Protection circuit for driver transistor of piezoelectric crystal - has capacitor between crystal and transistor with resistor providing discharge path for crystal self capacitance

摘要

A transistor (2) in the final stage of a drive circuit for a piezoelectric crystal (1) of a recording head, is protected against overvoltages by a protection circuit. The emitter-collector current path of the transistor is in series with a variable resistor (3) across the supply voltage (UB), and in parallel with the load circuit of the crystal. The load circuit includes a storage capacitor (4) which is connected to the supply voltage by a part of the variable resistance, and to earth by a diode (5). The switching state of the transistor determines the charge in the piezoelectric crystal due to its self-capacitance. This self-capacitance can discharge through a resistor (7), the discharge rate being limited by a parallel diode (6).