Category: electronic Measuring Instruments

OHMMeters The instrument, which is used to measure the value of resistance between any two points in an electric circuit is called ohmmeter. It can also be used to find the value of an unknown resistor. The units of resistance are ohm and the measuring instrument is meter. So, the word “ohmmeter” is obtained by combining the words “ohm” and “meter”. Types of Ohmmeters Following are the two types of ohmmeters. Series Ohmmeter Shunt Ohmmeter Now, let us discuss about these two types of ohmmeters one by one. Series Ohmmeter If the resistor’s value is unknown and has to be measured by placing it in series with the ohmmeter, then that ohmmeter is called series ohmmeter. The circuit diagram of series ohmmeter is shown in below figure. The part of the circuit, which is left side of the terminals A & B is series ohmmeter. So, we can measure the value of unknown resistance by placing it to the right side of terminals A & B. Now, let us discuss about the calibration scale of series ohmmeter. If $R_{x}= 0 :Omega$, then the terminals A & B will be short circuited with each other. So, the meter current gets divided between the resistors, $R_{1}$ and $R_{2}$. Now, vary the value of resistor, $R_{2}$ in such a way that the entire meter current flows through the resistor, $R_{1}$ only. In this case, the meter shows full scale deflection current. Hence, this full scale deflection current of the meter can be represented as $0 :Omega$. If $R_{x}= infty :Omega$, then the terminals A & B will be open circuited with each other. So, no current flows through resistor, $R_{1}$. In this case, the meter shows null deflection current. Hence, this null deflection of the meter can be represented as $infty Omega$. In this way, by considering different values of $R_{x}$, the meter shows different deflections. So, accordingly we can represent those deflections with the corresponding resistance value. The series ohmmeter consists of a calibration scale. It has the indications of 0 $Omega$ and $infty :Omega$ at the end points of right hand and left hand of the scale respectively. Series ohmmeter is useful for measuring high values of resistances. Shunt Ohmmeter If the resistor’s value is unknown and to be measured by placing it in parallel (shunt) with the ohmmeter, then that ohmmeter is called shunt ohmmeter. The circuit diagram of shunt ohmmeter is shown in below figure. The part of the circuit, which is left side of the terminals A & B is shunt ohmmeter. So, we can measure the value of unknown resistance by placing it to the right side of terminals A & B. Now, let us discuss about the calibration scale of shunt ohmmeter. Close the switch, S of above circuit while it is in use. If $R_{x}=0 :Omega$, then the terminals A & B will be short circuited with each other. Due to this, the entire current, $I_{1}$ flows through the terminals A & B. In this case, no current flows through PMMC galvanometer. Hence, the null deflection of the PMMC galvanometer can be represented as $0 :Omega$. If $R_{x}=infty :Omega$, then the terminals A & B will be open circuited with each other. So, no current flows through the terminals A & B. In this case, the entire current, $I_{1}$ flows through PMMC galvanometer. If required vary (adjust) the value of resistor, $R_{1}$ until the PMMC galvanometer shows full scale deflection current. Hence, this full scale deflection current of the PMMC galvanometer can be represented as $infty :Omega$ In this way, by considering different values of $R_{x}$, the meter shows different deflections. So, accordingly we can represent those deflections with the corresponding resistance values. The shunt ohmmeter consists of a calibration scale. It has the indications of $0 :Omega$ and $infty :Omega$ at the end points of left hand and right hand of the scale respectively. Shunt ohmmeter is useful for measuring low values of resistances. So, we can use either series ohmmeter or shunt ohmmeter based on the values of resistances that are to be measured i.e., high or low. Learning working make money

