TRIAC
Triac is a three terminal AC switch which is different from the other
silicon controlled rectifiers in the sense that it can conduct in both the
directions that is whether the applied gate signal is positive or negative, it
will conduct. Thus, this device can be used for AC systems as a switch.This is
a three terminal, four layer, bi-directional semiconductor device
that controls AC power. The triac of maximum rating of 16 kw is available in
the market. Figure shows the symbol
of triac, which has two main terminals MT1 and MT2
connected in inverse parallel and a gate terminal.
Construction of Triac
Two SCRs are connected in inverse parallel with gate terminal as common.
Gate terminals is connected to both the N and P regions due to which gate
signal may be applied which is irrespective of the polarity of the signal.
Here, we do not have anode and cathode since it works for both the polarities
which means that device is bilateral. It consists of three terminals namely,
main terminal 1(MT1), main terminal 2(MT2), and gate
terminal G. Figure
shows the construction of a triac. There are two main terminals namely MT1
and MT2 and the remaining terminal is gate terminal.
Operation of Triac
The triac can be turned on by applying the gate voltage higher
than break over voltage. However, without making the voltage high, it can be
turned on by applying the gate pulse of 35 micro seconds to turn it on. When
the voltage applied is less than the break over voltage,
we use gate triggering method to turn it on. There are four different modes of
operations, they are-
1. When MT2
and Gate being Positive with Respect to MT1 When
this happens, current flows through the path P1-N1-P2-N2.
Here, P1-N1 and P2-N2 are forward
biased but N1-P2 is reverse biased. The triac is said to
be operated in positively biased region. Positive gate with respect to MT1
forward biases P2-N2 and breakdown occurs.
2. When MT2
is Positive but Gate is Negative with Respect to MT1 The
current flows through the path P1-N1-P2-N2.
But P2-N3 is forward biased and current carriers injected
into P2 on the triac.
3. When MT2
and Gate are Negative with Respect to MT1
Current flows through the path P2-N1-P1-N4.
Two junctions P2-N1 and P1-N4 are
forward biased but the junction N1-P1 is reverse biased. The triac is said to
be in the negatively biased region.
4. When MT2
is Negative but Gate is Positive with Respect to MT1 P2-N2
is forward biased at that condition. Current carriers are injected so the triac
turns on. This mode of operation has a disadvantage that it should not be used
for high (di/dt) circuits. Sensitivity of triggering in mode 2 and 3 is high
and if marginal triggering capability is required, negative gate pulses should
be used. Triggering in mode 1 is more sensitive than mode 2 and mode 3.
Characteristics of a Triac
The triac characteristics is similar to SCR but it is
applicable to both positive and negative triac voltages. The operation can be
summarized as follows-
First Quadrant Operation of Triac
First Quadrant Operation of Triac
Voltage at terminal MT2 is positive with respect to
terminal MT1 and gate voltage is also positive with respect to first
terminal.
Second Quadrant Operation of Triac
Voltage at terminal 2 is positive with respect to terminal 1 and
gate voltage is negative with respect to terminal 1.
Third Quadrant Operation of Triac
Voltage of terminal 1 is positive with respect to terminal 2 and
the gate voltage is negative.
Fourth Quadrant Operation of Triac
Voltage of terminal 2 is negative with respect to terminal 1 and
gate voltage is positive. When
the device gets turned on, a heavy current flows through it which may damage
the device, hence in order to limit the current a current limiting resistor
should be connected externally to it. By applying proper gate signal, firing
angle of the device may be controlled. The gate triggering circuits should be
used for proper gate triggering. We can use diac for triggering the gate pulse.
For firing of the device with proper firing angle, a gate pulse may be applied
up to a duration of 35 micro seconds.
Advantages of Triac
1. It can
be triggered with positive or negative polarity of gate pulses.
2. It
requires only a single heat sink of slightly larger size, whereas for SCR, two
heat sinks should be required of smaller size.
3. It
requires single fuse for protection.
4. A safe
breakdown in either direction is possible but for SCR protection should be
given with parallel diode.
Disadvantages of Triac
1. They
are not much reliable compared to SCR.
2. It has
(dv/dt) rating lower than SCR.
3. Lower
ratings are available compared to SCR.
4. We need
to be careful about the triggering circuit as it can be triggered in either
direction.
Uses of Triac
1. They
are used in control circuits.
2. It is
used in High power lamp switching.
3.
It is used in AC power control.
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