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Megohm Bridge Method for Measurement of High Resistance – Electronic Instrumentation and Measurement

October 21, 2019


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 will study the make ohm method which is used for the measurement of high resistances high resistances are those resistance which are greater than the value of 100 kilo ohm or 0.1 maga ohm so let us start with that topic. As i have said that the high resistances are those resistances which are having the resistance value greater than 100 kilo ohm or point 1 mega ohm these high resistances there are various methods for their measurement like loss of charge method direct deflection method mega method and the mega method here we are studying the make o method now if we talk about the high resistances there are various examples of high resistance like the insulation resistance of the cables and the machines and the leakage resistance of the capacitors also the high resistance circuits like the vacuum tubes volume resistivity and surface resistivity of a material all these are the examples of high resistances now this may go method it measures the high resistances in the form of a bridge Network if we talk about bridge the bridge consists of four arms and in the four arms we have four resistances and one of the resistance is the unknown resistance which we want to measure now the popular a bridge network which is used for the measurement of resistance is the Wheatstone bridge now Mako method it also uses the some modification of this vide stone bridge in that also we will have the four resistances but one of the arm of the bridge it consists of a mega ohm resistor or a high resistance which consists of three terminals and in that three terminals what two terminals are the main terminals and one is the guard terminal so let us first studied the resistor or the megohm bridge that what are the four arms of those bridge so Megan Bridge just like the wheatstonebridge it will also consist of four arms and in one of the arm it will consist of one three terminal high resistance and this is located in one of the arm of the bridge now let us see how this is the three terminal resistance you so in the Macomb bridge we will have four resistances out of those four resistance one resistance is the unknown resistance or we can say the high resistance which we want to measure now this high resistance it will consist of three terminals one a and B are the main terminals and one of the terminal is the guard terminal which is designated as G so here a and B are the main terminals and in between these main terminals the high resistances connected and the third terminal is termed as the guard terminal now if we simplify this it will be like we have the three terminals and in between these three terminals we have three resistances it will be treated as three resistances between a and B the main terminals are high resistance is connected that is our a/b between a and G we have another resistance whittles are a G and R B G okay so these are the equivalent resistances between the three terminals so this resistance will be connected in one of the arm of the bridge so if we draw the socket for the Megane bridge it will be like so in the Wheatstone bridge also we were having four arms so here also we will have four arms a B C and the D arm in between B and D we have the galvanometer which will show us the null deflection when the four resistances ratio of these resistances will be equal that is P by Q equals to R by s now here one of the arm in one of the arm we have connected this resistance this resistance we want to measure are a B okay and re G and B G these are the internal resistances of this resistor s is the standard resistance which can be varied this is a variable standard resistance and here the battery is connected which is providing the EMF now when we measure this resistance by using the wheatstonebridge we are using this high resistance using the wheatstonebridge so let us take an example that this Rab is equal to 100 mega ohm okay and this we are measuring with the help of the wheatstonebridge now this is also 100 mega ohm this is also 100 this is also 100 so this guard terminal is there are a G and RB g they will come in series with each other so total resistance will be 100 plus 100 200 and then that will be in parallel with the are a b so 200 and this hundred they will be in parallel with each other so equivalent resistance will be 100 into 200 upon 100 plus 200 so from this this formula the value of the resistance will come out to be 67 mega ohms but the actual value is what it is 100 mega ohm but through this we are getting the value of 67 mega ohm so there is an error of 33 percent in this okay so to remove this error some modification is done in this bridge so that the correct value of the unknown resistance can be measured let’s see what is the modification you so instead of been in the first is we have the galvanometer and in that the galvanometer is checking the null condition and this is the gut terminal where the resistances are here connected now this bad terminal is left alone in this circuit but now here this god terminal is connected to the point B okay so in that case if we measure the resistance value that resistance value will come out to be more accurate as compared to the earlier circuit so this modification is done the gut terminal and the terminals where the P and the Q resistances they are meeting they will be taken as the common okay and the galvanometer it again measures the knot deflection condition now this maycon bridge it can measure the value of the resistances in the range from zero point one mega ohm to ten raised to the power six mega ohms so this is the range of the resistances which can be measured with the help of this megohm bridge when we are measuring point one mega ohm resistance then the accuracy is within three percent whereas when we are measuring the resistances which are of the order from 10 raised to the power 4 to 10 raised to the power 6 then the accuracy is about 10 percent okay so you can see that the accuracy which we are getting from the beach stone bridge that is increased in the case of Magoon bridge here you can see that they are we were getting or 33 percent of accuracy but here we are getting an accuracy of 3 percent and 10 percent okay so more accurate results are obtained with this modification again let’s see so what this instead of that galvanometer here we are having the guard terminal and that God terminal will be connected to the meeting point of P and the Q resistances okay here is standard resistance that is also variable resistance so here we will have different values for it the range will be like from point 1 1 point 0 then 10 and 100 1000 in this way we have 5 dial switches here and through that die switches we can vary the value of this standard resistance the Q resistance it is also its value can also be changed and this dial on the Q it gives us the range from 1 to 10 and 100 mm mega ohm so its value of Q it can be in the range from 1 to 10 100 mm and here also different ranges are provided on the dial switches of the standard resistance and the terminal of P and Q where they are meeting that is termed as the guard terminal and here the deflection will be checked and this unknown resistance can be measured so Accord bend this bridges balance the value of this unknown resistance is given by just like the wheatstonebridge unknown resistance R will be equal to P by Q into s so this is how the value of an high resistance unknown high resistance can be measured with the help of the make own method in the mega method it is just similar to the Wheatstone bridge but the difference is that we are having a three terminal hydra’s since connected in one of the arms of the Wheatstone bridge and in that three terminals two are the main terminal one is the guard terminal and that got terminal is connected to the meeting point of the P and the Q resistances and using the same balanced equation as that of the weed Stonebridge the unknown resistance can be measured so in this video we studied the make a method which is used for the measurement of high resistances and I hope that this topic is clear to you thank you

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4 Comments

  • Reply phanindra reddy November 17, 2018 at 6:51 am

    nice explanation ..thank you

  • Reply Gaurav Adhav March 13, 2019 at 1:52 pm

    explain megger method plz

  • Reply Ekeeda May 18, 2019 at 10:13 am

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  • Reply Akash Sharma August 4, 2019 at 7:53 pm

    Good explaination mam

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