power electronics

1.ULRASONIC

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Light operated relay using LDR & SCR

Explain working of light operated relay using LDR & SCR

• Principle:

                      When light is emitted on LDR then internal resistance of LDR is decrease.

     We can control high voltage supply.

• Circuit diagram:

          

• Construction:

                       The circuit consists of one SCR and a load which is connected with SCR .instead of load we can use relay to which either AC&DC load can be connected a variable resistance and LDR are connected in series and the combination is connected to the gate of the SCR AC supply is given to circuit.

• Working:

                       Now as we know internal resistance of the LDR is high. when light is not falls or emitted .at that timer when supply is given and sufficient light is not available  to the LDR the resistance of the, LDR is high so a very small current flow through the LDR and to gate of SCR which is enable to turn on SCR. so the SCR is off and load current does not flow .to turn on SCR ,gate pulse is required to this high current is required to flow through LDR to gate .when light is falls on LDR, the applications of the LDR becomes low ,so large current flows in to the gate of the SCR as a result the SCR is on and load current is states to flow since it if is a DC supply than commutation circuit is required to turn off supply and if SCR is working on AC supply. the SCR is automatically turned off after  +ve half cycle so in the next cycle again light should falls on the SCR to trigger the SCR thus the LDR can be used to trigger the SCR .

                       Now, when relay is connected instead of load then when light is emitted on LDR than SCR turned on and relay is energized due to large current flow through relay.

• Application:

•   To light switch.

•   Missile system.
•   Light operated alarm.
•   Camera exposure setting.
•   Street light controller.

Ultrasonic waves and Megnetostractive Generator

 Write Properties of Ultrasonic waves.

Ans.  1. The ultrasonic waves are high frequency sound waves.

2. They are having smaller wavelength.

3. They produce heating effect when passes through the medium.

4. They get reflected, refracted, interference, diffraction and absorbed by the medium similar to the ordinary sound waves.

5. They act as catalytic agents to accelerate chemical reactions.

6. They produce stationary wave pattern in the liquid while passing through it.

o   This is known as magnetostriction. The change in length depends on the intensity of the applied magnetic field and nature of the ferromagnetic material. The change in length is independent of the direction of the field. When the rod is placed inside a magnetic coil carrying alternating current; the rod suffers a change in length for each half cycle of alternating current.


Explain working of Megnetostractive Generator.

Ans.  Principle:

o   The general principle involved in generating ultrasonic waves is to cause some dense material to vibrate very rapidly. The vibrations produced by this material than cause air surrounding the material to begin vibrating with the same frequency.

o   These vibrations then spread out in the form of ultrasonic waves. When a magnetic field is applied parallel to the length of a ferromagnetic made of material such as iron or nickel, a small elongation or contraction occurs in its length.

o   That is, the rod vibrates with a frequency twice that of the frequency of A.C. The amplitude of vibration is usually small, but if the frequency of the A.C. coincides with the natural frequency of the rod, the amplitude of vibration increases due to resonance.

Construction:

o   The ends of the ferromagnetic rod A and B is wound by the coils L1 and L2. The coil L is connected to the collector of the transistor and the coil L1 is connected to the base of the transistor as shown in the figure.

o   This is known as magnetostriction. The change in length depends on the intensity of the applied magnetic field and nature of the ferromagnetic material. The change in length is independent of the direction of the field. When the rod is placed inside a magnetic coil carrying alternating current; the rod suffers a change in length for each half cycle of alternating current.o   The frequency of the oscillatory circuit (LC) can be adjusted by the condenser C and the current can be noted by the millimeters connected across the coil L2.

o   The battery connected between emitter and collector provides necessary biasing i.e., emitter is forward biased and collector is reverse biased for the NPN transistor. Hence, current can be produced by applying necessary biasing to the transistor with the help of the battery.

Working:

o   The rod is permanently magnetized in the beginning by passing direct current. The battery is switched on and hence current is produced by the transistor.

o   This current is passed through the coil L2, which causes a corresponding change in the magnetization of the rod. Now, the rod starts vibrating due to magnetostriction effect.

o   When a coil is wounded over a vibrating rod, then e.m.f. will be induced in the coil called as converse magnetostriction effect. Due to this effect an e.m.f. is induced in the coil L1.

o   The induced e.m.f. is fed to the base of the transistor, which act as a feedback continuously. In this way the current in the transistor is built up and the vibrations of the rod is maintained.

o   The frequency of the oscillatory circuit is adjusted by the condenser C and when this frequency is equal to the frequency of the vibrating rod, resonance occurs.

o   At resonance, the rod vibrates longitudinally with larger amplitude producing ultrasonic waves of high frequency along both ends of the rod.