What is the circuit diagram equivalent to a resistance meter?

Discussion in 'General Electronics Chat' started by StuckStudent123, Dec 7, 2018.

  1. StuckStudent123

    Thread Starter New Member

    Sep 23, 2016
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    If a voltmeter can be represented as a very large resistance and an ammeter can be represented as a very low resistance then what is the equivalent of a multi-meter on a resistance setting? I'm asking because I'm trying to make an oscillator circuit and currently the frequency is too low but I have noticed that when I measure the value of one of the resistors with a multi-meter when the resistor is still in the circuit, it gives me the frequency I want. I've tried changing the value of the resistor to a lower one (I thought maybe it would have been like resistors in parallel thus lowering the overall resistance) but this didn't help so I'm wondering how I can recreate the multi-meter's effect with components.
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    On Resistance measurement the probes apply a voltage across the resistance being tested.
    You are injecting a voltage into your circuit.
    E
     
  3. Ian Rogers

    Well-Known Member

    Dec 12, 2012
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    If you are slightly increasing the voltage by having the multimeter in circuit, you will need a higher value resistor ( if the resistor in question is common ground )...

    This is an assumption... I'm assuming the resistor makes up a potential divider!
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    It seems that my multimeter puts a current through the device to be tested while the voltage across the device is measured. R = E/I
     
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  5. OBW0549

    Distinguished Member

    Mar 2, 2015
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    Ummm... no, it's as @DickCappels said: the DVM injects a controlled constant current into the unknown resistance and measures the resulting voltage. If the meter were to apply a voltage and measure the resulting current, shorting the meter leads to verify zero or when checking continuity would be... problematic.
     
  6. BillB3857

    AAC Fanatic!

    Feb 28, 2009
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    It depends upon the type meter being used. Older (Simpson 260 for example) applied a voltage in series with the unknown resistance and measured the amount of current flowing through it. More modern meters use a constant current source and measure the voltage developed across the unknown resistance. Either type would tend to inject some voltage into an operating circuit. Try reversing the meter leads when you make your test. One way will probably raise the freq and the other way would probably lower the freq. Use a diode to see which lead is positive when checking resistance and that answer may answer your initial question.
     
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  7. WBahn

    Moderator

    Mar 31, 2012
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    First off, this isn't a useful way to make a resistance measurement -- whatever reading you get will almost certainly be impossible to extract any meaningful information from and if it seems like you can, then you are probably leading yourself down a rabbit hole.

    But I understand that, validity of the measurement or not, you are trying to replicate it's impact on the circuit. As others have said, the equivalent circuit depends on the type of meter and even the brand and specific model as well as the range it is set on.

    Do you have another meter handy? If so, then you can take a stab at measuring the DC parameters by noting the reading with it in the circuit and then removing it from the circuit and connect it to that value resistor and then use another meter, preferably a high-input impedance digital meter, to measure the voltage across the resistor. Then do the same with a resistor that is twice the size and again with half the size. See if these measurements agree with a model of the meter that consists of a voltage source in series with a resistor and, if so, figure out wha the values are -- we can help.

    But this may not be sufficient -- in fact it probably won't be. Your oscillator is interacting with the meter at the frequency of oscillation and the appropriate model at that frequency might be very different from the DC model.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    In summary, the answer to your question "What is the circuit diagram equivalent to a resistance meter?" for a typical digital multimeter, it's a constant-current source with a low voltage compliance limit in parallel with the meter input capacitance.
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you look at the schematic of the simpson 260 multimeter, you will see R21 in the Ohms path.
    This is used to "null" the ohms measurement. Just short the pins and set the 0 Ohms on the scale.

    simpson_260-7_7m_analog_multimeter_sch.pdf_1.png

    Also the scale will not be linear in the ohms range.

    simpson_260-7m_02.jpg

    Bertus
     
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