Triacs, diacs, and quadracs are ac power control semiconductor devices used in line frequency applications such as lighting control, motor speed control, and temperature modulation. They can be found in consumer and industrial settings. Triacs are a member of the thyristor family and have a four-layer structure. Diacs (DIode AC switch) are used to trigger triacs into the on state, and quadracs are a merger of a triac and diac in a single device. This FAQ begins with a review of basic triac operation, looks at how a diac can be used to trigger a triac, describes quadracs, and closes by looking at optotriacs.

A triac (sometimes referred to as a triode for alternating current, bilateral triode thyristor, or bidirectional triode thyristor) is a bidirectional three-terminal power switching device. Triacs are related to silicon controlled rectifiers (SCRs), but triacs conduct in both directions while SCRs conduct in a single direction. A triac can be triggered (turned on) by a positive or negative voltage applied to the gate, while an SCR requires a positive gate voltage to turn on.

In some designs, SCRs are used to trigger triacs. Once turned on, both triacs and SCRs continue to conduct until the current falls below the holding current, at which time the devices turn off. They are not turned off using gate control. Gate turn-off thyristors (GTOs) are similar to triacs and SCRs but can be turned off by shutting down the gate signal, resulting in a higher level of control.

The combination of bidirectional conduction and the ability to control the phase angle when the device is triggered makes triacs well-suited for use in ac power applications. The use of phase control enables control of the average current flow, which can be used to control the speed of motors, dimming of lamps, and the temperature of heaters.

It’s possible to control the brightness of an LED lamp with a very simple triac circuit. This basic implementation uses a triggering capacitor, a fixed resistor, a potentiometer (to control the light level), and a triggering device. In this design, two inverse parallel SCRs are used as the triggering device (Figure 1). The circuit can produce a wide range of brightness from the LED. Of course, this basic design has no power factor control and can be relatively inefficient. A more complex but still relatively simple triac light dimmer implementation can deliver 86% efficiency with a power factor over 0.95.

Adding a diac

Like a triac, a diac is a bidirectional, or full-wave, switching device that can conduct in forward and reverse polarities. Diacs are sometimes referred to as symmetrical trigger diodes. They can be used in place of two inverse parallel SCRs to trigger a triac in an ac switch, such as a residential light dimmer or the starter circuit for a fluorescent lamp (Figure 2). This circuit provides an improved user experience, compared with simpler implementations with poor hysteresis, by adding the steering diodes around the firing capacitor (C1).

A diac switches on when its breakdown voltage is exceeded. It remains on until the current through the device drops below the holding current when it turns off and returns to a high resistance state. Its switching and conducting characteristics are mostly symmetrical. In some designs, a diac can be used to trigger an SCR. Diacs can be obtained as discrete devices in surface mount packages. Higher power diacs can be bolted to a chassis for better thermal dissipation. Since they are most often used with triacs, it’s common to find the devices co-packaged or they are integrated onto a single die. In either case, the resulting device is referred to as a quadrac.

Diac + Triac = Quadrac

Replacing the diac and triac in the previous design example with a quadrac in a TO-220 isolated mounting tab package further reduces component and solution size compared with a discrete diac and discrete triac (Figure 3). This implementation has a lower full turn-on voltage resulting from the higher breakover voltage (V_BO) of the diac element in the quadrac. It can produce light outputs from 175° to <90° of each AC half-cycle.

Key specifications for quadracs include:

• Maximum repetitive peak forward and reverse voltages, V_DRM and V_RRM, respectively, are the maximum voltage the device can repetitively block.
• RMS on-state current is the maximum continuous RMS current, I_T(RMS)
• Diac breakover voltage, V_BO
• Peak non-repetitive surge current, I_TSM
• Critical rate of increase of on-state current, di/dt, usually in A/µs
• Fusing requirement, I²t

Optotriacs

Optotriacs–also called phototriacs or solid-state relays–are ac switches used in industrial and process control systems. Optotriacs provide electrical isolation between the driver and the load. Isolation can be needed for various reasons, including safety, ground-loop breaking, and mitigation, of electromagnetic interference. Optotriacs are available as non-zero crossing (NZC) and zero crossing (ZC) devices. Depending on the situation, various VDE and UL requirements apply to optotriacs and the systems in which they are deployed.

NZC Phototriacs

Applications that need phase angle independent fine switching control use NZC triacs. Applications that can benefit from using NZC phototriacs include lighting dimmers, where brightness is dependent on when in a half cycle the phototriac is triggered, and in motor control, where fine control can be used to produce smooth and uninterrupted motion. The use of NZC phototriacs often results in sharp di/dt transients that cause significant levels of EMI (Figure 4).

Figure 4: NZC phototriacs can turn on at any point in the sine wave cycle, often resulting in a high di/dt and large amounts of undesirable EMI. (Image: Vishay)

The sharp di/dt transitions are the source of EMI concerns. Electromagnetic energy can be radiated into space and travel down the power lines. In addition, the power lines can carry lower frequency harmonics that are not readily radiated in free space. Expectations for minimizing and controlling EMI to comply with various standards can eliminate the use of NZC phototriac solutions in many instances.

ZC Phototriacs

ZC phototriacs are suited to a different class of applications with a larger control time constant such as solenoid drivers, some types of solid state relays, and heater controls. EMI reduction is a key benefit of using ZC phototriacs. Since the device is triggered as the zero crossing point of the sine wave, the di/dt transition is low and well-controlled. In addition, since the trigger always takes place at the zero crossing point, it allows the longest time possible for current to build up in an inductive load, reducing system stresses.

Summary

Triacs and quadracs are useful for ac line frequency power applications such as lighting, motor speed, and temperature controls. Triacs can be driven using dual SCRs or a single diac device. A quadrac is produced by integrating a triac and diac in a single package, reducing component count. In addition to conventional triacs, phototriacs are available for use in applications that benefit from their inherent electrical isolation between the drive and power circuits. Phototriacs are available with and without zero crossing switching to meet the performance needs of specific applications.

References

Controlling LED Lighting Using Triacs and Quadracs, Littelfuse
Phototriac Basics, Vishay
Quadrac, Wikipedia
Triac, Wikipedia
Triac-Based Motor Controller, Renesas