30-01-2013, 12:03 PM
EE362L, Power Electronics Triac Light Dimmer
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Abstract
This document presents the construction of a light
dimmer circuit that utilizes the current-regulating properties of a
triac. Prior to being triggered, the triac provides a barrier in the
circuit, preventing current flow from a 70V AC source through
the light bulb. During this time voltage across a capacitor within
in the circuit builds up until it exceeds the breakover voltage of a
diac. Once the breakover voltage is exceeded, the diac “fires” the
triac into a conducting state and current flows through the light
bulb. The amount of voltage seen over the light bulb is
determined by the firing angle of the triac which is set by the RC
time constant of the circuit. This process then repeats every half
cycle. Finally, it can be seen that this switching introduces high
order harmonics into the system.
INTRODUCTION
This document presents the construction, operation, and
testing of a light dimmer circuit. Using simple devices
such as a diac and a triac, power flow is regulated to a light
bulb by intermittently applying a 70V AC source across the
load during each half cycle. The intensity of the light is
determined by the proportion of the sine wave that is applied
across the load. This document contains the circuit operation,
testing results, and the observations that were made during the
experiment.
LIGHT DIMMER CIRCUIT OPERATIONS
Figure 1 shows the construction of the light dimmer circuit
with the triac. The circuit is designed such that only a portion
of the 60Hz sine wave from the Variac passes through the
light bulb. To accomplish this, a bilateral diac is used to
control the current flow to the triac. Initially the capacitor is
discharged, and during each half cycle of the input from the
Variac, Van, the capacitor is charged to its breakover voltage.
When this occurs the diac begins to conduct and discharges
the capacitor through the triac gate. This point in time is
known as firing and it allows the voltage on Van to be applied
to the light bulb. For most of this experiment the Variac was
set to 70V AC.
RESULTS
To ensure the safety of ourselves and those around us, all
experiments were performed using an isolation transformer
between the wall outlet and the Variac.
For the first part of the experiment, the potentiometer was
set for full brightness, or minimum resistance. As discussed
earlier, this corresponds to a minimum RC time constant, and
thus allows the greatest portion of the input wave to be seen
across the bulb. At this point the minimum firing angle, α,
could be determined. A plot of Vab vs time can be seen in
Figure 4 below.
CONCLUSION
We have successfully demonstrated the operation of the
triac light dimmer circuit. With this circuit we have shown
that one can regulate the intensity of a light using an AC wall
outlet as a source, and varying the RC time constant used to
determine the point in time that the triac conducts the AC
wave through the light during each half cycle. In addition we
have demonstrated the concept of a firing angle, α, and its
relation to the RMS voltage applied to the light bulb. The one
main drawback of this circuit is the vast amount of harmonics
that are present in the system, however for its simplicity, the
design is quite impressive.