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Inductance meter adapter for a DMM

Discussion in 'General Electronics Discussion' started by OhMy, May 19, 2014.

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  1. OhMy

    OhMy

    63
    0
    Apr 5, 2014
    Hi!
    I'm going to make some sort of inductance metering device, but not sure, which one would be better (if any) of these two- one attached, the other one from here.
    I have a DMM UNI-T UT61E with capacitance and frequency measuring capabilities. But I don't really have any known inductances to calibrate with, so it would be easier to make the one from the link with LM311.
    Any experience with these kinds of devices?

    I know, I should just buy one from eBay, but as it is in most cases- it's all about the money...

    Thanks
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    The circuit with the LM311 is probably more robust than the one you attached in the word doc. By measuring the frequency directly, you eliminate any uncertainties that result from the frequency-to-voltage conversion in the attached circuit. Also, the 74HC132 is not, as the document states, an operational amplifier, but a logic gate which is "abused" in this circuit. The output voltage of this circuit will depend on the stability of the power supply and the thresholds of the logic gates. It will all be rather sensitive to changes in temperature.

    Note that with any of these circuits the accuracy is a direct function of the accuracy of the capacitors. Use precision capacitors for good results.
     
  3. OhMy

    OhMy

    63
    0
    Apr 5, 2014
    Thanks, Harald. You pretty much repeated my own thoughts (except for the abuse of 74HC132- my mind does not reach that far :D) about the choice.
    What else I don't really get, is the phrase: "If an oscillation is visible or audible (f < 16kHz) the inductance can be calculated according to the formula...". Why such a limitation? It would be difficult to measure higher frequencies with a sound cart, but I don't see any other reason. Would accuracy suffer above these 16kHz?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    If you know the frequency, you can calculae the inductance, no matter whether it is "visible or audible". I think what the author meant is that you need to be able to measure the frequency, either by visible means on a scope or of the frequency falls into the audio raneg b y means of a PC soundcard.
    Since you have a multimeter that can measure frequencies, all that is required is that the generated frequency falls into any of your instrument's measuring ranges.
     
  5. OhMy

    OhMy

    63
    0
    Apr 5, 2014
    OK.
    Will let you know the results, when LM311 arrives.
     
  6. OhMy

    OhMy

    63
    0
    Apr 5, 2014
    So...
    The LM311 finally arrived (shipping time gets only worse, as time centuries change :mad:). The circuit kinda worked right away; here are some test results, using couple of salvaged and marked inductances:
    Inductance marked as;__calculated value with 268nF capacitor, uH;__calculated value with 2.505uF capacitor, uH.

    682J____6533____601
    3R3J____3.48____3.57
    6R8J____6.96____7.11
    330____33.24____14.8
    101____97.9____20.3
    10 ____10.4____2.46
    270____26.42____11.6
    471J____457____438
    472J____4610____515
    330 ____32 ____32.6
    100____10.44____10.69
    220____22____8.4

    Question: Why some measurements with the bigger capacitors are so wrong, but others are pretty close? Calculations were made in Excel, using the same formula, and frequency was measured with a UNI-T multimeter.
    I don't have any bigger and known inductances.

    Some pictures of this creation:
     

    Attached Files:

  7. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Try to shorten wire of test probe for better accuracy. Maybe solder the test clip directly to pcb. Also keep your Inductance meter from fluorescent or magnetic inducing devices. I have encountered this problem before.
     
  8. OhMy

    OhMy

    63
    0
    Apr 5, 2014
    It looks to be a random error, because I measured some other unknown inductances, and mostly the readings came out quite close one to another, but there were times, when the one with 2.505uF cap was way lower, than the other. All else was the same- placement, power source and test lead length.
    I'm thinking, could it be, that there are some parasitic different frequency waves at the output, which DMM picks up as the main frequency? But it doesn't happen with the lower value capacitor...

    Either way it's a really handy device, even if it requires some calculation :).
     
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