Energy is product of power of time. Electrical energy can be measured in watt second. But the convenient reading unit in practical world is kilo watt hours i.e. kWh, while quantifying energy it is expressed in units consumed.

1 unit of energy = 1 kWh

The electricity supply company supplies electrical power to consumers (utility). This power consumed by consumer over a period of time is measured i.e. energy is measured in terms of units. The rate of cost per unit is decided by the supplier. The meter which measures the electrical energy is known as Electricity Meter.

The LT (Low Tension) consumers have to use single phase electricity meters and HT (High Tension) consumers have to use three phase electricity meter. The electrical connection of electricity meter is similar to wattmeter.

Figure 2: Elements in electricity meter.

There are two elements in electricity meter:

1. Current coil or series coil
2. Voltage coil or shunt coil

The current coil is in series with the load and voltage coil is in parallel to the load and supply (See Figure 2).

Working Principle of Electricity Meter

Fig. 3: Constructional details of single phase induction type electricity meter.

An alternating current is passed through the coils of two electromagnets-shunt magnet and series magnet. These two electromagnets produce their own magnetic flux. The magnetic flux of shunt magnet is proportional to supply voltage and magnetic flux of series magnet is proportional to load current. These two fluxes are alternating in nature. The interactions of two fluxes produce a resultant flux. An aluminium disc is suspended between the shunt magnet and series magnet. The resultant flux is also alternating in nature. This resultant flux links with the aluminium disc and induces eddy currents in the disc. These eddy currents interact with the resultant flux and a torque is produced on the disc. The disc thus starts rotating. The rotation of disc is proportional to power i.e. N ∝ VI cos φ. These revolutions can be measured over a period of time and thus energy can be quantified.

Construction of Electricity Meter

The entire parts of single phase induction type electricity meter can be divided into four different systems.

• Driving system
• Moving system
• Braking system
• Registering mechanism

Moving system and registering mechanism

It consists of aluminium disc suspended between shunt and series magnet as shown in Fig. 3. The aluminium disc is mounted on spindle which is supported at both ends by jewel bearings. The disc rotates and causes rotation of spindle. The spindle transfers its motion to gears connected to registering mechanism. This registering mechanism counts number of revolution over a period of time i.e. energy. The units consumed are displayed in terms of numbers.

Braking system

Fig. 4: Circuit diagram / Connection diagram of single phase induction type electricity meter

This system is for speed adjustment of disc. A permanent magnet known as braking magnet is provided at the edge of the disc as shown in Fig. 3. If speed of the disc is to be reduced, the braking magnet is taken towards the edge of the disc. If speed of the disc is to be increased, the braking magnet is taken towards centre of the disc. The connections of electricity meter are similar to wattmeter (see Figure 4). The current coil terminals are designated as 1S – 1L. The pressure coil terminals are designated as 2S – 2L.

Driving system

It consists of two electromagnets shunt magnet and series magnet. Shunt magnet is E-shaped and series magnet is C shaped. Shunt magnet is wound with pressure coil (PC). Shunt magnet is E-shaped and series magnet is C shaped. Shunt magnet is wound with pressure coil (PC) of thin conductor with large number of turns.

Series magnet is wound with current coil (CC) of thick conductor with few numbers of turns. The electromagnets are made up of silicon steel stampings. Copper shading bands are provided on central limb of shunt magnet. Due to this copper shading band is for lag adjustment. Hence it is also known as lag adjustment device. It brings the shunt magnet flux at 90° lagging position w.r.t. supply voltage. For satisfactory operation of electricity meter the phase angle (φ) between supply voltage V and pressure coil flux should be 90°.

V = KVI cos φ sin α = KVI cos φ sin (90°)

N = KVI cos φ

where,

V pressure coil voltage

I = load current

φ = p.f. angle

α = phase angle or angle of lag

The Cu shading bond adjusts angle α to 90°

Thus the disc revolutions are proportional to VI cos φ i.e. power. These revolutions are measured over a period of time to measure energy.

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Clamp Meter (or Clamp on meter) is a current transformer device to measure the high values of currents without disturbing (interrupting) the circuit.

Special features of Clamp Meter

Clamp Meter can be used instantaneously for the measurement of current with no physical connection with the HV circuits. It is portable and used in HV or LV line current measurements. Its core can be opened (split) with strigger switch to accommodate the cable, wire, bus bar, fuse link etc. for current measurement.

Construction & Working of Clamp Meter

Figure 1: Clamp Meter Parts.

