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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
method 1, measure physical size: L(uH)=((turns*radius in inches)^2)/((9*radius in inches)+(10*height in inches)) L(uH)=((18.75*1.75)^2)/((9*1.75)
+(10*2.15))=28.9uH
method 2, connect it in series with a pot to a signal generator and scope the voltage across both and adjust untill equal. Then measure the pot to find impedance: measured: 28 ohms at 80Khz (4 cycles in 10 divisions at 5uS/div) Xl=2*PI*F*L L(H)=Xl/f/PI/2 L(H)=28/80000/
PI/2=55.7uH
difference of 50% so which one is it? or is it none? should i do some other sort of test?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I think you should check your size formula because I think you may have a centimeters/inches error. I could be wrong about that, but when I do the size calculations they're usually pretty close, like 90%.
Your measured is almost guaranteed to be right with the provision that the probe isn't affecting your measurements too much. I'd suggest go back and measure at 1kHz so capacitive effects don't play a large role, and then see how it deviates from the 55.7uH measurement.
Thought comes to mind, this is a single layer solenoid right? Because the formula is different for multiple layer solenoids.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
i used a ruler with inches on it..... though i took the 3.5 inch diameter and halved it (1.75) for the radius measurment
actually i just use a bit of wire then proper oscilloscope probe, but i did that on both channels. at 1Khz the inductor is going to have some ridiculously low impedance that i would not trust the accuracy of my dmm.
yes, this is a single layered cylindrical coil, same top and bottom diameter too.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I'd say say your frequency measurement might be off aswell, 4 cycles is a bit low for determining the frequency, you then have an accuracy of 20% (assuming your counting whole cycles). Looking at more cycles will give you more accuracy.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
well, yeh, but i did line it up against a flat line on the other channel so there was exactly 4 lots of 360 degrees on the screen. ok i will count it again with a different time/div
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The most difficult test to get wrong (I think that means what I meant) is to resonate the coil with a known capacitor and then find the frequency. You can use them in parallel and measure the frequency of highest voltage rise by feeding it and measuring it with a high impedance. Or you can have them in series and measure where their impedance is lowest, perhaps by putting them across the output of your signal generator and tuning the frequency for the lowest terminal voltage.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Great, now you know that you don't know the answer
Are you planning to use this as a TC or something where it will be used with a cap as a resonant circuit? If so, do you have the cap you plan to use? Measuring the true resonant frequency is more useful than knowing the inductance. Anyway, I'd try again with a few different frequencies, preferably over a bigger range than 80-90KHz Other things to worry about are distortion (does the 'scope trace look like a decent sine wave?) and loading; is the signal generator struggling to feed the load or is the scope loading it down? With a measured impedance of just 28 ohms I think the former is more likely to be a problem than the latter.
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finding the inductance
