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  1. #1
    Join Date
    Jan 2008
    Location
    KC Metro Area
    Posts
    314

    Default Measure inductance wtih a multimeter

    I searched the forum and found some answers but I thought I'd start a new thread. I've got a pair of AU265s coming and I need to measure the inductors, and I don't have an LCR meter. I can be a cheapskate at times, so I figure I can measure the inductors without having to shell out money for a LCR meter using this method:
    http://technologyinterface.nmsu.edu/...ct/induct.html

    I don't have a tone generator, but I have a PC hooked up to my stereo. I can use a software tone generator on the PC to send sine waves to my stereo. So please double check my plan:

    1) Set a tone on the PC
    2) Hook up the circuit detailed above, measure across the terminals (A to B) and set the volume until I read 1.0v.
    3) Measure from C to B and crunch the numbers

    My questions:
    -What frequency should I use?
    -In the tutorial linked above, it is mentioned to use a resistor of "between 100 S and 1 KS". I assume he means 100 ohm and 1 k ohm but the ohm symbol got mangled and is showing up as "S"?

  2. #2
    Join Date
    Oct 2005
    Location
    ZZ9 Plural Z Alpha; 2nd home in Wisconsin
    Posts
    549

    Default Re: Measure inductance wtih a multimeter

    I've got the cheapskate answer for you right here. Look around, there might be cheaper and/or better ones. My experience with these cheap meters (and I've compared a few with calibrated $1K+ meters) is that they are good enough for DIY speaker use except for small inductor values under .1 mH. And often in those cases you can just short the leads and get a reading, then subtract that from the inductor reading. This would actually be cheaper than a pair of large inductors, so it pays off right away.

  3. #3
    Join Date
    Jan 2008
    Location
    KC Metro Area
    Posts
    314

    Default Re: Measure inductance wtih a multimeter

    Hmm, so you are suggesting that I buy a new toy instead of hooking up various wires, equipment, and cobbling together parts while my wife scratches her head at the mess I'm making in the basement? Where's the fun in that?

    But you are right. $22 to be able to re-use some inductors and not buy new ones pays for itself quickly. Thanks for the info.

  4. #4

    Default Re: Measure inductance wtih a multimeter

    My hand held LCR meter uses a 1KHz tone.
    If I use it to try and measure a speaker's resistance, I'm actually measuring the speaker's impedance at 1,000 Hertz!

  5. #5

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by jkrutke View Post
    I've got the cheapskate answer for you right here. Look around, there might be cheaper and/or better ones.
    I own one of those meters and it works great.

  6. #6
    Join Date
    Sep 2005
    Location
    Kokomo, Indiana
    Posts
    9,066

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by View Post
    My hand held LCR meter uses a 1KHz tone.
    If I use it to try and measure a speaker's resistance, I'm actually measuring the speaker's impedance at 1,000 Hertz!
    With my LCR meter if I switch it to resistance then it measures DC. I have to switch to measure C and L and then it is a 1khz sin wave. Doesn't your work like that?

  7. #7

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by Jeff B. View Post
    With my LCR meter if I switch it to resistance then it measures DC. I have to switch to measure C and L and then it is a 1khz sin wave. Doesn't your work like that?
    No.
    Besides, I use my best True RMS DMM to measure resistances down to .01 ohms.

  8. #8

    Default Re: Measure inductance wtih a multimeter

    To add a new twist to this question, I have a small collection of unlabelled toroid forms
    they might be iron power or ferrite material, don't know and didn't care until I tried to calculate how many turns of wire I need to use to create a 24 uH crossover inductor.
    Apparently depending on material it could be 20 turns or several hundred.
    Don't have a lcr meter, but do have a scope, signal generator and a few lab grade dvm to wire up...

  9. #9

    Default Re: Measure inductance wtih a multimeter

    Actually, with a resistor in series with the driver, a DMM, and a way to drive your amp with tones, you can measure the impedance curve, not just a nominal value at one frequency like a meter. A lot slower than using WT3 however.

  10. #10

    Default Re: Measure inductance wtih a multimeter

    I see I left out the root problem I am facing,
    what is the inductance value of my custom wound toroids.

    One of the web sites I visited have me enter the physical parameters of the toroid and then after entering the desired uH value returned a value of n turns needed.
    It didn't inquire what the magnetic properties of the form were, and it never occurred to me that there are many different magnetic variations to a toroid form..
    So I carefully calculate the 30 turns will require about 6 ft of wire to away I go.
    Days later and brief research yields either powered iron or ferrite material, and each material then has many variations. Odds are my inductor is not a 25 uH after all.

    But the process to measure using existing test equipment is pretty easy...

  11. #11
    Join Date
    Sep 2010
    Location
    Houston, Texas
    Posts
    393

    Default Re: Measure inductance wtih a multimeter

    The process described in the link posted by the OP seems obtuse. Using the equipment you have, send a signal of frequency f though the inductor. Measure the voltage across the inductor and the current through the inductor. Divide the voltage by the current to get the impedance. Divide the impedance by 2πf to get the inductance.

  12. #12
    Join Date
    Oct 2005
    Location
    in front of a computer
    Posts
    1,078

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by Herman Trivilino View Post
    The process described in the link posted by the OP seems obtuse. Using the equipment you have, send a signal of frequency f though the inductor. Measure the voltage across the inductor and the current through the inductor. Divide the voltage by the current to get the impedance. Divide the impedance by 2πf to get the inductance.
    My inexpensive meters have relatively high shunt impedance, they should be a near short, but an ohmmeter across the other meter in amp mode reveals nearly an ohm. not a very good situation when the impedance we are trying to read isn't all that much higher.

