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Transcript
EE462L, Spring 2014 Triac Light Dimmer 1 Triac Light Dimmer Light bulb a b Triac (front view) 3.3kΩ + Van (from Variac) – c Bilateral trigger diode (diac) n + Van – n Light bulb + 0V – •Efficient Triac G MT1 0.1µF a •Simple MT2 250kΩ linear pot + Van – 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. •Inexpensive MT1 MT2 G a b •Ingenious + Van – n Light bulb + Van – b + 0V – After firing, the triac is a closed switch, so that practically all of Van is applied across the light bulb. 2 ! Triac Open When the voltage across the diac reaches about ±35V, it self-fires and its voltage collapses to about ± 5V Light bulb 3.3kΩ + Van (from Variac) – + Van (from Variac) – 250kΩ linear pot 0.1µF Triac Closed Bilateral trigger diode (diac) Capacitor discharges into triac gate Light bulb 3.3kΩ 250kΩ linear pot 0.1µF Bilateral trigger diode (diac) • Light bulb resistance is a few ohms when cold, and about 100-200Ω 2 when bright (use P Vrms / R 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 3 #6-32, ½” machine screw, flat washer, split washer, and hex nut Remove this center screw Flat rubber washers between #8 x 1” screws and porcelain #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 4 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 Remove this center screw 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. 5 Hookup Variac (plugged into isolation transformer) Isolation transformer Light dimmer 6 Connection to Variac Variac knob set to zero Light dimmer connected to black and white terminal posts 7 ! No-Firing Condition – Actual Variac voltage Capacitor voltage • 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 8 No-Firing Condition – Simulated (EE362L_Triac_Light_Dimmer.xls) Source Vrms 70 Voltage 150 Denom-mag 3.874785 100 Denom-ang 75.0439 50 VCrms 18.06552 VCang 0 -75.0439 0 Tau -50 0.00993 -100 Fixed R kohm 3.3 Freq Hz 60 30 60 90 Potentiometer kohm 96 C uF 0.1 Diac breakover V 35 Diac on volts V 5 120 150 180 210 240 270 300 330 360 -150 Angle Source voltage Capacitor voltage Diac breakover 9 ! Firing Condition – Actual Vcn Van Diac conducts when Vcn reaches 32-35V (diac breakover voltage). The capacitor then discharges through the triac gate. α = 90° ≈ 16.67ms ÷ 4 • 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 10 Firing Condition – Simulated (EE362L_Triac_Light_Dimmer.xls) Source Vrms 70 Voltage 150 Denom-mag 2.210759 100 Denom-ang 63.10652 50 VCrms 31.66333 VCang 0 -63.1065 0 Tau -50 0.00523 -100 Potentiometer kohm Fixed R kohm 3.3 Freq Hz 60 49 C uF 0.1 Diac breakover V 35 Diac on volts V 5 EE362L_Triac_Light_Dimmer.xls Van and Vcn waveforms with potentiometer adjusted for α = 90° 30 60 90 120 150 180 210 240 270 300 330 360 -150 Angle Source voltage Capacitor voltage Diac breakover 11 Current ! 0 Power to Light (per unit of sinewave, Power to Resistive Load versus Firing Angle i.e., α = 0º) 30 60 90 120 150 180 210 240 270 300 330 360 Angle 1 0.9 0.8 0.7 P Current 0.6 0.5 0.4 0 30 60 90 120 150 180 210 240 270 300 330 360 0.3 0.2 Angle 0.1 0 0 30 60 90 120 150 180 Current Alpha 0 30 60 90 120 150 180 210 240 270 300 330 360 sin 2 2 2 Vab V an, rms 1 , rms 2 Angle 12 Remember to Calibrate Your Scope Probe 13 Magnitude of Voltage Harmonics Depends on Alpha ! 60Hz component 180Hz component 100Hz Save screen snapshot #3 Measuring the dB difference between 60Hz and 180Hz components of Vab V 32.81db 20 log 10 60 Hz 1Vrms V 26.87db 20 log 10 180 Hz 1Vrms V60 Hz 32 .81 1Vrms 10 20 V180 Hz 26 .87 20 1Vrms 10 = 43.7Vrms = 22.1Vrms 14