EE462L, Spring 2014 Triac Light Dimmer

1. EE462L, Spring 2014Triac Light Dimmer

2. Light bulb a b + Van – + Van – + Van – + 0V – + 0V – + Van – Light bulb a b n 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. After firing, the triac is a closed switch, so that practically all of Van is applied across the light bulb. n Triac Light Dimmer Light bulb a b • Ingenious • Simple • Efficient • Inexpensive Triac (front view) 3.3kΩ MT2 + Van (from Variac) – c Triac 250kΩ linear pot G MT1 Bilateral trigger diode (diac) 0.1µF MT1 MT2 G n

3. Triac Closed When the voltage across the diac reaches about ±35V, it self-fires and its voltage collapses to about ± 5V Light bulb Light bulb 3.3kΩ 3.3kΩ + Van (from Variac) – + Van (from Variac) – 250kΩ linear pot 250kΩ linear pot ! Bilateral trigger diode (diac) Bilateral trigger diode (diac) 0.1µF 0.1µF • Light bulb resistance is a few ohms when cold, and about 100-200Ω when bright (use to get light bulb resistance R) Capacitor discharges into triac gate Triac Open • The light bulb resistance is small compared to the 3.3kΩ and potentiometer combination and can be ignored when analyzing the RC electronic circuit • The circuit resets and the process repeats every half-cycle of 60Hz

4. #6-32, ½” machine screw, flat washer, split washer, and hex nut Flat rubber washers between #8 x 1” screws and porcelain Remove this center screw #8 x ½” screws for corner brackets #8 x 3/4” screws for terminal blocks 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 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 9.6°C rise above ambient air per Watt

6. Hookup

7. Connection to Variac

8. Variac voltage Capacitor voltage ! 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 • Capacitor voltage lags variac voltage almost 90º for large potentiometer resistance

9. No-Firing Condition – Simulated(EE362L_Triac_Light_Dimmer.xls)

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

11. Firing Condition – Simulated (EE362L_Triac_Light_Dimmer.xls)

12. ! Power to Light (per unit of sinewave, i.e., α = 0º)

13. Remember to Calibrate Your Scope Probe

14. ! 60Hz component 180Hz component 100Hz Save screen snapshot #3 Measuring the dB difference between 60Hz and 180Hz components of Vab Magnitude of Voltage Harmonics Depends on Alpha = 43.7Vrms = 22.1Vrms