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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!