    My favorite method is to resonate with a series capacitor and resistor (which you know the values of accurately) and look for the dip in voltage across the pair of the inductor and capacitor. The minimum frequency (2*pi*f)^2=1/(LC) solving for L is left as an exercise.

  13. #13
    Join Date
    Sep 2010
    Location
    Houston, Texas
    Posts
    393

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by rone View Post
    My inexpensive meters have relatively high shunt impedance, they should be a near short, but an ohmmeter across the other meter in amp mode reveals nearly an ohm. not a very good situation when the impedance we are trying to read isn't all that much higher.
    As you say, when used as an ammeter, it should have a low resistance (ideally zero). When used as a voltmeter the opposite is true, it should have a very high resistance.

    My favorite method is to resonate with a series capacitor and resistor (which you know the values of accurately) and look for the dip in voltage across the pair of the inductor and capacitor. The minimum frequency (2*pi*f)^2=1/(LC) solving for L is left as an exercise.
    I like it! I wonder if that's the method used by my LRC meter? It spends a few seconds doing something while I'm waiting for it to give me a steady reading. It could very well be sweeping looking for the resonant frequency, which is, by the way, independent of the resistance, so the DC resistance of the inductor is not relevant.

  14. #14
    Join Date
    Jul 2009
    Location
    canada
    Posts
    1,966

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by jkrutke View Post
    I've got the cheapskate answer for you right here. Look around, there might be cheaper and/or better ones. My experience with these cheap meters (and I've compared a few with calibrated $1K+ meters) is that they are good enough for DIY speaker use except for small inductor values under .1 mH. And often in those cases you can just short the leads and get a reading, then subtract that from the inductor reading. This would actually be cheaper than a pair of large inductors, so it pays off right away.
    I wasn't planning on buying an lcr meter but I didn't realize they were so affordable. The unit linked in this quote is available on ebay for under $20 shipped to Canada. But I took a quick look at the specs and IIRC it's only a 3 digit display with the lowest inductance range being 20 mH. That's not very good resolution compared to...

    http://cgi.ebay.ca/DM4070-2000uF-3-1...item27b2ea5df2
    That's the second cheapest lcr meter on ebay, about $34 shipped. Also only 3 digit display but this one has way more ranges, as low as 200 uH, and more ranges on both ends for capacitance.

    The second lcr meter looks like it might be worth $14 more but I don't really want to pay any more than that.

    Opinions? Is the second one worth the extra $$$?
    Don't even try
    to sort out the lies
    it's worse to try to understand.

  15. #15
    Join Date
    Sep 2010
    Location
    Houston, Texas
    Posts
    393

    Default Re: Measure inductance wtih a multimeter

    A speaker-builder is going to need that wider range, so, yes. Another factor to consider is zeroing. The LCR and a lot of the other digital meters that I have will not let me zero them. This can be a problem.

  16. #16
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    6,645

    Default Re: Measure inductance wtih a multimeter

    Use a standard resistor network/sinewave generator as used to measure impedance. Measure an inductor of known value to establish a baseline measurement for volts as mH rather than volts as ohms. Preferably measure a known value close to the estimated of the unknown. If you already have the gear to measure impedance you have the gear to measure inductors.

  17. #17
    Join Date
    Jul 2009
    Location
    canada
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    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by Herman Trivilino View Post
    A speaker-builder is going to need that wider range, so, yes. Another factor to consider is zeroing. The LCR and a lot of the other digital meters that I have will not let me zero them. This can be a problem.
    Thanks.

    Both those meters can zero (I think). The cheaper ones claim auto zeroing and the other one has an "adj" knob which (I assume) is used for zeroing.
    Don't even try
    to sort out the lies
    it's worse to try to understand.

  18. #18
    Join Date
    Jul 2009
    Location
    canada
    Posts
    1,966

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by billfitzmaurice View Post
    Use a standard resistor network/sinewave generator as used to measure impedance. Measure an inductor of known value to establish a baseline measurement for volts as mH rather than volts as ohms. Preferably measure a known value close to the estimated of the unknown. If you already have the gear to measure impedance you have the gear to measure inductors.
    Thanks Bill. I'll look into this but I'll at those prices I will probably buy a meter anyway.

    I have recently been acquiring passive xo components and now that I have a few hundred of them, the convenience and automatic nature of a meter is going to save me a lot of time.
    Don't even try
    to sort out the lies
    it's worse to try to understand.

  19. #19
    Join Date
    May 2008
    Location
    Fort Wayne, Indiana
    Posts
    758

    Default Re: Measure inductance wtih a multimeter

    FYI, I used to use a known value, non-inductive resistor in series with components and a signal generator to measure capacitance and inductance. I also used to measure T/S parameters with that resistor, a signal generator, and an oscilloscope. Now, I use Speaker workshop. After building a simple jig, found here, you can measure impedance of drivers, T/S parameters, passive components, and much more. I believe this setup is very similar to the WT3, but the software is open source and when I built the jig, I think it only ran me about $20 in parts. Granted, I had access to a 4-wire ohm meter to measure all my calibration resistors, but it's super easy to measure inductors and capacitors.

  20. #20
    Join Date
    Nov 2006
    Location
    New Hampshire
    Posts
    6,645

    Default Re: Measure inductance wtih a multimeter

    Quote Originally Posted by diy speaker guy View Post
    Thanks Bill. I'll look into this but I'll at those prices I will probably buy a meter anyway.
    I don't use that setup for impedance anymore, I just suggested it in case you did. I now use WT3, and it does inductance as well. I find that WT3 is a bit off, so I still calibrate it with a coil of known value.

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