Build a Zero Crossing Switch Circuit Diagram

This is a simple Zero Crossing Switch Circuit Diagram. The zero-crossing is important for systems which send digital data over AC circuits, such as modems, X10 home automation control systems, and Digital Command Control type systems for Lionel and other AC model trains. When switching loads with the aid of a thyristor, a large amount of RFI can be generated unless some form of zero crossing switch is used. 

 Zero Crossing Switch Circuit Diagram

 The circuit shows a simple single transistor zero crossing switch. Rl and R2 act as a potential divider. The potential at their junction is about 10% of the ac voltage. This voltage level is fed, via R3, to the transistor`s base. If the voltage at this point is above 0, the transistor will conduct, shunting any thyristor gate current to ground. When the line potential is less than about 2 V, it is possible to trigger the thyristor. 

The diode Dl is to remove any negative potential that might cause reverse breakdown. If electrical power is to be switched, no electrical interference is generated if switched at an instant when there is no current a zero crossing. Early light dimmers and similar devices generated interference; later versions were designed to switch at the zero crossing.