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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 !