DC Ammeters Current is the rate of flow of electric charge. If this electric charge flows only in one direction, then the resultant current is called Direct Current (DC). The instrument, which is used to measure the Direct Current called DC ammeter. If we place a resistor in parallel with the Permanent Magnet Moving Coil (PMMC) galvanometer, then the entire combination acts as DC ammeter. The parallel resistance, which is used in DC ammeter is also called shunt resistance or simply, shunt. The value of this resistance should be considered small in order to measure the DC current of large value. The circuit diagram of DC ammeter is shown in below figure. We have to place this DC ammeter in series with the branch of an electric circuit, where the DC current is to be measured. The voltage across the elements, which are connected in parallel is same. So, the voltage across shunt resistor, $R_{sh}$ and the voltage across galvanometer resistance, $R_{m}$ is same, since those two elements are connected in parallel in above circuit. Mathematically, it can be written as $$I_{sh}R_{sh}=I_{m}R_{m}$$ $Rightarrow R_{sh}=frac{I_{m}R_{m}}{I_{sh}}$ (Equation 1) The KCL equation at node 1 is $$-I+I_{sh}+I_{m}=0$$ $$Rightarrow I_{sh}=I-I_{m}$$ Substitute the value of $I_{sh}$ in Equation 1. $R_{sh}=frac{I_{m}R_{m}}{I-I_{m}}$(Equation 2) Take, $I_{m}$ as common in the denominator term, which is present in the right hand side of Equation 2 $$R_{sh}=frac{I_{m}R_{m}}{I_{m}(frac{1}{I_{m}}-1)}$$ $Rightarrow R_{sh}=frac{R_{m}}{frac{I}{I_{m}}-1}$(Equation 3) Where, $R_{sh}$ is the shunt resistance $R_{m}$ is the internal resistance of galvanometer $I$ is the total Direct Current that is to be measured $I_{m}$ is the full scale deflection current The ratio of total Direct Current that is to be measured, $I$ and the full scale deflection current of the galvanometer, $I_{m}$ is known as multiplying factor, m. Mathematically, it can be represented as $m=frac{I}{I_{m}}$(Equation 4) $R_{sh}=frac{R_{m}}{m-1}$(Equation 5) We can find the value of shunt resistance by using either Equation 2 or Equation 5 based on the available data. Multi Range DC Ammeter In previous section, we discussed about DC ammeter which is obtained by placing a resistor in parallel with the PMMC galvanometer. This DC ammeter can be used to measure a particular range of Direct Currents. If we want to use the DC ammeter for measuring the Direct Currents of multiple ranges, then we have to use multiple parallel resistors instead of single resistor and this entire combination of resistors is in parallel to the PMMC galvanometer. The circuit diagram of multi range DC ammeter is shown in below figure. Place this multi range DC ammeter in series with the branch of an electric circuit, where the Direct Current of required range is to be measured. The desired range of currents is chosen by connecting the switch, s to the respective shunt resistor. Let, $m_{1}, m_{2}, m_{3}$ and $m_{4}$ are the multiplying factors of DC ammeter when we consider the total Direct Currents to be measured as, $I_{1}, I_{2}, I_{3}$ and $I_{4}$ respectively. Following are the formulae corresponding to each multiplying factor. $$m_{1}=frac{I_{1}}{I_{m}}$$ $$m_{2}=frac{I_{2}}{I_{m}}$$ $$m_{3}=frac{I_{3}}{I_{m}}$$ $$m_{4}=frac{I_{4}}{I_{m}}$$ In above circuit, there are four shunt resistors, $R_{sh1}, R_{sh2}, R_{sh2}$ and $R_{sh4}$. Following are the formulae corresponding to these four resistors. $$R_{sh1}=frac{R_{m}}{m_{1}-1}$$ $$R_{sh2}=frac{R_{m}}{m_{2}-1}$$ $$R_{sh3}=frac{R_{m}}{m_{3}-1}$$ $$R_{sh4}=frac{R_{m}}{m_{4}-1}$$ The above formulae will help us find the resistance values of each shunt resistor. Learning working make money

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Discuss Electronic Measuring Instruments This tutorial is meant to provide our readers conceptual knowledge about various electronic measuring instruments and how to choose a specific measuring instrument based on their requirement. There are two types of measuring instruments: one is the type of measuring instruments that show the values on the scale of the meter, and other are type of measuring instruments that displays the waveforms. Learning working make money

