1. Principles of Digital Instruments Ms. Rokhaiya Banu, Asst. Professor
Department: B.E. Electrical Engineering
Subject: Electrical & Electronics Measurements
Semester: III
Teaching Aids Service by KRRC Information Section

2. PRINCIPLES OF DIGITAL INSTRUMENTS
MODULE :2
PRINCIPLES OF DIGITAL INSTRUMENTS

3. Analog and digital meters.
ANALOG METERS: An instrument which measures and indicates values by means of a continuous scale within which any value may be specified. Most of the time a pointer is used to indicate readings. DIGITAL METERS: A digital meter is a device used by technicians to test and measure electronic circuits. Most of them are portable, battery-powered units. They show measurements as numbers and symbols on an electronic display.

4. WHY ANALOG WAS REPLACED BY DIGITAL METERS?
Accuracy. Robustness. It has moving parts. These can get affected by an accidental drop to floor. It has relatively less input resistance especially in low voltage ranges
Advantages of digital meters
1 Output Display It is easy to gauge a reading through digital multi-meter since the output is automatically displayed in numbers through a Seven Segment Display.
2. Accuracy
A digital meter takes a precise, computer-generated reading and displays it on the screen.

5. Auto Polarity Placing the probes into the opposite polarity will result into a negative output.
Stable Calibration
We should only have to calibrate our digital meter once – when you first turn it on.
Current Protection
Most digital meters feature protection from sudden shifts in current.
6. Precision.
7. It can hold the reading in memory .
8. The reading speed is increased as it is easier to read.
9. With the advent of Integrated circuits, the size, cost and power requirements of digital multimeters has been drastically reduced.

6. DIGITAL VOLTMETERS
DVM displays measurements of dc or ac voltages as discrete numerals instead of a pointer deflection on a continuous scale as in analog devices.
Types of DVM
1.Ramp type DVM
2.Integrating DVM
3.Continuous-balance DVM
4.Successive-approximation DVM

7. RAMP DIGITAL VOLTMETER
Input voltage is converted into digital equivalent by counting the time taken for the ramp wave to decrease from the magnitude of input voltage to 0V.

8. Contd..
Ramp-type ADC can be divided into two sections as follows:       1. Voltage to time conversion section       2. Time measurement section
The analog input voltage is fed to the attenuation circuit. The attenuated signal is compared with the the ramp signal generated by the ramp generator given in the block diagram by the input comparator 'C1'.
Similarly, The ramp signal generated is compared with 0V via a zero-crossing detector 'C2'.
A sample rate multivibrator is connected to the ramp generator whose purpose is to provide an initiating pulse for the ramp generator to start the next ramp voltage for the next measurement.
It is also used to reset the counter before generating the next ramp voltage.

9. Contd…
In the time measurement section, there is counter which is triggered by a gating pulse.
The inputs of the gating pulse are (i) Output of 'C1' (ii) Output of 'C2' (iii) Clock pulse from the oscillator.
The counter is reset after each successful completion of time measurement by a control signal from the sample rate multivibrator.
The count produced is displayed by connecting suitable display device.

10. Operation
The attenuated signal is compared with a negative going ramp signal generated by the ramp generator.
When the ramp voltage coincides with the input signal, the output of 'C1' becomes low. This point is called coincidence point. This initiates the counting process ( start of count ).
The counter continues to count until the ramp voltage reduces and crosses zero (0V). This is detected by zero crossing detector 'C2'.
The output of 'C2' becomes high which ends the counting process (end of count).

12. Merits:. low cost. simple, easy to design
Merits: * low cost * simple, easy to design * long distance transmission of output pulse is possible Demerits: * accuracy of output greatly depends on linearity of the ramp. (since only one ramp is used) * input filter are needed for filtering noise from input signal.

13. Dual Slope Integrating type DVM

14. Block Diagram of Integrator type DVM

15. Energy meters are the basic part to measure the power consumption
Energy meters are the basic part to measure the power consumption. It is used everywhere, no matter how big or small consumption it is. It is also known as watt-hour meter. Here we discuss the construction and working principle of induction type energy meter. To understand the structure of watt-hour meter, we must understand the four essential components of the meter.
These components are as follows:
1)Driving system
2)Moving system
3)Braking system
4)Registering system

16. Driving System
The components of this system are two silicon steel laminated electromagnets. The upper electromagnet is called shunt magnet and it carries a voltage coil consisting of many turns of thin wire. The lower electromagnet is called series magnet and it carries the two current coils consisting of a few turns of thick wire. Current coils are connected in series with the circuit and load current passes through it.
Where as voltage coil is connected to the supply mains and produce a high ratio of inductance to resistance. There is copper bands in the lower part of shunt magnet which provides frictional compensation so that the phase angle between shunt magnet flux and the supply voltage is exactly 90°.

