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Re: [TCML] Flat strip primary inductance



Hi Finn,

Something else. As your measuring F and finding it's value high, probably pointing out lead inductance in the meter was too quick of a reply. But, lead inductance in the L to C wiring can cause this situation with this particular coil. Although I see the photo, it's hard to see the actual C to L connections and wiring. So I guess what I'm saying is if you can minimize any possible lead inductance, this may help solve the riddle. Easy enough to test for.

I'm unsure about the speaker wire coil. Wire coils do have more L for the same spacing and turns, but as you get to only a couple turns, the difference would be minimal. And like I mentioned, make sure there's no metal surface under the measurement. Well, there can be, but give it some distance (make sure it's not directly under the coil). It only takes a few inches before a metallic surface no longer affects the coil inductance.

Take care,
Bart

bartb wrote:
Hi Finn,

The 1.5uH sounds correct (and I'm pretty sure it is). The ribbon is only 2 turns and small. The inductance is therefore very small. I would bet that you have nearly 0.8uH of lead inductance from the meter affecting the measurement. It doesn't take much lead inductance to add to the total inductance of the LC on such a small coil inductance (and the value your off is about what you would see with the leads). If you have some old leads around that will fit in the Amprobe, cut those down to something short and measure again.

Also, metal sheeting directly under the coil can affect the inductance (shorted turn affect - but this would cause L to decrease). I doubt that is the issue but it can happen. This is easy to see (place aluminum foil directly under coil base, measure L, then slowly slide the foil out from under the coil and watch L change in the process.

Take care,
Bart


Finn Hammer wrote:
All,

I know this has been up before, but here goes:
A flat strip primary has lower inductance than one made out of round conductors. This is not new, but here is an extreme example, I think: I kept hanging tank capacitors on to the primary assembly, without reaching the desired res. frequency. At last I had to investigate further.

The primary is an internal flat strip primary, for a BRISG coil, and it consists of 2 turns 180mm dia with 5mm spacing.

All simulators, INCA, javatc, wintesla predict 1.5µH and since the primary cap is 7.5µF, the anticipated resonanse was 47kHz.

However, the assembly resonated at 63kHz which corresponds to 0.8µH

It baffels me that such an extreme difference could exist, so I made up a sanity check:

2Turns of thich speaker wire, and a 7.5µF cap resonates at 43kHz.

Has anybody experienced this before?

A picture posted to see the primary, and the sanity check:
http://www.hammertone.com/sanity.jpg

Cheers, Finn Hammer
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