Parallel LC Circuit

[Jessica Shaw]

Hi,

Link 1 (Helmholtz coil Specifications) : http://img600.imageshack.us/img600/7020/hhcf.jpg

Link 2 ( Block System Diagram): http://img836.imageshack.us/img836/536/systemblockdiagram.jpg


I am trying to power up the following 22 uH inductance coil (secondary
coil) with 115nF capacitor in parallel at the resonant frequency of
100KHz using Helmholtz coil. The load connected to the secondary
requires 100 mA at 3.3 V.

The resonant frequency is 100 KHz. The problem is that the secondary
coil gets really hot after like fifteen to twenty minutes.

How can I minimize this heating? Should I lower the Q of the secondary
coil system ?

I do not have the means to measure the magnetic field generated by the
Helmholtz coil.
Helmholtz coil is getting around 4A of current measured with an
ammeter.

I have also tried coils with values less than 22uH, smaller radius
and different resonant capacitor. I am driving the Helmholtz coil
using Full
H bridge.


Thanks
jess

[jsscs...@gmail.com]

I forgot to mention the specs. of the secondary coil. I am using the following solenoid

http://www.allelectronics.com/make-a-store/item/CR-224/22-UH-4-AMP-COIL/1.html


jess

[amdx]

On 5/16/2012 11:36 AM, Jessica Shaw wrote:
> Hi,
>
> Link 1 (Helmholtz coil Specifications) : http://img600.imageshack.us/img600/7020/hhcf.jpg
>
> Link 2 ( Block System Diagram): http://img836.imageshack.us/img836/536/systemblockdiagram.jpg
>
>
> I am trying to power up the following 22 uH inductance coil (secondary
> coil) with 115nF capacitor in parallel at the resonant frequency of
> 100KHz using Helmholtz coil. The load connected to the secondary
> requires 100 mA at 3.3 V.
>
> The resonant frequency is 100 KHz. The problem is that the secondary
> coil gets really hot after like fifteen to twenty minutes.
>
> How can I minimize this heating? Should I lower the Q of the secondary
> coil system ?

>

> Thanks
> jess

You need to reduce the losses in your coil.
You have losses in both the core material and the wire.
Look for core material that has low loss at 100khz.
Wind the coil with larger wire or find the proper Litz
wire for 100khz and use it.
This site has data on sizes of Litz to use for different frequencies.
http://www.litz-wire.com/applications.html
For 100khz it says to use either 38 of 40 gauge wire. Then you need to
see how many strands of wire to put in the bundle. That will be
determined by the area available and how much current you're pushing.
You might try removing the wire from the coil you have and rewinding
it with larger wire or two strands of smaller wire that have a larger
circular diameter.
Mikek

[Jamie]

My guess is that you have core saturation or, the bridge rectifier you
are using isn't fast enough? is the bridge a schottky type or some UF
type? Indication would show this if the bridge, too, is getting hot!

For a basic test, resistor load the secondary with nothing else to put
it to maximum load. If the coil is still getting hot, then you need to
select a coil with different core material designed to operate at those
speeds.

The core most likely is experiencing eddy currents and thus, is the
same thing as induction heating.

Jamie

[jsscs...@gmail.com]

I am using the following diode

http://search.digikey.com/us/en/products/TMMBAT46FILM/497-3273-1-ND/654651


What other core materials are available?

jess

[John S]

Are you serious? That is a *signal* diode. Ifrm is .35A (350 mA). You're
pushing 2A average thru them? Jeeze! It's hard to believe you haven't
smelled the odor of defeat.

[Bill Sloman]

On May 17, 12:47 am, jsscsha...@gmail.com wrote:
> I am using the following diode
>

> http://search.digikey.com/us/en/products/TMMBAT46FILM/497-3273-1-ND/6...

>
> What other core materials are available?

The posters were assuming that you had some kind of high permeability
core somewhere in your magnetic set-up, which isn't usual in a
Helmholtz set-up.