The Clamp Meter has the following parts (see Figure 1),

1. Split core
2. Secondary winding
3. Rectifier unit
4. Strigger switch (mechanism for opening core)
5. Ammeter

The meter consists of split core (1) which can be opened to accommodate primary (i.e. cable, bus bar, conductor, feeder) by pressing a strigger switch (4). On the bottom of the core secondary winding (2) is accommodated, ac voltage induced in this winding is converted into d.c by a rectifier unit (3). D’arsonval movement ammeter (5) is connected to the rectifier to receive d.c. for operation.

The arrangement of conductor (feeder) is primary, coil on core is secondary, forms a step down transformer circuit. Though the meter is connected to secondary, its scale is calibrated to read primary current (i.e. current to be measured of H.T. feeder, cable).

Advantages of Clamp Meter

1. Clamp on meter is generally used for HV line currents.
2. Accuracy is about 3
3. The instrument can be operated by one hand.
4. It is useful for measuring AC current only and not DC.

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While analyzing an analog circuit, the waveform on a CRO is observed at the test point. By comparing this waveform with the actual waveform expected in a healthy circuit, we can identify the fault. In digital circuits, this process will not work as the train of pulses representing binary codes (0, 1) look very similar to each other. Hence, while fault location in digital circuits, a train of pulses at the test point is converted into a recognizable form such as in Hexadecimal number. The recognizable form is called a “signature” and the technique is called as “signature analysis”. Therefore the fault location is reduced just to feed the appropriate input data and testing for the “signature” obtained at the output or test point.

Block diagram of Signature Analyzer

Fig. 1. Block diagram of Signature Analyzer.

The block diagram of a signature analyzer is shown in Fig. 1, giving the START signal. A register begins the process of capturing data bits. By giving STOP signal, this process is stopped after a time determined by CLOCK. The data bits captured by the register are displayed by a “hexadecimal signature.”

Test programs are written for various parts of the circuit. Now the signature analyzer is used to test the correct signature. The signature analyzer may have facility to store good “signature” to make comparison. An incorrect signature will indicate a fault.

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A logic probe is an equipment, which can indicate the logic state of a circuit, i. e., high logic (1) or low logic (0). A logic probe can view only one logic signal at a time and cannot store a record of the signal.

Block diagram of logic probe

The logic probe is powered by the circuit under test by connecting its leads to the DC supply lines of the board under test. Note that interconnection between two or more logic devices in a circuit is called a Node. The tip of the probe is placed in contact with the “node” under test. The selector switch is used to set for appropriate level of TTL (Transistor Transistor Logic) or CMOS (Complementary Metal Oxide Semi-conductor Logic). This setting can also be done automatically.

Fig. 1. Block diagram of logic probe.

Fig. 1. shows block diagram of the logic probe. The input logic level obtained from the probe tip is fed to a voltage detector through an over voltage protector. The voltage detector is set for appropriate voltage level for TTL or CMOS logics (manually or automatically). The detected voltage level say HIGH is fed to the appropriate memory (high) and the logic state is fed to the LED indicator till it is cleared by “reset” circuit. The process is repeated for LOW logic level. Recall that logic probe can detect one signal at a time.

Notes:

1. Usually, the logic probe has 3 indicators: high logic, low logic, and pulse. The “Pulse” indicator flashes for pulsating output i.e., when transition takes place from one logic level to another.
2. The logic probe can also detect the frequency pulses of shorter duration, which is difficult for a CRO.
3. The logic probe can be used to detect faulty ICs and gates by observing one or zero outputs for all input combinations.

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The spectrum analyzer is an instrument, which is capable to display graphically, the “amplitude as a function of frequency” in a portion of the RF spectrum and covers a frequency range of 10 MHz to 40 GHz. The instrument is widely used for the measurement of attenuation, FM deviation, and frequency in pulse modulation techniques.

The spectrum analyzer gives us the display of frequency spectrum of the input signal on a CRO screen. This display is very helpful in the analysis of any input signal; as it gives information about the location and strength of all the frequency components of the input signal.

Block Diagram of Spectrum Analyzer

Figure 1: Block Diagram of Spectrum Analyzer.

Most spectrum analyzers operate on the principle of “Heterodyne Wave Analyzer”. The Fig. 1 shows block diagram of a basic spectrum analyzer. The input signal is passed through an attenuator and then mixed in a mixer with a signal from a variable frequency (tunable) oscillator. The mixed signal is then filtered in a fixed frequency filter. The output of the filter is then detected, amplified and supplied to the vertical deflection plates of the CRO. A sawtooth signal generator generates signal for the oscillator, and frequency of the oscillator is controlled by the instantaneous value of the sawtooth voltage.

The oscillator frequency sweeps linearly and increases from its minimum to maximum value, as the sawtooth voltage rises from its minimum to the maximum value. The same sawtooth signal is supplied to the horizontal deflection plates of the CRO. So we can say that when sawtooth signal starts to rise from its minimum value, two events occur at a time: the frequency of the oscillator starts to increase and simultaneously the spot on the CRO screen travels in the horizontal direction.

