Q Meter – Working Principle and Construction – Electronic Instrumentation and Measurement
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Do subscribe to Ekeeda channel and press bell icon to get updates about latest Engineering, HSC and IIT-JEE Mains and Advanced videos. Hello Friends,In this video we are going to study about the instrument queue meter which is also known as the RLC meter this is used for the measurement of inductance, resistance ,capacitance and the quality factor of the coins so in this video we are going to study about its working principle of working its construction and its applications so let us start with a topic. The queue meter or the quality factor meter it is an instrument which is used for the measurement of quality factor of coils and for the measurement of inductance resistance capacitance of the coils at the radio frequency you now as this Q meter or the quality factor meter it is used for the measurement of inductance capacitance and resistance so it is measuring the L C and R of an electric circuit so it is also known as the RLC Needham now what is the quality factor of a point quality factor is defined as the ratio of the inductive reactance to the effective resistance of the coin you okay so if we write its formula it will be Q equals to ratio of inductive reactance to the effective resistance so inductive reactance is XL and the effective resistance is odd now we know that we are measuring the inductive reactance and the effective resistance at the radio frequencies so Excel will be equals to Omega naught L upon R so this is defined as the quality factor or the Q factor of a coil now this quality factor it is also known as the storage factor so if we are defining the storage factor of a coil it will be Q equals to the power dissipated by the coil divided by the power stored in the coil okay so if you want to find out the storage factor of any of the element storage elements which are storing energy in them like the inductors and the capacitors so for them if we are defining the storage factor or the quality factor or Q factor it will be the power dissipated divided by the power stored in the element so we can say that this is storage factor it is defining us the goodness of the coil or the performance of the cone okay and we can say that it is a kind of figure of Merit of a coin that how good is the performance of the coil and how much energy the coil or the element can stored in it okay so this is about the quality factor now while we are as I’ve said that this Q meter which is the instrument we used to measure the quality factor or Q factor of the coil so this Q meter it can measure the inductance it can measure the capacitance and it can measure the resistance of a electric circuit so why as I’ve said that it is measuring it at a radio frequency so why we are not directly measuring it the difference is that the effective resistance the value of the effective resistance in a DC circuit and in an AC circuit they are different okay so if they are using the same socket for measuring the resistance or effective resistance in an AC circuit and in a DC circuit it will give a different value because the value of the resistance it depends upon the frequency in an AC circuit so instead of directly measuring the effective resistance we are measuring it indirectly by first determining the quality factor and as the quality factor is equals to the ratio the inductive reactance to the effective resistance we can measure the effective resistance often a mentor offer circuit you so disaffected resistance it is not measured directly instead of that we are measuring it indirectly by first measuring the quality factor the Q factor and through that because Q is equal to XL upon R so through this we can measure the value of effective resistance as Excel by Q okay now this Q factor or the quality factor it is used for the measurement of the Q okay so let’s see the principle of working of this Q meter or the RLC meter so the principle of working of the cue meter it is based upon the well-known characteristic of a series resonant circuit which is having the are L and C which are connected in series with each other you so let’s see the socket through which we can understand the principle of working of the cue meter now this is the series resonant circuit in which we are having the resistance inductance inductor and a capacitor which are connected in series with each other now this circuit is having the voltage supply which is providing the voltage e to the circuit the voltage drop across the resistance and the inductor it is collectively called the e L and the voltage drop across the capacitor it is called EC the current flowing in the circuit is the I naught now but this is a series resonance circuit so at the resonance frequency we have a condition that at resonant frequency if not we are having X C is equals to X L this is the condition for a series resonance circuit that at the resonant frequency the inductive reactance and the capacitive reactance they are equal to each other okay now we know that the value of XC is what 1 upon 2 pi F naught C and the value of XL is 2 pi F naught L so if we put the value of XC and Excel in this equation where F naught is our resonant frequency we will get XL is equals to X C so 2 pi F naught L will be equal to 1 upon 2 pi F naught C ok so f naught Square will be equals to 1 upon 4 pie square L C so if we find out F naught it will be 1 upon 2 pi under root of L C so this is the value of the resonant frequency okay now if we draw the phasor diagram for this circuit so we know that the voltage drop across the resistance and the current they are in same phase with each other if we are talking about an inductor so voltage is going to lead the current by 90 degree and in the case of capacitor the voltage is going to lag the current by 90 degree so we’ll draw the phasor diagram which is showing the relationship between the voltage drop across the each of the elements and the current flowing through there so if we take the current as our reference phaser which is I know so if we are talking about the resistance the voltage drop that is I naught R will be in same phase with each other if we are talking about the inductor so voltage is going to lead the current by 90 degree so it