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EE462L, Spring 2013
Triac Light Dimmer
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Triac Light Dimmer
Triac
(front view)
MT1 MT2 G
+
Van
(from Variac)
Light
bulb
G
MT2
MT1
0.1F
3.3k
250k
linear
pot
Triac
Bilateral
trigger diode
(diac)
a
c
n
b
Light
bulba
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 3.3k
resistor is ignored in this diagram.
+ 0V+
Van
+
Van
Ingenious
Simple
Efficient
Inexpensive
After firing, the triac is a closed
switch, so that practically all of Van
is applied across the light bulb.
Light
bulba
n
b
+ Van+
Van
+
0V
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Triac Open
Light bulb resistance is a few ohms when cold, and about 100-200
when bright (use to get light bulb resistance R)RVP rms /2
The light bulb resistance is small compared to the 3.3k and
potentiometer combination and can be ignored when analyzing the
RC electronic circuit
When the voltage across thediac 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
+
Van
(from Variac)
Light
bulb
0.1F
3.3k
250k
linear
pot Bilateral
trigger diode
(diac)
Triac Closed
+
Van
(from Variac)
Light
bulb
0.1F
3.3k
250k
linear
pot Bilateral
trigger diode
(diac)
!
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Flat rubber washers between
#8 x 1 screws and porcelain
Remove
this
center
screw
#6-32, machine screw,
flat washer, split washer, and
hex nut
#8 x 3/4
screws for
terminal blocks
#8 x
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
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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
9.6C 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 into
isolation transformer)
Light dimmer
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Connection to Variac
Light dimmer
connected to black
and white terminalposts
Variac knob set to zero
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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 90 for large
potentiometer resistance
!
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No-Firing Condition Simulated
(EE362L_Triac_Light_Dimmer.xls)
Fixed Potentiometer Diac Diac
Source Freq R kohm C breakover on volts
Vrms Hz kohm uF V V
70 60 3.3 96 0.1 35 5
Denom-mag
3.874785
Denom-ang75.0439
VCrms
18.06552
VCang
-75.0439
Tau
0.00993
-150
-100
-50
0
50
100
150
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
Voltage
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 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
VanVcn
= 90 16.67ms 4
Diac conducts when Vcnreaches 32-35V (diac
breakover voltage). The
capacitor then discharges
through the triac gate.
!
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Firing Condition Simulated
(EE362L_Triac_Light_Dimmer.xls)
Fixed Potentiometer Diac Diac
Source Freq R kohm C breakover on volts
Vrms Hz kohm uF V V
70 60 3.3 49 0.1 35 5
Denom-mag
2.210759
Denom-ang63.10652
VCrms
31.66333
VCang
-63.1065
Tau
0.00523
-150
-100
-50
0
50
100
150
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
Voltage
Source voltage Capacitor voltage Diac breakover
EE362L_Triac_Light_Dimmer.xls
Van and Vcn waveforms with
potentiometer adjusted for = 90
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0 3 0 6 0 9 0 1 20 1 50 1 80 21 0 24 0 2 70 3 00 33 0 36 0
Angle
C
urrent
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
2
2sin12 ,
2, rmsanrmsab
VV
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
Curren
t
0 30 60 90 120 150 180 210 240 270 300 330 360
Angle
Current
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 180Hz
components of Vab
60Hz component
180Hz component
100Hz
Save screensnapshot #3
Vrms
V
db
Hz
1log2081.3260
10
20
81.32
60 101VrmsV Hz
Vrms
Vdb
Hz
1log2087.26 18010
2087.26
180 101VrmsV Hz
= 43.7Vrms
= 22.1Vrms
Magnitude of Voltage Harmonics Depends on Alpha!