Voltmeter design problem, please help

Discussion in 'Homework Help' started by soliloquy, Aug 16, 2009.

  1. soliloquy

    Thread Starter New Member

    Aug 16, 2009
    2
    0
    This is our question :
    Design an analog multimeter with a panel meter that has a full scale deflection dc voltage rating of 15V and an internal coil resistance fo 100Ω. The multimeter must be able to measure :
    1) voltage range from 1mV to 1000V
    2) current range from 10mA to 5A and
    3) resistance range from 0.1Ω to 100MΩ
    Describe in detail all the designing steps and calculations

    MY problem is
    For the Voltmeter design
    http://www.allaboutcircuits.com/vol_1/chpt_8/2.html
    the site only shows us how we can add mutiplier resistors to measure bigger voltages, but how about smaller voltages ;
    in my case 1.5 mV to 0.15V ?
    tried adding parallel resistors, but this only divides the current and the voltage remains the same,
    we need a I = 0.15Ampere for full scale deflection (fsd) but adding parallel, the current that passes through the internal coil resistance of meter is too small therefore only a small deflection will be seen (not sensitive)
    how do i solve this? diagrams would be much appreciated =)
     
  2. mauro.laurenti

    Active Member

    May 8, 2009
    68
    0
    Hello,

    If you want to go beyond the resolution (in terms of lower voltage/current) you need to fool the instrument.
    You must amplify the signal before you feed it to the instrument.

    Ciao,

    Mauro
     
  3. soliloquy

    Thread Starter New Member

    Aug 16, 2009
    2
    0
    means i need to use an op amp 0.o
     
  4. mauro.laurenti

    Active Member

    May 8, 2009
    68
    0
    ...yes,
    A transistor as well, but I would say that that an op amp can make your life easier.

    Ciao,

    Mauro
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    What is the full scale deflection of panel meter (ammeter) you are using?
     
  6. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > a panel meter that has a full scale deflection dc voltage rating
    > of 15V and an internal coil resistance of 100Ω

    It sounds like there is something missing in this description, namely
    the series resistor. A panel meter by itself is a microammeter, for
    which a resistance of 100Ω sounds typical. In order to get the meter
    to read 15V, you are supposed to connect a fairly large resistor in
    series with the meter movement. To proceed further, you need to
    find out what the value of this resistance is. With that information
    in hand, you can then calculate how many microamps it takes to
    deflect the needle fully, then figure out what sizes of resistors are
    needed to provide the ranges.

    As a mater of fact, this sounds a lot like the Radio Shack panel
    meter. Looking at the back of the package of such a meter, I
    see it says the following:

    15 VDC full scale deflection
    85 Ω internal resistance
    15 kΩ resistor should be connected between meter and voltage being measured

    By Ohm's law, 15 V across a 15 kΩ resistor makes 1 mA flow, so
    that means that the full scale deflection of the meter movement
    occurs at 1000 μA. (By the way, this means that this is not a
    very sensitive meter --- a value like 50 μA or less would be
    typical for a more sensitive meter. Put another way, this meter
    has 1 kΩ/V, which limits its use to circuits with large currents
    flowing through them and makes it worthless for fine electronics
    without amplification.)

    Have another look at your homework to locate information
    which would let you figure out how many microamps are
    needed to deflect the needle across the full scale. If no such
    information is provided, point out to your teacher that you can't
    finish the assignment because of inadequate specs.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    It the amperage of the meter isn't indicated, we certainly can infer it from the lowest voltage scale and the coil resistance. From there, you can work through all the details one by one.

    I posed the question so the OP is aware of a starting point, inferred or not.
     
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