Soft iron cores are too conductive to work at 100kHz, but Mn/Zn
ferrites work pretty well up there. Ni/Zn ferrites are used at higher
frequencies.

http://en.wikipedia.org/wiki/Ferrite_%28magnet%29

Amdx is more likely to be right in suggesting that you may need to go
to Litz wire, but the skin depth in copper is 0.2mm at 100kHz,
so you'd need to be using fairly thick wire for this to be a problem

http://en.wikipedia.org/wiki/Skin_effect

Have you worked out what the resistance of your 22uH secondary is, and
the current that is circulating around it?

--
Bill Sloman, Nijmegen

[jsscs...@gmail.com]

Hi,

I measured the DC resistance of the coil. Its 0.2 Ohm. So, if 40 V peak to peak is appearing across the coil than current in the coil is 200A. How could it be true?

I am not using any core for Helmholtz coil pair. I am trying to power up the secondary coil ( LC circuit) using Helmholtz coil pair. The secondary coil is set up with the magnetic ferrite material.


jess

[John S]

Hi, Jess -

First, I suspect that the DCR of the coil is less than .2 ohm from the
link to the picture you posted. Have you checked the ohmmeter reading
with the probes shorted?

If the coil is really .2 ohm, then the power dissipated in that much
resistance would be about 16*.2 or about 3.2 watts if your current
measurement you posted earlier is correct.

Third, the current through the coil is not determined by the DCR, it is
set by the inductance, voltage, and frequency. That is, the current is
mostly determined by the inductor's reactance.

John S

[John S]

On 5/17/2012 12:08 PM, John S wrote:
> On 5/17/2012 11:39 AM, jsscs...@gmail.com wrote:
>> Hi,
>>
>> I measured the DC resistance of the coil. Its 0.2 Ohm. So, if 40 V
>> peak to peak is appearing across the coil than current in the coil is
>> 200A. How could it be true?
>>
>> I am not using any core for Helmholtz coil pair. I am trying to power
>> up the secondary coil ( LC circuit) using Helmholtz coil pair. The
>> secondary coil is set up with the magnetic ferrite material.
>>
>>
>> jess
>
> Hi, Jess -
>
> First, I suspect that the DCR of the coil is less than .2 ohm from the
> link to the picture you posted. Have you checked the ohmmeter reading
> with the probes shorted?
>
> If the coil is really .2 ohm, then the power dissipated in that much
> resistance would be about 16*.2 or about 3.2 watts if your current
> measurement you posted earlier is correct.

My mistake. I just re-read your original post and you say that that is
the current through the Helmholtz coil. However, based on your
schematic, the current through the 22uH coil should result in only about
.26W. It shouldn't be getting hot.

Measure the current through the coil (solenoid).

[jsscs...@gmail.com]

Hi,

I measured the coil resistance using ohmmeter. It is 0.2 Ohm. Where did you get the number 16?

jess

[Robert Macy]

it is not unusual for the impedance to skyrocket in a coil caused by
skin depth in magnetic fields. increase of 4 to 10 times is highly
likely even at low frequencies of 100kHz.

Check. Get a copy of FREE femm 4.2 and analyze your coil in
axisymmetric mode [accurate 3D]
the program will calculate the fields, the impedance

[Jim Thompson]

On Thu, 17 May 2012 10:05:55 -0700 (PDT), Robert Macy
<robert...@gmail.com> wrote:

>On May 17, 9:39 am, jsscsha...@gmail.com wrote:
>> Hi,
>>
>> I measured the DC resistance of the coil. Its 0.2 Ohm. So, if 40 V peak to peak is appearing across the coil than current in the coil is 200A. How could it be true?
>>
>> I am not using any core for Helmholtz coil pair. I am trying to power up the secondary coil ( LC circuit) using Helmholtz coil pair. The secondary coil is set up with the magnetic ferrite material.
>>
>> jess
>
>it is not unusual for the impedance to skyrocket in a coil caused by
>skin depth in magnetic fields. increase of 4 to 10 times is highly
>likely even at low frequencies of 100kHz.

Clear back in the late '70's I was making switchers running at only
20kHz. The high temperatures of the transformers made me look into
skin effect... all the way up to the 9th harmonic produced significant
dissipation. So I changed to Litz.

