Automatic_shutdown_for_abnormal_inputs110_V_220_V

Published:2009/7/24 4:20:00 Author:Jessie | From:SeekIC

This circuit is useful in systems or instruments that can be powered from 110 V or 220 V (with a changeover switch). If a regulator is powered from 220 V with the switch set at 110 V, the regulator is forced to dissipate abnormal power, which causes thermal shutdown and possible damage. This circuit senses abnormally high input voltages and shuts down the regulator before thermal shutdown occurs. Normally, the input voltage is low enough to keep the transistor on, pulling the enable pin toward the auxiliary output and maintaining regulator output. If the circuit is inadvertently powered from 220 V, with the switch at 110V, VIN rises. This cuts off the transistor, and the 10-kΩ resistor pulls the enable pin to ground and shuts down the regulator. The diode prevents VBE from zenering during the reverse-bias condition, which exists during shutdown. For the values shown, the circuit functions over ranges of 88 to 135 Vac and 180 to 260 Vac (110 to 220 Vac ±20%).



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