TRIAC: A Three Terminal Semiconductor Device for Controlling Current
I. Introduction
a. Back ground of the Study
Triac
The TRIAC is a three terminal semiconductor device for controlling current. It gains its name from the term TRIode for AlternatingCurrent.
It is effectively a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the TRIAC is a bidirectional device.
TRIAC
The TRIAC is an ideal device to use for AC switching applications because it can control the current flow over both halves of an alternating cycle. A thyristor is only able to control them over one half of a cycle. During the remaining half no conduction occurs and accordingly only half the waveform can be utilised.
Typical / idealised TRIAC & thyristor switching waveforms
The fact that the TRIAC can be used to control current switching on both halves of an alternating waveform allows much better power utilisation. However the TRIAC is not always as convenient for some high power applications where its switching is more difficult.
TRIAC symbol
The circuit symbol recognises the way in which the TRIAC operates. Seen from the outside it may be viewed as two back to back thyristors and this is what the circuit symbol indicates.
TRIAC symbol for circuit diagrams
On the TRIAC symbol there are three terminals. These are the Gate and two other terminals are often referred to as an "Anode" or "Main Terminal". As the TRIAC has two of these they are labelled either Anode 1 and Anode 2 or Main Terminal, MT1 and MT2
LDR
Light dependent resistor symbol
The circuit symbol used for the light dependent resistor or photoresistor combines its resistor action while indicating that it is sensitive to light. The basic light dependent resistor symbol has the rectangle used to indicate its resistor action, and then has two incoming arrows - the same as those used for photodiodes and phototransistors to indicate its light sensitivity.
Light dependent resistor symbol used in circuit diagrams
For most
a. Back ground of the Study
Triac
The TRIAC is a three terminal semiconductor device for controlling current. It gains its name from the term TRIode for AlternatingCurrent.
It is effectively a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the TRIAC is a bidirectional device.
TRIAC
The TRIAC is an ideal device to use for AC switching applications because it can control the current flow over both halves of an alternating cycle. A thyristor is only able to control them over one half of a cycle. During the remaining half no conduction occurs and accordingly only half the waveform can be utilised.
Typical / idealised TRIAC & thyristor switching waveforms
The fact that the TRIAC can be used to control current switching on both halves of an alternating waveform allows much better power utilisation. However the TRIAC is not always as convenient for some high power applications where its switching is more difficult.
TRIAC symbol
The circuit symbol recognises the way in which the TRIAC operates. Seen from the outside it may be viewed as two back to back thyristors and this is what the circuit symbol indicates.
TRIAC symbol for circuit diagrams
On the TRIAC symbol there are three terminals. These are the Gate and two other terminals are often referred to as an "Anode" or "Main Terminal". As the TRIAC has two of these they are labelled either Anode 1 and Anode 2 or Main Terminal, MT1 and MT2
LDR
Light dependent resistor symbol
The circuit symbol used for the light dependent resistor or photoresistor combines its resistor action while indicating that it is sensitive to light. The basic light dependent resistor symbol has the rectangle used to indicate its resistor action, and then has two incoming arrows - the same as those used for photodiodes and phototransistors to indicate its light sensitivity.
Light dependent resistor symbol used in circuit diagrams
For most