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Probably a stupid question...
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Dave
electronics forum addict


Joined: 20 Jan 2006
Posts: 79

PostPosted: Fri Jul 14, 2006 5:09 pm    Post subject: Probably a stupid question... Reply with quote

Anybody have any idea what to do when you have two forms of the same
equation, using the same variables and numbers for those variables so that
one checks the other, only they don't give the same answers? Case in point,
the calculation for inductance of a single-layer air-core inductor using the
diameter/length of that coil and number of turns, and it's counterpart to
calculate the number of turns in that coil given the inductance and
diameter/length. Very simple and straightforward, only I get different
answeres from the two. What could I be doing wrong? I have measured and
remeasured the length of the coil, and done the same with the diameter, even
going to the point of wrapping ten loops of wire around it before removing
same and measuring/dividing first by ten and then by pi to make sure I was
being accurate. Still, I get inconsistant answeres from the two equations,
these being L=d^2*n^2/(18d + 40 l) and (sqrtL*18d+40l)/d. The first tells
me 8.35 microhenries with a diameter of 1.75 and 9.5 turns, but the second
then tells me 7.9 turns with the same diameter and a length calculated by
the number of turns that will fit in an inch ( .04 inches for #24 plastic
coated copper wire). I even ran all this through a different calculator.
And both of these equations come from the ARRL handbook (p 6.22 of the 2004
edition), so I have to trust them... Am I just stupid? What is going on
here? I feel like an idiot, and am probably going to find out I am right,
but I need to know... What am I doing wrong?

Many thanks for any help. Even insults are welcome, so long as you care to
share why you are insulting me. :)

Dave
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Brian O
electronics forum beginner


Joined: 31 Mar 2006
Posts: 6

PostPosted: Fri Jul 14, 2006 6:21 pm    Post subject: Re: Probably a stupid question... Reply with quote

I looked at my ARRL 2003 handbook. Just from first glance of what you have
written, the way you may be calculating the second formula may be incorrect.
You wrote:
(sqrtL*18d+40l)/d

where it should be
(sqrtL*(18d+40l))/d

You may be multiplying the sqrtL by 18d BEFORE you add 18d and 40 l . It
will give you different results.
--
Brian

"Dave" <db5151@hotmail.com> wrote in message
news:12bfk6gjkvhk0cd@corp.supernews.com...
Quote:
Anybody have any idea what to do when you have two forms of the same
equation, using the same variables and numbers for those variables so that
one checks the other, only they don't give the same answers? Case in
point,
the calculation for inductance of a single-layer air-core inductor using
the
diameter/length of that coil and number of turns, and it's counterpart to
calculate the number of turns in that coil given the inductance and
diameter/length. Very simple and straightforward, only I get different
answeres from the two. What could I be doing wrong? I have measured and
remeasured the length of the coil, and done the same with the diameter,
even
going to the point of wrapping ten loops of wire around it before removing
same and measuring/dividing first by ten and then by pi to make sure I was
being accurate. Still, I get inconsistant answeres from the two
equations,
these being L=d^2*n^2/(18d + 40 l) and (sqrtL*18d+40l)/d. The first tells
me 8.35 microhenries with a diameter of 1.75 and 9.5 turns, but the second
then tells me 7.9 turns with the same diameter and a length calculated by
the number of turns that will fit in an inch ( .04 inches for #24 plastic
coated copper wire). I even ran all this through a different calculator.
And both of these equations come from the ARRL handbook (p 6.22 of the
2004
edition), so I have to trust them... Am I just stupid? What is going on
here? I feel like an idiot, and am probably going to find out I am right,
but I need to know... What am I doing wrong?

Many thanks for any help. Even insults are welcome, so long as you care
to
share why you are insulting me. :)

Dave


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Dave
electronics forum addict


Joined: 20 Jan 2006
Posts: 79

PostPosted: Fri Jul 14, 2006 7:19 pm    Post subject: Re: Probably a stupid question... Reply with quote

My God. That was it. Now I really do feel like an idiot, but I have been
working on this for days and couldn't think of anything else. My deepest
thanks to you for your insight and courtesy.

Indeed, much appreciated. Now I know which numbers to trust.

