Does a coil also have a capaciotance?

Yes. Its referred to as distributed capacitance. It has resistance too. Likewise capacitors have L & R too. Welcome to the real world which they didn't teach you about. Art

Absolutely. Always. Lots of coils are spec'd to have an SRF, self-resonant frequency. The impedance peaks at the SRF, and drops above that frequency as the capacitance starts to dominate. Given SRF and the inductance, you can calculate the equivalent capacitance, as though all the C were connected to the ends of the inductor. Of course, in real life the capacitance is distributed all over the place.

The turns also have capacitance to the universe; and to the core, and to other windings, if any.

John

"Joe Snodgrass"

** As others had said, any practical coil of wire has a self resonant frequency due to it's capacitance. By measuring this frequency, one can find the effective parallel capacitance - which may be from a few pF upwards.

Likewise, all practical capacitors have lead inductance and hence a self resonant frequency - this time it's series resonance so the impedance value drops to a minimum then rises. For most caps with wire leads each end, the inductance is some value between 10nH and 40nH producing self resonant frequencies of a few kHz to tens of MHz, depending on the cap value.

In both cases, the usual formula F = 1/( 2.pi. sq rt L.C ) applies.

.... Phil

Joe Snodgrass schrieb:

Hello,

every real component has all three properties, resistance, inductance and capacitance. There is no component with only one or two of these properties.

Bye

Though you can often get away with ignoring the resistance of a super- conducting inductor.

-- Bill Sloman, Nijmegen

Bill Sloman schrieb:

Hello,

even in this case there is resistance, the isolation resistance of the conductor to ground.

Bye

"Uwe Hercksen"

** Hey Uwe,

Bill is Dutch and a colossal pedant.

But you sound like an much bigger, German one.

Show the old fool who is boss.

Go for it - boy.

.... Phil

Actually I'm Australian (like Phil, though I don't take any pride in him as a specimen of the Australian population) even if I'm currently resident in the Netherlands.

Getting a Ph.D. is the kind of thing that instills pedantic habits - but the comment about super-conducting inductors was made for comic, rather than pedantic effect, as someone with a more highly developed theory of mind than Phil Allison might have been able to detect.

-- Bill Sloman, Nijmegen

That was comedy?

John

er-

he

For those with the right kind of sense of humour - and it helps if you didn't skip the relevant lectures back when you were an undergraduate.

-- Bill Sloman, Nijmegen

I doubt there were any lectures on superconductive inductors for me to miss. And I rarely cut class.

I do work around superconductive magnets, well past $1e6 and 200 kgauss, in NMR and MRI and FTMS systems. They behave *almost* as though they have zero resistance. Your statement somehow bypassed my sense of humor.

How about you? Work with any interesting superconductor gadgets lately?

John

I don't think it really goes against everything they taught you in class, it's more like they just might not have ever mentioned it explicitly. :-)

I do recall an exercise in an electromagnetics class where you determined the capacitance of some coil so as to demonstrate that, at low frequencies, the positive (inductive) reactance was a couple orders of magnitude higher than the negative (capacitive) reactiance and hence could be ignored.

As others have mentioned, any arrangement of wires (and perhaps a dielectric) will have all of resistance, capacitance, and inductance, but the goal is usually to arrange them in a manner where one of these aspects dominates its behavior, at least at the "frequencies of interest" where you intend to use it. Dielectrics do play a large part in this too -- air core inductors are much more commonly seen than air core capacitors, for instance (although this is partially due to the fact that magnetic dielectric materials tend to have non-negligible loss, hysteresis, etc. whereas there are a number of truly excellent electric dielectric materials available than make for really good capacitors).

---Joel

super-

f the

No. Did have an interesting conversation with a couple of guys at an Analog Devices presentation earlier in the week - they are developing an infra-red sensing array which basically runs on the edge of super- conduction, with super-conducting quantum interference devices to amplify the sensor output. The whole thing is going into a satellite with a closed-circuit liquid helium refrigeration system.

I was deeply envious.

-- Bill Sloman, Nijmegen

Superconducting bolometers are pretty amazing. At IBM a colleague of mine had an FTIR spectrometer with one of those in it--it was better than any available photodiode in the 3-5 um region.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

super-

Some nasty vibrations from that cryocooler, I'll bet.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Take your own advice, Phyllis.

--
You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.

Yes they do..

this is where Self resonance frequency (SRF) comes into play. The coil will actually resonate at a freq with out any help from external caps.

Coil designers have to keep this in mind.

Jamie

It's actually called: "Self Resonant Frequency (SRF)"

--
You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.

In sci.electronics.design Spehro Pefhany wrote: : On Fri, 13 May 2011 12:04:32 -0700 (PDT), the renowned Bill Sloman : wrote: : >No. Did have an interesting conversation with a couple of guys at an : >Analog Devices presentation earlier in the week - they are developing : >an infra-red sensing array which basically runs on the edge of super- : >conduction, with super-conducting quantum interference devices to : >amplify the sensor output. The whole thing is going into a satellite : >with a closed-circuit liquid helium refrigeration system.

: Some nasty vibrations from that cryocooler, I'll bet.

Sounds like the SAFARI instrument for the SPICA mission. Actually, EMI causes more headache than mechanical vibrations, currently.

Regards, Mikko