I am studying a sample motion detection circuit which uses a D203S PIR sensor and a BISS0001 motion detector IC. After sensing motion, BISS0001 gives output which switches SCR and SCR switches TRIAC. I don't understand the role of some of the components.
The circuit uses a 7.5V Zener diode across triac gate terminal. What is the purpose of using this Zener diode? When SCR switches the triac the voltage across the Zener varies a lot with respect to the load. It shows different voltages at different load conditions.
We know that a Zener diode always has a fixed voltage drop when supplying greater voltage then the specified one. What's happening in this case?