Another desirable feature of regulated voltage supplies is overcurrent protection. When overcurrent is drawn from the output of a regulated current source, or when there is a short circuit, an overcurrent protection circuit is added so that the regulator circuit and the power supply are not damaged. Such a circuit is given in Figure-1.8. In this circuit, transistors R6 and Q4 are added in addition to the circuit in figure 1.7.
Other parts of the cycle are the same. These new members give us two options.
1. The output current IL is limited to a predetermined current value.
2. If the output current IL exceeds a predetermined value, the output voltage is reduced to zero.
The operation of the buck is briefly as follows: The IL current from the output is passed through the RAK resistor and Q3 transistor. This current IL causes a voltage drop across the RAK resistance. The value of this voltage determines the IL current and RAK value. When the voltage on RAK reaches the base-emitter voltage of Q4 transistor, Q4 conducts and Q3 limits the base voltage of the transistor. Thus the current remains at a constant value and is calculated as follows.
So we can set the current limit by setting the resistor R6 to the desired value. Other electronic circuit elements can be used to protect overcurrent and short circuit. (Such as SCR, Opamp, Flip-flop) depends on the choice of this user.