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,fan speed regulators, 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.
TRIAC 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.
DIAC and TRIAC connected together
Basic Circuit:
This is the circuit
diagram of the simplest lamp dimmer or fan regulator.The circuit is based on
the principle of power control using a Triac.The circuit works by varying the
firing angle of the Triac . Resistors R1 ,R2 and capacitor C2 are associated
with this. The firing angle can be varied by varying the value of any of these
components. Here R1 is selected as the variable element . By varying the value
of R1 the firing angle of Triac changes (i.e. how much time should Triac
conduct) changes. This directly varies the load power, since load is driven by
Triac. The firing pulses are given to the gate of Triac T1 using Diac D1.
The most basic wavefor(i.e ignoring all losses and harmonics) is shown below.
The waveform shown below demonstrates the output voltage of TRIAC before and after rectification.
Alpha is firinf angel of thyristers.
From the two figures shown below we can see the output waveform by changing
firing angel. In the first figure the
output will be half power of the input power.
In the second figure as firing angel is zero ,therefore
output power will be same as input.
