TRIAC
The TRIAC
is a three terminal semiconductor device for controlling current. It gains its
name from the term TRIode for Alternating Current.
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 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.
Figure 5.13 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.
TRIAC basics
The TRIAC
is a component that is effectively based on the thyristor. It provides AC
switching for electrical systems. Like the thyristor, the TRIACs are used in
many electrical switching applications. They find particular use for circuits
in light dimmers, etc., where they enable both halves of the AC cycle to be
used.
This
makes them more efficient in terms of the usage of the power available. While
it is possible to use two thyristors back to back, this is not always cost
effective for low cost and relatively low power applications.
It is
possible to view the operation of a TRIAC in terms of two thyristors placed
back to back.
Figure
5.14 TRIAC symbol, equivalent as two thyristors
One of
the drawbacks of the TRIAC is that it does not switch symmetrically. It will
often have an offset, switching at different gate voltages for each half of the
cycle. This creates additional harmonics which is not good for EMC performance
and also provides an imbalance in the system
In order
to improve the switching of the current waveform and ensure it is more
symmetrical is to use a device external to the TRIAC to time the triggering
pulse. A DIAC placed in series with the gate is the normal method of achieving
this.
Figure 5.15 DIAC and TRIAC
connected together
Operation
With switch S open, there will be no gate current
and the triac is cut off. Even with no current the triac can be turned on
provided the supply voltage becomes equal to the breakover voltage.
When
switch S is closed, the gate current starts flowing in the gate circuit.
Breakover voltage of triac can be varied by making proper currnt flow. Triac
starts to conduct wheather MT2 is positive or negative w.r.t MT1.
Figure 5.16 TRIAC operation under biasing
If terminal MT2 is positive w.r.t MT1 the TRIAC is
on and the conventional current will flow from MT2 to MT1. If terminal MT2 is
negative w.r.t MT1 the TR IAC is again turned on and the conventional current
will flow from MT1 to MT2.
Characteristics
Figure 5. 16 The V-I
Characteristics curve for TRI AC
The V-I curve for triac in the Ist and IIIrd
quadrants are essentially identical to SCR in the Ist quadrant.The triac can be
operated with either positive or negative gat e control voltage but in normal
operation usually the gate voltage is positive in quadrant I and n egative in
quadrant III. The supply voltage at which the triac is ON depends upon gate
current. T he greater gate current and smaller supply voltage at which triac is
turned on. This permits to u se triac to control a,c. power in a load from zero
to full power in a smooth and continuous man ner with no loss in the
controlling device.
Advantages and disadvantages
When
requiring to switch both halves of an AC waveform there a re two options that
are normally considered. One is to use a TRIAC, and the other is to use two th
yristors connected back to back - one thyristor is use d to switch one half of
the cycle and the second connected in the reverse direction operates on the
other half cycle.As there are two opti ons the advantages and disadvantages of
using a TRIAC must be weighed up.
Advantages
·
Can switch both halves of an AC waveform
·
Single component can be used for full AC switching
Disadvantages
·
A TRIAC does not fire symmetrically on both sides
of the waveform
·
Switching gives rise to high level of harmonics due
to non-symmetr ical switching
·
More susceptible to EM I problems as a result of
the non-symmetrica l switching
·
Care must be taken to ensure the TRIAC turns off
fully when used w ith inductive loads.
Applications
TRIACs are used in a number of applications.
However they tend not to be used in high power switching applications - one of
the reasons for this is the non-symmetrical switching characteristics. For high
power applications this creates a number of difficulties, especially with
electromagnetic interference.
However
TRIACs are still used for many electrical switching applications:
·
Domestic light dimmers
·
Electric fan speed controls
·
Small motor controls
·
Control of small AC powered domestic appliances
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