will be I naught XL across the inductor and if you are talking about the capacitor voltage is going to relax the current by 90 degree so it is I naught X single now we know that the sum of the voltage drop across the resistance and inductor collectively their sum is il so I know Todd and I not Excel if we extend them their resultant will be the e n okay so this is our input voltage our input voltages what E and this input voltage is equal to the voltage drop across the resistance now here the inductor is also present in capacitor is also present so input voltage is measured across the resistance and output voltage is measured across the capacitor so this I know dot which is the voltage drop across the resistance it will be equal toward input voltage II so this is the phasor diagram of the cunita now as we have got the value of the resonant frequency and we know that the input voltage is e and output voltage is the voltage across the capacitor that is easy now the current at resonance it will be I naught and this ion is equal to the voltage across the the input voltage that is e divided by the ah okay now through this equation we can get the value of the input voltage input voltages e so e will be equals to I naught into R so here we have got the input voltage now if we get the ratio of the input voltage and the output voltage it will be equal to EC is our output voltage and input voltage is e so it will be equals to EC is the voltage across the capacitor so it will be equal to I naught X E and E is the voltage across the resistance which we have calculated I naught um now at resonance frequency we know that X is equals to X L so here we can write it as I not Excel upon so it will be equals to I not what is the value of XL Omega naught L upon here we are having I naught R okay so we will write it as I naught now I naught and I know they will be cancelled so the ratio of the output voltage upon the input voltage it is equals to Omega naught L by now we know that what is our quality factor we know that quality factor Q is defined as the ratio of the inductive reactance to the effective resistance inductive reactances Omega naught L upon R and here we have take we have got the ratio of easy upon e as Omega naught l by r so we can say that EC upon e is equals to the quality factor Q or we can say our output voltage EC is equal to Q times E now this was the voltage across a capacitor and voltage across capacitor is our output voltage so we can replace this by e naught which is the output voltage and it is equal to Q E so this relationship says that the output voltage it is Q times the input voltage so if we keep the input voltage as constant we can directly read the value of the quality factor from the output voltage or we can say that we can if we are measuring the voltage across the capacitor so that will be directly equal to the value of the quality factor of the circuit okay so this is the principle of working of the instrument q needham so we can say that the input voltage it is magnified few times and if this output voltage we know that it is the voltage across the capacitor so if we are connecting of voltmeter across the capacitor so that one region is going to give us the reading of the capacitor voltage or we can say the output voltage so we can if sister input voltage escape is constant so the voltmeter it is going to give us the reading which is the region of the quality factor of the in a point okay so this is the principle which is used in the Q meter or the RSC meter so let’s see the construction or the circuit of the few neat instrument you so this is the diagram for the Q meter in this humidor there are various components like it is having a variable RF oscillator a baby locally you offer shunt resistance will be our havoc on thermocouple voltmeter and the coil under is the coil whose quality factor or the resistance inductance we want to measure that foil undertaste is connected between the darkness T 1 and T 4 and between the terminals T 3 and T 4 a variable standard capacitor is connected electronic voltmeter is connected across this capacitor which is going to measure the capacitor voltage so the principle is same that we are having a series resonance of it RLC and in this series resonant circuit we are measuring the voltage the output voltage the voltage across the capacitor and that voltage will be V naught will be equals to Q times of the input voltage and input voltage is this e okay and this EMF or the this input voltage is induced due to this shunt resistance so if we talk about the components of the few meter we will be having a variable RF oscillator variable radio frequency oscillator then we will be having 2 volt meters of thermocouple voltmeter and an electronic voltmeter then we are having of shunt resistance and the value of this shunt resistance is very low means that it is of the order of hundreds of the Omani is it is approximately zero point zero ohms so as the value of this shunt resistance shunt means it is connected in pattern with the circuit so the value of this shunt resistance is very low so this is hardly going to introduce any resistance at the circuit how do you do this shunt resistance and even with key is can used in the socket and this EMF E is measured through this form of a pill voltmeter okay so this EMF E is our input voltage at the same voltage is going to cause a current to flow in the circuit and you do that current at the radio frequency this series resonance of it will be resonating and we will be happy XL is equal to XE we are going to adjust the people sit through this variable RF oscillator the frequency is adjusted so that we are getting this condition xn equals to AC because at this resonant frequency only we are going to get this condition that T naught is equal to Q e okay now because we are having the output voltage connected across we have to measure the output voltage across this capacitor so they are going to calibrate this electronic voltmeter such that it is going to give us directly the readings of the quantity factor or the Q factor of the form so if we talk about the measurement of the quality factor of Q meter what will be the steps for that first the RF oscillator it will be adjusted at the desired frequency and the tuning capacitor it is adjusted for the maximum