2 ee462l triac light dimmer ppt
TRANSCRIPT
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EE462L, Fall 2012
Triac Light Dimmer
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Triac Light Dimmer
Triac(front view)
MT1 MT2 G
+Van
(from Variac)
Lightbulb
G
MT2
MT1
0.1F
3.3k
250klinear pot
Triac
Bilateraltrigger diode
(diac)
a
c
n
b
Lightbulba
n
b
Before firing , the triac is an openswitch, so that practically no voltageis applied across the light bulb. Thesmall current through the 3.3kresistor is ignored in this diagram.
+ 0V +Van
+Van
Ingenious
Simple
Efficient
Inexpensive
After firing , the triac is a closedswitch, so that practically all of V an is applied across the light bulb.
Lightbulba
n
b
+ Van +Van
+0V
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Triac Open
Light bulb resistance is a few ohms when cold, and about 100- 200when bright (use to get light bulb resistance R)R V P rm s /
2
The light bulb resistance is small compared to the 3.3k andpotentiometer combination and can be ignored when analyzing theRC electronic circuit
When the voltage across thediac reaches about 35V, itself-fires and its voltagecollapses to about 5V
The circuit resets and the process repeats every half-cycle of 60Hz
Capacitor dischargesinto triac gate
+Van
(from Variac)
Lightbulb
0.1F
3.3k
250klinear
pot Bilateraltrigger diode
(diac)
Triac Closed
+Van
(from Variac)
Lightbulb
0.1F
3.3k
250klinear
pot Bilateraltrigger diode
(diac)
!
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Flat rubber washers between#8 x 1 screws and porcelain
Removethis
center screw
#6-32, machine screw,flat washer, split washer, andhex nut
#8 x 3/4screws for terminal blocks
#8 x screwsfor corner
brackets
The potentiometer is connected so that turning it clockwise lowers the resistanceof the firing circuit, fires the triac sooner, and makes the light brighter
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The back of the triac fitsfirmly against the heat sink,
with maximum surfacecontact. The flat washer,
then split washer, then hexnut fit on the other side of the
corner bracket.
Removethis
center screw
9.6C rise above ambientair per Watt
To make it easy to connect an oscilloscope
probe, it helps to put an extra spadeconnector, bent upwards at a 90 angle,under the back terminal block screws
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Hookup
Isolation transformer
Variac (plugged intoisolation transformer)
Light dimmer
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Connection to Variac
Light dimmer connected to black
and white terminal posts
Variac knob set to zero
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No-Firing Condition Actual
When potentiometer resistance is large, there is no firing because thecapacitor voltage never exceeds (positive or negative) the diacbreakover voltage
Variac voltage
Capacitor voltage
Capacitor voltage lags variac voltage almost 90 for large
potentiometer resistance
!
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No-Firing Condition Simulated(EE362L_Triac_Light_Dimmer.xls)
Fixed Potentiometer Diac DiacSource Freq R kohm C breakover on voltsVrms Hz kohm uF V V
70 60 3.3 96 0.1 35 5
Denom-mag3.874785
Denom-ang75.0439
VCrms18.06552
VCang-75.0439
Tau0.00993
-150
-100
-50
0
50
100
150
0 30 60 90 120 1 50 180 2 10 240 270 3 00 330 3 60
Angle
V o
l t a g e
Source voltage Capacitor voltage Diac breakover
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Firing Condition Actual
Capacitor voltage Vcn does not go into steady state AC right away asVan crosses the zero axis. There is a time delay due to the RC timeconstant.
The RC time constant delay plus phase shift of the AC solution for
Vcn determines the point at which the diac breakover is achieved
Van Vcn
= 90 16.67ms 4
Diac conducts when V cn reaches 32-35V (diac
breakover voltage). Thecapacitor then dischargesthrough the triac gate.
!
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Firing Condition Simulated(EE362L_Triac_Light_Dimmer.xls)
Fixed Potentiometer Diac DiacSource Freq R kohm C breakover on voltsVrms Hz kohm uF V V
70 60 3.3 49 0.1 35 5
Denom-mag2.210759
Denom-ang63.10652
VCrms31.66333
VCang-63.1065
Tau0.00523
-150
-100
-50
0
50
100
150
0 30 60 90 120 1 50 1 80 2 10 2 40 270 3 00 3 30 3 60
Angle
V o
l t a g e
Source voltage Capacitor voltage Diac breakover
EE362L_Triac_Light_Dimmer.xlsVan and V cn waveforms with
potentiometer adjusted for = 90
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0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
C u r r e n t
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 30 60 90 120 150 180
Alpha
P
22sin
12 ,2
, rmsanrmsabV V
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
C u r r e n
t
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
C u r r e n t
Power to Light (per unit of sinewave,i.e., = 0)
!
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Remember to Calibrate Your Scope Probe
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Measuring the dB difference between 60Hz and 180Hzcomponents of V ab
60Hz component
180Hz component
100Hz
Save screensnapshot #3
Vrms
V
dbHz
1log2081.3260
10
2081.32
60 101VrmsV Hz
Vrms
V db Hz
1log2087.26 18010
2087.26
180 101VrmsV Hz
= 43.7Vrms
= 22.1Vrms
Magnitude of Voltage Harmonics Depends on Alpha !