Audio millivolt meter

Discussion in 'General Electronics Chat' started by soda, Oct 11, 2009.

  1. soda

    Thread Starter Senior Member

    Dec 7, 2008
    Hi to all the members,
    My project is a audio millivolt meter. In this project i need a 200uA panel meter for the readout.I only have a 100uA and a 100mA meter in my scrap box.Can anybody please tell me how i can change these meters to get a 200uA reading on one of them.

    Thanks for all your help
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    Good Evening Soda, welcome to AAC.

    An audio millivoltmeter? Not so common these days. Tell us more.

    You have two approaches.

    1) It may be possible to alter the cricuit to accomodate a different meter. This could work with either of your meters.

    2) You could simply make the 100μ meter into a 200μ by adding a resistor in parallel. I recommend using a pot you can trim to be exactly the same value as your meter. Don't try to measure the meter movement with an ohmmeter you could well damage it.
    You cannot directly use the 100 mA meter.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    All dc meters use a coil to deflect the needle. The coil has an effective Ohmic resistance. You need to determine the meter's internal resistance.

    Grab a fresh AA, C or D Alkaline battery. As a check, read the open circuit battery voltage; it should read 1.54V.

    Hook a 10K resistor in series with your 100uA meter, and connect them across the battery. The meter should read almost full scale. Actual reading is not important, but make a note of what it reads.

    Now go get a 1K pot, set it to max resistance, and connect it as a two-terminal rehostat across the meter terminals. Twiddle the pot until the meter reads exactly half of the original reading.

    Disconnect the pot, and measure the pot's resistance. Get a 1% resistor closest to the measured pot resistance, and permanently wire it across the meter terminals. Whalah, you now have a 200uA meter, and now know what the meter's internal resistance is...
  4. soda

    Thread Starter Senior Member

    Dec 7, 2008
    Hi Mike,

    Thanks a lot for your help. I will do as you said.

  5. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    The method suggested by Mike is standard in determining meter internal resistance.

    For fresh 1.5V battery, 10KΩ value could be a bit on the low side as the current will then be about 150uA and the 100uA meter needle will peck to the high stopper, making the subsequent determination of meter internal resistance invalid.

    One can try using 15K, 18K or 20K as series resistor in this case.