The circuit in this figure is protecting the circuit and the system with power supplies that may exceed safe limits. One example is small consumer products that use external ac adapters; it's easy to mistakenly plug in the wrong adapter. Another example is a portable system that uses a rechargeable battery pack. If the battery pack is absent or fails to open during recharging, a high-compliance charger can deliver excessive voltages to the system.
The circuit works using LM4041 adjustable shunt-voltage regulator as a voltage detector. When it operates as a reference, the LM4041 develops a voltage across its positive and negative terminals. This signal forces the voltage across R1 to equal 1.24V. In this circuit, however, R3 prevents this servo action. With R3 in the circuit, VG is near ground when the voltage across R1 is less than 1.24V, and VG is approximately 1V below the positive rail when the voltage across R1 is greater than 1.24V. You can, therefore, set a threshold voltage by selecting appropriate values of R1 and R2. When the supply voltage exceeds the threshold, VG goes high, thereby turning off Q1 and removing power from the load. Select R1 and R2 according to:
It where VSHUTOFF is the supply voltage that causes shutoff. With the values shown, the circuit removes power from the load when the supply voltage reaches approximately 6V. R4 provides hysteresis to prevent chattering when the supply voltage is near the shutoff value. IC1 can accommodate shutoff voltages as high as 10V; clamping IC1's supply voltage with another inexpensive shunt reference or zener diode (across the positive and negative terminals) allows higher maximum shutoff voltages. Maximum supply voltage with the components is approximately 50V.
