Download File - Go ELECTRONICS

Chapter no.5
Thyristor
application &
photosensitive
control circuits
Contents:










Ac power control using TRIAC.
Light dimmer.
Automatic battery charger.
Emergency light system.
Temperature controller.
Fan speed regulator.
Opto coupler.
Burglar alarm .
Batch counter.
Smoke detector.
Ac phase control using triac:
Phase control is also called as firing angle control. so phase
control is basically the control of firing angle of the triac. The
phase control is used to precisely control the amount of
power delivered to the load such as fans or lamp load etc..
Operation:
Operation in positive half cycle:
 In the positive half cycle , live point (L) is positive
with respect to the neutral point (N).
 The charging current for the capacitor C₁ flows
through R₁ as shown in figure.
 As soon as the voltage across C₁ reaches the
break-over voltage of the Diac, it is turn on to
supply gate current for the Triad and the Triad
will be turned on.
 The conducting Triac is equivalent to a closed
switch. So the R₁C₁ and Diac short circuited.
The load voltage is equal to the instantaneous
supply voltage.
Operation in negative half cycle:






In the negative half cycle, the live point (L) is negative with
respect to the neutral (N).
The Triac and Diac both are in the off state. The charging current
for the capacitor C₁ flows through R₁ as shown in fig.
Voltage on C₁ is now negative. As soon as this voltage reaches
the break-over voltage of the Diac, it is turned on and supplies
gate current to the Triac.
The Triac is then turned on. The Diac and R₁C₁ circuit is short
circuited.
The load current reverses its direction and the voltage across
the load will be negative equal to the instantaneous ac supply
voltage.
Thus Diac being a bi-directional device can turn on the Triac in
both the half cycles of the input ac supply. The capacitor C₁ must
be a non-polarized capacitor, being capable of charging to
positive as well as negative voltages.
The firing angle (α) i.e. the instant at which the Triac is
turned on in the positive as well as negative half cycles of
the ac supply voltage can be controlled by making the
resistance R₁ variable.
 This variable resistance would then decide the charging
rate of capacitor C₁ and hence firing angle (α) in each half
cycle. As the control circuit operates directly on AC supply,
it is automatically synchronized with the supply. The load
voltage waveforms for a resistive load are shown above.

Light dimmer using triac:

Light dimmer
using triac:
Battery charger:
Waveforms of
battery charger:
Operation:







The input transformer T is a step down transformer reduces the
230V AC mains 15V.
The secondary voltage rectified by the full wave rectifier circuit.
The zener diode Z1 maintains a constant voltage 15V, at point “x”
The rectifier voltage waveform at “A” .The dotted line in this figure
indicates the battery voltage.
When voltage at point A is greater than the battery voltage the
SCR1 is forward biased and can conduct if the gate junction is a
forward biased.
Thus SCR1 conducts from P to R as shown in figure and charges 12
volt battery connected in the circuit.
As the battery accumulates more and more charge, the dotted line
goes up and the point P and R come closer to Q in figure, thus a
reducing the conduction time for SCR1, and hence increasing the
charging time , of battery.
When the battery is fully charged say about 14 volts the cathode of
SCR1 is at 14V and the gate is at 14.3V. This difference of 0.3V
between the gate and cathode can not forward biased as gate
junction and will not be triggered. Thus the battery is cut off from
the supply and charging will stop automatically.
Emergency light system:
The basic emergency lightning system is as shown in figure. This
system includes the facility to charge the 6 volt battery and switches
automatically from the AC supply failure takes place.
Operation:
Mode 1 (when AC supply is on) :
The diode D1and D2 along with the center tap transformer
T1 from full wave rectifier. They provide DC voltage for the 6V lamp
load when the AC supply is ON. Diode D3 and R1 supply the battery
charging current which can be varied by R1. The anode gate of SCR1
is kept at the battery voltage. While the cathode of SCR1 is kept at a
higher potential by C1. Therefore as long as the AC supply is ON, the
SCR1 remains reverse biased.
Mode 2 (when AC supply is OFF):
As soon as the AC supply is interrupted, the output of the
rectifier formed by D1 and D2 goes to zero. The cathode potential of
SCR1 falls below battery voltage. The gate current is supplied to
SCR1 through R3 and the SCR is triggered. This connects the 6v
battery across the lamp. When the AC input reappears the SCR1 is
turned OFF automatically and the charging of the battery will begin.
Temperature controller:
Phase control circuits may also be used for regulating
temperature. Fig. shows the connection diagram of temperature
controller.
Operation:








It is a simple full-wave phase control circuit.
By adjusting resistance R₁ and pot R₂ we can fix the resistance
temperature for the load.
Z₁ is a zener diode which gives a fixed voltage across it. this voltage
appears across the thermistor R₄.
When the voltage across the thermistor R₄ is sufficient to charge
the capacitor C₁ to a voltage equal to or more than the break-over
voltage of the Dias, the Dias triggered and sends a trigger pulse to
the gate of the Triac.
The Triac starts conducting, thus connecting the heater element in
the circuit.
As the temperature increases, the thermistor resistance decreases
and as such, the voltage across capacitor C₁ is reduced. This
increases the firing angle of the Triac thus reducing the voltage
across the heater element accordingly and consequently reduction
of heat takes place.
Gradually, a stage comes when the voltage across the capacitor C₁
becomes insufficient to trigger the Diac and the Triac is
automatically switched off.
This results in the disconnection of the heater element from the
circuit.

R1 is limiting resistor.

The function of Z₁ is to avoid the effect of the supply fluctuations
on the performance of the circuit.

For a 220V, 50Hz A.C. supply, R₁ may be chosen 47k, 2W and the
capacitor C₁ of 0.1μF value.

The rating of zener diode Z₁ may be decided by the break-over
voltage of the Dias. The Triac specification depends on the load to
be controlled.
Fan speed regulator using triac:


The fan motor is a single phase induction motor, the speed of
which depends on the rms value of voltage applied to it.
Fig shows how a triac can be used to control the speed of fan
motor.
Operation:








In both the half cycle of the input supply voltage, the capacitor C will
charge through the resistor R.
As soon as the voltage across the capacitor reaches the breakdown
voltage of the diac, a triggering pulse is generated and the triac is
turned on.
As soon as the triac is turned on, the voltage between points A and N
will drop down to on state voltage drop of the triac.
The voltage on the C is negligibly small .The capacitor can charge again
after the triac is turned off.
The charging rate of the capacitor C is decided by the variable resistor
R.
If the value R is low, the capacitor will charge to the diac breakdown
voltage in less time, that is the firing angle will be small, the rms voltage
applied to the motor will be higher and fan speed increases.
With increase in the value of R, the capacitor will charge slowly
increasing the firing angle (∝)of the triac thus reducing the speed.
In this way, the fan speed can be regulated by controlling the firing
angel of the triac in both half cycles.
Opto-coupler:
In electronics, an opto-isolator (or optical isolator, optocoupler,
photocoupler, or photoMOS) is a device that uses a short optical
transmission path to transfer a signal between elements of a circuit,
typically a transmitter and a receiver, while keeping them electrically
isolated — since the signal goes from an electrical signal to an optical
signal back to an electrical signal, electrical contact along the path is
broken.
Operation:




A common implementation involves a LED and a phototransistor,
separated so that light may travel across a barrier but electrical
current may not. When an electrical signal is applied to the input of
the opto-isolator, its LED lights, its light sensor activates, and a
corresponding electrical signal generated at the output. Unlike a
transformer, the opto-isolator allows for DC coupling and provides
significant protection from serious overvoltage conditions in one
circuit affecting the other.
With a photodiode as the detector, the output current is
proportional to the amount of incident light supplied by the
emitter. The diode can be used in a photovoltaic mode or a
photoconductive mode.
In photovoltaic mode, the diode acts like a current source in
parallel with a forward-biased diode. The output current and
voltage are dependent on the load impedance and light intensity.
In photoconductive mode, the diode is connected to a supply
voltage, and the magnitude of the current conducted is directly
proportional to the intensity of light.






