AMERICAN INTERNATIONAL UNIVERSITY-BANGLADESH (AIUB)

Faculty of Engineering

Department of Electrical and Electronic Engineering

Industrial Electronics and Drives- LAB

SECTION: F LAB REPORT NO: 03 DATE: 04/03/2021

TITLE: AC Voltage Controller (using DIAC and TRIAC).

Submitted by:

Group Members Name ID

SANJID ISLAM 18-38984-3

SHABUJ DASGUPTA AMI 18-37119-1

MD NUR NOBI 18-37137-1

A. G. M. WAZED PARVEJ 17-35726-3

Md. Mizanur Hasan 17-35654-3

Nasir Uddin 17-34568-2

Submitted To

MEHEDI HASASN

FACULTY OF ENGINEERING
DEPERTMENT OF EEE
Title: AC Voltage Controller (using DIAC and TRIAC).

Abstract:

In this experiment the Light dimmer characteristic using DIAC and TRIAC was explained. The
Objective of this experiment is to control AC voltage using DIAC and TRIAC. We have
implemented the basic TRIAC triggering circuit using DIAC. In this circuit, we noticed that
the light illumination is controlled using TRIAC and DIAC. The DIAC seemed as a
bidirectional triggered diode. The circuit we have implemented and was simulated later. The
simulated circuits run successfully and gave accurate waveform. But the practical implement
was not good at all due to human error. However, we managed to know the mechanism of
controlling TRIAC using DIAC through the simulation and theoretical study.

Introduction:

The term DIAC stands for the Diode for Alternating Current (DIAC), it is a bidirectional
semiconductor switch that can be turned ON in both forward and reverse direction. The device
is a member of the Thyristor family and it is mostly used in triggering TRIAC and other
Thyristor based circuits. Triac’s are electronic components that are widely used in AC power
control applications. They are able to switch high voltages and high levels of current, and over
both parts of an AC waveform. This makes triac circuits ideal for use in a variety of
applications where power switching is needed.
Light dimmer circuit is used to adjust the illumination of lamp. The circuit in figure 6 shows
the basic TRIAC triggering circuit using DIAC. Here in this circuit the light illumination is
controlled using TRIAC and DIAC. The DIAC is a bi-directional trigger diode used mainly in
firing triac’s and thyristor’s in AC control circuits. [1]

Theory and Methodology:

DIAC (Diode for Alternating Current):

A DIAC is a three-layer, two-junction, two terminal (labeled A1 and A2) bidirectional
semiconductor device which can be switched from OFF state to ON state for either polarity of
applied voltage. If a positive or negative voltage is applied across the terminals, only a small
amount of leakage current flow through it. As the applied voltage is increased, the leakage
current will continue to flow until the voltage reaches the break over voltage VBO. At this point
avalanche breakdown occurs and the device exhibits negative resistance, i.e. current through
the device increases with the decreasing values of applied voltage.

Fig 1(a): Symbol of a DIAC. Fig 1(b): Basic structure of a DIAC. Fig 1(c): DIAC.[2]
 Fig. 2: I-V characteristics of DIAC. [2]

TRIAC (Triode AC switch):

A TRIAC is a 3 terminal semiconductor switching device which can control alternating

current in a load. A Triac behaves just like two conventional SCRs connected together in
inverse parallel (back-to-back) with respect to each other and because of this arrangement
the two SCRs share a common Gate terminal all within a single three-terminal package. Its
characteristics in I (First) and III (Third) quadrants are essentially identical to those of an
SCR in the I quadrant. The TRIAC can be operated with either positive or negative gate
control voltage, but in normal operation usually the gate voltage is positive in I quadrant
and negative in III quadrant. The supply voltage at which the TRIAC is turned ON depends
upon the gate current. The greater the gate current, the smaller the supply voltage at which
the TRIAC is turned on.

Since a triac conducts in both directions of a sinusoidal waveform, the concept of an Anode
terminal and a Cathode terminal used to identify the main power terminals of a SCR are
replaced with identifications of: MT1, for Main Terminal 1 and MT2, for Main Terminal 2
with the Gate terminal G referenced the same. [2]

Fig 3(a): Symbol of a TRIAC Fig 3(b): Equivalent circuit of Fig 3(c): Internal construction
a TRIAC
aaaaa

I-V characteristics of TRIAC:

Fig. 5: I-V Characteristics of TRIAC [4]

Apparatus:
1. DIAC [BTA134 (1)] 7. Capacitor [0.02uF]
2. Trainer Board 8. Transformer [220V:12V]
3. Regular power supply 9. Light
4. Oscilloscope
5. Resistors [1.0K(1)]
6. POT [100K]
Precautions:
The circuit setup contains 220V. Students should be cautious about this.