Lissajous Figures Lissajous figure is the pattern which is displayed on the screen, when sinusoidal signals are applied to both horizontal & vertical deflection plates of CRO. These patterns will vary based on the amplitudes, frequencies and phase differences of the sinusoidal signals, which are applied to both horizontal & vertical deflection plates of CRO. The following figure shows an example of Lissajous figure. The above Lissajous figure is in elliptical shape and its major axis has some inclination angle with positive x-axis. Measurements using Lissajous Figures We can do the following two measurements from a Lissajous figure. Frequency of the sinusoidal signal Phase difference between two sinusoidal signals Now, let us discuss about these two measurements one by one. Measurement of Frequency Lissajous figure will be displayed on the screen, when the sinusoidal signals are applied to both horizontal & vertical deflection plates of CRO. Hence, apply the sinusoidal signal, which has standard known frequency to the horizontal deflection plates of CRO. Similarly, apply the sinusoidal signal, whose frequency is unknown to the vertical deflection plates of CRO Let, $f_{H}$ and $f_{V}$ are the frequencies of sinusoidal signals, which are applied to the horizontal & vertical deflection plates of CRO respectively. The relationship between $f_{H}$ and $f_{V}$ can be mathematically represented as below. $$frac{f_{V}}{f_{H}}=frac{n_{H}}{n_{V}}$$ From above relation, we will get the frequency of sinusoidal signal, which is applied to the vertical deflection plates of CRO as $f_{V}=left ( frac{n_{H}}{n_{V}} right )f_{H}$(Equation 1) Where, $n_{H}$ is the number of horizontal tangencies $n_{V}$ is the number of vertical tangencies We can find the values of $n_{H}$ and $n_{V}$ from Lissajous figure. So, by substituting the values of $n_{H}$, $n_{V}$ and $f_{H}$ in Equation 1, we will get the value of $f_{V}$, i.e. the frequency of sinusoidal signal that is applied to the vertical deflection plates of CRO. Measurement of Phase Difference A Lissajous figure is displayed on the screen when sinusoidal signals are applied to both horizontal & vertical deflection plates of CRO. Hence, apply the sinusoidal signals, which have same amplitude and frequency to both horizontal and vertical deflection plates of CRO. For few Lissajous figures based on their shape, we can directly tell the phase difference between the two sinusoidal signals. If the Lissajous figure is a straight line with an inclination of $45^{circ}$ with positive x-axis, then the phase difference between the two sinusoidal signals will be $0^{circ}$. That means, there is no phase difference between those two sinusoidal signals. If the Lissajous figure is a straight line with an inclination of $135^{circ}$ with positive x-axis, then the phase difference between the two sinusoidal signals will be $180^{circ}$. That means, those two sinusoidal signals are out of phase. If the Lissajous figure is in circular shape, then the phase difference between the two sinusoidal signals will be $90^{circ}$ or $270^{circ}$. We can calculate the phase difference between the two sinusoidal signals by using formulae, when the Lissajous figures are of elliptical shape. If the major axis of an elliptical shape Lissajous figure having an inclination angle lies between $0^{circ}$ and $90^{circ}$ with positive x-axis, then the phase difference between the two sinusoidal signals will be. $$phi =sin ^{-1}left ( frac{x_{1}}{x_{2}} right )=sin ^{-1}left ( frac{y_{1}}{y_{2}} right )$$ If the major axis of an elliptical shape Lissajous figure having an inclination angle lies between $90^{circ}$ and $180^{circ}$ with positive x-axis, then the phase difference between the two sinusoidal signals will be. $$phi =180 – sin ^{-1}left ( frac{x_{1}}{x_{2}} right )=180 – sin ^{-1}left ( frac{y_{1}}{y_{2}} right )$$ Where, $x_{1}$ is the distance from the origin to the point on x-axis, where the elliptical shape Lissajous figure intersects $x_{2}$ is the distance from the origin to the vertical tangent of elliptical shape Lissajous figure $y_{1}$ is the distance from the origin to the point on y-axis, where the elliptical shape Lissajous figure intersects $y_{2}$ is the distance from the origin to the horizontal tangent of elliptical shape Lissajous figure In this chapter, welearnt how to find the frequency of unknown sinusoidal signal and the phase difference between two sinusoidal signals from Lissajous figures by using formulae. Learning working make money