>
>Check. Get a copy of FREE femm 4.2 and analyze your coil in
>axisymmetric mode [accurate 3D]
>the program will calculate the fields, the impedance

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

[jsscs...@gmail.com]

I do not know how to use FEMM4.2. I also need to simulate the magnetic field generated by Helmholtz coils . I will appreciate if you can help with both issues.

jess

[John S]

It is 4A squared. As I said, I was mistaken about the current in the
solenoid coil. My apologies.

John S

[Robert Macy]

On May 17, 10:39 am, jsscsha...@gmail.com wrote:
> I do not know how to use FEMM4.2. I also need to simulate the magnetic field generated by Helmholtz coils . I will appreciate if you can help with both issues.
>
> jess

Get a copy, install. Join users group.

You can import/export .dxf files, too. Best way to learn is to jump
in, give me an email address to send a 'sample' Helmhotlz coil to. It
is also a text file and can be opened with any editor.

To run femm 4.2:
Exercise the original to learn terms/features.
Then copy and rename, and make appropriate changes:
Start with 'Problem' dimensions, frequency
re-Draw geometry.
Include a line around the hwole problem to 'terminate' the
calculations.
Define the BLOCKS - AIR, COPPER
Define the CIRCUIT - DRIVE, 1A makes it easy to get constants.
Define the BOUNDARY [set to infinite space - read manual, 1/(uo*r)]
Calculate, look at results
you can click on circuit symbol and get inductance impedance etc.
Explore capabilities of this tool.

femm deefaults to very coarse, and therefore useless for any accuracy,
mesh, but calculates fast and is still educational. To get useful
results, you need to make the 'mesh' fine enough. I always calculate
skin depth and then make certain the mesh has at least three nodes
within that dimension. Mesh can be 'adjusted' within any BLOCK
material, or along any contour line. I usually use both with the
finest mesh along contours, after all, all the interesting things
happen near boundaries.

[jsscs...@gmail.com]

jsscs...@gmail.com

[Robert Macy]

On May 17, 2:36 pm, jsscsha...@gmail.com wrote:
> jsscsha...@gmail.com
didn't comethrough
spell it out slowly
like this
jsscsha .AT. gmail .DOT. com

[Bill Sloman]

On May 17, 6:39 pm, jsscsha...@gmail.com wrote:
> Hi,
>
> I measured the DC resistance of the coil. Its 0.2 Ohm. So, if 40 V peak to peak is appearing across the coil than current in the coil is 200A. How could it be true?

I hope you used a Kelvin connection - contact resistance is usually
around 0.1 Ohm. If you know the diameter of the copper wire used to
wind the coil, the diameter of the coil and the number of turns you
can look up the resistance of the wire per metre and calculate the
length of wire and its resistance, which can get you a good-enough
resistance rather more easily.

> I am not using any core for Helmholtz coil pair. I am trying to power up the secondary coil ( LC circuit) using Helmholtz coil pair. The secondary coil is set up with the magnetic ferrite material.

That core may concentrate the field generated by the Helmholtz coils,
but not much - the permeability Manganese/Zinc ferrites is of the
order of 1000, so it represents something close to a dead short in the
flux path, but it's not going to represent much of the flux path nor
snaffle a large proportion of the total flux generated.

The 40V peak to peak is being generated across the impedance of the
coil, which - as John S. has pointed out - is the vector sum of the
resistance of the coil and its inductive reactance (which is
presumably rather higher at the frequency of interest).

As Jim Thompson has pointed out, if you are exciting the secondary
with a square wave you have to take into account that a square wave
includes all the odd harmonics of the fundamental, and while a skin
depth of 0.2mm at 100kHz isn't likely to be a problem, the skin depth
at 900kHz - the nineth harmonic - is down to 0.022mm.

--
Bill Sloman, Nijmegen

[Robert Macy]

Estimating resistance by calculating diameter times length is just
that. From experience winding coils, I've found that estimation is
always low. Just accepted it as cheap manufacturers were cutting the
wire size down, until I carefully wrapped a coil as tight as I could,
layering it into the windings for maximum stacking factor. When I
measured that coil's resistance it was one of those 'duh!' moments,
because the resistance was 45% higher than expected. Obviously
stretching the wire, eh?

In a coil skin depth effect 'lumps' the current into a tiny side
portion of the wire, thus the resistance is way high compared to the
resistance if the wire is NOT in a field. And, you guessed it, the
stronger the field, the more that skin effect lumps those conductors -
and the higher the resistance.

