Can I measure inductor like this?

Discussion in 'General Electronics Chat' started by bug13, Jan 8, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I will need to measure some inductors I got from some old electronic.

    So can I hook it up to a AC say 9VAC from a transformer, and measure the AC current, then calculate the L with XL=2*PI*f*L?

    the value of the inductors I need to measure is in mH range (guessing), so will this give me a good indication of the actually value of those inductors?

    if not, what else can I do? I only have scope and DDM.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You will also need to know the DC resistance of the coil because X(sub L) is the total impedance. Also, frequency in your application should be roughly in the same range as your 60Hz transformer. That is, it may not be valid for use in a 100kHz oscillator since eddy current and other losses will start to get very big - it will depend on the quality of your core materials. You should also have a resistor in series with your coil so you can measure a voltage drop (otherwise you will only measure the source voltage which should not change).

    EDIT: I guess you said you would measure current. I recommend measuring the voltage drop and calculating based on voltage drop instead of current. There are internal resistances in current meters that would have to be corrected for and all can be avoided by measuring voltage drop.
     
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  3. Markd77

    Senior Member

    Sep 7, 2009
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    Probably the best way is to make an oscillator with them. SgtWookie gave me a circuit using an inverter. I'll have a look tomorrow if you like.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    we used a square wave switcher and switch a known voltage across it then look at the rate the current through the inductor rises.

    V = L di/dt
     
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  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi Markd77, that will be great if you can have a look tomorrow, thanks a lot
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    How exactly do you look at the rate the current through the inductor rises? can you give a bit more details?
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You use a current loop. You could probably also use a low value resistance.
     
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  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    OK, my original plan doesn't work for my inductors, my main is 50Hz and XL is too small, I can't get a useful reading with my $10 DMM
     
    Last edited: Jan 9, 2013
  9. Mustafatarhan49

    Member

    Aug 16, 2012
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    You are talking about AC. What if the input is DC step?
    do you think this is the right formula to find out the RL value for DC? iL(t) = (Vmax/L)*(1-e^-(t*RL/L))
     
  10. Markd77

    Senior Member

    Sep 7, 2009
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  11. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi Markd77,

    I don't have 74HC04, so I look around and found this:

    http://www.falstad.com/circuit/e-colpitts.html, Colpitts oscillator

    using the formula you provided, it works good enough for me, thanks for your help.
     
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