An opto-isolator can also be constructed using a small incandescent
lamp in place of the LED; because the lamp has a much slower
response time than an LED, will filter out noise or half-wave power in
the input signal. In so doing, it will also filter out any audio- or higherfrequency signals in the input. It has the further disadvantage, of course,
(an overwhelming disadvantage in most applications) that incandescent
lamps have finite life spans.
The optical path may be air or a dielectric waveguide. The transmitting
and receiving elements of an optical isolator may be contained within a
single compact module, for mounting, for example, on a circuit board;
in this case, the module is often called an opto-isolator or optoisolator.
The photosensor may be a photocell, phototransistor, or an optically
triggered SCR or Triac.
An opto-coupler, also called opto-isolator, is an electronic component
that transfers an electrical signal or voltage from one part of a circuit
to another, or from one circuit to another, while electrically isolating
the two circuits from each other.
It consists of an infrared emitting LED chip that is optically in-line with
a light-sensitive silicon semiconductor chip, all enclosed in the same
package.
The silicon chip could be in the form of a photo diode, photo
transistor, photo Darlington, or photo SCR.
Burglar alarm:
Fig. shows simple burglar alarm circuit which makes enclosure
or on the door for the purpose of protection against burglary ,
scr is connected in the circuit as shown in fig. limiting resistor R
and the micro switch are connected at the gate of the scr. The
micro (door) switch remains in the OFF position when the
door is closed, whereas the switch is put on automatically when
the door is opened. With the reset switch S closed, working of
the circuit as follows:
Operation:
Condition (1):
when the door is closed. The d.c. supply is available at
input terminals of the alarm. The micro switch being in the off
position, does not allow any signal at the gate of the scr. The scr
does not conduct in this condition and therefore return path is
not available for the current to flow through the alarm and the
alarm is not energized.

Condition (2):
when the door is opened. micro switch in this
condition, being in the ON position allows the required signal
at the scr gate. Resistor R is for limiting the current. As soon as
,the gate signal is received, the scr starts conducting and the
alarm is energized. For putting the alarm off, the scr is deenergized by opening the reset switch s. the alarm may be
replaced by lamp, as shown in the dia.

Batch counter:
Batch counter is special purpose counter, which
is used to count the number of opaque object moving
on a conveyer belt.The block diagram of a batch
counter system is shown in fig.
Operation:
This system is used for counting the number of
objects that are passing on a conveyer belt.
 A light beam is focused continuously on LDR and the
reverse photocurrent is flowing through it.
 As soon as an object passes and interrupts the beam
of light, the photocurrent flowing through the LDR
will reduce to zero.
 Hence corresponding to every object passing through
we get low going pulse as shown in fig.
 An electronic counter counts these low going pulse
and displays them on seven segment display.
 As each counted pulse correspond to an object, the
displayed number corresponds to the number of
objects.

Smoke detector:
Operation:








Operation in presence of smoke:
When the smoke is present between the light source and LDR, it
interrupt the light falling on LDR. The resistance of LDR increases.
This increase the base voltage of Q1 and turn it on. The collector
voltage of Q1 now reduce to a low value.
Due to this the base voltage of Q2 will reduce to a very low value and
Q2 is turn off.
The relay is de-energized and the N.C. contact is closed to connect
supply to the alarm and the alarm will be operated to indicate the
presence of smoke.
Operation in absence of smoke:
When the smoke is not detected, the light from the light source falls
on LDR without any interruption. The resistance of LDR is low and the
base voltage of transistor Q1 is low.
i.e. Q1 is in the off state. This raises the collector voltage of Q1 to Vcc
Due to high collector voltage of Q1, the base of Q2 receives a
sufficiently high voltage and Q2 is turned on.
The collector current of Q2 flows through the relay coil to energies
the relay. The N.C. contact of the relay is open circuited and the alarm
does not sound.