component can be modified to give different current limiting range.
Here is the function of each components. The resistor R1 is used to sense the current. At R2, potentiometer at minimum resistance. if the current drawn by the load reach 1.2A then the voltage across R1 reach 0.6V and Q2 begin conducting, thus shorting the base voltage of Q4 to ground. This shorting action reduce the base current and therefore reduce the output voltage sensed by the load, and prevent the current to flow further. If you need the current limiter to limit at lower threshold range, you can change the R1 to 1R and you’ll get about 0.7A to 4.2A adjustment range. Because of the power dissipation capability of 2N3055 transistor, at the worst case that the load is shorted to ground (zero resistance), if you limit the current to 8.4 A then the circuit can handle maximum source voltage of 14V, while limiting the current at 4.2A can handle up to 27V source voltage. The maximum voltage can be handled by this circuit is 60 volt, but at that maximum voltage you can only safely set the current limit at 1.9A in the extreme condition, when the load is shorted to ground. Please make sure the Q1 transistor has sufficient heat sink. [freecircuitdiagram.com]
