high series L reading from a capacitor(!?)

[dai h.]

I recently found some info on old style tubular or "dogbone" ceramic caps (as follows) :

1. they have a bit more L(inductance) than the disks.
>
>2. more labor was involved/necessary in making them--specifically a lead on each end needed to be twisted around the tubular body and soldered (maybe cut too).
>
>3. (as far as some of the reasons for their obsolescence here) demand for the disks went up when Japanese makers began producing American type MIL comm. equipment domestically (probably licensed production). (The specs for the equipment included those for disk ceramics) Disk ceramic construction was also more suited to automation.
>
>(source: the Japanese Murata site:
>http://www.murata.co.jp/company/enkaku/03.html)

...and in the course of a conversation with someone was asked how much L, and I didn't know but I was presuming (based on bits I could remember reading about cap L) that L would be quite miniscule, then I thought I should try taking a reading, and (reading the series L with an LCR meter) I got a seemingly extraordinarily high series L reading (as you can see in one of the attached pics). I then tried measuring several other caps for series L, and again the L seemed to be rather high. I suppose for a film cap it sort of makes sense if it's rolled up, but a disk ceramic and micas gave the high L readings as well and (as far as the few I took readings on) lower cap values gave higher, and higher value caps gave lower. I tried websearching for info to explain this but thus far nada. I suppose this could very well be another situation where I deserve a big "DUHHHH!" and experience a "DOH!", but presently I am in a (very) state.

[cbarrow7625]

This might help:

http://www.csgnetwork.com/capinductancecalc.html

It just shows that what you are seeing is normal. It's not normal at 1kHz but the same behavior is there.

Or try this:

http://www.avx.com/docs/techinfo/parasitc.pdf

Tech paper on parasitic inductance in multilayer ceramic capacitors.

Chris

[dai h.]

thanks! (After ruminating a bit...) so the opposition to AC this cap has (expressed as an L value) when putting 1kHz through the cap is 50.72Henries? The cap measured 489.6pF, and inserting that and 0.01MHz(10kHz) into the calculator gives 517,367,067.17nH = 517mH (1000uH=1nH=0.001mH). If I insert 489.6pF and 50.72H into this LC Tuned Circuit Resonant Frequency Calculator:

http://www.deephaven.co.uk/lc.html

...I get 1kHz(1.0099730950250025kHz). Then if I put 489.6pF and 517mH, I get 10kHz. So when putting X freq. there is a max. opposition expressible as an inductance (cap and inductor are opposites) which with the cap value can be used to calculate the resonant frequency? Arggh... Confused. In the pdf. file it says, "the resonant point will give the most signal attenuation". Something like the most signal attenuation = most impedance to the signal = most inductance (opposition)?

[Enzo]

Dai, your two initial measurements had four significant digits, so any calculations done can have no more than four as well. Meaning that you shouldn't have results like 1.0099730950250025kHz That should be 1.009 or in this case 1.01. Save yourself the trouble.

When I first looked at your photo I wondered what the default units were on the meter. Certainly not 50 Henries. I'd be guessing more like 50 microhenries.

[dai h.]

Dai, your two initial measurements had four significant digits, so any calculations done can have no more than four as well. Meaning that you shouldn't have results like 1.0099730950250025kHz That should be 1.009 or in this case 1.01. Save yourself the trouble.

okay thanks Enzo. Sorry for the ridiculous number, lol (cut and paste job).

When I first looked at your photo I wondered what the default units were on the meter. Certainly not 50 Henries. I'd be guessing more like 50 microhenries.

sorry I'm not sure what you mean here. Do you mean the max. it can measure?

[Enzo]

No, I mean what units? All I see is a number, what units does the meter measure in unless otherwise noted? Henries, millihenries, microhenries, Oh Henry's? Not worried about the range or resolution. 50 is just a number, without units it is meaningless. If your ohm meter read 102, you'd want to know if that was ohms or k-ohms. My ohm meter for example has ohms, k ohms, and M ohms on the readout, depending on scale.

Meters with scale select switches can get away without units on the screen because the selector switch designate the units.

Unles that H on the right actually means Henries instead of Inductance. In that case I'd have to guess the capacitance is confusing the meter or something. I have a hard time imagining that little cap has the same inductance as something that usually is a large iron cored transformer-like thing.

[dai h.]

yes that is Henries and yes that did seem pretty weird to me too hence the question. The meter is autoranging(though it can be switched to manual). For 1kHz freq. max. reading is 99.99Henries.(For me) it was an expensive piece of kit!

[Enzo]

I am not familiar with it, so I can't comment on how it works, but I wonder if it is intended more to measure what I might call pure values. In other words it will measure the inductance of inductors fine but the inductance of caps not so fine. or something like that. AFter all, most inductors have continuity through them, not so your cap. SInce it must make a reading by some sort of inference, the cap would affect the freq variation the component makes different that a plain old inductor would.

[dai h.]

right, maybe this is the wrong method to measure L from a cap or something. I don't know enough to say.