What is Triac
Table of Contents
Triac full form – Triode for alternative current
The term “thyristor” is used frequently to refer to a wide range of semiconductor parts used as electronic switches. Thyristors only have two states: on (conductive) and off (non-conductive), just like a mechanical switch. In addition to switching, they can also be used to modify the power supplied to a load.
Thyristors are typically used in high voltage and current applications. The most widely used thyristor devices are silicon controlled rectifier (SCR) and triode for alternative current. The creation, traits, and uses of TRIACs are examined in this article.
What is a TRIAC?
A TRIAC is a three-electrode, bidirectional AC switch that enables electrons to move either way. It is comparable to two SCRs connected in reverse parallel with their gates coupled together. A gate signal similar to that of an SCR initiates the conduction of a TRIAC in both directions. To facilitate the creation of better AC power controls, TRIACs were created.
TRIACs come in a range of packaging configurations. They are capable of handling various current and voltage levels. In comparison to SCRs, TRIACs typically have lower current capabilities; they are typically limited to less than 50 A and cannot take the place of SCRs in high-current applications.
As a result of their capacity to function with either positive or negative voltages across their terminals, TRIACs are regarded as being versatile. Controlling low power in an AC circuit is better served by the use of a Triode for alternative current because SCRs have the drawback of only conducting current in one direction.
TRIAC Construction (triac symbol)
Despite having similar external appearances, TRIACs and SCRs have different schematic symbols. The gate, terminal 1, and terminal 2 are the TRIAC’s terminals. Look at Figure 1.
Figure 1. TRIAC terminals include a gate, terminal 1 (T1), and terminal 2 (T2).
Anode and cathode are not identified. The main switch terminals, T1 and T2, allow current to flow in either direction. The reference terminal for all voltages is terminal 1. The case or metal mounting tab at Terminal 2 is where a heat sink can be attached.
TRIAC Triggering Circuit and its Advantages
Between T1 and T2, TRIACs obstruct current in either direction. A brief positive or negative pulse delivered to the gate can initiate conduction in either direction in a TRIAC.
The TRIAC gate conducts electricity when the proper signal is applied to it. Until the gate at point A is opened, the Triode for alternative current is not turned on. Look at Figure 2.
Figure 2. A TRIAC remains off until its gate is triggered.
The trigger circuit pulses the gate at point A, turning on the TRIAC and enabling current flow. The forward current is brought to zero at point B, and the TRIAC is then turned off. The trigger circuit can be built to produce pulses at any point that switch between the positive and negative half-cycles. The average current delivered to the load can therefore change. The fact that almost no power is lost to heat generation is one benefit of the TRIAC. Not when the current is turned off, but rather when it is obstructed, is heat produced. Either fully ON or fully OFF describes the Triode for alternative current. Never does it restrict current in part.
The absence of a reverse breakdown condition of high voltages and high currents, like those found in diodes and SCRs, is another crucial component of the TRIAC. The Triode for alternative current is activated if the voltage across it rises too high. The TRIAC can carry a reasonably high current once it is turned on.
The Characteristic Curve of TRIACs
A TRIAC’s characteristics are based on T1 being used as the voltage reference point. The voltage and current polarities represented are T2’s relative to T1 polarities.
The polarities shown for the gate are also with respect to T1. See Figure 3.
Figure 3. A TRIAC characteristic curve shows the characteristics of a TRIAC when triggered into conduction.
Once more, a gate current (IG) of either polarity can cause the TRIAC to start conducting in either direction.
Because of their adaptability, TRIACs are frequently utilized in place of mechanical switches. In addition, TRIACs are more cost-effective than back-to-back SCRs in low-amperage applications.
Single-Phase Motor Starters
In situations where mechanical cut-out start switch arcing is undesirable or even dangerous, a capacitor-start or split-phase motor must frequently operate. In these circumstances, a TRIAC can take the place of the mechanical cut-out start switch. Look at Figure 4.
Figure 4. A mechanical cut-out start switch may be replaced by a TRIAC.
Because it does not produce an arc, a TRIAC can function in such hazardous conditions. Through a current transformer, the gate and cut-out signals are delivered to the TRIAC. The current transformer stops triggering the TRIAC as the motor accelerates because its current is reduced. The start windings are disconnected from the circuit once the TRIAC has been turned off.
Testing Procedures for TRIACs
An oscilloscope should be used to test TRIACs while they are in use. With the TRIAC removed from the circuit, a DMM can be used to perform a rough test. Look at Figure 5.
Figure 5. A DMM may be used to make a rough test of a TRIAC that is out of the circuit.
To test a TRIAC using a DMM, the following procedure is applied:
- Set the DMM on the Ω scale.
- Connect the negative lead to the main terminal 1.
- Connect the positive lead to the main terminal 2. The DMM should read infinity.
- Use a jumper wire to short-circuit the gate to main terminal 2. Nearly 0 should appear on the DMM. When the lead is taken out, the zero reading ought to hold.
- So Placing the positive lead on main terminal 1 and the negative lead on main terminal 2 will reverse the DMM leads. The DMM ought to display infinity.
- So Using a jumper wire, short-circuit the TRIAC’s gate to main terminal 2. Nearly 0 should appear on the DMM. After the lead has been taken out, the zero reading ought to hold.