1. Triac Light Dimmer

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

3. Triac Open Triac Closed+
Van
(from Variac) –
Light
bulb
0.1µF
3.3kΩ
250kΩ linear pot
Bilateral trigger diode (diac)
Triac Closed +
Van
(from Variac) –
Light
bulb
0.1µF
3.3kΩ
250kΩ linear pot
Bilateral trigger diode (diac)
When the voltage across the diac reaches about ±35V, it self-fires and its voltage collapses to about ± 5V
Capacitor discharges into triac gate
Light bulb resistance is a few ohms when cold, and about Ω 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
The circuit resets and the process repeats every half-cycle of 60Hz

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. 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
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

6. Hookup

7. Connection to Variac

8. No-Firing Condition – Actual
Variac voltage
Capacitor voltage Vcn +
Van
(from Variac) –
Light
bulb G
MT2
MT1
0.1µF
3.3kΩ
250kΩ linear pot
Bilateral trigger diode (diac) a c n b
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
MT1
Light
bulb G
MT2
0.1µF
3.3kΩ
250kΩ linear pot
Bilateral trigger diode (diac) a c n b
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. Magnitude of Voltage Harmonics Depends on Alpha
Measuring the dB difference between 60Hz and 180Hz components of Vab
60Hz component
180Hz component
100Hz
Save screen snapshot #3
= 43.7Vrms
= 22.1Vrms

22. Thyristor, or Silicon Controlled Rectifier (SCR)
Blocks forward conduction until gate current breaks down inner n1-p2 junction, where it then behaves like a diode.
Conducts until forward current drops below the holding level, then turns off.