Question about measuring coil inductance.

[UFO]

Maybe you guys can help me with this.

I am going to be winding a few PI coils so I thought I would practice one in the time being.

I made a rig to measure inductance as per shown on this page. http://www.coilgun.info/theoryinduct...inductance.htm
Now, I'm sure this would be a suitable test for a 60Hz device, but I don't know if the frequency is suitable for detectors.

I thought I would try it anyways.

I used the calculator here to get the specs say a stock Baracuda coil at 453uH
Now the final coils will be wound with regular insulated wire but I just wanted to verify a testing procedure so I made the practice coil with enamel coated wire.

Inner Radius - 77.5 mm
Wire thickness - .46mm
Number of turns - 33
Results
Mean Radius 78.82 mm
Coil thickness 2.64 mm
Inductance - 0.462mH

So I spin a coil and test it, around 2-3 layers thick.
I could not use me resistor decade box as the test shows as it only goes down to 10ohms. I used a 25ohm rheostat instead.

I turned the rheostat until I got equal voltage across the rheostat and the coil, 2.6v.
I then got the measured the impedance on the rheostat which was 1.8ohms.(Lx)

Using the formula based on my 60Hz transformer,
L = Lx / (2 pi f)
L = Lx / 377
L = 1.8 /377

=0.004774

No matter how I move the decimal place I can't get it to line up with the "477", so I assume this number is just a coincidence for now. (The "477" would have been pretty close to the 462uH that the calculator predicted)

So I don't know if it's because of the frequency I'm using, my math or the testing rig itself. It seemed like a reasonable approach to measuring inductance with a higher voltage. (6v).

Any suggestions?

Thanks for your time.

[Carl-NC]

The problem is that 60Hz is too low for the inductance you are trying to measure. For 453uH @60Hz, the XL = 0.17 ohms; the wire resistance is probably higher than this, so you will not be able to get an accurate reading.

Do you have a signal generator? Try 10kHz, the XL is about 28.5 ohms which is more reasonable. If you make f too high, the capacitance will start causing problems.

- Carl

[UFO]

Thanks Carl.

I will look into maybe using my computer as a signal generator, with a little amp.

[UFO]

OK, I put a 10kHz signal into a LM3875 audio amplifier. I used samples from these two sites. I set the volume half way and measured 4.6v or 5v depending on which source tone I was using.
http://www.freesound.org/people/klan.../sounds/28637/
http://www.youtube.com/watch?v=wVoF_aDPxWw

Anyhow the coil and the rheostat balance out now at 25.4 ohms.(Lx)

So f = 10 000
L = Lx / (2 pi f)
L = 25.4 / 62 800
L = 4.044585987261146e-4 (I'm assuming Henry?)

So I'm not sure what to do from here with this number, here is what I did:

-If I punch in 25.4 / 62 800 = 4.044585987261146e-4 x 1 000 000 in that exact order into my calculator, I get 404, which seems like it may be a result, 404uH?

[chemelec]

Based on the numbers given,

Inner Radius - 77.5 mm = 6.1" Dia
Wire thickness - .46mm = 25AWG??
Number of turns - 33

I would think your coil will be about 330uH.
Or Less, if it is Not Tightly bundled together.

[Carl-NC]

Yes, 404uH is your result. This method is not terribly accurate because of series R and shunt C parasitics. Based on your parameters, you probably have a wire R of about 1.7 ohms, which puts the actual L closer to 377uH. And, as Gary notes, this could still be off.

[wallker]

Hi chemelec and Carl, which calculator or which formula did you use to calculate the inductance?
With Quzahi coil calculator and UFO data about diameter, wire and windings inductance is indeed 462uH.
Regards Marko.
http://www.geotech1.com/forums/showthread.php?t=17909

[UFO]

OK thanks, great. I am happy with reasonable results, to get me in the ballpark, something to compare coils against. I can always fine tune the coil by hand.

I can't help but wonder if the profile of the coil has much of an effect. The coil you see pictured above is relatively flat, like the coil you see pictured below. It is 26 turns of 22AWG in two flat rows of 13. 7 1/2" diameter(19cm)
I have not measured it yet.

I assume the calculator posted here is generating a round profile?

The next coils I wind I will try to make more of a square profile, closer to a round shape.

I found that I can wind a layer, then put a layer of masking tape on the coil to hold the layer intact, then I can wind a neat layer on top of the tape without disturbing the layer below.

I made this turn table this morning to spin my templates. (Picture below)

What are your thoughts on setting the coil in epoxy? I read where it may add unwanted capacitance, however it is my understanding that some factories set their coils in epoxy.
Should I avoid epoxy? I will be winding the coils with insulated wire(not enamel)to reduce capacitance, but ideally I would like to set the coils in epoxy for water resistance.

What about wax?

I can however keep the coil stable with the masking tape and aluminum shield like you see below.

Thanks again for your help.

[chemelec]

Sorry, I screwed up BADLY on my calculations.
So I made one!

Inner Radius - 77.5 mm = 6.1" Dia
Wire thickness - .46mm = Actually 26 AWG with the Insulation
Number of turns - 33

And Winding the Same coil, I measure an inductance of 465uH

[UFO]

I measured these two coils.

Note: I expect my measured results to be lower, because I am using thicker insulation rather then the thin enamel the calculator assumes.

-The "flat" 7 1/2" (19cm) coil measured 294uH compared to the calculated 396uH. @ 1.1ohm
Within 25%

-The new 10" (25cm) coil that is wound more round (actually square), measured 377uH compared to the calculated 396uH. @ 1ohm
Within 5%. Perfect.

So it appears that the calculator is better suited to the denser round, or in my case square profile. That is why the first coil I measured was low, is because it is "flat". It still might be a good coil profile though, I just can't use the calculator for it and will have to measure.

It appears I can wind my coils, using the thicker insulation and be within 5% of the calculator. Good stuff. Thanks.

Any thoughts on potting material? (Epoxy wax etc.)