VOM battery problem

Discussion in 'General Electronics Chat' started by legac, Jul 20, 2008.

  1. legac

    Thread Starter Well-Known Member

    May 4, 2005
    54
    0
    Hi all
    I use a vintage US Army VOM. Its Ohm range is supplied by 1.35 volt. I have tried 1,5 volt cell but its needle always exceeds meter scale. How can I deliver 1.35 volt from 1.5 volt? Your ideas are highly appreciate. Thank you in advance
    Legac
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You might try a rechargeable battery. They generally put out around 1.2v-1.3v when charged.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Was the original battery a mercury cell?

    Pentax, Cannon, and Wein all make 1.35V zinc-air batteries for older cameras which took the mercury cells. The zinc-air batteries are expensive, though.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Measure how much current the meter draws when you apply its full range voltage. Then place a resistor in series with the meter with a value that will be

    0.15/Im

    where Im measured current drawn by the voltmeter when it has a applied a full scale voltage.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    You need a 0.2 volt zener diode drop in series with your 1.5 volt cell.

    As these don't exist, try the forward voltage drop of a germanium diode or perhaps a germanium transistor with its collector and base leads strapped together.
     
  6. legac

    Thread Starter Well-Known Member

    May 4, 2005
    54
    0
    Thank all of you. I will try the options and get back to you once I have resolved the problem. Perhaps I will try Ge transistor first. I am thinking about another option, that is to use two small 1.5 volt cells in series to have 3 volts. It would be easier to make it down to 1.35 volt.
    @ mik3: Resistor in series is not workable because each range draws a different current.
    Rgds
    Legac
     
  7. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    Is the v.o.m compatable with testing transistor curcuits ?
     
  8. legac

    Thread Starter Well-Known Member

    May 4, 2005
    54
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    No. The model is ME-70A-PSM6. It looks like for Navy because it is sealed carefully.
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Use one resistor for each scale or the best solution is to replace the old voltmeter with a cheap and nice digital multimeter.
     
  10. legac

    Thread Starter Well-Known Member

    May 4, 2005
    54
    0
    Hi
    An old 2SB Japanese audio transistor fitted the low range. The Hi range was overloaded a little bit. Finally I replaced the resistor in series with the pot to extend the adjusment scale. It is OK now.
    @ mik3: I do have a DVOM. However I still use the military VOM for tube amplifier because of the high risk potential. The DVOM is for final check only.
    Thank you.
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Very Wise.
     
  12. theamber

    Active Member

    Jun 13, 2008
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    0
    Most solid state D'arsonval meters are current sensing, your meter probably has the D'arsonval movement. All that I have seen has some type of adjustment outside and inside for controlling the full scale deflection.
     
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