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Author Topic: How to calculate capacitance in indcutor  (Read 1411 times)
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Tahmeed
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« on: January 05, 2011, 07:11:02 07:11 »

In some resonator circuit, the capacitance of an inductor causes problem.. anyone please inform me about how to calculate this capacitance?
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oldvan
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« Reply #1 on: January 05, 2011, 08:19:58 08:19 »

I found the following with a bit of help from Google:

Self-capacitance Of Inductors (PDF Document)

Not an easy set of calculations!!
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carbontracks
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« Reply #2 on: January 06, 2011, 12:39:51 00:39 »

Unless you have a very specific winding geometry, trying to accurately predict self capacitance if pretty much an exercise in futility.  If you're dealing with a simple solenoid, flat spiral, or helical coil, then there are free pograms available that can estimate it.  They're often used by people who build tesla coils.  One I use sometimes is Inca:
http://www.coe.ufrj.br/~acmq/programs/inca.zip
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solutions
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« Reply #3 on: January 06, 2011, 03:04:09 03:04 »

If it is a passive resonator circuit, I think you are saying your assumed C for resonant frequency is not what you calculate with L and your external C.  A lot of inductor manufacturers will specify the self resonant frequency.  Not knowing all the details, it sounds to me like you could work your L and SRF into the standard resonance (assume parallel) equation (f is the inversion of 2 pi root (LC)) and you'll get close enough in an estimation of winding capacitance.  

If you can't find the SRF spec, you can see it on a scope in the ringing when you apply a fast risetime square wave to the inductor.  Another quick empirical method is to TUNE the circuit by varying C until you get what you want out of it.

Sometimes we are so bent on getting it perfect, we don't see the forest from the trees...your caps might have 20% tolerance, for instance.  If you are running such a high frequency that winding cap is relevant, I'd be more worried about core loss if it is not an air core (you didn't say) or if the L is big (you didn't say).
« Last Edit: January 06, 2011, 03:08:30 03:08 by solutions » Logged
Tahmeed
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« Reply #4 on: January 06, 2011, 06:39:33 06:39 »

@ solutions...........

actually I faced the problem during studying an AM receiver circuit. i didn't found needed inductors in local market so i made it by myself with copper wires ...... so tuning stations was kind of tough..... and of course it was a passive resonator...I have a digital multimeter with capacitance measuring function not for inductance.Sad.....can u pls give me link for a circuit to measure inductance.... i wanna try myself to make one inductance meter..Smiley

@others......thanx for links
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solutions
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« Reply #5 on: January 06, 2011, 11:48:10 11:48 »

The homemade inductors for AM are usually air core and are tweaked with a movable slug.  You will need to tune the RF and IF when all is said and done.  

You can make your own inductors, as you probably know, by using a pencil as a mandrel and winding the copper around it.  Probably easiest to just use one of the many javascript calculators for "solenoids" - carbontracks posted one already so I'm not going to clobber his contribution.  It will not predict parasitics, so it's only a roughout at best.

The easiest way I know to measure is a known capacitance in parallel with the inductor, then sweep an RF generator and look at where the peak output of that filter occurs.  Then use the parallel resonance equation. Finding it with the meter you have is tough, and most LCR meter circuits use low frequencies that are not suitable for RF.  RF generator and scope, unfortunately.  The other option is to get a super fast risetime chip (which you probably can't get either, but have a look) and get the inductor or overshoot...it will ring at its self resonance.  Here you need a scope.  Use your cap meter and then derive the inductance, again using the parallel resonance equation.

It really sucks to not be able to access components you need - I found that out when I was overseas last month.
« Last Edit: January 06, 2011, 11:50:20 11:50 by solutions » Logged
Tahmeed
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« Reply #6 on: January 07, 2011, 04:51:39 16:51 »

@sol.n.........yah your recommended process can work....... but this accuracy factor affects more in high frequency as fm....I think from this sense AM is easy than FM.... now I am trying to make a super regenerative fm receiver...... there are many circuits in websites but i cud not found any one telling the basic mechanism... the basic theory. heterodyne is IC based n expensive... but super regeneratives can be designed with cheap parts.. can u help?
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solutions
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« Reply #7 on: January 08, 2011, 10:51:47 10:51 »

If you are receiving one frequency super regens are the way to go if I remember it right.

I built a full AM band superhet in school using vacuum tubes.  I can assure you that it is not going to work if you plan on using fixed inductors - as you have noted, the frequencies are much higher.  But, you don't need ICs - can use transistors as well.  

We had tuning slugs in each stage.  I suspect things have not changed much on the passives, despite PLL and all the other modern "solid state' stuff.  

I'd help, but I am backlogged about two months in self-inflicted work (and been sick, still am half the time), and that's with 18 hour days. I pop in here to help when I can, but I'm pretty flakey right now so I cannot be counted on.

There are a bunch of nice old radio circuits on this site: http://vintageradio.me.uk/radconnav/transsuperhet/

Google for "pocket radio" and "portable radio" as a start
« Last Edit: January 08, 2011, 10:55:53 10:55 by solutions » Logged
Tahmeed
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« Reply #8 on: January 09, 2011, 07:20:59 19:20 »

@sol....n.hey thanx buddy...... for a bunch of circuits........ thats great........ but i actually wanna know the basic mechanism.....i cudn't found any webpage describing the fundamentals......if u know such kinda' page then can suggest me.... super regen are perfect to research on i think......Cheesy
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solutions
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« Reply #9 on: January 09, 2011, 07:53:18 19:53 »

The FIRST radios were regens, so you might want to look at some really old tube stuff for insight on how it all works.  Please post any good books here that you find in your travels.

cheers & good hunting
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