Dave

"Brian O" <briano1812@lispwest.com> wrote in message
news:e98n4001nm7@enews2.newsguy.com...
Quote:
I looked at my ARRL 2003 handbook. Just from first glance of what you have
written, the way you may be calculating the second formula may be
incorrect.
You wrote:
(sqrtL*18d+40l)/d

where it should be
(sqrtL*(18d+40l))/d

You may be multiplying the sqrtL by 18d BEFORE you add 18d and 40 l . It
will give you different results.
--
Brian

"Dave" <db5151@hotmail.com> wrote in message
news:12bfk6gjkvhk0cd@corp.supernews.com...
Anybody have any idea what to do when you have two forms of the same
equation, using the same variables and numbers for those variables so
that
one checks the other, only they don't give the same answers? Case in
point,
the calculation for inductance of a single-layer air-core inductor using
the
diameter/length of that coil and number of turns, and it's counterpart to
calculate the number of turns in that coil given the inductance and
diameter/length. Very simple and straightforward, only I get different
answeres from the two. What could I be doing wrong? I have measured and
remeasured the length of the coil, and done the same with the diameter,
even
going to the point of wrapping ten loops of wire around it before
removing
same and measuring/dividing first by ten and then by pi to make sure I
was
being accurate. Still, I get inconsistant answeres from the two
equations,
these being L=d^2*n^2/(18d + 40 l) and (sqrtL*18d+40l)/d. The first
tells
me 8.35 microhenries with a diameter of 1.75 and 9.5 turns, but the
second
then tells me 7.9 turns with the same diameter and a length calculated by
the number of turns that will fit in an inch ( .04 inches for #24 plastic
coated copper wire). I even ran all this through a different calculator.
And both of these equations come from the ARRL handbook (p 6.22 of the
2004
edition), so I have to trust them... Am I just stupid? What is going on
here? I feel like an idiot, and am probably going to find out I am
right,
but I need to know... What am I doing wrong?

Many thanks for any help. Even insults are welcome, so long as you care
to
share why you are insulting me. :)

Dave




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Charles Schuler
electronics forum Guru


Joined: 04 Feb 2005
Posts: 335

PostPosted: Fri Jul 14, 2006 9:15 pm    Post subject: Re: Probably a stupid question... Reply with quote

That was a good catch on Brian's part.

I'd like to add that many "equations" of this ilk are actually
approximations.
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Dave
electronics forum addict


Joined: 20 Jan 2006
Posts: 79

PostPosted: Sat Jul 15, 2006 1:17 am    Post subject: Probably a stupid question... Pt II Reply with quote

"Charles Schuler" <charleschuler@comcast.net> wrote in message
news:cJydnbIh9bfDlCXZnZ2dnUVZ_tOdnZ2d@comcast.com...
Quote:
That was a good catch on Brian's part.

I'd like to add that many "equations" of this ilk are actually
approximations.


Yes, I understand that at least. I am still having some trouble making the
counterpart to the equation Brian mentioned work however. L=d^2*n^2/(18d +
40 l) doesn't give me the same answer, or anything nearby, as the one I
fixed with parentheses after Brian's advice. I am workintg with a TI 89
Titanium, which reshuffles parentheses according to whether or not it thinks
they are needed where you put them. This calculator is messing with my
head. I am about to go back to my "basic" TI 30-XA, which I at least know
how to use. To bed, and to scratch my head. More tomorrow maybe.

Thanks for reading my ramblings.

Dave
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David
electronics forum addict


Joined: 19 Feb 2005
Posts: 97

PostPosted: Sat Jul 15, 2006 3:05 am    Post subject: Re: Probably a stupid question... Pt II Reply with quote

The only stupid question is the one that wasn't asked.



Kind Regards

David
-----------------------------------------------------------
ORBIT COMMUNICATIONS Pty Ltd - Wireless Solutions that Work
(Telemetry, Control, Monitoring, Security, HVAC ...)
A.C.N. 107 441 869


Website : http://www.orbitcoms.com
PO Box 4474 Lakehaven
NSW 2263, AUSTRALIA
Phone: 61-2-4393-3627
Fax : 61-2-4393-3685
Mobile: 61-413-715-986

Dave wrote:
Quote:
"Charles Schuler" <charleschuler@comcast.net> wrote in message
news:cJydnbIh9bfDlCXZnZ2dnUVZ_tOdnZ2d@comcast.com...
That was a good catch on Brian's part.