Circuit Diagram:

Fig 6: Complete experimental setup (Light dimmer circuit). [2]

Experimental Procedure:

1. The circuit was constructed as shown in Figure 6.

2. The wave shapes were drawn across TRIAC and lamp.
3. The output voltages was measured across the lamp and Complete table 1.
4. The graphical representation of the conduction angle (180o-delay angle) vs. output
voltages was shown.

Simulation and Measurement:

Fig 7: Complete experimental setup (Light dimmer circuit [6]

 Result /Output graph:

Fig 8: Output of Simulation work. [6]

Experimental Data:

NO Value of R2 (KΩ) Delay angle (Degree) Conduction Output (Vp)

angle (Degree)
1 100 12 75 45
2 100 12 75 45

Questions for report writing: [5]

1. Illustrate the basic differences in between DIAC and TRIAC along with the I-V
characteristics.

Ans: TRIAC

As we know, thyristor is a half wave device similar to the diode and hence will provide only
half power. A Triac device consists of two thyristor’s connected in parallel but in opposite
directions and is controlled by the same gate.
Triac is a two dimensional thyristor which is triggered on both halves of the input a.c. cycle
using positive or negative gate pulses. Triac connecting terminals are MT1, MT2 and gate (G)
as shown. The triggering pulses are applied between G and MT1 terminals. The gate current to
handle 100A from triac is not more than 50mA or so.

DIAC

Diac is made of two zener diodes connected back to back. One of the application of these
devices is shown in the figure-3 as lamp dimmer circuit. It conducts in either direction when
the voltage across C1 reaches it breakdown value of approx. 30 volt. A current pulse form C1
triggers the triac and power is supplied to the load (here lamp). The conducting period of triac
depends on value R1. The more is the value of R1, C1 will get charged very slowly. This makes
triac turn on very late in the cycle and hence light will be dimmer.

2. Why a DIAC is called a bidirectional device?

Ans: Because they will conduct in either direction, if the breakdown voltage is reached.

3. Does the DIAC conduct equally during the positive & negative alternations?

Ans: Yes, it is a bidirectional device. They are symmetrical.

4. When is the DIAC turned - on?

Ans: A DIAC behaves like many gas discharge tubes. It does not conduct until a ~high voltage
is reached where it "fires" and remains conducting until the current drops to a very low value.
Polarity doesn't matter. Typically used to trigger a triac from a RC delay, the Diac dumps the
capacitor into the triac gate at some phase after 0° and 180°. Because the Diac waits for the
capacitor voltage to build up, the triac can be triggered with a high value resistor and small
capacitor, ie low power.
5. What is the main function of TRIAC?

Ans: TRIAC, from Triode for Alternating Current, is a generalized trade name for an electronic
component that can conduct current in either direction when it is triggered
6. How is a TRIAC equivalent to two anti-parallel SCRs? Is it exact anti-parallel?

Ans: No, a triac only has 3 terminals and they can be triggered by gate currents in either
direction regardless of the anode to cathode voltage.

7. How do you get the phase controlled ac output voltage in this experiment?

Ans: TRIACs directionality makes them convenient switches for alternating-current (AC). In
addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows
control of the average current flowing into a load (phase control). This is commonly used for
controlling the speed of induction motors, dimming lamps, and controlling electric heaters.

8. Explain why a TRIAC can replace a SCR but a SCR cannot replace a TRIAC?

Ans: TRIAC is equivalent to two SCR join together back to back in reverse polarity from each
other. Therefore, TRIAC is a bidirectional parts whereas SCR is unit directional parts

9. Why do not we use UJT relaxation oscillator instead of DIAC?

Ans: UJTs operate at relatively low voltages, and diacs operate at higher voltages. Also the on
resistance of UJTs is much larger than a diac.

Analysis:
In this experiment we have successfully learn about AC voltage controller using DIAC and
TRIAC. The circuit was constructed carefully. The apparatus are carefully used. By using an
oscilloscope the output shape was gotten. The output shape was similar to our expected
shape. On the whole all the things were learnt.

Conclusion:
In conclusion we can easily say that our motive or objective of the experiment is full filled and
we prove by the experimental and simulated graph and other values.

References:

[1] https://electronicspost.com/draw-and-explain-the-v-i-characteristics-of-an-diac and triac/

[2] American International University IED Lab Manual-3.

[3] “Introduction to Power electronics”. Dr. Robert Erickson, University of Colorado Boulder
and University of Colorado System.

[4] http://www.academia.edu/15381333/EXPERIMENT_IV_SILICONCONTROLLED_
REC TIFIER.

[5] https://forums.ni.com/t5/Multisim-and-Ultiboard/Thyristor-SCR-testing-
circuit/td- p/2281842.

[6] Multisim software.