#3 THE LOOPHOLES BEHIND VOLTAGE MEASUREMENT USING A MULTIMETER

Being an electronic engineer, we must know how to use a multi-meter and on what basis, multi-meters are chosen while measuring voltage across certain load resistor. Let me start this topic with a simple question. Just have a look at the picture below and give me the voltage across R2.

 Most of us will say that the drop across R2 is 6V because both the resistors are of equal value 'R'. But many of us don't know that the voltage is a function of 'R', when measured using a multi-meter. For example see the simulation results below.

So, from the above figure, we clearly notice that the voltage across R2 is not equal to 6V, although both the resistors .i.e., R1 and R2 are of same value. And we see that the voltage level is decreasing with the increase in the 'R' value. (Refer to 1st figure to see what 'R' is). Why is this happening ?

Simple reason - 

• When we use an ideal voltmeter/ multi-meter, its internal resistance will be infinite. So, when an ideal voltmeter is placed parallel to a resistor, no current flows through the meter and all the available voltage across the resistor will be measured by it. 
• When we use a practical one, it will have a finite (but large) internal resistance. So, if 'R' is very small compared to the internal resistance of practical multi-meter, it will show a perfect reading, just as the multi-meter has given a value of 6V in 2nd figure. Note that, even though the a 22k resistor is used, the value isn't 6V - it might be some nano volts less than 6V. But that difference is neglected and 6V is displayed as the output.

Note - Even though the multi-meter shows a different value of voltage across R2 for different values of 'R', the voltage is always 6V. But we don't have an ideal multi-meter that shows this value. The answer for the question I asked at the beginning of this topic is 6V itself and is independent of R, if and only if we have an ideal voltmeter.

So the conclusions for this topic are-

1. The multi-meter selected, should have it's internal resistance to be atleast 10^6 times of that of the 'R' value to have an accurate reading of voltage. We should select the multi-meter, which has got such internal resistance. 
2. Irrespective of 'R', the voltage is 6V. But the problem lies within the measurement (within multi-meter)
3. If ideal multi-meter is available (hypothesis), we would have got 6V in all the cases of 'R'

If you have any queries, post a comment. You can as well contact me at +Prajwal Kotamraju  , pj.kotamraju225@gmail.com