I'd like to add that many "equations" of this ilk are actually
approximations.


Yes, I understand that at least. I am still having some trouble making the
counterpart to the equation Brian mentioned work however. L=d^2*n^2/(18d +
40 l) doesn't give me the same answer, or anything nearby, as the one I
fixed with parentheses after Brian's advice. I am workintg with a TI 89
Titanium, which reshuffles parentheses according to whether or not it thinks
they are needed where you put them. This calculator is messing with my
head. I am about to go back to my "basic" TI 30-XA, which I at least know
how to use. To bed, and to scratch my head. More tomorrow maybe.

Thanks for reading my ramblings.

Dave

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jasen
electronics forum Guru Wannabe


Joined: 18 Jun 2006
Posts: 204

PostPosted: Sat Jul 15, 2006 5:13 am    Post subject: Re: Probably a stupid question... Reply with quote

On 2006-07-14, Dave <db5151@hotmail.com> wrote:
Quote:
Anybody have any idea what to do when you have two forms of the same
equation, using the same variables and numbers for those variables so that
one checks the other, only they don't give the same answers? Case in point,
the calculation for inductance of a single-layer air-core inductor using the
diameter/length of that coil and number of turns, and it's counterpart to
calculate the number of turns in that coil given the inductance and
diameter/length. Very simple and straightforward, only I get different
answeres from the two.

I can't guess what you're doing wrong.

what formulas are you using ?
what values are you putting into them etc...

Bye.
Jasen
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HKJ
electronics forum beginner


Joined: 15 Feb 2005
Posts: 33

PostPosted: Sat Jul 15, 2006 6:49 am    Post subject: Re: Probably a stupid question... Reply with quote

Dave wrote:
Quote:
I have measured and
remeasured the length of the coil, and done the same with the diameter, even
going to the point of wrapping ten loops of wire around it before removing
same and measuring/dividing first by ten and then by pi to make sure I was
being accurate. Still, I get inconsistant answeres from the two equations,
these being L=d^2*n^2/(18d + 40 l) and (sqrtL*18d+40l)/d. The first tells
me 8.35 microhenries with a diameter of 1.75 and 9.5 turns, but the second
then tells me 7.9 turns with the same diameter and a length calculated by
the number of turns that will fit in an inch ( .04 inches for #24 plastic
coated copper wire).

For more exact coil calculations use
http://www.miscel.dk/MiscEl/miscelAirCoil.html
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Dave
electronics forum addict


Joined: 20 Jan 2006
Posts: 79

PostPosted: Tue Jul 18, 2006 2:38 am    Post subject: Re: Probably a stupid question... Pt II Reply with quote

Dumb question update:

After putting things aside for a couple days, and sleeping on it two or
three times, I now realize that besides not using my parentheses correctly,
I was inputing the value of L incorrectly. Inputing it in Henries rather
than microhenries. (shaking head.) I had just been staring at it so long
that I couldnt see it anymore. With all your help (and especially Brian's)
and a couple days away from it, it is a simple thing again.

Thnaks.

Dave

"Dave" <db5151@hotmail.com> wrote in message
news:12bgggt7pa8v1a1@corp.supernews.com...
Quote:

"Charles Schuler" <charleschuler@comcast.net> wrote in message
news:cJydnbIh9bfDlCXZnZ2dnUVZ_tOdnZ2d@comcast.com...
That was a good catch on Brian's part.

I'd like to add that many "equations" of this ilk are actually
approximations.


Yes, I understand that at least. I am still having some trouble making
the counterpart to the equation Brian mentioned work however.
L=d^2*n^2/(18d + 40 l) doesn't give me the same answer, or anything
nearby, as the one I fixed with parentheses after Brian's advice. I am
workintg with a TI 89 Titanium, which reshuffles parentheses according to
whether or not it thinks they are needed where you put them. This
calculator is messing with my head. I am about to go back to my "basic"
TI 30-XA, which I at least know how to use. To bed, and to scratch my
head. More tomorrow maybe.

Thanks for reading my ramblings.

Dave

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