Working of Spectrum Analyzer

As the oscillator frequency increases, first of all, the fundamental frequency component i.e., harmonic of the input signal is filtered out from the fixed frequency filter. This filtered signal is then detected, amplified and supplied to the vertical deflection plates of the CRO, so the spot on the CRO screen is deflected to the left say at point A, and deflection of the spot in the vertical direction shows the amplitude of that particular frequency component.

As voltage of the sawtooth signal rises further, frequency of the oscillator increases. so second frequency component (i.e., second harmonic) is filtered out and after detection and amplification reaches to the vertical deflection plates of the CRO, the spot moves in horizontal direction towards right and say comes to point B, at this point, deflection of the spot in the vertical direction will show amplitude of the second harmonic component.

Similarly, all the harmonic components appear on the CRO screen. The scanning range of this instrument is equal to the tuning range of the oscillator.

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Arbitrary waveform can be defined as a waveform which does not have a particular predefined shape or characteristics. The amplitude and frequency of an arbitrary waveform vary in a random manner. An arbitrary waveform may possess periodicity at some times and non-periodicity at other times. It may also include transients, noise components etc.

An Arbitrary waveform can be generated by superimposing either noise or DC offset voltages upon a standard signal or by introducing gaps between waveform bursts or by performing various modulations (such as amplitude, frequency, phase modulations) on a standard signal.

Arbitrary waveforms are used as test signals to determine whether the test equipment is functioning properly and also to detect any faults if present in the equipment. In this respect, arbitrary waveform is applied as input to the test equipment and its response is analyzed as the arbitrary waveform progresses through the equipment.

Block Diagram of Arbitrary Waveform Generator

Arbitrary Waveform Generator is a waveform generator, which generates waveforms based on digital data stored in RAM. This digital data gives the detail information of the constantly varying voltage levels of an AC signal without or with DC content. The basic block diagram of arbitrary waveform generator is shown in the below figure 1.

Figure 1: Block Diagram of Arbitrary Waveform Generator

In this type of waveform generator, digital data is stored in waveform random access memory. In this type, a cathode ray oscilloscope is used to measure a waveform in which the data is sampled. A digital to analog converter shown in figure 1, is used to read back the memory locations and feeding the data points thereby reconstructing the signal at any time. From Nyquist sampling theorem we know that,

f_s = 2f_m

Where,

f_s = Sampling frequency

f_m = Maximum frequency component of the sampled signal

If the above condition is satisfied then we can achieve better fidelity. Therefore, the details of the needed signal discovered which requires as many points as necessary for digital data stored in RAM. Usually, the stored data points are read by arbitrary waveform generator whose frequency limits are specified. But, the instrument can operate at a finite maximum frequency. Usually, the operating frequency or sample rate of this instrument is specified in terms of M samples/s or G samples/s.

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Working Principle of Digital Frequency Meter

In digital frequency meter, the unknown frequency signal is first converted into a train of pulses and then continuously supplied to one of the two inputs of the AND gate.

Figure (1): Basic Principle digital frequency meter.

A pulse having 1’s duration is applied to another input terminal of the gate is shown in figure (1). The total number of pulses passed through the gate during this time period is counted by an electronic counter. Here each pulse (in a train of pulses) indicates one cycle of the unknown signal. Therefore, total number of pulses counted by the counter is directly proportional to the frequency of the applied unknown signal.

Block Diagram and Working of Digital Frequency Meter

Figure (2): Basic Block Arrangement of Digital Frequency Meter

The block diagram of digital frequency meter is shown in figure (2). The unknown frequency signal is applied to the amplifier, where the signal is strengthened or amplified to a suitable level, and then applied to input of Schmitt trigger circuit. The function of the Schmitt trigger is to change the amplified input signal into square wave signal with fast rise and fall times. This converted square wave is properly differentiated and then clipped (by means of differentiator and clipper circuits). Thus a train of pulses appears at the output of Schmitt trigger and each pulse represents one single (1’s) cycle of the input signal. These train of pulses are then applied to the start/stop gate. When the start/stop gate is enabled or opened, the input pulses are allowed to pass through it and appears at the input of the counter. Then, the counter begins to count the pulses.

When the start/stop gate is disabled or closed, the pulses are not allowed to pass to the counter and then the counter stops counting. During the time period of enabling and disabling the gate, the pulses are counted arid the count is displayed on the digital readout (display unit). The measurement of this time period gives the value of unknown frequency signal. Then the value of unknown frequency is calculated by using the formula,

\[f=\frac{N}{t}\]

Let,

f – Unknown frequency of the applied signal.

t – The time period between enabling and disabling of the gate.

