Basics of Triac
Silicon controlled Rectifier or SCR is a unilateral device where the control of the supply to the load
is only half waveform. In order to achieve full waveform control, one with bilateral control is used.
It consists of two SCR connected in inverse parallel where the gates are connected together. The
connections are labelled as MT1(Main Terminal 1), MT2(Main Terminal 2) and G(Gate).
Current can flow in either direction between MT1 and MT2 terminals when a small gate current is
applied between MT1 and the gate terminal. It is turned ON by triggering a positive or negative
current between MT1 and the gate.
The holding current is the minimum current required to hold it on. As with SCR, the control of
large load current can be triggered by miliampere gate current, hence making it a convenient
switch for AC circuits. It can also do phase control where a certain percentage of power from the
mains is supplied to the load.
It is used in applications such as light dimmers and other motor control devices.
The setback of using this device is that it causes electromagnetic interference due to the sudden
rise in the line current when it is gated ON. This chopping of the mains supply causes harmonics
and may cause disturbances to other devices nearby.
It could cause interference in radio and television receivers as well as microcontroller based
equipment. As such, care must be taken to filter out these harmonics to an acceptable level.
To study the VI characteristics of switching device in Power Electronics.
Characteristics of TRIAC
An SCR is a unidirectional device as it can conduct from anode to cathode only and
not from cathode to anode. A TRIAC can, conduct in both the directions. A TRIAC is
the bidirectional thyristor with three terminals. It is used extensively for control of
power in AC circuit. When in operation, a triac is equivalent to two SCRs connected
in antiparallel. The device symbol and V1 characteristics are shown in figure.
As the TRIAC can conduct in both the directions, the term anode and cathode are
not applicable to TRIAC. It is three terminals are usually designated as MT1 (main
terminal 1), MT2 and the gate by G as in thyristor.
With no signal the gate, the TRIAC will block both half cycles of applied voltage in
case peak value of this voltage is less than the brake over voltage of VBO1 or VBO2
of triac. The triac can however turns ON in each half cycle of the applied voltage by
applying a positive or negative voltage to the with respect to terminal MT1. For
convenience terminal MT1 is taken as the point for measuring the voltage and
current at the gate and MT2 terminal.
The Turn-on Process of a Triac can be Explained as under
1) MT2 is Positive and Gate Current is Positive.
In this mode MT2 is positive with respect to MT1, junctions P1N1, P2N2 are forward
biased but junction N1P2 is reverse biased. When gate terminal is positive with the
respect to MT1, gate current flows mainly through P2N2 junction like an ordinary
SCR, Fig.6(a). When gate current has injected sufficient charge into P2 layer,
reverse biased junction N1P2 breaks down just as in a normal SCR. As a
result, TRIAC starts conduction through P1N1P2N2 layers. This shows the when
MT2 and gate terminals are positive with respect to MT1, triac turns on like a
conventional thyristor. Under this condition, TRIAC operates in the first quadrant.
The device is more sensitive in this mode. It is recommended method of triggering if
the conduction is desired in the first quadrant.
2) MT2 is Positive and Gate Current is Negative.
In this mode gate terminal is negative with respect to MT1, gate current flows
through P2N3 junction Fig 6(b) and reverse biased junction N1P2 is forward biased
as in a normal thyristor. As a result, triac starts conducting through P1N1P2N3 layers
initially. With the conduction of P1N1P2N3, the voltage drop across this path falls but
potential of layer between P2N3 rises towards the anode potential of MT2. As the
right hand portion of P2 is clamped at the cathode potential of MT1, a potential
gradient exists across layer P2, its left hand region being at higher potential than its
right hand region. A current shown dotted is thus established in layer P2 from left to
right. This current is similar to conventional gate current of an SCR. As a
consequence, right-hand part of TRIAC consisting of main structure P1N1P2N2
begins to conduct. The device structure P1N1P2N3 may be regarded as pilot SCR
and the structure P1N1P2N2 as the main SCR. It can then be stated that anode
current of pilot SCR serves as the gate current for the main SCR. As compared with
turn-on process discussed in (I) above, the device with MT2 positive but gate current
negative is less sensitive and therefore, more gate current is required.
3) MT2 is Negative and Gate Current is Positive
The gate current Ig forward biases P2N2 junction. Layer N2 injects electrons into P2
layer as shown by dotted arrows. As a result, reverse biased junction N1P1 breaks
down as in a conventional thyristor. Eventually the structure P2N1P1N4 is
completely turned on. As usual, the current after turn-on is limited by the external
load. As the triac is turned on by remote gate N2, the device is less sensitive in the
third quadrant with positive gate current.
4) MT2 is negative and Gate Currents is Negative
In this mode, N3 acts as a remote gate. The gate current Ig flows from P2 to N3 as in
a normal thyristor. Reverse-biased junction N1P1 is broken and finally, the structure
P2N1P1N4 is turned on completely. Though the triac is turned on by remote gate N3
in third quadrant, yet the device is more sensitive under this condition compared with
turn-on action with positive gate current discussed in (iii) above.