[jsscs...@gmail.com]

jsscshaw88 at gmail dot com

[Bill Sloman]

Seems likely. It still beats measuring 0.2 ohms with anything short of
a proper Ohm-meter with Kelvin (four terminal) connections.

> In a coil skin depth effect 'lumps' the current into a tiny side
> portion of the wire, thus the resistance is way high compared to the
> resistance if the wire is NOT in a field.  And, you guessed it, the
> stronger the field, the more that skin effect lumps those conductors -
> and the higher the resistance.

The guesswork is all yours. Skin effect depth is purely frequency
dependent.

--
Bill Sloman, Nijmegen

[Spehro Pefhany]

On Fri, 18 May 2012 08:44:34 -0700 (PDT), Robert Macy
<robert...@gmail.com> wrote:

>Just accepted it as cheap manufacturers were cutting the
>wire size down, until I carefully wrapped a coil as tight as I could,
>layering it into the windings for maximum stacking factor. When I
>measured that coil's resistance it was one of those 'duh!' moments,
>because the resistance was 45% higher than expected. Obviously
>stretching the wire, eh?

Maybe. I might expect magnet wire to be on the high side of the
tolerance band- it's sold by weight and they can sell more rolls if
they make it a bit thicker- since it's used (pretty much) by linear
measure (in fact the linear use goes up with thickness as you fill the
bobbin).

[Jamie]

It is always advisable to be micro measuring your wire if you plan on
using it for specifics, do to its expected characteristics.

I know this because I happen to work for a company that makes wire..
Copper reduction is getting so bad now that instead of the OD being
slightly near the odd size on the min side, it has been reaching over
towards the next smaller gauge.

Something to think about..

Jamie

[Robert Macy]

What? that's combining a lot here

skin effect is an effect caused by current carriers doing something
different because they're in a field. Could even be their own field.

from memory: skin depth is a concept that is useful only and is
defined as where the current density has dropped to 1/e within a
PLANAR surface with a PLANAR field impinging upon it. There in lies
some key information: planar this and planar that are some kind of
restrictions.

Now more confusion with skin depth being planar and people refer to
skin depth of a wire. Are they the same dimension? No. Same origin.
From the same effect, but different values.

The reason I cannot recall the EXACT definition? It's because skin
depth has too many assumptions applied to the calculation which pretty
much renders it useless for anything of value. However, it is a good
staritng point.

Plus, the equation is super easy to remember in MKS units
skin depth = sqrt ( 2 / (w*perm*cond) ), where
w is radians per second
perm is absolute permeability
cond is in S/m which for copper is around 58e6 S/m

Of major importance, if you have ANY gradient in the field, that
magnetic field will 'punch' right through. I've seen a lot of people
calculate skin depth through a shield, and then wonder why the shield
looks transparent! the magnetic field punches right through. Again,
skin depth is a PLANAR concept, and should be used carefully.

In a plain wire in free space skin effect causes the carriers to go
towards the outside of the wire, uniformly distributiing themselves
about the wire. For that situation a good estimation of conducting
cross sectional area is skin depth times pi times diameter. HOWEVER,
that same wire coiled upon itself suddenly has its own field pushing
the conductors around even more and REALLY bunches them, rather
tightly, and with more turns it gets worse. They really end up so
tight, it makes it look like your wire is 54 Awg, no longer the 20 Awg
you started with.

But you knew all this, right?

[Tim Williams]

"Robert Macy" <robert...@gmail.com> wrote in message
news:c34b218c-377c-47c9...@eh4g2000vbb.googlegroups.com...

>In a coil skin depth effect 'lumps' the current into a tiny side
>portion of the wire, thus the resistance is way high compared to the
>resistance if the wire is NOT in a field. And, you guessed it, the
>stronger the field, the more that skin effect lumps those conductors -
>and the higher the resistance.

In fact, not just skin but also proximity effect.

Incidentally, the oft-quoted formula for skin effect (an inverse
exponential) only applies when the skin is very thin relative to the
diameter, or on a flat surface, well away from edges. In general, the skin
depth on a cylinder follows a Bessel function.