value of the output voltage so first step will be to oscillator a set to the desired frequency second step is tuning capacitor you now under resonance condition we will be having Q e naught will be equals to Q times of e that is output voltage is equals to Q times of the input voltage so the voltmeter across capacity value of the Q factor directly as we are reading the value of the Q factor directly from the instrument so this hue meter is also known as the direct reading instrument okay so this is the measurement of the Q factor through the Q meter now in the circuit when the measured value of the Q it is different from the true value there will be an error you do the distributed capacitance of the coil and you do this shunt resistance so corrections has to be made you know due to this shunt resistance for this shunt resistance and for this distributed capacitance so we can say that the measured value of the Q factor from this Q meter it is less than the true value of the quality factor you now why this is occurring why the measure value is different from the true value because the cube which we are measuring from the instrument that is regarded as the few or the purity factor of the complete or the whole circuit but we only want to measure the quality factor of the coin okay so that is why there is a difference between the measured value and the true value and there is an error in the instrument and this error has occurred due to the shunt resistance and the distributed capacitance of the coin so corrections are made for the shunt resistance also and for the distributed capacitance so first we will see the correction for the shunt resistance you because shunt resistance is also connected in series with the RLC circuit so this shunt resistance will also be added in the effective resistance of the coil so as we know that the quality factor it is an issue of the inductive reactance that is Omega naught L upon the effective resistance so the shunt resistance is also added in it so it will be R plus RS h okay because here as a when we have studied this circuit we have told that this shunt resistance is very low but if the value of this shunt resistance is comparable to the effective resistance it will go it is going to affect the readings okay so our essence is also added in the effective resistance so the measured value of the Q is omega naught L upon R plus R SH but true value is Omega naught L by r so if we calculate the error that is the difference between the true value and the measured value you will see that the true value is equal to the measured value multiplied with 1 plus R SH by okay so we can say that the measured value it is smaller than the true value by this factor 1 plus R SH by r and so there is a difference between them and an error has occurred you now if you aren’t measuring the coils which are having very high resistance so we can say very low value of the quality factor then this difference between the measured value and the true value it can it is negligible so we can use this cue meter for the measurement of the coils which are having very low value of the quality factor you so if we are having quality factor is very low means we can ER using is for low Q coins but if we are using the same instrument for high Q coils the error will be more now next correction will be the correction for the distributed capacitance you now in the circuit you have seen that across this the coil which is under test we have connected it between the terminals T 1 and T 2 and it is happy or distributed capacitance also related to it so as in the diagram the circuit diagram of the Q meter we have seen that the coil under test is connected between the terminals T 1 and T 2 and this coil is also having a distributed capacitance some capacitance will also be there in the coil so this capacitance is going to be added with the value of the capacitor okay so the measured value of the quality factor or the Q factor it will be smaller than the true value okay you so the measured value is less than the true value by a factor which depends upon the value of the distributed capacitance so we know that the true value is xn upon R okay and at resonance frequency Excel is equals to XC okay now measured value will be equal to the XC plus the distributed capacitance C D so we can say X C D okay upon R so if we see the true value is equal to the measured value 1 plus CD by C so we can say that the measured value is smaller than the true God you by a factor 1 plus CD by C and this factor it depends upon the value of the distributed capacitance so these are the two connections which are made in the readings of the Q meter first Direction was due to the shunt resistance and second direction is for the distributed capacitance now this Q meter it is available in two forms one is the series connection and second is the parallel connection you so in the series connection the L R and the coil they are connected in series with each other and in the parallel you so in the parallel connection you can see that R and L and the coil whose resistance and inductance we want to measure they are connected in parallel with each other means we are having a parallel resonance circuit and here we are having a series resonant circuit okay now if we come up to the applications of the pew meter so this few meter can be used for the measurement of various quantities like it can measure the quality factor of the coil it can measure the effective resistance the inductance capacitance and also it can measure them back width and a frequency can be measured through it so let’s see that how this Q meter it is measuring the different quantities so first application is the measurement of Q that is quality factor which we have already seen that through the circuit how it is measuring the Q meter how the output voltage value of the output voltage it is equal to the quality factor so in this mail measure of this quality factor is measure P naught output voltage it is equal to Q times of the input voltage or we can say Q is equal to the ratio of the output voltage and the input voltage and the same circuit as we have studied that in the Q meter that is used for the measurement of Q second is the measurement of inductance now as we know that in the resonant circuit Excel is equals to XC