Missing pulse detector protects transformer

Designed as part of the controller of an astronomical telescope, this circuit delivers an ac drive waveform to the drive motor. The frequency of the ac output is set by an external oscillator that generates the desired drive rate.

But what happens if the oscillator fails or isn’t hooked up? Most circuits of this type would leave one side of the transformer continuously energized, causing it to saturate and damage one of the FETs.

To keep this from happening, a missing-pulse detector was designed using two CMOS NOR gates (see the figure). The input is capacitively coupled so that it will not be affected if the input gets stuck high or low. A 0.1-mF capacitor charges quickly through a diode and discharges slowly through a resistor. If the input signal is present, the charge on the capacitor is refreshed on every cycle. If not, the capacitor discharges, the output of the second gate goes high, and both FETs are held “off” until the input resumes.