DC Bridges DC bridges can be operated with only DC voltage signal. DC bridges are useful for measuring the value of unknown resistance, which is present in the bridge. Wheatstone’s Bridge is an example of DC bridge. Now, let us discuss about Wheatstone’s Bridge in order to find the unknown resistance’s value. Wheatstone’s Bridge Wheatstone’s bridge is a simple DC bridge, which is mainly having four arms. These four arms form a rhombus or square shape and each arm consists of one resistor. To find the value of unknown resistance, we need the galvanometer and DC voltage source. Hence, one of these two are placed in one diagonal of Wheatstone’s bridge and the other one is placed in another diagonal of Wheatstone’s bridge. Wheatstone’s bridge is used to measure the value of medium resistance. The circuit diagram of Wheatstone’s bridge is shown in below figure. In above circuit, the arms AB, BC, CD and DA together form a rhombus or square shape. They consist of resistors $R_{2}$, $R_{4}$, $R_{3}$ and $R_{1}$ respectively. Let the current flowing through these resistor arms is $I_{2}$, $I_{4}$, $I_{3}$ and $I_{1}$ respectively and the directions of these currents are shown in the figure. The diagonal arms DB and AC consists of galvanometer and DC voltage source of V volts respectively. Here, the resistor, $R_{3}$ is a standard variable resistor and the resistor, $R_{4}$ is an unknown resistor. We can balance the bridge, by varying the resistance value of resistor, $R_{3}$. The above bridge circuit is balanced when no current flows through the diagonal arm, DB. That means, there is no deflection in the galvanometer, when the bridge is balanced. The bridge will be balanced, when the following two conditions are satisfied. The voltage across arm AD is equal to the voltage across arm AB. i.e., $$V_{AD}=V_{AB}$$ $Rightarrow I_{1}R_{1}=I_{2}R_{2}$Equation 1 The voltage across arm DC is equal to the voltage across arm BC. i.e., $$V_{DC}=V_{BC}$$ $Rightarrow I_{3}R_{3}=I_{4}R_{4}$Equation 2 From above two balancing conditions, we will get the following two conclusions. The current flowing through the arm AD will be equal to that of arm DC. i.e., $$I_{1}=I_{3}$$ The current flowing through the arm AB will be equal to that of arm BC. i.e., $$I_{2}=I_{4}$$ Take the ratio of Equation 1 and Equation 2. $frac{I_{1}R_{1}}{I_{3}R_{3}}=frac{I_{2}R_{2}}{I_{4}R_{4}}$Equation 3 Substitute, $I_{1}=I_{3}$ and $I_{2}=I_{4}$ in Equation 3. $$frac{I_{3}R_{1}}{I_{3}R_{3}}=frac{I_{4}R_{2}}{I_{4}R_{4}}$$ $$Rightarrow frac{R_{1}}{R_{3}}=frac{R_{2}}{R_{4}}$$ $$Rightarrow R_{4}=frac{R_{2}R_{3}}{R_{1}}$$ By substituting the known values of resistors $R_{1}$, $R_{2}$ and $R_{3}$ in above equation, we will get the value of resistor,$R_{4}$. Learning working make money

AC Ammeter Current is the rate of flow of electric charge. If the direction of this electric charge changes regularly, then the resultant current is called Alternating Current (AC). The instrument, which is used to measure the Alternating Current that flows through any branch of electric circuit is called AC ammeter. Example − Thermocouple type AC ammeter. Now, let us discuss about Thermocouple type AC ammeter. Thermocouple Type AC Ammeter If a Thermocouple is connected ahead of PMMC galvanometer, then that entire combination is called thermocouple type AC ammeter. The block diagram of thermocouple type AC ammeter is shown in below figure. The above block diagram consists of mainly two blocks: a thermocouple, and a PMMC galvanometer. We will get the corresponding circuit diagram, just by replacing each block with the respective component(s) in above block diagram. So, the circuit diagram of thermocouple type AC ammeter will look like as shown in below figure. Thermocouple generates an EMF, $e$, whenever the Alternating Current, I flows through heater element. This EMF, $e$ is directly proportional to the rms value of the current, I that is flowing through heater element. So, we have to calibrate the scale of PMMC instrument to read rms values of current. So, with this chapter we have completed all basic measuring instruments such as DC voltmeters, AC voltmeters, DC ammeters and AC ammeters. In next chapter, let us discuss about the meters or measuring instruments, which measure resistance value. Learning working make money

Basics of Oscilloscopes Oscilloscope is an electronic equipment, which displays a voltage waveform. Among the oscilloscopes, Cathode Ray Oscilloscope (CRO) is the basic one and it displays a time varying signal or waveform. In this chapter, let us discuss about the block diagram of CRO and measurements of some parameters by using CRO. Block Diagram of CRO Cathode Ray Oscilloscope (CRO) consists a set of blocks. Those are vertical amplifier, delay line, trigger circuit, time base generator, horizontal amplifier, Cathode Ray Tube (CRT) & power supply. The block diagram of CRO is shown in below figure. The function of each block of CRO is mentioned below. Vertical Amplifier − It amplifies the input signal, which