HELP 12

Question

Answer 1

6.3)
The light-activated SCR (LASCR) is the light sensitive equivalent of the normal SCR.
Its state is controlled by the light falling on depletion layers like in a normal SCR gate current turns on the device.
Just as a normal SCR, the LASCR will continue to conduct even if the light source is removed.

Diagram for the characteristic curve of the SCR :

The characteristic curve of LASCR like as SCR.

Answer 2

6.2)

Block diagram of the closed loop motor speed controller :

The speed set value d and the detected voltage e of the speed generated by a tacho-generator is compared in the comparison amplifier block.

Then the level of the voltage signal a is determined.
The voltage signal a is low when the speed detected value to the speed set value rises, and is higher when the speed set value lowers.

Since the trigger signal is output at the point where the triangular wave b intersects with the voltage signal a, the timing (phase angle) when the triac is turned on with a level of the voltage signal a is determined.

When this timing is slow, the voltage applied to the motor becomes low and the rotational speed of the motor decreases.

The lowered rotational speed is fed back again and the control is repeated so that the difference between the speed detected value and the speed set value may always be constant.

Answer 3

6.1)

The diac and the triac are used together in a variety of applications, such as the room incandescent-light dimmer switch.

 A diac-triac combined package is called as quadrac.

Spped control of small AC motor using quadrac :

This quadrac-based AC motor speed controller circuit is designed for controlling the speed of small household motors like drill machines.

The speed of the motor can be controlled by changing the setting of P1.

The setting of P1 determines the phase of the trigger pulse that fires the triac.

The circuit incorporates a self-stabilizing technique that maintains the speed of the motor even when it is loaded.