chapter 8 - Voltmeter impact on measured circuit

Discussion in 'General Electronics Chat' started by ma_rine_sa, Apr 11, 2009.

  1. ma_rine_sa

    Thread Starter New Member

    Apr 11, 2009
    This topic kinda belongs in the feedback section but i also have a question about chapter 8 (Voltmeter impact on measured circuit) as i am still confused at some points after reading it.

    The question i will ask is probably so obvious and simple that i am having trouble to comprehend.

    The pictures below are from chapter 8 (Voltmeter impact on measured circuit):
    picture 1 below
    picture 2 below

    For the next question i can be right or i can be wrong, i just simply don't know.

    Is it possible for me to use the Voltmeter (picture 1) to measure the above circuit (picture 2)?

    If i can not use the voltmeter to measure the above circuit, why? and can someone draw the correct voltmeter to use for the above circuit?

    I'm not looking for any trouble. I just simply want to learn and get some confirmation.

    Thank you for your help.
    Last edited: Apr 11, 2009
  2. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    I don't really know why you have asked these questions in the first place. :confused:

    The chapter is explaining exactly why one cannot use that particular setup(meter shown in your picture 1 which has a low internal impedance) to measure voltage across a 250MΩ resistor and get a correct result. The internal impedance of the meter will "combine" with the 250MΩ to form a new resistor with much lower value, in this case that of the meter, and thus the whole circuit configuration has been altered.

    The chapter also offers possible solution of using a potentiometer setup to do the correct measurement.
  3. italo

    New Member

    Nov 20, 2005
    most analog meters use 50ua full scale that is one source of error most comecial digital voltmeters are very hi in input inpedance but 250m it looks like an open circuit i wander if ever a 250mega was ever made. eitherway it can easily be measured and calculated.
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    High value resistors. or series strings are found in practice, voltage multipliar tubes, 240M, ionization chambers etc; I just trusted Mfg. markings. A few years ago 10M was the common input Z for VTVMs, and 'scope probes. Could add a non-inverting fet input OP Amp for low V[o-15] to cheapic VM.
  5. RAH1379

    Well-Known Member

    Dec 13, 2005
    an easy way to tell if your meter is loading the circuit is to measure the voltage on two different ranges, if you get a lower reading on the lower range then it is loading the circuit under test. Also if you multiply the ohms per volt times the range selected you will get the total resistance of the meter.Loading has the most effect on high impedance circuits with low voltages.
    Last edited: Apr 15, 2009
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    This is quite true. The only way to measure very high resistance resistors is with a high voltage power supply. However, unless you know the high voltage power supply voltage accurately (which requires accurate high voltage resistors to accomplish) you are really in a catch 22! So, buy good quality standard resistors. :)