A tester is provided in which a test signal having a selected polarity and magnitude is applied across the main electrodes of a device under test. In the case of an SCR and TRIAC device under test, for example, pulses of a predetermined energy level and polarity are applied to the gate electrode to trigger the SCR or TRIAC into conduction. The tester operates to identify the anode and cathode terminals of the conducting device, as well as to determine without destructive failure of the device under test whether the device is open, shorted or exhibiting leakage. 

 

CHAPTER ONE
1.0                                                        INTRODUCTION
The silicon controlled rectifier and the triac are both solid-state electronic components that turn electrical currents on and off. Unlike some switches, which return to a stable "off" state, SCRs and triacs "latch" on or off, and remain that way until certain conditions change. Because of their switching and latching actions, both devices are called thyristors. While they have many similarities, important differences exist between their operation and use.
The thyristor or as it is otherwise called scr (silicon controlled rectifier) is a silicon diode which consists of three or more pn junctions and can either be conductive (on) or not (off).
There are many different types of thyristors but the most common of all is the one which has three leads, the anode (a) the cathode (k) and the gate (g). The gate is what makes the thyristor so special because a relatively small current through its gate can control much heavier currents which pass through the device itself.
In general if there is a voltage across the anode and the cathode of a  thyristor and we apply a low positive voltage at the gate the thyristor will turn on and even if we remove the trigger voltage it will stay on till the current which flows through it, is somehow interrupted. as you see the

Thyristor acts as an electronic switch. As it has already been mentioned the thyristor consists of a series of pn junctions and this gives it some of the characteristics of a silicon diode. From the moment the thyristor becomes conductive it behaves as a silicon rectifier and only allows current flow in one direction. Thyristors are used today in many devices such as ac rectifiers, dimmers, electronic car ignition circuits, overload protection devices, logic circuits, and also in industrial applications where they are used to control heavy currents with very much lower control currents.
The thyristor as we have already mentioned is sometimes referred at by the initials scr and its electronic symbol is the one you see in diagram 1. another similar to the thyristor device, is the triac. Its name comes from the tri-ode for alternating current. The triac as opposed to the thyristor will let the current flow in both directions without departing very much from reality. The triac can be considered as being two thyristors connected in parallel reversed with respect to each other and having a common gate.

The triac can be turned on by the application of either positive or negative pulses in its gate. Because of their ability to control ac the triacs are used in phase control to regulate the speed of ac motors, in dimmers, in heating applications and wherever it is needed to control ac current without rectifying it.
Occasionally   with these components   doubt arises we work correctly, especially when we are repairing an appliance and also sometimes when we try to recover any items of equipment   already disused or damaged.
The problem is that with the multimeter   We can only check if shorted or open not know   you switch   correctly   in the case of thyristors is unidirectional check   (CC)   and checking the triac   (CA) is bidirectional because the latter are designed to work with alternating current. The aim of this work is to design such an electronics instrument that measures the polarity as well as knowing the working condition of triac and scr.
In the testing of semiconductor SCR devices and triac, the testers apply a triggering signal or pulse to the gate electrode for turning on the device. However, when the polarity of the device and the power dissipating ability of the device are unknown, prior art testers tend to provide excessive power to electrodes of the device under test, destructively failing the device. Since the various devices to be tested have an unknown internal impedance, prior art testers tend to cause devices with relatively low internal impedance to draw excessive current burning out the device which otherwise might have been in acceptable condition. A tester of the type with which the focus of this is concerned should be able to identify not only the polarity of the electrodes of various devices under test whose characteristics are unknown, but also whether or not the devices meet or exceed certain minimum characteristics without destructive failure of the devices.
1.1                                   AIM AND OBJECTIVE OF THE PROJECT
The main aim of this work is to design an electronics instrument for testing THYRISTORS and TRIACS.
At the end of this work students involved will be able to:

1. Explain how an RC phase-shifting network can control the load current in an SCR circuit.
2.  Explain the differences and similarities between an SCR and a triac. • Explain the different operating modes of a triac and scr.
3. Explain the construction, operation, and applications of a scr and triac tester

1.2                                              PURPOSE OF THE PROJECT
The purpose of this work is design a tester that is capable of non-destructively testing semiconductor devices such as SCRs and Triacs.

1.3                                         SIGNIFICANCE OF THE PROJECT
This tester will put the component in a circuit which simulates its working conditions and will apply the right biasing voltages to trigger it and will also pass a current through the device to leave no doubt that it is working properly.

1.4                                                 SCOPE OF THE PROJECT
A tester for thyrister and triac semiconductor devices having two main electrodes and a control electrode includes first, second, third terminals adapted to be connected respectively to the main electrodes and the control electrode, and to a power source.
First switch means are connected between the source and the first and second terminals for selectively setting the value and the polarity of electrical energy provided from the source to the first and second terminals. Second switch means connect the source with the third terminal to determine selectively the polarity and value of electrical energy provided from the source to the third terminal. Indicating means are connected with the first and second terminals to indicate the presence of current flow upon the first and second switch means being set to provide a value and polarity of the electrical energy to the terminals according to the acceptable operating parameters of the particular device under test. The absence of the indication upon the switch means being set to provide the last-mentioned value and polarity of the electrical energy to the terminals indicates a failed device.

1.5                                          APPLICATION OF THE PROJECT
This instrument is mostly used

1. Electronics laboratory
2. Electronics hobbyist
3. And in industries for testing and troubleshooting electronics circuits such as motor circuits

1.6                                  DIFFERENCE BETWEEN SCR AND TRIAC
Following are main difference between SCR (Silicon controlled rectifier) and TRIAC (Triode for alternating current) :

SCR TRIAC SCR stands for silicon controlled rectifier TRIAC stands for triode for alternating current. The SCR is unidirectional device. The TRIAC is bidirectional device. It available in large ratings. It available in smaller ratings. The SCR control DC power. The TRIAC control DC as well as AC power. The SCR can be triggered by positive gate voltage only The TRIAC control DC as well as AC power. In SCR only one mode of operation is possible In TRIAC  four different modes of operation is possible.   1.7                          PROJECT WORK ORGANISATION The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows: Chapter one of this work is on the introduction to the study. In this chapter, the background, significance, aim/objective, scope, application, purpose of the study were discussed. Chapter two is on literature review of this study. In this chapter, all the literature pertaining to this work was reviewed. Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed. Chapter four is on testing analysis. All testing that result accurate functionality was analyzed. Chapter five is on conclusion, recommendation and references.

 

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