LED driver as fault monitor

Posted on Sep 30, 2012    3056

Using just four transistors and six resistors, this circuit provides switchable, constant-current drive for an LED and indicates both open- and short-circuit fault conditions (Figure 1). And there`s a bonus, too. Control signal VCONT switches the LED on and off. When VCONT is high, Q1 and the LED are off. When VCONT is as low as 0V, Q1 turns on and sources a constant current to the LED. Because most LEDs have a forward-voltage drop of at least 1.2V, adequate base-bias voltage exists for Q3, which turns on, thereby providing a conduction path for Q2. This conduction, in turn, provides bias for Q4, which turns on and pulls high, thus indicating a healthy LED.

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