Page 70 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Power control circuit using SCR,DIAC,TRIAC & IGBT

Objectives: At the end of this lesson you shall be able to
• explain the construction and working of SCR,DIAC,TRIAC & IGBT
• explain power control circuits using SCR
• explain power control circuit using DIAC & TRIAC
• explain the construction and using of IGBT.

Introduction to power electronics devices

Industrial electronics is concerned primarily with electronics applied to industries such as industrial equipments,
controls and processes. An important application of electronics in industries is in controlling of machinery.
In communication electronics, domestic & entertainment electronics, generally, the electronic devices operate
with currents in the order of Microampere to Milliampere. For industrial applications, most frequently, devices
are required to handle currents in the range of ampere to several thousands of ampere. This, therefore calls for
high power electronic devices. One such high power electronic device frequently used in industrial electronic
application is the SCR,TRIAC,IGBT and DIAC for associate triggering circuits.

This devices can be used to run, dc motors from an ac power source, control power tool speed,also to control
motor speeds of small appliances like, mixers and food blenders, illumination control, temperature control and
so on.
Silicon Controlled Rectifier (SCR)
Before Silicon controlled rectifiers were invented(1956), a glass tube device called Thyratron was used for high
power applications. Silicon Controlled Rectifier (SCR) is the first device of the thyristor family. The term thyristor
is coined from the expression Thyratron-transistor. SCR is a semiconductor device. SCR does the function of
controlled rectification. Unlike a rectifier diode, SCR has an additional terminal called the gate which controls the
rectification(gated silicon rectifier).
The basic principle application of SCRs is to control the amount of power delivered to a load(motor, lamp, etc.,).A
rectifier diode will have one PN junction. SCRs on the other hand will have two PN junctions (P-N-P-N layers).
Fig 1 shows the electrical symbol, basic construction and a typical SCR packages.

Fig 1

Basic operation of SCR
When a gate direct current is applied to the gate terminal, forward current conduction commences in the
SCR(latched into conduction). When the gate current is removed, the forward current through the SCR does not
cut-off. This means, once the SCR is latched into conduction, the gate loses control over the conduction. The
current through the SCR can be turned off only by reducing the current through it(load current) below a critical
value called the Holding current.

Fig 2 shows how an SCR can be gated into conduction or turned off.
In Fig 2a, with switched S1 open the SCR is in OFF state and no current is flowing through the load.
In Fig 2b, when S1 is closed, a small gate current(around 1/1000 or less compared to load current) turns-ON
(fires) the SCR. A heavy load current starts flowing through the SCR and load RL.

57

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