Operation of TRIAC and DIAC

TRIAC is an acronym for triode AC semiconductor. TRIACs have the same switching characteristics as SCRs but conduct both directions of AC current flow.

A TRIAC is equivalent to two SCRs connected in parallel, back to back (Figure 1). Because a TRIAC can control current flowing in either direction, it is widely used to control application of AC power to various types of loads.

Equivalent TRIAC
Fig. 1. Equivalent TRIAC.

It can be turned on by applying a gate current and turned off by reducing the operating current to below the holding level. It is designed to conduct forward and reverse current through the terminals.

Operation of TRIAC

Figure 2 shows a simplified diagram of a TRIAC. A TRIAC is a four-layer NPNP device in parallel with a PNPN device. It is designed to respond to a gating current through a single gate.

Fig. 2. Simplified TRIAC.

The input and output terminals are identified as main terminal 1 (MT1) and main terminal 2 (MT2). The terminals are connected across the PN junctions at the opposite ends of the device.

Terminal MT1 is the reference point for measurement of voltage and current at the gate terminal. The gate (G) is connected to the PN junction at the same end as MT1. From MT1 to MT2, the signal must pass through an NPNP series of layers or a PNPN series of layers. The schematic symbol for a TRIAC is shown in Figure 3.

Fig. 3

It consists of two parallel diodes connected in opposite directions with a single gate lead. The terminals are identified as MT1, MT2, and G (gate).

A TRIAC can be used as an AC switch (Figure 4). It can also be used to control the amount of AC power applied to a load (Figure 5). TRIACs apply all available power to the load.

TRIAC AC switch circuit.
Fig. 4. TRIAC AC switch circuit.

When a TRIAC is used to vary the amount of AC power applied to the load, a special triggering device is needed to ensure that the TRIAC functions at the proper time. The triggering device is necessary because the TRIAC is not equally sensitive to the gate current flowing in opposite directions.

TRIAC AC control circuit.
Fig. 5. TRIAC AC control circuit.

TRIACs have disadvantages when compared to SCRs.

  • TRIACs have current ratings as high as 25 amperes, but SCRs are available with current ratings as high as 1400 amperes.
  • A TRIAC’s maximum voltage rating is 500 volts, compared to an SCR’s 2600 volts. TRIACs are designed for low frequency (50 to 400 hertz) whereas SCRs can handle up to 30,000 hertz.
  • TRIACs also have difficulty switching power to inductive loads.

Operation of Diac

Bidirectional (two-directional) trigger diodes are used in TRIAC circuits because TRIACs have nonsymmetrical triggering characteristics; that is, they are not equally sensitive to gate current flowing in opposite directions. The most frequently used triggering device is the DIAC (diode AC).

The DIAC is constructed in the same manner as the transistor. It has three alternately doped layers (Figure 6). The only difference in the construction is that the doping concentration around both junctions in the DIAC is equal.

operation of diac
Fig. 6. Simplified DIAC.

Leads are only attached to the outer layers. Because there are only two leads, the device is packaged like a PN junction diode. Both junctions are equally doped, so a DIAC has the same effect on current regardless of the direction of flow.

One of the junctions is forward biased and the other is reverse biased. The reverse-biased junction controls the current flowing through the DIAC. The DIAC performs as if it contains two PN junction diodes connected in series back to back.

The DIAC remains in the off state until an applied voltage in either direction is high enough to cause its reverse-biased junction to break over and start conducting much like a zener diode. Break over is the point at which conduction starts to occur. This causes the DIAC to turn on and the current to rise to a value limited by a series resistor in the circuit.

Fig. 7. DIAC Symbol.

The schematic symbol for a DIAC is shown in Figure 7. It is similar to the symbol for a TRIAC. The difference is that a DIAC does not have a gate lead.

DIACs are most commonly used as a triggering device for TRIACs. Each time the DIAC turns on, it allows current to flow through the TRIAC gate, thus turning the TRIAC on.

The DIAC is used in conjunction with the TRIAC to provide full-wave control of AC signals.

Figure 8 shows a variable full-wave phase-control circuit.

operation of triac and diac
Fig. 8. Variable full-wave phase-control circuit.

Variable resistor R1 and capacitor C1 form a phase-shift network. When the voltage across C1 reaches the break-over voltage of the DIAC, C1 partially discharges through the DIAC into the gate of the TRIAC.

This discharge creates a pulse that triggers the TRIAC into conduction. This circuit is useful for controlling lamps, heaters, and speeds of small electrical motors.

Testing TRIAC with Ohmmeter

  1. Determine the polarity of the ohmmeter leads.
  2. Connect the positive lead to MT1 and the negative lead to MT2. The resistance should be high.
  3. With the leads still connected as in step 2, short the gate to MT1. The resistance should drop.
  4. Remove the short. The low resistance should remain. The ohmmeter may not supply enough current to keep the TRIAC latched if a large gate current is required.
  5. Remove the leads and reconnect as specified in step 2. The resistance should again be high.
  6. Short the gate to MT2. The resistance should drop.
  7. Remove the short. The low resistance should remain.
  8. Remove and reverse the leads, the negative lead to MT1 and the positive lead to MT2. The resistance should read high.
  9. Short the gate to MT1. The resistance should drop.
  10. Remove the short. The low resistance should remain.
  11. Remove the leads and reconnect in the same configuration. The resistance should again be high.
  12. Short the gate to MT2. The resistance should drop.
  13. Remove the short. The low resistance should remain.
  14. Remove and reconnect the leads. The resistance should be high.

Summary

  • TRIACs control current in either direction by either a positive or negative gate signal.
  • Because TRIACs have nonsymmetrical triggering characteristics, they require the use of a DIAC.
  • DIACs are bidirectional trigger diodes. They are mostly used as triggering devices for TRIACs.

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