Electronic Design

Simple Current-Limiter And Power Switch Offer Overcurrent Protection

Many applications that use a single power supply need some type of overcurrent protection. Such a device will help prevent the entire system from shutting down in the event of a single fault or short circuit. The high-current switch Q1, driver Q3, and the overcurrentsensing Q2 transistors with positive feedback form a bistable circuit suitable for this function (see the figure).

A logic-high pulse on the anode of diode D1 will turn the power switch on, provided that the cathode of diode D2 also is held at a logic-high level. If an overcurrent condition occurs at the output, Q2 conducts and instantly turns Q3 and Q1 off. A logic-low pulse on the cathode of D2 will always turn the switch off, while a steady logic-low level will force the switch to the off state. A steady high level on D1’s anode could force the switch on. But this is not recommended since, in the case of an overcurrent condition, the circuit would repeatedly turn on and off.

Capacitors in the output line cause the circuit to always power-up in the off position. A bypass resistor, R, may be added if the state of the output line is to be monitored by a microcontroller. The microcontroller’s D/A port may be used for this purpose. The value of resistor R should be great enough not to force the unloaded output into the on state. Yet it shouldn’t be so high that voltage, in the presence of proper load, is too low at the output. This would result in inconsistent voltage detection. D2 may be omitted if an open-drain output of the microcontroller is used.

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