N – Total number of counts displayed.

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Usually the temperature measurement is carried out by using thermocouples and RTDs. The conventional method of recording the measured temperature is very time consuming due to the non-linear characteristics of the sensors and may introduce human errors. Hence digital thermometers are developed to obtain the direct display of the temperature.

Block Diagram of Digital Thermometer

Fig. 1: Block Diagram of Digital Thermometer

The block diagram of a digital thermometer is as shown in Fig. 1. The linearizer used in digital thermometers is very important block. Using microprocessor based linearization technique, same unit can be used for different types of thermocouples. Fig. 1 shows the block diagram a digital thermometer.

Construction of Digital Thermometer

It consists of Temperature sensor like thermocouple. The compensating network to provide the reference temperature to the thermocouple.  Signal conditioner like amplifier. Analog to digital converter (ADC), and Electronic display unit for direct display of the temperature.

Working of Digital Thermometer

The thermocouple senses the temperature and produces proportional thermoelectric e.m.f. (voltage) in response to the changes in temperature. The compensating network provides the reference point for the thermocouple by producing a compensating voltage proportional to the reference temperature. The output of the thermocouple is further amplified by the amplifier. This amplified analog signal is fed as input to the ADC (Analog to digital Converter) which converts it into an equivalent digital form. The digital voltage from analog to digital Converter is linearized by the linearizing system and then used to drive the electronic display unit to give the digital display corresponding to the measured temperature.

Advantages of Digital Thermometer

1. The temperature readout is simple.
2. High accuracy.
3. Small in size

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Block Diagram & Working of Electronic Weighing Machine

Fig. 1: Block Diagram of Electronic Weighing Machine.

Fig. 1 shows the block diagram of an electronic weighing machine which mainly consists of load cell, amplifier, filter, analog to digital converter, processor and the display unit. The load is placed on the load cell pan. The load cell contains bounded strain gauge transducers which convert the applied weight into equivalent electrical output using Wheatstone bridge. Zero setting arrangement is provided for the load cell. The output voltage from the Wheatstone bridge is then amplified by the amplifier and further filtered to remove the unwanted noise. This amplified and filtered analog signal is then fed to the Analog to Digital Converter (ADC) to convert it into equivalent digital form. The processor processes this input data stores it in the memory and calculates the cost of the item. The display driver generates the control signals for the printer and displays the data on the digital display unit. The processor also supports various functions. This type of weighing machine is used in super markets.

Advantages of Electronic Weighing Machine

1. High accuracy.
2. Fast response.
3. Small in size.
4. High resolution.

Applications of Electronic Weighing Machine

These machines are utilized in almost all industries and commercial applications.

1. In grocery shops and super markets, for precision weight measurements like weighing gold, silver, diamond and other precious stones.
2. In platform weighers, animal and human weighing, truck weighing, crane weighing and textile industries, etc.

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Digital Voltmeter (DVM) is a voltage measuring device that represents A.C (or) D.C voltage measurement in digital form as discrete numerals.

Working Principle of Digital Voltmeter

The principle on which a DVM operates is analog to digital conversion. DVM converts the analog voltage into a digital display. The input to a DVM is analog voltage and output of a DVM is a digital representation of the input voltage.

Block Diagram of Digital Voltmeter

Figure 1: Digital Voltmeter Block Diagram.

A digital voltmeter consists of analog to digital converter, a signal processing and a digital display. The figure 1 shows the block diagram of a DVM.

Working of Digital Voltmeter

The analog input voltage is supplied to the Analog to Digital Converter (ADC). All ADC’s need reference signal which is generated internally. The circuit of the reference generator is based upon the type of ADC used in the digital voltmeter. The output from the ADC is in digital form and is being processed to the signal processing devices. Decoders, amplifiers, filters, rectifiers, modulators, voltage conversion device are used in signal processing device. Desired operations are performed in signal conditioning devices in order to make the signal suitable for display. Data transmission elements are also used for transmitting the data from signal processing device to display. It contains latches, counters, etc., based upon the requirement. A digital readout or the display indicates the readings of the voltage in digital form. A display may be LCD or LED’s.

Advantages of Digital Voltmeter

Digital readout is an important advantage of an digital voltmeter over an analog voltmeter in which a pointer deflects on a continuous scale to indicate the values of unknown voltages. This advantage eliminates the problem of parallax error and approximations by proving accurate digital values. Because of greater developments in the IC technologies. the power requirements, physical size and also cost of these devices are reduced. Therefore, Digital Voltmeters (DVMs) can actively compete with conventional analog instruments both in price and portability.

Specifications of Digital Voltmeter

Some of the general specifications of a digital voltmeter are given in the following table,

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