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BE CAREFUL WHEN BUILDING THESE CIRCUITS BECAUSE OF THE LETHAL VOLTAGES !
BUILD AT OWN RISK !
In the circuit below, a PUT is used as part of a zero-crossing detector.
Zero-crossing detectors are used to in phase control for triacs or SCR's to control power for motors or lighting applications.
The input is AC 220V/50Hz which is fed to the base of transistor Q1 and the emitter of Q2 via resistors R1 and R2, that limit the current.
The base of Q2 is connected to ground, so Q2 will conduct when the voltage at the junction of R2 and R3 is lower than -0.7V. Q1 will conduct when this voltage is higher than +0.7V.
So when the voltage at the junction of R2 and R3 is outside -0.7V to +0.7V, the output will be low and when the voltage is within -0.7V to +0.7V, the output will be high.
This results in a pulse of about 250us, in the middle of which is the zero-crossing of the AC input. The pulse width is determined by the values of R1, R2 and R3.
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