Obviously, this further says nothing about a bent cylinder (like wire going
around in a loop, or wound on a bobbin), let alone the effect of nearby
wires. When you have a bunch of wires near each other, carrying current the
same direction, the effect is magnified, so that current is forced towards
the inside of a turn. Where the free-space skin effect might seem suitable
(e.g., ~28AWG at ~100kHz), deep inside a transformer, only 10% of the cross
section might actually be carrying current. Litz wire is made with
particularly fine strands (40AWG stranding is usually used at 200kHz),
because all those strands carrying current in the same direction really
pinches the current.

Also incidentally, AC concentrates nicely in the corners of a rectangular
conductor. This makes rectangular tubing really suck for coils. The same
coil with square vs. round conductor might have, oh I don't know, half the
Q. (The same coil, with equivalent diameter litz of sufficiently fine
stranding, might have 5-10 times higher Q!)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

[Jamie]

Of course he did/does. He's just testing you!..

Jamie

[Jasen Betts]

it's not his fault, googles messing with what it displays, click the address.

--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to ne...@netfront.net ---

[Robert Macy]

On May 18, 10:28 pm, Jasen Betts <ja...@xnet.co.nz> wrote:

> On 2012-05-18, Robert Macy <robert.a.m...@gmail.com> wrote:
>
> > On May 17, 2:36 pm, jsscsha...@gmail.com wrote:
> >> jsscsha...@gmail.com
> > didn't comethrough
> > spell it out slowly
> > like this
> > jsscsha     .AT.  gmail .DOT. com
>
> it's not his fault, googles messing with what it displays, click the address.
>
> --
> ⚂⚃ 100% natural
>

> --- Posted via news://freenews.netfront.net/ - Complaints to n...@netfront.net ---

Already sent. But thank you. I keep forgetting that it's possible to
'see' an email address ...even with google access!

On the note of sending a .fem model of a Helmholtz coil, I had to
quickly recreate to meet my commitment to offer a sample. Couldn't
find a single one in archives.

I used 18 Awg wire, single turn in each coil, made it 3 ft diameter,
and ran it at 100kHz. However, gave me a chance to set it up for
default mesh, which produces awful looking results, and a very fine
mesh, which produces 'smooth' and also more accurate results.
Amazingly, skin effect was just starting to raise the impedance of the
wire. Clearly seen in the results plot.

There is another coil shape, which can't remember the name of, that
has a 'distributed' turns down its diameter, (spacing AND turns with
fixed diameter), but easy to construct accurately, which produces a
larger volume of uniform magnetic field than the Helmholtz coil does.
Kind of a physical interpretation of Tschebyshev vs Butterworth? Never
checked that, but kind of looked like the distributed poles.

Anybody wants copies of the femm, I'll send to your email address.

[Spehro Pefhany]

On Sat, 19 May 2012 03:33:30 -0700 (PDT), the renowned Robert Macy
<robert...@gmail.com> wrote:

>
>
>There is another coil shape, which can't remember the name of, that
>has a 'distributed' turns down its diameter, (spacing AND turns with
>fixed diameter), but easy to construct accurately, which produces a
>larger volume of uniform magnetic field than the Helmholtz coil does.

Probably a Maxwell coil (three coils inscribed on the surface of a
sphere). Four coils per axis are also used (again, on the surface of
an imaginary sphere)- as the centre coil in the Maxwell configuration
is kind of inconvenient-- but I don't think there is any standard name
for that configuration (Fransleau-Braunbeck coil is one name).

>Kind of a physical interpretation of Tschebyshev vs Butterworth? Never
>checked that, but kind of looked like the distributed poles.
>
>Anybody wants copies of the femm, I'll send to your email address.

Wouldn't mind, thnks.