is to be displayed on the screen of CRT. Delay Line − It provides some amount of delay to the signal, which is obtained at the output of vertical amplifier. This delayed signal is then applied to vertical deflection plates of CRT. Trigger Circuit − It produces a triggering signal in order to synchronize both horizontal and vertical deflections of electron beam. Time base Generator − It produces a sawtooth signal, which is useful for horizontal deflection of electron beam. Horizontal Amplifier − It amplifies the sawtooth signal and then connects it to the horizontal deflection plates of CRT. Power supply − It produces both high and low voltages. The negative high voltage and positive low voltage are applied to CRT and other circuits respectively. Cathode Ray Tube (CRT) − It is the major important block of CRO and mainly consists of four parts. Those are electron gun, vertical deflection plates, horizontal deflection plates and fluorescent screen. The electron beam, which is produced by an electron gun gets deflected in both vertical and horizontal directions by a pair of vertical deflection plates and a pair of horizontal deflection plates respectively. Finally, the deflected beam will appear as a spot on the fluorescent screen. In this way, CRO will display the applied input signal on the screen of CRT. So, we can analyse the signals in time domain by using CRO Measurements by using CRO We can do the following measurements by using CRO. Measurement of Amplitude Measurement of Time Period Measurement of Frequency Now, let us discuss about these measurements one by one. Measurement of Amplitude CRO displays the voltage signal as a function of time on its screen. The amplitude of that voltage signal is constant, but we can vary the number of divisions that cover the voltage signal in vertical direction by varying volt/division knob on the CRO panel. Therefore, we will get the amplitude of the signal, which is present on the screen of CRO by using following formula. $$A=jtimes n_{v}$$ Where, $A$ is the amplitude $j$ is the value of volt/division $n_{v}$ is the number of divisions that cover the signal in vertical direction. Measurement of Time Period CRO displays the voltage signal as a function of time on its screen. The Time period of that periodic voltage signal is constant, but we can vary the number of divisions that cover one complete cycle of voltage signal in horizontal direction by varying time/division knob on the CRO panel. Therefore, we will get the Time period of the signal, which is present on the screen of CRO by using following formula. $$T=ktimes n_{h}$$ Where, $T$ is the Time period $j$ is the value of time/division $n_{v}$ is the number of divisions that cover one complete cycle of the periodic signal in horizontal direction. Measurement of Frequency The frequency, f of a periodic signal is the reciprocal of time period, T. Mathematically, it can be represented as $$f=frac{1}{T}$$ So, we can find the frequency, f of a periodic signal by following these two steps. Step1 − Find the Time period of periodic signal Step2 − Take reciprocal of Time period of periodic signal, which is obtained in Step1 We will discuss about special purpose oscilloscopes in next chapter. Learning working make money

Active Transducers Active transducer is a transducer, which converts the non-electrical quantity into an electrical quantity. Let us consider the non-electrical quantities such as pressure, illumination of light and temperature. Hence, we will get the following three active transducers depending on the non-electrical quantity that we choose. Piezo Electric Transducer Photo Electric Transducer Thermo Electric Transducer Now, let us discuss about these three active transducers one by one. Piezo Electric Transducer An active transducer is said to be piezo electric transducer, when it produces an electrical quantity which is equivalent to the pressure input. The following three substances exhibit piezo electric effect. Quartz Rochelle salts Tourmaline The piezo-electric effect exhibited by these three substances is Tourmaline, Quartz, and Rochelle salts, in this ascending order. The ascending order of mechanical strength having by these three substances is Rochelle salts, Quartz, Tourmaline. Quartz is used as piezo electric transducer, as it exhibits the moderate piezo electric effect and having moderate mechanical strength among those three piezo electric substances. Quartz Transducer The circuit