Not a good experiment

Discussion in 'Feedback and Suggestions' started by tone_b, Nov 19, 2009.

  1. tone_b

    Thread Starter New Member

    Nov 8, 2009
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    In the Vol. VI - Experiments section under the Chapter 2: Basic Concepts and Test Equipment. And next on the Voltmeter Usage page, it reads about 3/4 of the way down:

    See attachments, since copy and pasting does not work.

    I have tried various analog meters and none of them are picking up voltage readings by shining a light onto a LED.
    I have also used some pretty bright lamps and various LED's so far nothing. What gives with this experiment?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Since I have not tried this experiment before today, it intrigued me, so here goes with my tests.

    I could just about get a high quality analog meter needle to shudder on the most sensitive range, and with a strong light.

    With a digital meter on the 200millivolt range I was reading about 15 - 20 millivolts dark and around 100 millivolts + in good light.

    Explanation - it's to do with the ohms per volt rating of analog meters.
    My digital meter has a fixed impedance of 10Megohm on all ranges, the analog one has 50,000 ohms per volt

    This rating multiplied by the max reading on the range concerned yields the impedance of the meter on that range.

    This means the input impedance and therefore load on the LED is 0.03 x 50000 = 1500 ohms.

    You should note that on very high voltage readings the analog meter actually has a higher input impedance than the digital.
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    studiot is exactly right -- it's input impedance, which is why I threw out my VOM 30 years ago. :)

    The text should certainly mention this limitation, as a beginner will likely get frustrated or confused.

    I have a photodiode that I keep potted in a chunk of PVC pipe with a BNC connector on the end. It makes a very good poor-man's optical radiation detector just plugged into a DMM. I can get acceptable readings in near darkness to outside when the diode is pointed right at the sun.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Specifically it is in this chapter...

    http://www.allaboutcircuits.com/vol_6/chpt_2/1.html

    3rd paragraph down...

    I suspect this paragraph is a bit dated. VOMs and DVMs are about the same prices nowadays, and VOMs are getting harder to find, while DVMs are getting easier. I expect this trend will only increase over time.

    BTW, I didn't have any problem cut and pasting, either with Firefox or IE.
     
    Last edited: Nov 19, 2009
  5. tone_b

    Thread Starter New Member

    Nov 8, 2009
    9
    0
    My analog meter has a 20K Ohm/V input impedance. So I guess I need a higher input impedence meter for this experiment to work. Thanks again.
     
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