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

[Robert Macy]

On May 19, 7:57 am, Spehro Pefhany <speffS...@interlogDOTyou.knowwhat>
wrote:

> On Sat, 19 May 2012 03:33:30 -0700 (PDT), the renowned Robert Macy
>

Not on a sphere, but on a cylinder, all same diameters

send request to my email address, and I'll send back to you

[John S]

On 5/20/2012 9:37 AM, Fred Abse wrote:

> On Thu, 17 May 2012 12:08:38 -0500, John S wrote:
>
>> Third, the current through the coil is not determined by the DCR, it is
>> set by the inductance, voltage, and frequency. That is, the current is
>> mostly determined by the inductor's reactance.
>

> It's stated to be a parallel resonant circuit. Current in the coil
> depends on inductance,frequency, voltage, *and Q* (ie. resistance).
> Current in the coil will be loaded Q times the current in the external
> circuit.
>

Her schematic shows the voltage across the coil to be 40 to 50V at
100kHz. It is a 22uH inductor with .2 ohms of resistance. The current
through it is 45V (P-P) and the impedance of the coil is .2 +j13.8 so
the current through the coil is 1.14A. That results in .26W of
dissipation in the coil which I posted elsewhere in this thread.

[John S]

On 5/20/2012 10:50 AM, Fred Abse wrote:

> On Sun, 20 May 2012 10:45:46 -0500, John S wrote:
>
>> The current through it
>> is 45V (P-P)
>

> Huh?
>

I meant "voltage across it"

[John S]

On 5/20/2012 12:06 PM, Fred Abse wrote:

> That makes more sense :-)
>
> I can't find any meaningful data on the coil described, other than its
> being rated 4 amps. It looks like the type of inductor intended for
> smoothing/decoupling, in which case, that would be 4 amps DC.
>
> Its equivalent series resistance at 100kHz is probably a lot more than 0.2
> ohms, with losses in the ferrite contributing, as well as skin effect.
>
> Ferrites used in decoupling chokes tend to be lossy, deliberately so. A
> choke intended for 4 amps DC with a volt or two of ripple is unlikely to
> perform well in a resonant circuit. That sort of choke usually has a Q
> around 10, which would put its ESR at 1.3 ohms, which puts dissipation at
> around a watt. Most of that dissipation will be in the ferrite.
>
> DC resistance measurements are useless in this application.
>

Agreed. In fact, I doubt her .2 ohms reading anyway because the coil is
rated for 4A which would be a dissipation of .2*4*4 or 3.2W. The choke
is only .4 inches in diameter and .63 inches tall. Although that's about
the size of a 3W resistor, the choke is not designed to run as hot as a
resistor.

In addition, I estimated that the flux density would be somewhere around
a couple hundred gauss. Nowhere near saturation. It would have to be
really crappy ferrite to be that hot at that flux density.

I think something else is wrong. Maybe the measurements are in error. Or
the load may not be as shown. If it were me, I would remove the bridge
and put a resistive load equivalent to the required output power across
the coil and see what happens.

[John S]

Oops! Looked at the wrong number. My estimate is a flux density of about
100 gauss or less.

[John S]

On 5/20/2012 1:03 PM, Fred Abse wrote:

> On Sun, 20 May 2012 12:44:32 -0500, John S wrote:
>
>> Oops! Looked at the wrong number. My estimate is a flux density of about
>> 100 gauss or less.
>

> It's not saturation that makes those chokes lossy, it's eddy currents.
> They make 'em lossy so as to avoid ringing in their intended application.
> They're just ripple absorbers. They do run warm in decoupling applications.
>
> I'll find something similar and measure its Q. Manufacturers quote around
> 10 for that class of choke.
>

I found a couple here. They are about 10uH and Q of about 7 or 8 at
10kHz. Looks like you're right. Even at very low B they are lossy. Very
interesting.

[jsscs...@gmail.com]

jsscshaw eight eight AT GMAIL .COM

[Robert Macy]

On May 20, 3:41 pm, jsscsha...@gmail.com wrote:
> jsscshaw eight eight AT GMAIL .COM

If this is for me I sent several days ago. In spam folder?

[jsscs...@gmail.com]

i have downloaded the FEMM software on my machine. My helmholtz coils has seven turns. I gave their dimensions in my first original post in a drawing. The pair is separated by 8.5 inches. The coil is generating a magnetic field in the area of 20 inches in length, 10 inches wide and 12 inches deep box. I want to simulate the field. How can I do that in FEMM.

