PUT as a DIAC (trigger diode)

BE CAREFUL WHEN BUILDING THESE CIRCUITS BECAUSE OF THE LETHAL VOLTAGES !

BUILD AT OWN RISK !

In the circuit below, a PUT is used as a replacement of a DIAC. DIAC's are mainly used as trigger devices for triacs or SCR's. A DIAC will start to conduct when the forward voltage or reverse voltage over the DIAC exceeds about 30V. The circuit below behaves as a DIAC, but only in one direction (unidirectional DIAC).
D2 is used to ensure that only positive voltages reach the PUT. When the voltage on node 2 exceeds 10V, the PUT will latch into conduction, because the current and thus the voltage over R1 will increase until the voltage over R1 reaches 0.7V, causing Q2 to start conducting. When the PUT is latched, it will stay latched until the current through the PUT transistors becomes low enough so the PUT unlatches again. This happens when the amplitude of the input waveform has become low enough.

Click here to download the LTSpice simulation for the PUT used as a DIAC

The circuit below is a practical example of the use of a PUT to emulate the behaviour of a unidirectional DIAC. Here, the PUT is used to trigger a triac. Phase control is provided by R3, R4 and C1. The diode bridge ensures that the PUT is always fed with a positive voltage because it only works with positive voltages.