Introduction

This dimmer switch is suitable for 230V incandescent lamps with a total power
of up to 300 Watts.
The circuit is switched in series with the lamp so it can directly replace the wall switch.

Circuit description

At first triac TR1 is not conducting; C1 is charged through R1/P1 until the trigger level of diac D1 is reached.
When the diac trigger level is reached (about 30 V), D1 fires and Triac TR1 is switched on.
The triac will remain in conductive mode until the mains current is lower than the triac hold current
at the end of the half mains period.
This works for both the negative and positive mains period because both the triac and the diac are bi-directional.
R1 is added to protect the potentiometer in the case of a short circuit. The fuse is absolutely
mandatory as the discharge arc in a failing incandescent lamp is virtually a short circuit.

If you do not have a diac you can replace it with a small neon lamp (without series resistor!)
but then you will have to use a smaller value for P1 because the "trigger" voltage of a small neon lamp
is higher: typically 70 V.

The circuit will also work at 120V/60Hz, P1 must be smaller in that case.

Dimmer switch schematic: