D6 Diac, LASCR, UJT

Question 1

What does Diac stand for?

Answer 1

bi-directional diode thyristor

also called the bilateral trigger diode

Question 2

Describe the main function of a Diac

Answer 2

to trigger SCRs and Triacs

Question 3

Why are Diacs used to trigger SCRS and Triacs

Answer 3

ensure consistent firing angles and more stable triggering

Question 4

Define: voltage-sensitive switch or Diac

Answer 4

as the voltage increases the Diac will not conduct until some threshold voltage

Question 5

Define: breakover voltage (VBO) as pertains to a Diac

Answer 5

the point or threshold voltage at which the resistance of the Diac goes low and the Diac conducts.

Question 6

Diacs are rated in

Answer 6

breakover volts

Question 7

list the components of the Diac oscillator cct or sawtooth generator

Answer 7

variable resistor
capacitor
Diac

Question 8

a diac-triac combination is commonly used in

Answer 8

a incandescent lamp dimmer

Question 9

LASCRs are frequently used to fire a

Answer 9

larger SCR

Question 10

What is the voltage across a conducting Diac

Answer 10

10 to 15V

Question 11

How is the light sensitivity of a LASCR accomplished?

Answer 11

by connecting a rheostat in the gate that diverts a portion of the light-energy produced to ground thus making the LASCR less sensitive.

Question 12

snubber circuit in dc motor control

Answer 12

is used to prevent firing of the Triac due to induced voltages caused by collapsing fields in the windings.

Question 13

the light in a dimmer cct

Answer 13

is on high with low resistance in the gate cct because the capacitor can charge quickly

Question 14

List the three terminals of a UJT

Answer 14

(E) emitter
(B1) base one
(B2) base two

Question 15

Diac terminals are labelled

Answer 15

MT1 and MT2