Summer 2016 – Q.6

6. (a) Compare : [6M]
(i) SCR and TRIAC
(ii) DIAC and TRIAC.

i) SCR and TRIAC

SCR	TRIAC
SCR stands for silicon controlled rectifier.	TRIAC stands for triode for alternating current.
The SCR is unidirectional device.	The TRIAC is bidirectional device.
It available in large ratings.	It available in smaller ratings.
The SCR control DC power.	The TRIAC control DC as well as AC power.
The SCR can be triggered by positive gate voltage only.	The TRIAC can be triggered either by positive or negative gate voltage.
In SCR only one mode of operation is possible.	In TRIAC four different modes of operation is possible.
It is more reliable.	It is less reliable.
The SCR conduct current in one direction only.	The TRIAC conduct current in both the directions.
It needs two heat sink.	It needs only one heat sink.

ii) DIAC and TRIAC.

DIAC	TRIAC
It is a two terminal device.	IT is a three terminal device.
It can be considered as two zener diodes connected back to back.	It can be considered a two thyristors connected in parallel but in opposite directions controlled by the same Gate.
It stands for Diode AC switch.	It stands for Triode AC Switch.
A diac is fabricated in three, four or five layer structure. The three layer diac is more common. The structure is very much similar to PNP transistor with no base connection.	A triac is a five layer, three terminal semiconductor device. The terminals are marked as MT1, MT2 as anode and cathode terminals in case of SCR. And the gate is represented as G similar to the thyristor.
It has much lower breakdown voltage typically 30V. So when the voltage across a diac reaches its breakdown voltage (in either direction) the diac spontaneously goes into conduction and the voltage across it drops to around 5V.	It can operate in both forward blocking mode and reverse blocking mode. Normally it is capable of handling large voltages.
Diac is mainly used as a trigger device to the triac.	Due to the bidirectional control of AC, triacs are used as AC power controllers, fan controllers, heater controllers, triggering devices for SCRs, three position static switch, light dimmers, etc.

Electronic weighing machine:-
It is a standard method of determining weight in many industrial applications.
Many of the weighing problems in the industrial process as well as in general. Weighing are solved by in corporating an electronic system.

Some of the main features of electronic weighing machine
i) Compact and small in size
ii) Easy to operate
iii) high resolution
iv) high stability and ruggedness

Fig. schematic block diagram of electronic weighing system

Fig. shows the schematic block diagram of electronic weighing system. It consists of load cell, suitable signal conditioner, analog to digital converter, and output recorders/indicators giving both the analog and digital outputs.

The load cells (which may be up to 4 or 8 cells) convert the weight data into electrical signal which is amplified to give an output of 0 to 5V or 20 mA. This signal can be used as input for analog recorder or servo indicator to print or indicate weight. But to get digital readouts or printing it is necessary to convert analog signal into digital signal. Analog to digital converter is used for this purpose. Interfacing circuit provides the necessary interface for different printing and indicating devices.

(c) Define : [2M]
(i) Active Transducer
(ii) Passive Transducer.

Active Transducer:
A transducer is a device that converts input energy into output energy. Typically the output energy is differing in kind but related to the input.

Active transducers generate electric current or voltage directly in response to environmental stimulation. Examples of active transducers are thermocouples and piezoelectric accelerometers. Thermocouples produce voltage difference on its terminals that is proportional to the temperature of the environment.

Passive transducer:
They convert one form of energy to other form without any use of energy. Passive transducers convert physical quantities like;temperature,pressure,speed etc.

Passive transducers produce a change in some passive electrical quantity, such as capacitance, resistance, or inductance, as a result of stimulation. These usually require additional electrical energy for excitation.

A simple example of a passive transducer is a device containing a length of wire and a moving contact touching the wire. The position of the contact determines the effective length of the wire, varying the resistance of the length of wire.
Other examples of passive transducers are strain gauges, resistance temperature detectors (RTDs), and thermistors.