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Old 2nd January 2013, 12:13 PM   #1
ODougbo is offline ODougbo  United States
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Default Help - Forgot how to measure coils

There's a neat way to measure coils - but forgot

See diag. by sweeping the generator and measuring in two places the volts will/can be the same.

Once the freq. is found, divide by number (again forgot) by the freq.

I usually test with a 1.0mH to get me going (and can compair to factory coils)

Can somebody refresh my memory plz? - I'll mark the dial on the Gen next time - .5 .6 .7. 8. 9. etc.... and/or make a chart.
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Old 2nd January 2013, 03:49 PM   #2
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I believe you are talking about f1 and f2 which are 0.7071 times the peak voltage which represents the resonant Z of the coil. However isnt it simples just to find the voltage at the peak instead?

Either way, you need to measure current too.

EDIT: your question befuddled me. Im quoting Qts measurement. Oops. I just measure V and I at a single given F. Divide V by Xl to calculate Z, if you take V and I then you have Z. Rearrange to get Xl. Then sub F and Xl into inductive reactance equation.
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Old 2nd January 2013, 04:12 PM   #3
ODougbo is offline ODougbo  United States
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Its some kink of text book thing e.g. 2x3.14 (Freq)

It seems really accurate; I also use 2 way toggle to speed it up.

Maybe voltage on each side of coil (while sweeping).

Yes need to measure current too.
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Old 2nd January 2013, 04:24 PM   #4
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XL = 2*pi*f*L and Z = V/I and XL = Z.
take a point at F, measuring V and I. Calculate Z.
Sub XL and F into 1st formula and rearrange to find L.

Thats correct i think (bearing in mind im writing this on the bus home from work after a xmas holiday of ONE day )

however it occurs to me that you may mean the pulsed voltage method, which i rarely use.
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Old 2nd January 2013, 08:12 PM   #5
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I use speaker workshop and use the passive measurement option. It seems pretty accurate. The other thing I have done is to parallel with a close tolerance cap and do an impedance sweep, and then work out the value of the coil based on the resonant frequency of the LC combo (knowing the value of the cap).

Tony.
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Old 2nd January 2013, 08:56 PM   #6
ODougbo is offline ODougbo  United States
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Give up yet?

It will never work as shown. It works a little bit if you use a 40ohm load resistor and divide 6.5 x freq.

Sorry folks..I didn't make this stuff up, somebody showed this to me

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Old 2nd January 2013, 09:43 PM   #7
benb is offline benb  United States
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When the voltage is the same across the resistor and the inductor, the inductive reactance is equal to the resistor value. Don't compare either one against the whole output of the amp, it won't be twice the other two measurements! (Extra credit if you can say why and what the amp's output voltage is relative to the voltage across the resistor or inductor).

For an 8 ohm resistor, you get:

8 = 2 * pi * f * l

l = 8 / (2 * pi * f)

l = 0.785 / f
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Old 2nd January 2013, 11:27 PM   #8
ODougbo is offline ODougbo  United States
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Thanks benb.

I found my note on volt meter - L = 43 / 2 * pi * f I see now that the 43 was my stack of resistors :-/

I give the 8ohm Resistor a try again.
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Old 2nd January 2013, 11:46 PM   #9
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First choice is just use an LCR meter.

If you dont have one use a capacitor is series wit ha resistor in series with the coil and find its resonant frequency. i.e. when voltage across LC is a minimum.
Then use:
L = 1/ 4 pi pi C f f
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Old 3rd January 2013, 02:35 PM   #10
ODougbo is offline ODougbo  United States
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Red face ugh

Brain Freeze

I see how it works now

Thanks all
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