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An Electronic Guide to TRIAC

TRIAC Overview

TRIAC (TRIode (three-terminal) AC semiconductor switch) is essentially a bidirectional thyristor, which is developed on the basis of ordinary thyristors, which can not only replace two reverse polarity paralleled thyristors, but also use only a trigger circuit. It is currently an ideal AC switching device. Although the TRIAC can be formally regarded as a combination of two ordinary thyristors, in fact it is a power integrated device composed of seven transistors and multiple resistors. Low-power bidirectional TRIACs are generally sealed in plastic, and some also have small heat dissipation poles. Typical products are BCM1AM (1A/600V), BCM3AM (3A/600V), 2N6075 (4A/600V), MAC218-10 (8A/800V) and so on. Most of the high-power TRIACs use RD91 packages, for example, the main parameters of BTA40-700 are: IT=40A, VDRM=700V, IGT=100mA. 

TRIAC Stucture and Working Principle

TRIAC is composed of NPNPN five-layer semiconductor material, equivalent to two ordinary thyristors in reverse parallel, it also has three electrodes, namely the main electrode T1, the main electrode T2 and gate G. Figure 1 is the structure and equivalent circuit of the TRIAC.

Figure 1: the structure and equivalent circuit of the TRIAC

TRIAC can be bidirectionally conducted, that is, the gate plus positive or negative trigger voltage can trigger the positive and negative conduction of the TRIAC. Figure 2 is its trigger state.

  Figure 2: trigger state

When the voltage of gate G and master electrode T2 is positive with respect to main electrode T1 (VT2> VT1, VG>VT1) or the voltage of gate G and main electrode T1 relative to main electrode T2 is negative (VT1< VT2, VG<VT2), the conduction direction of the TRIAC is T2→ T1 at this time T2 is the anode, T1 is the cathode.

When the gate G and the main electrode T1 are positive with respect to the main electrode T2 (VT1> VT2, VG>VT2) or the gate G and the main electrode T2 are negative with respect to the main electrode T1 (VT2<VT1, VG<VT1), the TRIAC conduction direction is T1→T2, at which time T1 is the anode and T2 is the cathode.

Between the main electrode T1 and the main electrode T2 of the TRIAC, regardless of whether the polarity of the applied voltage is forward or reverse, as long as there are trigger voltages with different positive and negative polarity between gate G and the main electrode T1 (or T2) to meet the necessary trigger current, the TRIAC can trigger the conduction to be in low-impedance state. At this time, the voltage drop between the main electrodes T1 and T2 is about 1V.

Once the TRIAC is turned on, it can continue to maintain its on state even if it loses the trigger voltage. When the current of the main electrodes T1 and T2 is reduced to below the maintenance current or the voltage between T1 and T2 changes the polarity, and there is no trigger voltage, the TRIAC blocks, and only by re-applying the trigger voltage can it be turned on again.

Differences between Unidirectional Thyristors and TRIACs

Compared with unidirectional thyristors, the main differences of bidirectional thyristors are:

(1) It is bidirectional conduction after triggering;

(2) The trigger voltage does not distinguish between polarity, as long as the absolute value reaches the trigger threshold value, the TRIAC can be turned on.

TRIACs can be widely used in industry, transportation, home appliances and other functions to achieve AC voltage regulation, AC speed regulation, AC switch, stage dimming, table lamp dimming and other functions. In addition, it is used in the circuits of solid state relays and solid state contactors.

Four Factors Need to Be Considered When selecting TRIAC

In the selection of TRIAC, the following four factors need to be considered:

1. Withstand voltage: Vdrm = 2-3 times the supply voltage, such as the power supply voltage AC = 220V, then Vdrm > = 600V.

2. Allowable current: the load current without inrush current (such as the heater load) must be at least > = 1.3-1.5 times the load current; However, when there is an inrush current (such as a motor load), the ambient temperature, the peak of the inrush current, and the size of the radiator must be considered.

3. CR absorption circuit: when controlling the inductive load, such as due to the role of current delay during the conversion, (di/dt)c and (dv/dt)c exceed a certain value, (di/dt)c and (dv/dt)c may not need the gate signal and Z directly into the on state, thus becoming uncontrollable. If the supply voltage AC=220V, C=0.01–0.47uF, R=47–100Ω are generally selected.

4. TRIAC can be opened through the gate signal when applying forward or reverse voltage, so it is necessary to pay attention to the design of the gate trigger circuit: trigger current, trigger voltage, gate resistance.

Determining the TRIAC Electrode Using Multimeter

The following describes the method of determining the TRIAC electrode using the multimeter R×1 gear, and also checking the triggering ability. 

1. Determine the T2 pole

As can be seen from Figure 3(a), the G pole is close to the T1 pole and farther away from the T2 pole. Therefore, the forward and reverse resistances between G-T1 are very small. When measuring the resistance between any two pins with R×1 gear, the low resistance is kept between G-T1 and the positive and negative resistances are only a few tens of ohms. The forward and reverse resistors between T2-G and T2-T1 are infinite. This suggests that if one pin cannot be conduct to the other two pins, it must be the T2 pole.

In addition, in TRIACs in a TO-220 package, the T2 pole is usually connected to a small heat sink. The T2 pole can also be determined from this.

2. Distinguish between G pole and T1 pole 

(1) After finding the T2 pole, first assume that one of the remaining two pin is the T1 pole and the other pin is the G pole. 

(2) Connect the black gauge to the T1 pole, the red gauge to the T2 pole, and the resistance is infinity. Then use the red gauge tip to short circuit T2 and G, add a negative trigger signal to the G pole, and the resistance value should be about ten ohms (see Figure 5(a)), which proves that the tube has been turned on, and the conduction direction is T1→ T2. The red gauge tip is then detached from the G pole (but still connected to T2), and if the resistance value remains unchanged, it means that the tube can maintain its on state after triggering (see Figure 3(b)). 

(3) The red gauge is connected to the T1 pole, the black gauge is connected to the T2 pole, and then the T2 and G are short-circuited, and the positive trigger signal is added to the G pole, the resistance value is still about ten ohms, and if the resistance value is unchanged after the G pole is separated, it means that the tube can also maintain the conduction state in the direction of T2 → T1 after the trigger, so it has a bidirectional trigger property. This proves that the above assumptions are correct. Otherwise, the assumption does not match the actual situation, and it is necessary to make a new assumption and repeat the above measurements. 

Obviously, in the process of identifying G and T, the triggering capacity of the TRIAC is also checked.

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