Transcript Light dimmer class notes

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