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Input impedance of Dick Smith Q-1420 multimeter (cheap)

Discussion in 'Hobby Electronics' started by Matthew Kirkcaldie, Jan 16, 2006.

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  1. Silly question, probably. I have a cheap DSE multimeter, model Q-1420.
    Documentation's long gone, but I wanted to know its input impedance when
    measuring DCV. I'm setting the idle current on an amplifier and the
    service manual specifies a VTVM (tube voltmeter) to make the measurement
    - presumably because of high input impedance. I assume if the DMM has
    10 meg input impedance it'll be OK - but I need to know!

    Cheers, MK.
     
  2. Phil Allison

    Phil Allison Guest

    "Matthew Kirkcaldie"

    ** Thats right - Doc.


    ** Nearly always, DMMs have 10 or 11 Megohms impedance on DC voltage ranges.

    Very cheap ones might have 1 Megohms.


    ** Err - must be a vacuum tube amp, right Doc ?


    ** Any DMM will do the job just fine.

    1M or 10 M.


    BTW:

    Old VTVMs are just vaguely interesting relics nowadays.

    DMMs are guilty of planet wide, VTVM genocide.





    ........ Phil
     
  3. If you had a second meter you'd be able to measure it!
    The DMM most likely has in the order of 10M so it should do just fine
    for your application.
    Yes, the VTVM was specified because of its higher input impedance than
    a normal analog meter (10-50Kohm/volt typical).
    Incidentally, some DMMs have a selectable "infinite input impedance"
    mode (in the order of 10's Gohms) for specilised high impedance circuit
    measurments. I think I saw a cheapie meter once that had no input
    impedance on the mV range and the usual 10Mohm on all others.

    Dave :)
     
  4. Andy Wood

    Andy Wood Guest

    How about using the meter in question to measure it:

    Say you have a DC power supply set to 10V. You measure this with the
    meter and (hopefully!) it shows 10.0V. Then you measure it again with
    a high value resistor, of say 10M in series and, for example if the
    reading is now 5.0V the impedance of the meter is 10M. You might have
    to try a few different resistors if you start out with no clue as to
    the answer.


    Andy Wood
     
  5. Mr.T

    Mr.T Guest

    Yes, I doubt anyone could build a VTVM and sell it for $10 :)

    I still have a FET-VOM floating around though, along with an AVO 8. They are
    still useful for peaking and nulling. Much better than a digital read out or
    LCD bar graph for that purpose IMO.

    MrT.
     
  6. Phil Allison

    Phil Allison Guest

    "Andy Wood" ..

    ** To see if a DMM is 10M or 1M impedance on the DC volts range - you only
    need is *any* battery and a 1M resistor.

    The answer is irrelevant to the OP in any case.



    ......... Phil
     
  7. Phil Allison

    Phil Allison Guest

    "Jasen Betts" >

    ** **** off - copy cat idiot.




    ....... Phil
     
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