Parallel LC Circuit

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

jess

solenoid

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

I am using the following diode

What other core materials are available?

jess

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.

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.

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

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

-- Bill Sloman, Nijmegen

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

o peak is appearing across the coil than current in the coil is 200A. How c= ould it be true?

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

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

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.

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

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).

Hi,

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

jess

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

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.

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.

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

get the number 16?

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

John S

eld generated by Helmholtz coils . I will appreciate if you can help with b= oth issues.

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.

snipped-for-privacy@gmail.com

didn't comethrough spell it out slowly like this jsscsha .AT. gmail .DOT. com

o peak is appearing across the coil than current in the coil is 200A. How c= ould 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.

the secondary coil ( LC circuit) using Helmholtz coil pair. The secondary c= oil 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

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

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

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.

jsscshaw88 at gmail dot com