inductance testing circuit

Discussion in 'The Projects Forum' started by lokeycmos, Dec 18, 2009.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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  2. SgtWookie

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    Jul 17, 2007
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    You would need to change S1 to a 2-pole rotary switch, and duplicate the R6/R8/C1 combination for each additional range, adjusting C1 up or down as appropriate.
     
  3. Wendy

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    Mar 24, 2008
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    That is actually a cool design. Not sure if I trust it very much, but I've been thinking about something like that for years.

    One of the real problems with inductors in general is they are much sloppier than capacitors. A capacitor is pretty simple, but inductors tend to have capacitance included with the part (especially DYI parts), that and their core reacts differently to various frequencies. The upshot is you may have an inductor that measures 470µH, and it is that value for that frequency, but something wildly different at a different frequency. Just a heads up.
     
  4. SgtWookie

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    Jul 17, 2007
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    Took another look at that circuit; I'm afraid you won't get very reliable results from it.

    Decided to run it in a simulation; it was decidedly non-linear even using ideal components.

    Take a look at Ronald Dekker's "Flyback Converters for Dummies" page. It's a really good resource.
    http://www.dos4ever.com/flyback/flyback.html
    You do need an oscilloscope for his test method, but it's much more accurate that you'll get with the circuit you're trying to use.

    [eta]
    Bill, we cross-posted; I must've just started replying as you were posting yours.
     
  5. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    correct me if im wrong, but the inductance stays the same at any frequency. It is the inductive reactance that changes.
     
  6. Wendy

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    Nope, what I was talking about is the core. Better cores are more consistent (which is why torrids are more popular). The material inside the inductor also has a freq response that has nothing to do with inductance. The built in capacitance on the inductor also screws with you, but that isn't what I'm talking about. Even capacitors have this problem to a much smaller degree, especially electrolytics.

    So an inductor that is 470µH at DC and low frequencies can be 47µH at 10Mhz (an extreme example). Soft iron cores such as iron may have this effect worse than powdered iron or graphite cores.

    Add the built in capacitance, and coils can be self resonant at some frequency. Some designs, such as Tesla Coils, use this fact.

    I'm not mentioning this to bring you down, but it is something you need to be aware of. Ain't electronics fun!

    I'd still use the coil measurement device, and just be aware of some of it's limitations. It will give you a good ballpark figure you can work with.
     
  7. bundick

    Active Member

    Dec 19, 2007
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    Inductance or the induced Voltage as a magnet passes a Ferrite core with wire wrapped around the core.

    So if the Rpm is slow, the Scope would see one sine wave, if the Rpm is much quicker, the sine wave would be shorter in amplitude and shorter in pulse width. IMHO!

    So the faster the Rpm, or frequency, the less Voltage the Scope would see at the output of the inductor.
    Thats how I see it...am I even close? :)
     
  8. DickCappels

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    Aug 21, 2008
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    If you need a lower range, you might find it necessary to reduce R3. This circuit is not particularly linear, but good enough for most work. And yes, stray parallel capacitance and core effects (mainly Q) can have a huge effect on accuracy. Best to calibrate it with the type of inductor you plan to test.
     
  9. Duane P Wetick

    Active Member

    Apr 23, 2009
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    I'm really suspicious of any inductance tester. There is just no substitute for using a signal generator to resonate a LC circuit, and watch for a voltage peak, along with a good reactance chart for determining a reliable inductance figure. Inductance standards are hard to find, but 1% film or mica capacitors are available everywhere.

    Regards, DPW [ Spent years making heaters out of op-amps.]
     
    Last edited: Dec 22, 2009
  10. Wendy

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    Mar 24, 2008
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    What does this have to do with the OP project? If you are really interested you need to start a new thread, I don't see any connection here.
     
  11. bundick

    Active Member

    Dec 19, 2007
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    My question had to do with induction and what it might look like on a scope at two different frequencies. IE: How could he transfer a switch to a different set of components to look at the same circuit at different speeds.
    Sorry, I wont interrupt again.
     
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