This simple lamp dimmer uses only a Triac, a Diac and some other passive
components to achieve power control on lamp . It is suitable for 230V
incandescent lamps with a total power of up to 300 Watts. 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.