A triac is a three-electrode, bidirectional AC switch that allows electrons to flow in either direction. The word “triac” is derived from the phrase “triode for alternating current.”
Triacs are the equivalent of two SCRs connected in a reverse-parallel arrangement with gates also connected to each other.
A triac is triggered into conduction in both directions by a gate signal in a manner similar to that of an SCR. Triacs were developed to provide a means for producing improved controls for AC power.
Triacs are available in a variety of packaging arrangements. They can handle a wide range of current and voltage.
Triacs generally have relatively low-current capabilities compared to SCRs. Triacs are usually limited to less than 50 A and cannot replace SCRs in high-current applications.
Triacs are considered versatile because of their ability to operate with positive or negative voltages across their terminals.
Since SCRs have a disadvantage of conducting current in only one direction, controlling low power in an AC circuit is better served with the use of a triac.
Although triacs and SCRs look similar, their schematic symbols are different. The terminals of a triac are the gate, terminal 1 (T1), and terminal 2 (T2). See Figure 1.
There is no designation of anode and cathode. Current may flow in either direction through the main switch terminals, T1 and T2. Terminal 1 is the reference terminal for all voltages. Terminal 2 is the case or metal-mounting tab to which a heat sink can be attached.
Figure 1. Triac terminals include a gate, terminal 1 (T1), and terminal 2 (T2).
Triacs block current in either direction between T1 and T2. A triac can be triggered into conduction in either direction by a momentary positive or negative pulse supplied to the gate.
If the appropriate signal is applied to the triac gate, it conducts electricity. The triac remains off until the gate is triggered at point A. See Figure 2.
At point A, the trigger circuit pulses the gate and the triac is turned on, allowing current to flow. At point B, the forward current is reduced to zero and the triac is turned off.
The trigger circuit can be designed to produce a pulse that varies at any point in the positive or negative half cycle. Therefore, the average current supplied to the load can vary.
Figure 2. A triac remains off until its gate is triggered.
One advantage of the triac is that virtually no power is wasted by being converted to heat. Heat is generated when current is impeded, not when current is switched off. The triac is either fully ON or fully OFF. It never partially limits current.
Another important feature of the triac is the absence of a reverse breakdown condition of high voltages and high currents, such as those found in diodes and SCRs.
If the voltage across the triac goes too high, the triac is turned on. When turned on, the triac can conduct a reasonably high current.
Triac Characteristic Curve
The characteristics of a triac are based on T1 as the voltage reference point. The polarities shown for voltage and current are the polarities of T2 with respect to T1.
The polarities shown for the gate are also with respect to T1. See Figure 3. Again, the triac may be triggered into conduction in either direction by a gate current (IG) of either polarity.
Figure 3. A triac characteristic curve shows the characteristics of a triac when triggered into conduction.
Triacs are often used instead of mechanical switches because of their versatility. Also, where amperage is low, triacs are more economical than back-to-back SCRs.
Single-Phase Motor Starters
Often, a capacitor- start or split-phase motor must operate where arcing of a mechanical cut-out start switch is undesirable or even dangerous. In such cases, the mechanical cut- out start switch can be replaced by a triac. See Figure 4.
A triac is able to operate in such dangerous environments because it does not create an arc. The gate and cut-out signal are given to the triac through a current transformer.
As the motor speeds up, the current is reduced in the current transformer and the transformer no longer triggers the triac. With the triac turned off, the start windings are removed from the circuit.
Figure 4. A mechanical cut-out start switch may be replaced by a triac.
Triacs should be tested under operating conditions using an oscilloscope. A DMM may be used to make a rough test with the triac out of the circuit. See Figure 5.
To test a triac using a DMM, the following procedure is applied:
- Set the DMM on the Ω scale.
- Connect the negative lead to main terminal 1.
- Connect the positive lead to main terminal 2. The DMM should read infinity.
- Short-circuit the gate to main terminal 2 using a jumper wire. The DMM should read almost 0 Ω. The zero reading should remain when the lead is removed.
- Reverse the DMM leads so that the positive lead is on main terminal 1 and the negative lead is on main terminal 2. The DMM should read infinity.
- Short-circuit the gate of the triac to main terminal 2 using a jumper wire. The DMM should read almost 0 Ω. The zero reading should remain after the lead is removed.
Figure 5. A DMM may be used to make a rough test of a triac that is out of the circuit.