Thanks

jess

[jsscs...@gmail.com]

The Q of the coil is equal to (wL) / r

Where
w = 2 * pi * F
L = inductance of the coil

r = AC resistance

How can I find the AC resistance of my coil ?


jess

[Robert Macy]

I thought a Helmholtz coil had the dimensions of
separation = diameter/2

diameter of 18 inches means the two coils should be separated 9
inches, not 9.5 ?

I usually measure the ac R by resonating at the frequency of interest
and find the Resr.

I calculate the expected Resr by using femm and the answer is part of
the the table shown when you click on the little 'inductor' symbol
while in answer

[jsscs...@gmail.com]

Hi,

Not in this case. The Helmholtz coils are rectangular. Plus I experiment with the placement and found the magnetic field strong at that distance.

Can I measure the AC resistance using resistance meter like measuring a resistance of a resistor? I guess, that I am little confused about measuring the AC resistance without using the software.

Thanks

jess

[jsscs...@gmail.com]

FEMM

[jsscs...@gmail.com]

Hi,

One more thing that I do not understand. The Helmholtz coil is generating a uniform magnetic field at the frequency of 100KHz.

Why do we need the secondary coil to resonate at the frequency of 100KHz to receive that magnetic field? Because the magnetic field is everywhere and it will pass through the secondary coil and the voltage will be generated.

jess

[jsscs...@gmail.com]

Hi,

Yes, I am designing it by i really do not why the secondary has to be tuned in a hint would help?

I found a following ferrite core material

http://www.newark.com/ferroxcube/rod6-20-3c90/ferrite-rod-20x6mm/dp/63R5830

The Data sheet of the core 3C94 is as follows

http://www.ferroxcube.com/prod/assets/3c94.pdf


I also found the following formula

Bop = ( E X 10^8 ) / ( (4.44 )x F x N x Ae )

Where

Bop = magnetic flux density in guass
E = maximum rms voltage across inductor in volts
f = frequency in hertz
N = number of turns
Ae = effective cross sectional area of the core in cm square

So, with 40 volts peak to peak at the across the inductor will give 28.2 rms voltage

F = 100KHz

N = 10

Ae = 0.6 cm

Bop = 1058.5 Guass which is equal to 105mT.

Now , data sheet is giving the value B = 470mT.

Am I right to conclude that this material will not heat up because 105mT is less than 470mT?

Thanks

jess

[Tim Williams]

<jsscs...@gmail.com> wrote in message
news:455145cd-e267-4f0d...@googlegroups.com...

> Now , data sheet is giving the value B = 470mT.
>
> Am I right to conclude that this material will not heat up because 105mT
> is less than 470mT?

Datasheet is also giving you power density data at various B, F points. I
think you'll find something like 100mT at 100kHz, or somewhere around that.

"Will not" is untrue because it has nonzero losses at any field or
frequency.

In practical terms, you'll be lucky to even feel it warm up.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

[pha...@aol.com]

I'm sure you haven't measured the DC resistance of your inductor
correctly.

Using the same ohmmeter, short the leads together while in ohms mode
and see if you don't get about .2 ohms. That is the resistance of the
ohmmeter leads. The actual resistance of your inductor should be much
less, perhaps around .02 ohms.

I have an inductor with a similar core and similar dimensions. I
don't know for sure that the ferrite material is the same, but it's
probably similar. Looking at the photo of your core, I estimate it's
wound with about 18 gauge wire, about 12 turns per layer.

I rewound my core with 18 gauge wire and about 2 1/2 layers to get
about 20 uH inductance.

I then used an impedance analyzer to measure and plot the AC
resistance of the inductor over the frequency range 100 Hz to 1 MHz.

This resistance is the combination of the increase in wire resistance
due to skin and proximity effect and the ferrite losses. It would be
nice to measure the wire loss alone to see how much of the resistance
is due to wire loss and how much due to the ferrite. One way to do
this if several inductors are available is to use a hammer to break
up and remove the ferrite; then the resistance due to wire alone can
be measured. Another way to do this with small inductors like this is
to use a neodymium iron magnet to fully saturate the ferrite; then the
effect is to have an "air core" inductor.

Here is an image of the analyzer scan of the AC resistance of the
inductor.

http://i49.tinypic.com/2cygps4.png

The top curve is the AC resistance with the ferrite core in place. The
bottom curve is the AC resistance with the ferrite "removed" by means
of a magnet saturating the ferrite.

