What is a TRAIC ? traic principle | traic advantage and disadvantages .

TRIAC 

The triac is a member of the thyristor family. Basically, a triac can be thought as two SCR connected in parallel and in opposite directions witha common gate terminal. Unlike the SCR, the traic can conduct current in either direction when it is triggered on, depending on the polarity of the voltage across its A1 and A2 terminals. The triac is a bi- directional semi- controlled devices.


        Fig. (a) and (c) show the circuit symbol and schematic cross section of a triac respective. As 
the Triac can conduct in both the directions the terms “anode” and “cathode” are not used for Triacs. The three terminals are marked as MT1 (Main Terminal 1), MT2 (Main Terminal 2) and the gate by G. As shown in Fig (b) the gate terminal is near MT1 and is connected to both 
N3 and P2 regions by metallic contact. Similarly MT1 is connected to N2 and P2 regions while MT2 is connected to N4 and P1 regions.
 The following TRIAC may be operate in four types-



Mode 1 ; MT2 positive with respect to MT1, G positive with respect to MT1.


Mode 2 ; MT2 positive with respect to MT1, G negative with respect to MT1
Mode 3 ; MT2 negative with respect to MT1, G negative with respect to MT1

Mode 4 ; MT2 negative with respect to MT1, G positive with respect to MT1.

Mode1 and  mode3 mainly used for operation in four method.

 V-I characteristics ; 



From a functional point of view a triac is similar to two thyristors connected in anti parallel. 
Therefore, it is expected that the V-I characteristics of Triac in the 1st and 3rd quadrant of the V-I 
plane will be similar to the forward characteristics of a thyristors. As shown in Fig. , with no 
signal to the gate the triac will block both half cycle of the applied ac voltage provided its peak 
value is lower than the break over voltage (VBO) of the device. However, the turning on of the 
triac can be controlled by applying the gate trigger pulse at the desired instance. Mode-1 
triggering is used in the first quadrant where as Mode-3 triggering is used in the third quadrant. 
As such, most of the thyristor characteristics apply to the triac (ie, latching and holding current). 
However, in a triac the two conducting paths (from MT1 to MT2 or from MT1 to MT1) interact 
with each other in the structure of the triac. Therefore, the voltage, current and frequency ratings 
of triacs are considerably lower than thyristors. At present triacs with voltage and current ratings 
of 1200V and 300A (rms) are available. Triacs also have a larger on state voltage drop compared 
to a thyristor. Manufacturers usually specify characteristics curves relating rms device current 
and maximum allowable case temperature as shown in Fig . Curves relating the device 
dissipation and RMS on state current are also provided for different conduction angles.

Triac switching and gate triggering circuit


Unlike a thyristor a triac gets limited time to turn off due to bidirectional conduction. As a result 
the triacs are operated only at power frequency. Switching characteristics of a triac is similar to 
that of a thyristor. However, turn off of a triac is extremely sensitive to temperature variation and 
may not turn off at all if the junction temperature exceeds certain limit. Problem may arise when 
a triac is used to control a lagging power factor load. At the current zero instant (when the triac 
turns off) a reverse voltage will appear across the triac since the supply voltage is negative at that 
instant. The rate of rise of this voltage is restricted by the triac junction capacitance only.  The 
resulting dv/dt may turn on the triac again. Similar problem occurs when a triac is used to 
control the power to a resistive element which has a very low resistance before normal working 
condition is reached. If such a load (e.g. incandescent filament lamp) is switch on at full supply 
voltage very large junction capacitance charging current will turn ON the device. To prevent 
such condition an R-C snubber is generally used across a triac. The triac should be triggered carefully to ensure safe operation. For phase control application, the triac is switched on and off in synchronism with the mains supply so that only a part of each half cycle is applied across the load. To ensure ‘clean turn ON’ the trigger signal must rise rapidly to provide the necessary charge. A rise time of about 1 μs will be desirable. Such a triac gate triggering circuit using a “diac” and an R-C timing network is shown in Fig.


In this circuit as Vi increases voltage across C1 increases due to current flowing through load, R1, 
R2 and C1. The voltage drop across diac D1 increases until it reaches its break over point. As D1
conducts a large current pulse is injected into the gate of the triac. By varying R2 the firing can 
be controlled from zero to virtually 100%.

There are some advantages of TRIAC which are given below,

  • The TRIAC need single fuse for protection.
  • It can be triggered with positive or negative polarities of gate pulses.
  • A safe breakdown in either direction is possible but for SCR protection should be given with parallel diode.
  • It needs only a single heat sink of slightly larger size where as for SCR two heat sinks should be required of smaller size.
  • When the voltage is reduced to zero the TRIAC turns OFF.

There are some disadvantages of TRIAC which are given below,

  • It can be triggered in any direction so we need to be careful about triggering circuit.
  • As compared to SCR (silicon controlled rectifier) it has low ratings.
  • The TRIACs are not much reliable as compared to SCR.
  • This is not suitable for DC applications.
  • The dv/dt rating is  very low as compared to SCR.
  • It has a very high switching delay

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