CRO and its Working

Hi friends, in this post we are going to learn about Basics of Cathode Ray Tube (CRO) and its function. We will also see the working of CRO.

Block Diagram of CRO:

CRO block diagram
CRO block diagram

Cathode Ray Tube (CRT):

CRT Produces a sharply focused beam of electrons, accelerated to a very high velocity. This electron beam travels from electron gun to the screen. The electron gun consists of filament, cathode, control grid, accelerating anodes and focusing anode. While travelling to the screen, electron beams passes between a set of vertical deflecting plates and a set of horizontal deflection plates. Voltages applied to these plates can move the beam in vertical and horizontal plane respectively. The electron beam then strikes the fluorescent material (phosphor) deposited on the screen with sufficient energy to cause the screen to light up in a small spot.

Vertical Amplifier:

The input signal is applied to vertical amplifier. The gain of this amplifier can be controlled by VOLT/DIV knob. Output of this amplifier is applied to the delay line.

Delay Line:

The delay Line retards the arrival of the input waveform at the vertical deflection plates until the trigger and time base circuits start the sweep of the beam. The delay line produces a delay of 0.25 microsecond so that the leading edge of the input waveform can be viewed even though it was used to trigger the sweep.

Trigger (Sync.) Circuit:

A sample of the input waveform is fed to a trigger circuit which produces a trigger pulse at some selected point on the input waveform. This trigger pulse is used to start the time base generator which then starts the horizontal sweep of CRT spot from left hand side of the screen.

Time base (Sweep) Generator:

This produces a saw – tooth waveform that is used as horizontal deflection voltage of CRT. The rate of rise of positive going part of sawtooth waveform is controlled by TIME/DIV knob. The sawtooth voltage is fed to the horizontal amplifier if the switch is in INTERNAL position. If the switch is in EXT. position, an external horizontal input can be applied to the horizontal amplifier.

Horizontal Amplifier:

This amplifies the saw – tooth voltage. As it includes a phase inverter two outputs are produced. Positive going sawtooth and negative going sawtooth are applied to right – hand and left – hand horizontal deflection plates of CRT.

Blanking Circuit:

The blanking circuit is necessary to eliminate the retrace that would occur when the spot on CRT screen moves from right side to left side” This retrace can cause confusion if it is not eliminate. The blanking voltage is produced by sweep generator. Hence a high negative voltage is applied to the control grid during retrace period or a high positive voltage is applied to cathode in CRT.

When a sawtooth voltage is applied to horizontal plates and an input signal is applied to vertical plates, display of vertical input signal is obtained on the screen as a function of time.

Power Supply:

A high voltage section is used to operate CRT and a low voltage section is used to supply electronic circuit of the oscilloscope.

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Tags: Working of CRO, How CRO generates waveforms, Working principle of CRO.

Construction and working principle of Megger for measurement of High resistances

We know that the ratiometer ohmmeters may be designed to cover a wide range of resistances. The principle of ratiometer ohmmeters is particularly adapted to application in portable instruments measuring insulation resistance. This principle forms the basis of insulation testing instrument known as Meggar.

Megger for measurement of high resistance
Megger for measurement of high resistance

Construction and working of Megger:

The main parts of the Megger are shown in the figure.

The current coil is same as that of permanent magnet moving coil instrument. V1 and V2 these are the two potential or voltage coils. The voltage coil V1 embraces the annular magnetic core. As shown in figure voltage coil V1 is in weak magnetic field when the pointer is at infinity and hence this coil exerts very little torque.

The torque exerted by this voltage coil increases as it moves into a stronger field and this torque will be maximum when it is under the pole face and under this condition the pointer will be at its zero end of the resistance scale.

In order to modify further the torque in the voltage circuit, another voltage coil V2 is used. This coil is also located in such a way it cam move from infinity to zero position of the resistance scale. The coil finally embraces the extension H of the pole piece.

The combined action of the two voltage coils V1 and V2 may be considered as though the coils constituted  a spring of variable stiffness ,being very stiff near the zero end of the scale where the current in the current coil is very small (on account of unknown resistance Rx is very large).

Thus this effect compresses the low resistance portion of the scale and opens up the high resistance of the scale. This is a great advantage since this instrument is meant to be used as “insulation tester” as the insulation resistances are quite high.

The voltage range of the Meggar can be controlled by voltage selector switch. This can be done by varying the resistance R connected in series with the current coil.