17. Moving System
As you can see in the figure, there is a thin aluminium disk placed in the gap between the two electromagnets and mounted on a vertical shaft. The eddy currents are induced in the aluminium disk when it cuts the flux produced by both the magnets. As a result of interference of eddy currents and two magnetic fields constitute a deflecting torque in the disk. As you start consuming power the disk slowly starts rotating and the several rotation of the disk displays the power consumption, in the particular interval of time. Normally it is measured in kilowatt-hours.

18. Braking System Registering System
The main part of this system is a permanent magnet called brake magnet. It is located near the disk so that eddy currents are induced in it due to movement of rotating disk through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.
Registering System
As its name suggest, it registers the number of rotation of the disk which is proportional to the energy consumed directly in kilowatt-hour. There is a disk spindle which is driven by a gear on the disk shaft and indicates the number of times the disk has turned.

19. Working Principle of Energy Meter
The working of single phase induction type energy meters are based on two main fundamentals:
1) Rotation of aluminium disk.
2)Arrangement of counting and displaying the amount of energy consumed.

20. Rotation of an Aluminium Disk
The rotation of metallic disk is operated by two coils. Both the coils are arranged in such way that one coil produces a magnetic field in proportion to voltage and the other coil creates a magnetic field proportion to current. The field produced by voltage coil is delayed by 90° so that eddy current is induced in the disk. The force exerted on the disk by the two fields is proportional to the product of the immediate current and voltage in the coils.
As a result of it, a light weight aluminium disk rotates in an air gap. But there is a need to stop a disk when there is no power supply. A permanent magnet works as a brake which opposes the rotation of the disk and balances the speed of rotation with respect to power consumption.

21. Arrangement of Counting and Displaying the Energy Consumed
In this system, the rotation of the floating disk has been counted and then displayed on the meter window. The aluminium disk is connected to a spindle which has a gear. This gear drives the register and the revolution of the disk has been counted and displayed on the register which has series of dials and each dial represent a single digit. There is a small display window in the front of the meter which displays the reading of energy consumed with the help of dials. There is a copper shading ring at the central limb of the shunt magnet. To make the phase angle between flux produced by shunt magnet and supply voltage

22. Digital multimeter is a test equipment which offers several electronic measurement task in one tool.
It is also known as the voltmeter or Ohm meter or Volt Ohm meter. The standard and basic measurements performed by multimeter are the measurements of amps, volts, and ohms.
Apart from that, these digital multimeters perform many additional measurements by using digital and logic technology. These may include temperature, frequency, continuity, capacitance etc.
The new improved integrated circuits of digital multimeter are more efficient, faster and work with a large accuracy as compared to an analogue multimeter.

23. A multimeter is a simple but useful device which has only three parts; Display screen, selection knob, ports.
Display screen-It has illuminated display screen for better visualization. It has five digits display screen; one represent sign value and the other four are for number representation.
Selection knob- As we know a single multimeter performs so many tasks like reading voltage, resistance, and current. The selection knob allows the user to select the different job.
Port- There are two ports on the front of the unit. One is the mAVΩ port which allows the measurement of all the three units: current up to 200 mA, voltage, and resistance. The red probe is plugged into this port. The other is COM port which means common and it normally connected to –ev of a circuit and black probe is plugged into it. There is one particular port is 10A, which is use to measures large current in the circuit.

24. Features of Digital Multimeter
Now a day, wide range of digital multimeters are available in the market. But their features make it a good multimeter. Some of the features which are very useful and should be there in a multimeter are:
Auto-Range This feature enables multimeter to change their internal range according to the requirement to detect the correct voltage, current, or resistance of the circuit. It has an ability to stop the meter at the particular range and prevent the overloading if the meter is in volt range. Such multimeters are of high quality and have more features.
A Back-Lit LCD It is a fancy feature, but felt very useful in dark situations when there is no other light source for watching reading in the multimeter. Auto-Off It is a great feature but normally comes in higher end version. It facilitates user to forget to turn the meter off when it is not in use.
Decent Probes Decent probes are bonus above all. In multimeters normally leads are used which get damage at flex point. So it is better to use probes which make work easier and easy to replace if get damaged.
Auto Polarity It is an important feature that shows the polarity of the current. When the voltage of current being measured shows positive reading (i.e. same polarity to meter connections) and if it shows negative (i.e. opposite polarity to meter connections). Such facility was not available in an analog multimeter and the meter deflects backward and reverses back to take the reading accurately.

