This circuit is design for provide automatic current limiting up to 8.4A. Unlike current limiter that uses only a resistor, this current limiting circuit doesn’t drop the voltage, or at least keep the voltage drop at minimum, until a certain current amount is exceeded. This current amount limit is adjustable from 1.4 A to 8.4A by a potentiometer. You can modify the component value to give different current limiting range. This is the simple figure of the circuit.

How is the circuit work? First, the resistor R1 is there to sense the current. At R2 potentiometer at minimum resistance (the center tap connected to R1), 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 to reduces 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), when 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 that can be handled the circuit is 60 volt, but you can only safely set the current limit at 1.9A in the extreme condition, when the load is shorted to ground. The Q1 transistor must use sufficient heat sink. [Schematic project source: Hasan Murod].

How is the circuit work? First, the resistor R1 is there to sense the current. At R2 potentiometer at minimum resistance (the center tap connected to R1), 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 to reduces 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), when 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 that can be handled the circuit is 60 volt, but you can only safely set the current limit at 1.9A in the extreme condition, when the load is shorted to ground. The Q1 transistor must use sufficient heat sink. [Schematic project source: Hasan Murod].

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