Basic electric theory #3

Discussion in 'Homework Help' started by xbox112005, Oct 31, 2011.

  1. xbox112005

    Thread Starter New Member

    Oct 30, 2011
    Please help me with this question.You are an electrician on the job.You have been given a multimeter that has the following ac voltage ranges:30,60 and 150.The meter state that it has a resistance of 5000ohm/v.You need to be able to measure a voltage of 277 volts.How much resistace should be inserted in series with the meter to make the 30-volt range indicate a full scale value of 300 volts?:confused:
  2. Georacer


    Nov 25, 2009
    Voltmeters traditionally measure voltage through the effect of current over a predefined resistor.

    For example, in the 30V scale, the internal resistance of 30*5kΩ would have a maximum current of I=V/R=30/(30*5k) excite its needle to the max of its scale.

    Now, for your case, you need to increase the scale by 300/30. The maximum current allowed to set the scale to the max is I'=Vmax/R'.
    We know that I=I', as the indicator is the same. We also know Vmax, it is 300V. But this time R' is equal to R'=Rinternal+Rseries.

    Can you calculate Rseries now?
  3. crutschow


    Mar 14, 2008
    This is really a nit, but it's the effect of current through a resistor, not over it.

    A meter that's rated in ohms/volt is normally a mechanical meter movement which has a very low intrinsic resistance and is basically a current operated device. Thus the resistor in series with the movement determines its sensitivity. If it has a sensitivity of 5000Ω/V, it's a simple calculation to determine the total resistance you need for 300V full scale.
    Georacer likes this.
  4. bertus


    Apr 5, 2008
  5. Georacer


    Nov 25, 2009
    Hey, if I'm a grammar Nazi, it's only reasonable for me to want to expand my party :p