Triacs and Diacs A power device with four layers conducts in one direction only. Bidirectional device may be obtained by connecting two of these back-to-back. The five layers, n1,p1,n2,p2,n3 can be combined into a single structure to form a new device. When T1 is positive with respect to T2 by voltage greater than VBO  (p2,n2,p1,n1) thyristor will be on. If Reverse polarity (p1,n2,p2,n3) will be on.

A five-layer device without agate can be designed for various breakdown voltages and current ratings. A diac is a five-layer gateless device. A diac is a device used to trigger other semiconductor power switches. Current and voltage ratings are determined by the type of device.

A five-layer device with gate is called triac.( four stages)

Unijunction Transistor (UJT) Complementary unijunction transistor (CUJT) Programmable unijunction transistor (PUT) The UJT has one pn junction and is used mainly as a triggering device in thyristor circuits and can also be used in oscillator circuits. The symbol is similar to a JFET. Note the angle of the emitter. The other terminals are called base 1 and base 2. The characteristics are quite different than any other transistor.

The resistive equivalent circuit of a UJT shown makes it easier to understand its operation. The emitter current controls the value of r B1 inversely. The total resistance or interbase resistance (r BB ) equals the sum of r B1 and r B2. The standoff ratio (  ) is the ratio r B1 / r BB.

UJT Operation: Unijunction transistor can trigger larger thyristors with a pulse at base B1.With the emitter disconnected, the total resistance R BB, a datasheet item, is the sum of R B1 and R B2. R BBO ranges from 4- 12kΩ for different device types. The intrinsic standoff ratio η is the ratio of R B1 to R BBO. It varies from 0.4 to 0.8 for different devices.

As V E increases, current I E increases up I P at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of R B1, the saturation resistance is lowest at the valley point.

I P and I V, are datasheet parameters; For a 2n2647, I P and I V are 2µA and 4mA, respectively. V P is the voltage drop across R B1 plus a 0.7V diode drop; V V is estimated to be approximately 10% of V BB. Peak votage of UJT Vp Vp=ηVbb +Vd

Complementary unijunction Transistor (CUJT) Like standard UJT except that the currents and voltages applied to it are of opposite polarity. Ideal for stable oscillators, timers, frequency dividers.

Programmable unijunction Transistor (CUJT) Although it has the same name as a UJT the programmable unijunction transistor’s structure is not the same. It is actually more similar to an SCR.

Programmable unijunction Transistor (CUJT) The PUT can be “programmed” to turn on at a certain voltage by an external voltage divider. This yields a curve similar to a UJT.

Programmable unijunction Transistor (PUT) External PUT resistors R1 and R2 replace unijunction transistor internal resistors R B1 and R B2, respectively. These resistors allow the calculation of the intrinsic standoff ratio η.

Programmable unijunction Transistor (PUT) VR is voltage divider (R1 and R2 can be specified) Vc capacitor voltage When Vc > VR the PUT will conduct

Programmable unijunction Transistor (PUT)

unijunction Transistor trigger circuits

LASCR (Light Activated SCR) End of Lecture