2 Triac Light Dimmer Triac (front view) MT1 MT2 G + V an (from Variac) – Light bulb G MT2 MT1 0.1µF 3.3kΩ 250kΩ linear pot Triac Bilateral trigger diode (diac) a c n b Light bulb a n b Before firing, the triac is an open switch, so that practically no voltage is applied across the light bulb. The small current through the 3.3kΩ resistor is ignored in this diagram. + 0V – + V an – + V an – Ingenious Simple Efficient Inexpensive After firing, the triac is a closed switch, so that practically all of V an is applied across the light bulb. Light bulb a n b + V an – + V an – + 0V –
3 Triac Open Light bulb resistance is a few ohms when cold, and about 100-200Ω when bright (use to get light bulb resistance R) The light bulb resistance is small compared to the 3.3kΩ and potentiometer combination and can be ignored when analyzing the RC electronic circuit When the voltage across the diac reaches about ±35V, it self-fires and its voltage collapses to about ± 5V The circuit resets and the process repeats every half-cycle of 60Hz Capacitor discharges into triac gate + V an (from Variac) – Light bulb 0.1µF 3.3kΩ 250kΩ linear pot Bilateral trigger diode (diac) Triac Closed + V an (from Variac) – Light bulb 0.1µF 3.3kΩ 250kΩ linear pot Bilateral trigger diode (diac) !
4 Flat rubber washers between #8 x 1” screws and porcelain Remove this center screw #6-32, ½” machine screw, flat washer, split washer, and hex nut #8 x 3/4” screws for terminal blocks #8 x ½” screws for corner brackets The potentiometer is connected so that turning it clockwise lowers the resistance of the firing circuit, fires the triac sooner, and makes the light brighter
5 The back of the triac fits firmly against the heat sink, with maximum surface contact. The flat washer, then split washer, then hex nut fit on the other side of the corner bracket. Remove this center screw 9.6°C rise above ambient air per Watt To make it easy to connect an oscilloscope probe, it helps to put an extra spade connector, bent upwards at a 90° angle, under the back terminal block screws
8 No-Firing Condition – Actual When potentiometer resistance is large, there is no firing because the capacitor voltage never exceeds (positive or negative) the diac breakover voltage Variac voltage Capacitor voltage Capacitor voltage lags variac voltage almost 90º for large potentiometer resistance !
10 Firing Condition – Actual Capacitor voltage Vcn does not go into steady state AC right away as Van crosses the zero axis. There is a time delay due to the RC time constant. The RC time constant delay plus phase shift of the AC solution for Vcn determines the point at which the diac breakover is achieved !
14 Measuring the dB difference between 60Hz and 180Hz components of V ab 60Hz component 180Hz component 100Hz Save screen snapshot #3 = 43.7Vrms = 22.1Vrms Magnitude of Voltage Harmonics Depends on Alpha !