The test voltages usually 500,1000 or 2500 V can be generated using hand driven generator G. A centrifugal clutch is incorporated in the generator drive mechanism which slips at a predetermined speed so that a constant voltage is applied to the insulation under test. This voltage provides a test of strength of low voltage insulation as well as a measure of its insulation resistance  since it is sufficient to cause breakdown at faults. Such breakdowns are indicated by sudden motion of the pointer off scale at zero end. As the same magnet system supplies magnetic fields for both instrument and generator, and as current and voltage coils moves in a common magnetic field, the instrument indications are independent of the strength of the magnet.

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Three Point Starter – best explained

Three point starter:


three point starter
three point starter

figure shows the schematic diagram of a three point starter for a shunt motor. It is so called because it has three terminals L, F and A. the starter consist of starting resistance divided into several sections and connected in series with the armature. The tapping points of the starting resistance are brought out to a number of studs. The three terminal L, F and A of the starter are connected respectively to the positive line terminal, shunt field terminal and armature terminal. The other terminal of the armature and shunt field windings are connected to the negative terminal of the supply   .the no volt coil is connected in the shunt field circuit. One end of the handle is connected to the terminal L through the over load release coil. The other end of the handle moves against a spiral spring and make contact with each stud during starting operation cutting out more and more starting resistance as it passes over each stud in clockwise direction.

Working of three point starter:

1)      Initially when a DC supply is switched ON with handle in the OFF position.

2)     The handle is now moved clockwise to the first stud, the shunt field winding is directly connected across the supply while the whole starting resistance is inserted in series with the armature circuit.

3)     As the handle is gradually moved over to the final stud, the starting resistance is cut out of the armature circuit in steps. The handle is now held magnetically by the no volt release coil which is energized by shunt field current.

4)     If the supply voltage is suddenly interrupted or if the field excitation is accidently cut, the no volt release coil is demagnetized and the handle goes back to the OFF position under the pull of the spring. If no volt coil were not used, then in case of failure of supply. The handle would remain on the final stud. If then supply is restored, the motor will be directly connected across the supply, resulting in an excessive armature current.

5)     If the motor is overloaded (or any fault occurs) it will draw excessive current from the supply. This current will increase the ampere turns of the overload release coil and pull the armature, thus short circuited the no volt release coil. The no volt coil is demagnetized and the handle is pulled to the OFF position by the spring .thus the motor is automatically disconnected from the supply

Limitations of the three point starter:

Unnecessary tripping of starter if a field regulator is connected in series with the field windings for speed control operation. This combination and magnetizing coil will carry same current if more and more resistance is added in field regulator then current in magnetizing coil is unable to hold the arm in attracted position and unnecessarily the arm will be released even if there is no over loading .this is avoided in four point starter.

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four point starter 

Operating principle of ammeters and voltmeters:

In our day today life, many times we require to measure different electrical quantities like current, voltage, resistance, etc. While doing experiment, there is necessity of multimeter. As we have already discussed about multimeter, how it measures different electrical quantities like electrical current, voltage, resistance, etc. But the basic instruments for the measurement of electric current and voltage are ammeters and voltmeters respectively.

Let us discuss these instruments one by one, operating principle or working principle of ammeters and voltmeters, finally major differences between ammeters and voltmeters.

Operating Principle:

Analog ammeters and voltmeters are classed together as there are no fundamental differences in their operating principles. The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current. In an ammeter this torque is produced by a current to be measured or by a fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. Thus all analog voltmeters and ammeters are essentially current measuring devices.

See also: Study & use of Digital Multimeter

The essential requirement of measuring instruments are (i) that its introduction into the circuit, where measurements are to be made, does not alter the circuit conditions ;(ii)the power consumed by them for their operation is small.

Working principle of Ammeters:



Ammeters are connected in the series with the circuit whose current is to be measured. The power loss in an ammeter is (I^2.Ra) where I is the current to be measured Ra is the resistance of the ammeter therefore ammeter should have low electrical resistance so that they cause a small voltage drop and consequently absorb small power.

Working principle Voltmeters:



Voltmeters are connected in parallel with the circuit whose voltage is to be measured .the power loss in voltmeter is (V^2/Rv), where V is the voltage to be measured and Rv is the resistance of the voltmeter. Therefore voltmeters should have a high electrical resistance, in order that the current drawn by them is small and consequently the power consumed is small.

Difference between Ammeters and voltmeters:




Connection It is to be connected in series mode It is to be connected in parallel mode
Resistance It has comparatively low resistance It has high resistance
Uses It is used to find the amount of current flowing in the circuit It is used to find the potential difference in the circuit
Circuit Circuit must be disconnected in order to attach the ammeter Circuit does not need to be disconnected
Accuracy Considered as less accurate Considered as more accurate compared to ammeter


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