25. DIGITAL FREQUENCY METER
Digital frequency meter is a general purpose instrument that displays the frequency of a periodic electrical signal to an accuracy of three decimal places.
It counts the number events occurring within the oscillations during a given interval of time. As the preset period gets completed, the value in the counter display on the screen and the counter reset to zero.
Various types of instruments are available which operates at a fixed or variable frequency. But if any frequency meter operates at a different frequency than specified range, it could carry out abnormally.
For measuring low frequencies, the deflection type meters are widely used. The deflection of the pointer on the scale shows the change in frequency. The deflection type instruments are of two types: one is electrically resonant circuits and other is ratio meter.

26. Operating Principle of Digital Frequency Meter
A frequency meter has a small device which converts the sinusoidal voltage of the frequency into a train of unidirectional pulses. The frequency of input signal is the displayed count, averaged over a suitable counting interval out of 0.1, 1.0, or 10 seconds. These three intervals repeat themselves sequentially. As the ring counting units resets, these pulses pass through the time-base-gate and then entered into the main gate, which opens for a certain period of time interval.

27. BLOCK DIAGRAM OF FREQUENCY METER
The time base gate prevents a divider pulse from opening the main gate during the display time interval. The main gate acts as a switch when the gate is open, pulses are allowed to pass. When the gate is closed, pulses are not allowed to pass that means the flow of pulses get obstructed. The functioning of the gate is operated by the main-gate flip-flop. An electronic counter at the gate output that counts the number of pulses passed through the gate while it was open. As the next divider pulse is received at main gate flip-flop, the counting interval ends and divider pulses are locked out. The resultant value displayed on a display screen which has the ring counting units of scale-of-ten circuits and each unit is coupled to a numeric indicator, which provides the digital display. As the reset pulse generator is triggered, ring counters get reset automatically and the same procedure starts again.

28. Tachometer
Device for indicating the angular (rotary) speed of a rotating shaft. Either by registering the total no. of revolution during the period of contact or by indicating the number of revolution per min.
Tachometers are classified as follows.
1)Mechanical tachometer
2)Electrical tachometer

29. What Are the Different Types of Tachometers?
Tachometers can be classified
1.On the basis of data acquisition
Contact
Non contact types
2. Classified as data type
Analog
Digital
3. On the basis of power .
Mechanical
Electrical

30. Comparison Non Contact Type Contact Type
The tachometer does not need to be in physical contact with the rotating shaft.
Preferred where the tachometer needs to be mobile.
Generally, laser is used or an optical disk id attached to rotating shaft and read by a IR beam or laser.
Contact Type
The tachometer has to be in physical contact with the rotating shaft.
Preferred where the tachometer is generally fixed to the machine.
Generally, optical encoder / magnetic sensor is attached to shaft of tachometer.

31. Photo-electric tachometer
It consists of a opaque disc mounted on the shaft whose speed is to be measured. The disc has a number of equivalent holes around the periphery. On one side of the disc there is a source of light (L) while on the other side there is a light sensor (may be a photosensitive device or photo-tube) in line with it (light-source).
On the rotation of the disc, holes and opaque portions of the disc come alternator in between the light source and the light sensor. When a hole comes in between the two, light passes through the holes and falls on the light sensor, with the result that an output pulse is generated. But when the opaque portion of the disc comes in between, the light from the source is blocked and hence there is no pulse output.
Thus whenever a hole comes in line with the light source and sensor, a pulse is generated. These pulses are counted/measured through an electronic counter.

32. Photo-electric tachometer
The number of pulses generated depends upon the following factors:
i. The number of holes in the disc;
ii. The shaft speed.
Since the number of holes are fixed, therefore, the number of pulses generated depends on the speed of the shaft only. The electronic counter may therefore be calibrated in terms of speed (r.p.m.)

33. Magnetic pickup tachometer
A coil wounded on permanent magnet not on iron core, this configuration enable us to measure rotational speed of the systems.
In the construction of variable reluctance sensor, we use ferromagnetic gearwheel. As the gearwheel rotates, change in magnetic flux take place in the pickup coil which further induces voltage. This change in magnitude is proportional to the voltage induced in the sensor.

34. Magnetic pickup tachometer

35. Pickup tachometer
Various pick-up devices can be used in conjunction with a digital counter to give a direct reading of speed.
An inductive pick-up tachometer is shown in Figure (a).
As the individual teeth pass the coil they induce an e.m.f. pulse which is appropriately modified and then fed to a digital counter.
A capacitive pick-up tachometer is shown in Figure (b). As the rotating vane passes between the plates a capacitance change occurs in the form of a pulse.
This is modified and then fed to the digital counter.

36. Pickup tachometer