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Multi-Way Conventional loudspeakers with crossovers

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Old 3rd January 2013, 05:07 PM   #11
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no worries. I only hope the method i use works well, others using the trap method have cast doubt in my mind. I also forgot to subtract DCR from the measured Z, before calculating L...
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Old 3rd January 2013, 05:15 PM   #12
ODougbo is offline ODougbo  United States
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Trap Method...aye.

I double check with factory/store bought (labeled) - on the flip side, could check/cailibrate generator; yes?

Best to make up chart, it can be very confusing :-|

Last edited by ODougbo; 3rd January 2013 at 05:29 PM.
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Old 4th January 2013, 05:00 AM   #13
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Quote:
Originally Posted by benb View Post
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).
When reactance of the coil equals resistance of the resistor or R, total impedance equals 1.414 times R. Then the equal voltage drops across the resistor and coil, by voltage division, that is R or Xl divided by the total impedance times the applied voltage of the amp or Ei, equals 0.707 times Ei. So Ei equals 1.414 times the voltage across resistor or inductor.

I like these kinds of quizzes.

The neat thing about this method, I believe, is that the voltmeter can be a "regular" one, that is, it can have a relatively narrow range of accurately measuring AC Voltage, usually not greater than 1 kHz, and the method still works.

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Pete
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Old 4th January 2013, 05:34 AM   #14
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Quote:
When reactance of the coil equals resistance of the resistor or R, total impedance equals 1.414 times R.
That is not correct.
And the voltage value isn't relevant to the measurement.
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Old 4th January 2013, 07:59 AM   #15
ODougbo is offline ODougbo  United States
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For the curious -

Tip of the day, if you try this, use banana plug (or two).
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Old 4th January 2013, 08:51 AM   #16
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What I learned, other than (and with less precision than) a Maxwell or Hay bridge,
a 60Hz AC signal can be applied to a variable resistance and the inductor DUT in series. When XL = R the inductance L is found by
L = R/376.8
where R is the final setting of the variable resistance.

Or, using any useful audio frequency with the same connection as above, but with a 1-ohm resistor in place of the variable one, measure the voltage across the resistor and the inductor. The inductance L is then found by
L = Rv / 2*pi*F*Lv
where Rv is the voltage across the resistor, F is the frequency of the AC signal, and Lv is the voltage across the inductor.

My extra credit is... can someone calculate the denominator in the formula for the first setup when using a 50Hz signal?

edit: and yeah denominator isn't the right word, but it's late and that's what came to mind easily. More easily than divisor.
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Old 5th January 2013, 10:30 AM   #17
ODougbo is offline ODougbo  United States
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Some feed back here: now that I got handle on this & used a speaker connector, it is fast. I.e. unwound 4 coils last night in a few minutes (.9 to 8.3ohm).

Sofaspud - you may have to answer the Q.

Thanks all.
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Old 5th January 2013, 03:48 PM   #18
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Quote:
Originally Posted by sofaspud View Post
That is not correct.
And the voltage value isn't relevant to the measurement.
Unless you are like Kirchhoff risen from the dead, you can't expect us to accept your judgement based on authority. If what I say is the total impedance is incorrect, than what is the correct value, and how did you arrive at it?

Zt = [(Xl)^2 + R^2]^1/2

Substitute Xl = R, and go from there. This is basic textbook electronics.

Regards,
Pete
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Old 5th January 2013, 07:37 PM   #19
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I'll look into it when I get a chance. Thanks.
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Old 5th January 2013, 10:16 PM   #20
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i agree. It is basic AC theory. Im still a basic person.

I see it as this:

Z = R+jX

So id just measure Z at a convenient frequency measuring mV across a shunt, say 10 Ohm, volts from the joint of coil and amp to ground. Measure DCR. Textbook stuff. Set volts, measure mV across shunt to get mA.

Accurate enough i think.

I use it for rotor impedance tests at work regularly

My DMM was cheap, 20. It measures to 400hz to a fraction of a percent. Thats plenty high enough, unless youre making 0.001 uH coils...
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