diagram of Quartz transducer is shown in below figure. As shown in the figure, quartz crystal is placed between base and force summing member. The output voltage can be measured across the metal electrodes, which are placed on two sides of quartz crystal. The output voltage, $V_{0}$ of above pressure transducer will be $$V_{0}=frac{Q}{C}$$ Photo Electric Transducer An active transducer is said to be photo electric transducer, when it produces an electrical quantity which is equivalent to the illumination of light input. The circuit diagram of photo electric transducer is shown in below figure. The working of photo electric transducer is mentioned below. Step1 − The photo electric transducer releases electrons, when the light falls on cathode of it. Step2 − The photo electric transducer produces a current, I in the circuit due to the attraction of electrons towards anode. We can find the sensitivity of photo electric transducer by using the following formula. $$S=frac{I}{i}$$ Where, $S$ is the sensitivity of photo electric transducer $I$ is the output current of photo electric transducer $i$ is the illumination of the light input of photo electric transducer Thermo Electric Transducer An active transducer is said to be thermo electric transducer, when it produces an electrical quantity which is equivalent to temperature input. The following two transducers are the examples of thermo electric transducers. Thermistor Transducer Thermocouple Transducer Now, let us discuss about these two transducers one by one. Thermistor Transducer The resistor, which depends on temperature is called thermal resistor. In short, it is called Thermistor. The temperature coefficient of thermistor is negative. That means, as temperature increases, the resistance of thermistor decreases. Mathematically, the relation between resistance of thermistor and temperature can be represented as $$R_{1}=R_{2}e^left ( beta left [ frac{1}{T_{1}}-frac{1}{T_{2}} right ] right )$$ Where, $R_{1}$ is the resistance of thermistor at temperature ${T_{1}}^{0}K$ $R_{2}$ is the resistance of thermistor at temperature ${T_{2}}^{0}K$ $beta$ is the temperature constant The advantage of Thermistor transducer is that it will produce a fast and stable response. Thermocouple Transducer Thermocouple transducer produces an output voltage for a corresponding change of temperature at the input. If two wires of different metals are joined together in order to create two junctions, then that entire configuration is called Thermocouple. The circuit diagram of basic thermocouple is shown below − The above thermocouple has two metals, A & B and two junctions, 1 & 2. Consider a constant reference temperature, $T_{2}$ at junction 2. Let the temperature at junction, 1 is $T_{1}$. Thermocouple generates an emf (electro motive force), whenever the values of $T_{1}$ and $T_{2}$ are different. That means, thermocouple generates an emf, whenever there is a temperature difference between the two junctions, 1 & 2 and it is directly proportional to the temperature difference between those two junctions. Mathematically, it can be represented as $$e alpha left ( T_{1}-T_{2} right )$$ Where, $e$ is the emf generated by thermocouple The above thermocouple circuit can be represented as shown in below figure for practical applications. The part of the circuit, which lies between hot & cold junctions including those two junctions is an equivalent model of basic thermocouple. A PMMC galvanometer is connected across the cold junction and it deflects according to the emf generated across cold junction. Thermocouple transducer is the most commonly used thermoelectric transducer. Learning working make money

Electronic Measuring Instruments Tutorial Job Search This tutorial is meant to provide our readers conceptual knowledge about various electronic measuring instruments and how to choose a specific measuring instrument based on their requirement. There are two types of measuring instruments: one is the type of measuring instruments that show the values on the scale of the meter, and other are type of measuring instruments that displays the waveforms. Audience This tutorial is meant for all the readers who are aspiring to learn the concepts of Electronic Measurements and Instrumentation. Prerequisites The fundamental concepts covered in Network Theory & Electronic Circuits tutorials will be useful for understanding the concepts discussed in this tutorial. Learning working make money