Discrete Thyristors (SCR and TRIAC)

Thyristor Voltage Suppressors
for Circuit Protection

A Thyristor is any semiconductor switch with a bi-stable action depending on p-n-p-n regenerative feedback. Thyristors are normally two- or three-terminal devices for either unidirectional or bi-directional circuit configurations.

Thyristors can have many forms, but they all have certain commonalities:

  • They are solid state switches that are normally open circuits (very high impedance)
  • They are capable of withstanding rated blocking/off-state voltage until triggered to on state
  • When triggered to on state, they become a low-impedance current path until principle current either stops or drops below a minimum holding level
  • After a Thyristor is triggered to on-state condition, the trigger current can be removed without turning off the device

Thyristors are used to control the flow of electrical currents in applications including:

  • Home appliances - lighting, heating, temperature control, alarm activation, fan speed
  • Electrical tools - for controlled actions such as motor speed, stapling event, battery charging
  • Outdoor equipment - water sprinklers, gas engine ignition, electronic displays, area lighting, sports equipment, physical fitness


  • High Voltage and Ampere ratings
  • Unidirectional and Bidirectional transient voltage protection
  • Automatically triggered "off" for specied periods of time
  • RoHS compliant
  • Glass-passivated junctions
  • High voltage capability up to 1000 V
  • High surge capability up to 950

Product Descriptions

Sensitive Triacs

Littelfuse's sensitive gate Triacs are AC bidirectional silicon switches that provide guaranteed gate trigger current levels in Quadrants I, II, III, and IV. Interfacing to microprocessors or other equipment with single polarity gate triggering is made possible with sensitive gate Triacs. Gate triggering currents of 3 mA, 5 mA, 10 mA, or 20 mA may be specified.

Sensitive gate Triacs are capable of controlling AC load currents from 0.8 A to 8 A rms and can withstand operating voltages from 400 V to 600 V.

Standard Triacs

Littelfuse's products are bidirectional AC switches, capable of controlling loads from 0.8 A to 35 A rms with 10 mA, 25 mA, and 50 mA IGT in operating Quadrants I, II and III.

Triacs are useful in full-wave AC applications to control AC power either through full-cycle switching or phase control of current to the load element. These Triacs are rated to block voltage in the "OFF" condition from 400 V minimum with selected products capable of 1000 V operation. Typical applications include motor speed controls, heater controls, and incandescent light controls.


Quadrac devices, originally developed by Littelfuse, are Triacs and Alternistor Triacs with a DIAC trigger mounted inside the same package. These devices save the user the expense and assembly time of buying a discrete DIAC and assembling in conjunction with a gated Triac.

The Quadrac is offered in capacities from 4 A to 15 A rms and voltages from 400 V to 600 V.

Alternistor Triacs

The Alternistor Triac is specifically designed for applications required to switch highly inductive loads. The design of this special chip effectively offers the same performance as two Thyristors (SCRs) wired inverse parallel (back-to-back).

This new chip construction provides the equivalent of two electrically-separate SCR structures, providing enhanced dv/dt characteristics while retaining the advantages of a single-chip device.

Littelfuse manufactures 6 A to 40 A Alternistor Triac with blocking voltage rating from 400 V to 1000 V. Alternistor Triacs are offered in TO-220, TO-218, and TO-218X packages with isolated and non-isolated versions.

Sensitive SCRs

Littelfuse's sensitive gate SCRs are Silicon-Controlled Rectifiers representing the best in design, performance, and packaging techniques for low- and medium-current applications.

Anode currents of 0.8 A to 10 A rms can be controlled by sensitive gate SCRs with gate drive currents ranging from 12 μA to 500 μA. Sensitive gate SCRs are ideally suited for interfacing to integrated circuits or in applications where high current load requirements and limited gate drive current capabilities exist. Examples include ignition circuits, motor controls, and DC latching for alarms in smoke detectors. Sensitive gate SCRs are available in voltage ratings to 600 V.


Littelfuse's SCR products are half-wave, Silicon-Controlled Rectifiers that represent the state of the art in design and performance.

Load current capabilities range from 1 A to 70 A rms, and voltages from 400 V to 1000 V may be specified to meet a variety of application needs.

Because of its unidirectional switching capability, the SCR is used in circuits where high surge currents or latching action is required. It may also be used for half-wave-type circuits where gate-controlled rectification action is required. Applications include crowbars in power supplies, camera flash units, smoke alarms, motor controls, battery chargers, and engine ignition.

Surge current ratings are available from 30 A in the TO-92 packaging to 950 A in the TO-218X package.


Littelfuse manufactures 15 A to 25 A rms Rectifiers with voltages rated from 400 V to 1000 V. Due to the electrically isolated TO-220 package, these Rectifiers may be used in common anode or common cathode circuits using only one part type, thereby simplifying stock requirements.


DIACs are trigger devices used in phase control circuits to provide gate pulses to a Triac or SCR. They are voltage-triggered bidirectional silicon devices housed in DO-35 glass axial lead packages and DO-214 surface mount packages.

DIAC voltage selections from 27 V to 70 V provide trigger pulses closely matched in symmetry at the positive and negative breakover points to minimize DC component in the load circuit.

Some applications include gate triggers for light controls, dimmers, power pulse circuits, voltage references in AC power circuits, and Triac triggers in motor speed controls.


SIDACs represent a unique set of Thyristor qualities. The SIDAC is a bidirectional voltage triggered switch. Some characteristics of this device include a normal 95 V to 330 V switching point, negative resistance range, latching characteristics at turn-on, and a low on-state voltage drop.

One-cycle surge current capability up to 20 A makes the SIDAC an ideal product for dumping charged capacitors through an inductor in order to generate high-voltage pulses. Applications include light controls, high-pressure sodium lamp starters, power oscillators, and high-voltage power supplies.

Thyristor Product Information Resources

Click on the links below for additional information about the Littelfuse selection of thyristor products:

Littelfuse Thyristor General Reliability MTBF Failure Rate FITS Report

Switching Thyristor Selection Guide

pdf icon Fundamental Characteristics of Thyristors Application Note

The thyristor family of semiconductors consists of several very useful devices. The most widely used of this family are silicon controlled rectifiers (SCRs), triacs, sidacs, and diacs. In many applications these devices perform key functions and are real assets in meeting environmental, speed, and reliability specifications which their electro-mechanical counterparts cannot fulfill. This application note presents the basic fundamentals of SCR, triac, sidac, and diac thyristors so the user understands how they differ in characteristics and parameters from their electromechanical counterparts. Also, thyristor terminology is defined.

AN1007 - Teccor® Thyristors Used as AC Static Switches and Relays

This application note describes circuits in which these thyristors are used to perform simple switching functions of a general type that might also be performed non-statically by various mechanical and electromechanical switches. In these applications, the thyristors are used to open or close a circuit completely, as opposed to applications in which they are used to control the magnitude of average voltage or energy being delivered to a load. These latter types of applications are described in detail in "Phase Control Using Thyristors"(AN1003).

AN1009 Teccor Thyristors Miscellaneous Design Tips and Facts

This application note presents design tips and facts on the following topics: -Relationship of IAV, IRMS, and IPK -dv/dt Definitions -Examples of gate terminations -Curves for Average Current at Various Conduction Angles -Double-exponential Impulse Waveform -Failure Modes of Thyristor -Characteristics Formulas for Phase Control

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