مهدی الکترونیک

Lamp dimmers using traics can be quite simple,  nothing more than a potentiometer, resistor, capacitor and triac with a built-in diac. (See Teccor's application note for examples and other technical data.) The circuit below is similar to designs using unijunction transistors to generate the triggering pulse. The unijunction is replaced by a two-transistor "flasher" circuit that drives a pulse transformer. This type of circuit gives a wide range of control while exhibiting little hysteresis or line voltage sensitivity. The two diodes rectify the line voltage such that the flasher sees a positive voltage pulse on each half-cycle and, after a delay set by R and the 0.1uF capacitor, the flasher circuit triggers the triac.  The capacitor discharge is deep so the dimmer starts fresh on the next half-cycle. Note that the triac always gets the same polarity of trigger pulse.