'Fast' coil

[pebe]

I have read that for a PI detector to find gold it needs a 'fast' coil.

If I wind one how can I measure how 'fast' it is?

[Qiaozhi]

I have read that for a PI detector to find gold it needs a 'fast' coil.

If I wind one how can I measure how 'fast' it is?

Read this -> Making a Fast Pulse Induction Mono Coil

[pebe]

Thanks Q. Interesting reading - though about a specific coil.
But reading between the lines it appears that a fast coil is obtained by minimum self capacity and stray capacity, and critical damping.
The adjustment for critical damping shown seemed a bit hit-and-miss, though. I had hoped that it could have been set up with a 'scope.

[Qiaozhi]

Thanks Q. Interesting reading - though about a specific coil.
But reading between the lines it appears that a fast coil is obtained by minimum self capacity and stray capacity, and critical damping.
The adjustment for critical damping shown seemed a bit hit-and-miss, though. I had hoped that it could have been set up with a 'scope.

The best way to adjust the damping resistor for critical damping is to attach a scope to the output of the preamp. Then use a damping resistor tool to find the optimum value. The damping resistor tool consists of a 10k pot in series with a 200 ohm resistor, and both in parallel with a 2k resistor. After adjustment, measure the resistance of the network and replace it with a fixed resistor of the same value. See page 155 of "Inside the METAL DETECTOR - Edition 2".

[pebe]

The best way to adjust the damping resistor for critical damping is to attach a scope to the output of the preamp. Then use a damping resistor tool to find the optimum value. The damping resistor tool consists of a 10k pot in series with a 200 ohm resistor, and both in parallel with a 2k resistor. After adjustment, measure the resistance of the network and replace it with a fixed resistor of the same value. See page 155 of "Inside the METAL DETECTOR - Edition 2".

What indication is there that the optimum value has been reached?

[kt315]

What indication is there that the optimum value has been reached?

pic the.

[pebe]

OK, I assume that he bottom trace 'mit R21' means without damping and the top one with oscillations means 'without R21'.
Is the value of the resistance when the oscillations just disappear called the critically damped point? Or is some other measurement more relevant?

[kt315]

I see you are Scotland. why are you not able to buy ITMD and learn this electronics yourself? no money on great book? reminder there is second edition.

[WM6]

OK, I assume that he bottom trace 'mit R21' means without damping and the top one with oscillations means 'without R21'.

Those "mit R21" mean: with proper selected damping resistor.
When damping resistor is correctly selected, then you need to hold decay time (depending on the characteristics of the coil) as short as possible and decay curve as straight as possible (without oscillations).
Observed without target.

[pebe]

kt315, I spent almost my entire working life in electronics and I don't need to buy a book to re-learn the principles.

But I have noted several cases where the terminology used in this forum is not the same that I was taught. I was therefore really looking for confirmation that the facts stated in the article were in accordance with my interpretation of them.

Anyway, isn't that what this forum is about - helping others?

[pebe]

Thanks WM6 for your reply. That's just what I was looking for.

BTW, the 'without' was a slip on my part. After writing the sentence, I rearranged it and that's when the error crept in!

[WM6]

What indication is there that the optimum value has been reached?

Indication is detecting distance of selected target sample (1 euro coin par example). Greater achieved distance mean better (more optimal) tunning.

Using scope, you do not need testing sample to check how damping resistor is tuned. In this case, you observe decay slope (curve) as per answer in post #9. Be sure no metal particle near to tested coil (at least 3 diameters of coil). For every coil, damping resistor should be selected/tuned separate, if you go for optimal.