and we were getting the resonant frequency as 1 upon 2 pi under root of LC so if we take square of these if we will get F naught square is equals to 1 upon 4 pie square L C so through this equation we can get the value of the inductance L as 1 upon 4 pie square F PI square F naught square C so through this formula we can measure the inductance also now next is the measurement of capacitance you now we know that we are having the frequency we know the value of the frequency resonant frequency it is equals to 1 upon 2 pi under root of LC so for the measurement of self-capacitance what we do we make two measurement one it has frequency f1 and second at the frequency f2 such that the second frequency f2 it is twice of f1 okay this is the relationship between them so if we talk about f1 its value will be 1 upon 2 by under root of L C 1 plus CD due to the distributed capacitance this distributed capacitance will be added in the value of the capacitance of the coil and if you talk about f2 it will be 1 upon 2 by under root of L C 2 plus C D okay now we know that the relationship between F 2 and F 1 is that F 2 is twice of F 1 so just put the value of F 2 it is 1 upon 2 pi under root of L C 2 plus C B is equals to 2 into 1 upon 2 pi under root of L C 1 plus C D so through this we will get the value of the distributed capacitance as C 1 minus 4c 2 by 3 so this is the value of the self capacitance so we can say the distributed capacitance which can be measured through the Q media because through Q meter we are getting the value of the resonant frequency and we have used the condition we have made two measurements at the different frequencies F f 2 such that f 2 is equals to twice of f 1 now the next quantity which can be measured through the Q meter is magnet can also be measured in the same way that we are measuring the capacitance we will take the values at the different frequencies and through that we will get the relationship between the Q factor and the bandwidth as Omega naught upon Omega 1 minus Omega 2 this is the relationship Q factor we can measure through the 2 meter Omega naught is the resonant frequency and Omega 1 and Omega 2 other frequencies where the different measurements are taken so through this formula we can measure the bandwidth we can say Omega 1 minus Omega 2 is the banker Tony because bandwidth is the range of frequencies okay so Omega 2 is our higher frequency and Omega 1 is our lower frequency so if we are measuring the difference between them we can measure the bandwidth through them next is the measurement of the resistance so how we can measure the resistance we are measuring the quality factor through the Q meter so we know that the measured value of the quality factor is equal to Omega naught l by r now L we already know that L we can measure through by the formula 1 upon 4 pie square F naught square C so the value of L is known Omega naught we know that it is equals to 2 pi F naught so the value of R will be 2 pi F naught L upon the measured value of the quality factor so through this we can measure the resistance of the cone next is measurement of capacity now for the measurement of capacitance we are going to use your dougie coil and this dummy coil is connected between the terminals t1 and t2 okay under t1 and t2 we were connecting the coil under the test now here we are connecting our dummy coil and we are going to vary the value of the tuning capacitor suppose that we are getting the value of the tuning capacitor we are making via varying its value tuning capacitor so that the resonant frequency is obtained the socket as resonator and the value of the capacitor or the tuning capacitor we are getting as c1 now again we are going to remove this dummy coil and we are going to connect our coil under test and the capacitor whose value we want to measure it is connected across the terminals t3 and t4 now this capacitor will be in parallel with the tuning capacitor and suppose that again we are resonating the circuit we are varying the value of the tuning capacitor and now it’s value is c2 okay so in the second case this was the first case and in the second case we are connecting the capacitor which we want to measure across the tuning capacitor so they are connected in parallel with each other and this is our second reading c2 so are the value of the capacitance which we want to measure it is the difference between the two readings c2 and c1 because in the first reading they are taking only the tuning capacitor and in the second case we are taking both tuning capacitor and the terminal capacitor or the capacitor we want to measure capacitor under test okay so the value of C T will be c2 – even the difference between the two readings now other than these quantities the way other applications of the cue meter are like we can measure the frequency through it you like f naught is equal to 1 upon 2 pi under root of LC through this formula we can measure the frequency also also unknown impedance can be measured and another application is the characteristic impedance of a transmission line so these are all the applications of the Q meter so you’re in this video P studied about that what is a cube meter why it is known as Q meter and rnc meter what is the principle of working of this instrument and then we studied that what are the quantities which can be measured through this Q meter and in the last to be studied it’s applications in different areas so I hope that this topic Q meter is now clear to you thank you

12 thoughts on “Q Meter – Working Principle and Construction – Electronic Instrumentation and Measurement

  1. How confidently you are teaching wrong. Definition of Q-facor is the ratio of "energy stored per cycle to energy dissipated", however in your explanation you just reversed it. Have you read before you teach. Stop acting of teaching.

  2. This is not the first time you teaching wrong things… But in the video of digital multi meter also you were uttering blunders. Please stop doing this. It is okay that you don't teach, but don't let people learn wrong things just because of you.

  3. I don't know Who are you professonaly but for me you are the one who made me score heighest marks in EI,although i was afraid of getting Failed.THANKS A LOT MAM!

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