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What is a Phototriac?

Phototriac motor control

There are a variety of electrically switchable devices that allow for safe control over current flow and voltage distribution in an electronic system. Relays, SCRs, analog switches, and physical switches are all useful for controlling current flow in a design and they are available in packages that can be mounted to a PCB. There is one type of electrically controlled switching element that can be electrically or optically toggled: a triac. In the case of an optically toggled version, we call it a phototriac.

Triacs and phototriac operate in similar ways and have very similar construction. The primary difference is how they are toggled (electrically vs. optically) and their sensitivity to the input signal. In addition, phototriacs can have a more complex detector construction than a typical phototransistor. Finally, phototriacs can operate very well in power electronics circuits where delivery of a line voltage to a load needs to be toggled.

Triacs vs. Phototriacs

Triacs give designers a simple way to implement switching with an AC circuit, but without using a physical switch on an AC line. This creates a major benefit in terms of safety. These components are also available in small packages, so they do not take up as much space as a comparable relay. Both types of components can withstand high voltages and moderate currents, but when higher current protection is needed a relay may be a better option.

Phototriac Packages

Phototriacs are triggered using an input optical signal, usually a continuous IR signal. The IR signal is provided by an LED. The image below shows a typical schematic symbol used to represent a phototriac package. This component would accept an input DC current or switching signal from a GPIO on the left side, and the output would be used to control a standard triac or modulate current delivery to a load on the right side.

Phototriac circuit example

DC voltage or a signal being used to turn on a phototriac in an optotriac IC.

Phototriacs are normally used in small through-hole or surface-mount packages; they are not available as elements that can be directly modulated with an external light source. These components include an LED inside the component package, and it is this LED that is used to toggle the phototriac component inside the package. These components are also known as optotriacs and they fall under the general category of optoisolators.

To use this type of component successfully, and to eliminate switch bounce and transients, some additional components may be needed:

  • A series resistor on the LED input to limit forward current (R in the above circuit)
  • A small capacitor across the LED input to dampen turn-on
  • Multiple passives on the output to smooth power delivery to a load

If your system will use an external light source to trigger switching (such as a laser pointer or discrete IR laser diode/LED), the better option is a phototransistor or photodiode. A phototransistor would typically be the best choice when simple conduction of a signal is needed upon illumination with an optical signal. These components can also be used in a kind of depletion mode, where they are modulated into an ON state and they divert current away from a load.

Example: Driving a Standard Triac With a Phototriac

One of the common applications of a phototriac in motor control and power electronics is its use as a modulator for a standard triac. In this example, the phototriac is triggered with an input signal on the left side of the component. The input signal can be modulated with the logic gate on the low side of the input. Next, this toggles the phototriac ON, and this delivers voltage to the main triac on the output side. Once the main triac turns on, it begins conducting and the motor receives power.

Phototriac circuit motor control

DC voltage or a signal being used to turn on a phototriac in an optotriac IC.

In this example, the ground net on the input side is disconnected from the AC side, meaning there is galvanic isolation between these two regions in the board. In the PCB layout, there should be two separate ground regions in the design, one of these will follow the standard set of digital design practices and the other could be allocated to earth or chassis.

Other applications phototriacs include:

  • Static switching
  • Inrush current limiting
  • Optical solid state relays

In each of these applications, there may be no isolation, meaning galvanic isolation is not required for a phototriac to work properly. In some application notes or datasheets, you may see the ground nets on each side of a phototriac connected. Make sure you understand and identify whether galvanic isolation is needed in your system when designing a phototriac circuit.

When you’re ready to create and simulate your optically isolated circuits using phototriacs, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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