The resistance at low frequency is about .0137 ohms, and this is also
the DC resistance.

At 100 kHz, the AC resistance with the ferrite in place is about .3
ohms. With the ferrite "removed", the AC resistance at 100 kHz is
about .113 ohms; this is the wire resistance alone.

Here's another image showing the AC resistance versus frequency of a
length of 18 gauge wire in 3 different configurations.

http://i45.tinypic.com/2uro5n8.png

The lowest curve is the AC resistance of a straight piece of 18 gauge
wire; its increasing AC resistance with frequency is due to skin
effect only. The next curve up is the AC resistance with the wire
wound into a single layer solenoid with a 1/4 inch ID; now we get some
proximity effect. The top curve is the same length of wire wound into
a 3 layer coil. Notice how the top curve exhibits a change of slope
at about 50 kHz; this is due to the so-called layer effect, or "eddy
current screening", as Snelling calls it.

[jsscs...@gmail.com]

Hi,

So, the following calculations that I did was right? I am heading in a right direction? The inductor will not heat up. I have no problem if it warms up a little and stay there. Please confirm !

jess

I also found the following formula

Bop = ( E X 10^8 ) / ( (4.44 )x F x N x Ae )

Where

Bop = magnetic flux density in guass
E = maximum rms voltage across inductor in volts
f = frequency in hertz
N = number of turns
Ae = effective cross sectional area of the core in cm square

So, with 40 volts peak to peak at the across the inductor will give 28.2 rms voltage

F = 100KHz

N = 10

Ae = 0.6 cm

Bop = 1058.5 Guass which is equal to 105mT.

[Robert Macy]

You are confirming exactly why I never trust 'bottled' formulas.

Did you try modeling your structure using femm 4.2?

[Jamie]

Formulas are like Models/theories of how different aspects of science
work that interact down to the atom level. It seems that for every
field out there where atoms get into the mix, a formula has been created
to work in that field and the explanation of how the ATOM interacts
gives a totally different meaning of how the ATOM in general,
works.

Since I have not seen a ATOM model, yet! that will fit every ones
idea of how and why the atom interacts the way it does with the current
subject at hand, One model that makes since for all aspects of science
that is. I am convinced that we really don't know and it's been one big
guessing game. As long as you can make it sound good and show some
tricky effects that is hard to explain with mechanical theories, you
have all kinds of funding coming your way.. Because some one always
wants to be the first to say, "We got it and it's ours"

Of course we all know how first time discoveries has been handled
over the years by big business, one theft after another.

Jamie

[P E Schoen]

wrote in message news:t0i0s759i0suhonok...@4ax.com...

I thought I would reply to this old post because the thread contains much
background information that helps explain some of the mystery of Jessica's
circuit. But also, your information on skin effect and the AC resistance of
wires at high frequency is something that I was not really very familiar
with, and your explanation and images were very helpful. I usually deal with
high currents (10s of thousands of amps) and line frequency, but even there
skin effect has some influence, which is one reason for laminated bus bars.

But I am starting to work on some projects that use higher frequency, and I
was surprised to see how much effect there was even at audio frequencies of
around 10 kHz, and of course if that's a square wave then harmonics will be
affected even more. And at 50-100kHz or more, as I am considering, I realize
that I will need to seriously take this into account.

Thanks!

Paul
http://www.enginuitysystems.com/pix/

[josephkk]

Thank you Paul. I knew that i wasn't the only one who has had an
interesting wake up call on this issue.

?-)

[Robert Macy]

On Jul 2, 10:36 pm, "P E Schoen" <p...@peschoen.com> wrote:
> wrote in messagenews:t0i0s759i0suhonok...@4ax.com...

if you don't already have, get free femm 4.2 [finite element analysis
software]

Then you can characterize currents in conductors, self inductance, etc
and plot the distribution of currents through your conductors - very
educational.

Heads up, a metal washer is a 'shorted turn' of a transformer so if
you have thousands of amps at 100kHz, you're likely to burn up that
washer, burn the bolt in two, and completely lose the structure.

If Doug S. previously of Applied Materials, is lurking, he can
describe in better detail.