Foil inductors, stray capacitance?

[Mark.Clappers]

Hello All,

I am currently busy designing a series notch filter. I am planning to use a copper foil inductor for this application mainly because of it's low DCR and stable inductance up to ~ 100KHz ( I am planning to use a Goertz CF.15 inductor in parallel with a Solen 6uF capacitor).

But I am wondering if anyone actually measured the capacitance of such a coil. Because of it's large surface area I would expect more capacitance then with a "normal" air core inductor. But how much? and is it significant at all?

Thanks!

[mondogenerator]

I wouldve thought any C would be extremely minimal, UNLESS the foil is broken halfway through the coil.

[Mark.Clappers]

mmm, you're right off course. If the foil is broken it would behave like a capacitor.

My gut feeling say's the capacitance is probably in the nF range and is of no effect in my application, but it would be nice to have some confirmation of this.

I do have a LCR meter at my disposal but then I would first have to buy the coil and maybe end up with a part I can't use. So if anyone has some experience with this and would like to share, please do .

[AllenB]

Although I don't have numbers, I might point out audio transformers such as output transformers for valve equipment. Interwinding capacitance is typically an issue that commands compromise. Of course we are talking about different values of inductance/turns of wire, but it makes the point.

[kevinkr]

Actually it is significant when you look at it as a lumped capacitance..(It is not stray capacitance which won't be that bad, but inter-winding capacitance which is unknown at this point) There is capacitance to the adjacent winding on either side of the foil, and it may be a lot more than a few nF overall depending on the dielectric constant of the insulating material used.

The only way to figure it out would be to determine the self resonance of the inductor by direct measurement, since you already know the inductance calculating the capacitance should not be difficult. A signal generator and appropriate value of series R is all that is required. IIRC At resonance the capacitance value should be the reciprocal of the inductance value you already know.. (From 2piFL = 1 / 2piFC)

Once you know the self capacitance of the inductor you can adjust the value of the shunt capacitor if necessary to compensate. You'll have to purchase the inductor and do some experimentation to get it tuned right.

[speakerdoctor]

Originally Posted by Mark.Clappers

Hello All,

I am currently busy designing a series notch filter. I am planning to use a copper foil inductor for this application mainly because of it's low DCR and stable inductance up to ~ 100KHz ( I am planning to use a Goertz CF.15 inductor in parallel with a Solen 6uF capacitor).

But I am wondering if anyone actually measured the capacitance of such a coil. Because of it's large surface area I would expect more capacitance then with a "normal" air core inductor. But how much? and is it significant at all?

Thanks!

You're in luck. Well, sort of. I happened to have a 0.15 mH ERSE copper foil inductor. I ran a scan on it using my WT2 testing device with calibrated test leads and got the attached. Looking over to the far right column you see the mH output of the test. EVERY data point over the 10 hz to 20 kHz scan is listed as uH or, micro-henry. If there had been any stray capacitance, one or more of the data points would have registered as a "C" instead of uH.
In this test the inductor ranged from 0.167 mH to 0.150 over the entire range.
The second column from the left is the data set for the red curve. Here you see how the inductor's reactance increases with frequency from 0.075 ohms to almost 19 ohms at 20 kHz.

[mondogenerator]

Originally Posted by Mark.Clappers

mmm, you're right off course. If the foil is broken it would behave like a capacitor.

wow! whilst i claim to be amateur, a little respect wouldve been nice. Yes a broken foil would turn it into a capacitor. As is transformers, interwinding C will exist, but is in all probability fairly meaningless.

My gut feeling say's the capacitance is probably in the nF(if not pF) range and is of no effect in my application, but it would be nice to have some confirmation of this.

I do have a LCR meter at my disposal , so do i...and I have a 1.2mH 12 Ga foil inductor to hand, maybe ill post the capacitance, if i can be arsed.

personally, I would worry about more significant issues, especially in an inductance so small (0.15?), maybe it would be just as worthwhile to use a resistance bridge to check ALL your solder/crimp joints are less than a predetermined R value, especially seeing as your amplifier MAY have an output Zobel, with more C then the interwinding C on your inductor anyway.....and also in parallel. Your point about area of the conductor is just, but neglecting the insulation thickness (which is significant compared to a purpose built capacitor) and the effect THIS will have.

[Conrad Hoffman]

IMO, you won't find any great difference in practical performance between a cheap hand wound 0.15 mH inductor made of wire and a boutique foil component. Also, I'd never buy a foil inductor from anybody that wouldn't tell me the Q and the self-resonant frequency up front without having to buy the thing and measure it.

[Mark.Clappers]

Hello All,

Thank you for the info!

It pretty much confirms what I was thinking. I'll just buy a couple of coils and hook them up to my Agilent 4263B LCR meter and measure the inductance and capacitance at various frequencies (and the Q factor of course). From the data already provided it looks like I'm worrying about a non issue.

wow! whilst i claim to be amateur, a little respect wouldve been nice. -> Sorry I didn't mean to offend anybody in any way, maybe a problem in translation from Dutch to English? I just wanted to confirm you're statement, that's all. Again sorry if I offended you.

[wintermute]

No problem in the translation from my perspective Mark... personally I've no idea why mondo reacted that way to your comment, I read it exactly as you said. ie acknowledgement of what he had said.

Tony.