Page 74 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Semi-conductor based light dimmers: Two light dimmers for incandescent light bulbs are discussed below.
Both these dimmer circuits control light intensity by adjusting the angle of conduction of a triac connected in series
with the bulb. The first dimmer uses a very simple circuit that is ideal for highly compact applications requiring
minimum cost. The second dimmer features soft starting for low in rush current and consequent long lamp life.
Soft start lamp dimmers are especially useful with expensive lights with short lives, such as projection lamps and
photo-graphic bulbs.
Simple light dimmer: The circuit shown in Fig 5 is a wide range light dimmer using very few parts. The circuit can
be operated using any mains supply source (240V, 50Hz) by choosing appropriate value of circuit components.
The circuit can control upto 1000watts of power to incandescent bulbs.
The power to the bulbs is varied by controlling the conduction angle of Triac. Many circuits can be used for phase
control, but the single Triac circuit used is the simplest and is therefore chosen for this particular application.
The control circuit for this Triac must function as shown in Fig 5b. The control circuit must create a delay between
the time voltage is applied to the circuit and the time it is applied to the load. The Triac is triggered after this
delay and conducts current through the load for the remaining part of each alteration. This circuit can control the
conduction angle from 0° to about 170° and provides better than 97% of full power control.

Fig 5

Light dimmer with soft-start option: The circuit at Fig 6 is a light dimmer with soft start option. Soft starting is
desirable because of the very low resistance of a cold lamp filament compared to its hot resistance. At the time of
initial switching on, the low resistance of the lamp causes very high inrush currents which leads to short filament/
lamp life. Lamp failures caused by high inrush currents is eliminated by the soft start feature, which applies
current to the bulb slowly enough to eliminate high surges.

Operation of the circuit at Fig 6 begins when voltage is applied to the diode bridge consisting of D through D .
4
1
The bridge rectifies the input and applies a dc voltage to resistor R and zener diode D . The zener provides a
1
5
constant voltage of 20volts to unijunction transistor Q , except at the end of each alternation when the line voltage
1
drops to zero. Initially the voltage across capacitor C is zero and capacitor C cannot charge to trigger Q . C
2
1
1
1
will begin to charge, but because the voltage is low, C will have adequate voltage to trigger Q1 only near the end
2
of the half cycle. Although the lamp resistance is low at this time, the voltage applied to the lamp is low and the
inrush current is small. Then the voltage on C rises, allowing C to trigger Q earlier in the cycle.
1
2
1
At the same time the lamp is being heated by slowly increasing applied voltage and by the time the peak voltage
applied to the lamp has its maximum value, the bulb has been heated sufficiently so that the peak inrush current
is kept to a reasonable value.
Resistor R4 controls the charging rate of C2 and provides the means to dim the lamp. Power to the load can be
adjusted manually by varying the resistance of R4. T1 is a pulse transformer. In addition to supplying the trigger
to Triac, this transformer isolates the high current load circuit from the low power triggering circuit (gate isolation
methods for Triac is discussed in further paragraphs).
A simple lamp dimmer cum Universal motor speed controller: In the lamp dimmer cum universal speed
controller circuit is in Fig 7, a Triac is used as control device. Phase control technique is used to control conduction
angle of the triac which inturn control the power fed to the lamp.
A lamp L is connected in series with ac mains supply to the Triac. The trigger pulses to Triac gate is given through
Diac. The Diac is triggered at the same breakover voltage level (30V) during both positive and negative half
cycles.

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CITS : Power - Electrician & Wireman - Lesson 60-69