SEMICONDUCTORS Triacs and Diacs

SEMICONDUCTORS The Triac has the same switching characteristics as an SCR but in both directions, it is the equivalent of two SCRs in parallel connected in opposite directions. Bidirectional Triode Thyristor or Triac

SEMICONDUCTORS In a triac each main terminal (at opposite ends of the device) and the gate are connected to a PN junction. The triac is a 4 layer NPNP device in parallel with a PNPN device.

SEMICONDUCTORS The equivalent circuit of a triac shows two SCRs connected in parallel but in opposite directions. The triac is designed to respond to the currents that flow through it’s single gate terminal to control both SCRs.

SEMICONDUCTORS The triac controls current flowing in either direction, this design is used to control AC power to various types of loads or circuits. The triac operates in a similar fashion to an SCR, a small gate current triggers to on state and is turned off by reducing their operating anode currents below their respective holding values.

SEMICONDUCTORS The schematic symbol below is sometimes labeled as MT1 and MT2 (main terminal 1 and 2). MT1 MT2

SEMICONDUCTORS The V-I characteristic curve is shown below, this shows current flows in both directions (-I & +I) and voltage flows in both direction (-V & +V).

SEMICONDUCTORS An SCR always requires a positive gate voltage but the TRIAC can be triggered by either a positive or a negative gate voltage. In order to create a triggering current, a positive or negative voltage has to be applied to the gate with respect to the MT1.

SEMICONDUCTORS Once triggered, the device continues to conduct until the current drops below a certain threshold, called the holding current (IH). The bi-directionality makes TRIACs very convenient switches for AC circuits, allowing them to control very large power flows with milli-ampere-scale gate currents.

SEMICONDUCTORS In addition, applying a trigger pulse at a controlled phase angle in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (phase control), which is commonly used, for example, in controlling the speed of low-power induction motors, in dimming lamps and in controlling AC heating resistors

SEMICONDUCTORS Below is a typical triac circuit used to vary the amount of AC power applied to a load. The triac is triggered into conduction on both the positive and negative alterations of each AC input cycle.

SEMICONDUCTORS A special triggering device is used to insure that the triac turns on at the proper time, capacitor C charges through R1 in one direction and then again in the other direction, during each alteration the triac is turned on when the voltage across C rises to the required level.

SEMICONDUCTORS One disadvantage of triacs is that they have lower current ratings (25A) than SCRs (700 to 800A). Triacs also have difficulty in switching the power applied to inductive loads. Triacs are designed for low frequency applications (50 to 400Hz) while SCRs can be used in frequencies up to 30,000Hz.

SEMICONDUCTORS We saw the use of a triggering device used with a triac, these triggering devices are referred to as bi-directional triggering diodes. The diac (bi-directional trigger diode) is similar to a bipolar junction transistor except that the doping concentrations around both junctions are equal and there are only two leads, each connected to the outer layers.

SEMICONDUCTORS The diac has no middle lead and resembles a regular PN junction diode. A diac will always be forward biased while the other is reverse biased, the reverse biased junction controls the current flowing through the diac.

SEMICONDUCTORS The diac remains in an off state until the applied voltage in either direction is high enough and the reverse biased junction reaches it’s breakdown voltage. The diac then turns on and current rises until it is limited by a series resistance, so the diac acts like a bi-directional switch.

SEMICONDUCTORS The diac is used in conjunction with a triac as a triggering device to provide full wave control of AC signals.

SEMICONDUCTORS The combination of the diac and triac are used to control the speed and direction of electric motors or control the temperature of heating elements. SCHEMATIC SYMBOLS