Source Impedance II

Hi,

I calculated K = 0.06 when two coils were approximately 8 inches apart. The voltage across the secondary coil is approximately 40 volts peak to peak ( desirable). Than I reduced the distance between the coils to almost zero and found K = 0.3. The secondary coil gets 10 V peak to peak if it gets really close to the coils.

1. How can I calculate the K between the secondary coil in the field and the primary coils?

1. How can I calculate the flux cutting the secondary coil when primary coils are 8 inches apart?

2. Can someone recommends a flus sensor or a magnetic field sensor that I can use to get the feed back from the plastic enclouser?

jess

Depends on what you mean by "good transformers". I've had tens of watts through coupled tuned circuits with less than 1dB of insertion loss at up to around 450MHz.

Field-plotting software might be a place to start. I've never use such a program - they tend to be expensive - but they are commercially available.

You/ve got one primary coil and one secondary coil. How can the primary coils - you've suddenly got two of them - be eight inches apart?

There shouldn't be any current induced in a plastic enclosure. It's dielectric constant won't be the same as air - or a vacuum - but it seems unlikely to make much difference to the interaction between your coils.

-- Bill Sloman, Nijmegen

much

But did you do that to transfer power? Or was the tuning action paramount?

-- Bill Sloman, Nijmegen

Please look at this

jess

Where is the secondary coil?

One primary coil is wrapped around the top of the plastic enclouser and one on the bottom of the plastic enclouser and the secondary is sitting inside the enclouser. Both primary coils are connected in series.

jess

What is the purpose of the secondary coil?

cell phone chargers

Oops. Sorry. It's been a while since I've had to think about Helmholtz coils.

It might help if you told us why you have put this arrangement together.

The point about Helmholtz coils is that they give an area of more or less uniform magnetic field half-way between the coils - it's more nearly uniform is you use circular coils, so one has to wonder what you are trying to achieve with your rectangular coils.

If this is going to be a cell-phone charger, the argument might be that a Helmholtz arrangement of coils gives you a fairly large area inside the box (plastic enclosure) where the charging rate will be much the same.

One then has to wonder why you have two receiving coils in your secondary ...

George Herold is right to point out that you can calculate the magnetic field at any point around (including in) the box - you know the size and direction of the current passing through every bit of the primary coil and you know where every bit of that coil is, so you can get the field at any point by applying the Biot=96Savart Law - see

-- Bill Sloman, Nijmegen

Both. Antenna filters for medium power transmitters. Design parameters minimum insertion loss with a defined bandpass characteristic.

You haven't been paying attention ;-)

It's been made abundantly clear over the last couple of months that the "primary" is a Helmholz pair.

Jessica seems to want to plot the field intensity within the Helmholz field.

With the bulk, degree of complexity, and likely cost you propose, I guess I'll stick to using a wall wart.

It's in every electrical engineering textbook I've read. Even radio ham books.

EE101 stuff.

Hi,

I read this article

Is he saying on page 409 that four coils if nested can provide magenetic field in all three axes?

I want to measure the field strength. Can any one recommend a deivce to do it at the frequency of 100KHz.

jess

Wind, say, 10 turns of some gauge wire onto a pill bottle or something like that. Connect it to a oscilloscope or whatever you have that can show amplitude in the millivolts or less. Experiment by moving it around in your area of interest. If the amplitude is sufficient, try to calculate what it means. If the amplitude is insufficient, try more turns of wire.

No guarantee that it will be successful, but it is easy to try.

John S

How can I calculate dI1 / dt and M?

jess

The rate of change of the current through the windings should be easy enough to work out.

M=3Dk.square root L1.l2 where k is always less than one, but close to it for a transformer. It essentially the proportion of the magnetic flux that goes through both coils.

-- Bill Sloman, Nijmegen

A test coil - a circular loop of wire of a known diameter. The rate of change of the flux threading the loop will generate a voltage between the ends of the loop. If you don't get enough volts to measure from a single turn coil, add a few more turns.

-- Bill Sloman, Nijmegen