Parallel LC Circuit

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).

peak is appearing across the coil than current in the coil is 200A. How could it be true?

secondary coil ( LC circuit) using Helmholtz coil pair. The secondary coil is set up with the magnetic ferrite material.

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

peak to peak is appearing across the coil than current in the coil is 200A.= How could it be true?

er up the secondary coil ( LC circuit) using Helmholtz coil pair. The secon= dary coil is set up with the magnetic ferrite material.

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 =3D 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?

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

peak is appearing across the coil than current in the coil is 200A. How could it be true?

the secondary coil ( LC circuit) using Helmholtz coil pair. The secondary coil is set up with the magnetic ferrite material.

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

Jamie

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

ess.

t.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.

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).

Wouldn't mind, thnks.

Best regards, Spehro Pefhany

"The Journey is the reward"

eff.com

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

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

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.

The only sure way is to measure inductance and Q *at the frequency of interest*, and calculate losses (tan delta) from that, which will include core hysteresis losses.

That's why I still keep an ancient Q meter hanging around.

Always assuming that the core isn't being driven into saturation.

Her schematic shows the voltage across the coil to be 40 to 50V at

Huh?

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

I meant "voltage across it"

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.

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

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

I found a couple here. They are about 10uH and Q of about 7 or 8 at