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Circuit Detector and Disconnecting Over Voltage Schematic

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; its 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, V_G is near ground when the voltage across R₁ is less than 1.24V, and V_G 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, V_G goes high, thereby turning off Q₁ and removing power from the load. Select R₁ and R₂ according to:

It where V_SHUTOFF 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 IC₁s 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.