Protection circuit for high power transistor equipment - uses Zener diodes and resistors to limit current in each transistor independently

摘要

The transistor circuit includes a pair of power transistors (10, 11) connected in parallel to supply a load (12). A respective series resistor (13, 14) senses the current in its power transistor. Independently operated protection transistors (18, 19) are controlled by the voltage drop across the sensing resistors to limit the current in each power transistor to a safe level. Both power transistors are protected from over-voltage by a common zener diode (32) connecting their collectors to the negative rail. Zener diodes (26, 27, 30, 31) and resistors (24, 25, 28, 29) are connected to vary the current limit in accorance with the voltage drop across the power transistors. In a modification temp. dependent resistors may be used to bary the limit current with temp.