TRIAC
Triacs are widely used in AC power control applications. They are able to switch high voltages and high levels of current, and over both parts of an AC waveform. This makes triac circuits ideal for use in a variety of applications where power switching is needed.
One particular use of triac circuits is in light dimmers for domestic lighting, and they are also used in many other power control situations including motor control.
A Triac behaves just like two conventional thyristors connected together in inverse parallel (back-to-back) with respect to each other and because of this arrangement the two thyristors share a common Gate terminal all within a single three-terminal package.
Since a Triac conducts in both directions of a sinusoidal waveform, the concept of an Anode terminal and a Cathode terminal used to identify the main power terminals of a thyristor are replaced with identifications of: MT1, for Main Terminal 1 and MT2 for Main Terminal 2 with the Gate terminal G referenced the same.
TRIAC Symbol
The symbol consists of three terminals which are Gate, Anode or Main terminal. The two terminals are labeled as an anode1, or anode2, main terminal MT1 or MT2. From the outside view, the symbol is viewed as a back to back Thyristors which we can observe in the symbol.
Triggering a TRIAC
Usually 4 modes of triggering is possible in TRIAC:
1. A positive voltage at MT2 and a positive pulse at the gate.
2. A positive voltage at MT2 and a negative pulse at the gate.
3. A negative voltage at MT2 and positive pulse at the gate.
4. A negative voltage at MT2 and a negative pulse at the gate.
I-V Characteristics
The following figure shows the typical TRIAC characteristics. The Triode For Alternating Current consists of ON and OFF state characteristics which are similar to the SCR. Now the characteristics are applicable to both the voltages of positive and negative.
This we can predictably because the Triode For Alternating Current consists of two SCRs which are connected in parallel and they are in opposite directions. In the first quadrant the MT2 is positive with respect to the MTX and in the fourth quadrant, it is in the negative.
The gate trigger may occur in any four quadrant modes of operation. If the device is in ON state the conduction permits a huge amount of current to flow through it.
A large amount of current is controlled by the resistance, otherwise, the device may be damaged. With the device, the gate is a control terminal and the proper signal is applied to the gate, therefore the firing angle of the device is controlled.
The circuit which we have used in the gate for triggering the device is called the gate triggering circuit. For the TRIAC the gate triggering circuits are similar to the SCRs. Generally, the triggering circuits will generate the triggering pulses for firing the devices.
The triggering pulses will have sufficient magnitude and duration, hence the firing of the device is assured. To sustain the firing of the device duration of 35 us is required.
Working
When the voltage on the MT1 is positive with regard to MT2 and a positive gate voltage is applied, one of the thyristors conducts. When the voltage is reversed and a negative voltage is applied to the gate, the other thyristor conducts. This is provided that there is sufficient voltage across the device to enable a minimum holding current to flow.
Advantages of TRIAC
In the TRIACs, there will be single gate control conduction in both the directions.
If the voltage is decreased to zero the TRIAC turns OFF.
Basically, it is bidirectional device and in the both the directions it will conduct.
High voltage rating are available
Disadvantages of TRIAC
It doesn’t fire symmetrically on both sides of the waveforms.
It doesn’t appropriate for the DC power
It has very high switching delay.
Applications of TRIAC:
TRIACs are used in numerous applications such as light dimmers, speed controls for electric fans and other electric motors and in the modern computerized control circuits of numerous household small and major appliances. They can be used both into AC and DC circuits however the original design was to replace the utilization of two SCRs in AC circuits.There are two families of TRIACs, which are mainly used for application purpose, they are BT136, BT139.