tightly wound copper coil is bad ?

[Tec]

On my system using a scope.

Tightly wound enameled copper takes longer to discharge
Than loosly wound coil made from insulated hookup wire.

The inductance of the enameled copper coil is 314 uH
The inductance of the loosely wound coil is 343uH

So, even though the loosely wound coil has a larger inductance,
It discharges quicker and can be used to detect smaller targets.

And also the damping resistor that gives critical damping on both coils is around 350ohms.
nothing like the damping resistor values I have seen in other circuits for similar sized coils.


So:
If tightly wound copper coil is so useless, why are they used in all the DIY PI designs I have seen.

am I missing something ?

Nothing makes sense.

[Carl-NC]

The loosely wound coil has a lower C and higher self-resonance, so it will have a faster transient response. You probably had to add some turns to get the inductance equal to the tightly wound coil. I would guess that the loose coil will tolerate a higher damping resistor for critical damping if you were to optimize it.

[Tec]

Hi Thanks for the reply,

Yes the loosely wound coil has more turns.

I also tried having less turns on the tightly wound coil, but was unable to get it as fast as the loose coil (even with just 15 turns 20cm diameter).

Scope traces attached of the output of the preamp.

No tightly wound copper coil I can make will let me detect a 5c coin because the curve doesn't get down fast enough.
Notice on the loosely wound (hookup wire) coil it gets down well before 20uSec so I can detect small targets, by small I just mean a aus 5c coin.

I know I should consult the formulas, but I don't know how to measure the capacitance of the coil (and lead).
I can measure inductance and resistance.
And look at its' actual performance.

[Qiaozhi]

Hi Thanks for the reply,

Yes the loosely wound coil has more turns.

I also tried having less turns on the tightly wound coil, but was unable to get it as fast as the loose coil (even with just 15 turns 20cm diameter).

Scope traces attached of the output of the preamp.

No tightly wound copper coil I can make will let me detect a 5c coin because the curve doesn't get down fast enough.
Notice on the loosely wound (hookup wire) coil it gets down well before 20uSec so I can detect small targets, by small I just mean a aus 5c coin.

I know I should consult the formulas, but I don't know how to measure the capacitance of the coil (and lead).
I can measure inductance and resistance.
And look at its' actual performance.

That's very strange, as I have no problem detecting either an AUS 5c or UK 5p coin at 8"+ with a loud definite response. This is with a coil made with 0.56mm enamelled copper wire and a damping resistor of 560 ohms.

Your problem may be with the low damping resistor you are using. It's possible the signal is heavily overdamped.

[6666]

Tec which MD design are you using ?

[Qiaozhi]

Tec which MD design are you using ?

These tests were performed with a Hammerhead detector.

[Carl-NC]

Agreed, try increasing the damping R to 470, 560, and 680 and see what happens.

[Alexismex]

.

The inductance of the enameled copper coil is 314 uH
The inductance of the loosely wound coil is 343uH

So, even though the loosely wound coil has a larger inductance,
It discharges quicker and can be used to detect smaller targets.

Hello Tec,
It is strange that you have these Inductance because normally..... it is the contrary ,
Coil made with enameled copper are more inductive that coil made from loosely insulated wire!!!!!

[Carl-NC]

...Coil made with enameled copper are more inductive that coil made from loosely insulated wire!!!!!

The loose-insulated wire coil has more turns.

[Tec]

The loose coil benefits me more

Yes,

Sorry for the confusion.
I am using my fandanlged detector..
The curve DOES change with a 5c coin, and it would be detectable with a standard PI.

But I am trying to detect the peak response early..
see the picture, I found that what happens really early on is affecting my discrimination.

The loose coil gets down quicker.

I can decrease the gain, but it still seems that the tightly wound coil is much steeper at the top.
Again, the picture should explain.
The way the small target bulges out the curve is very different between the loosely wound and tightly wound, because of what is happening early in the curve.

Anyway, I think I have fixed the issue by measuring the V1, outside the op amp ..i.e way up on the curve...(I can do that because I am triggering an op amp(comparator) rather than taking a sample.))

So...I hope this explains, why the seemingly fast decay of the loose coil (due to what ever mystery property) is affecting my detector...

Note the sample delay is there to represent a standard PI, mine doesn't have a sample delay

[Tec]

*I am assuming it is because of Lower C as Carl-NC said...lower C on the loose coil.

Note: When finding the damping resistor, I try a range of resistances eg 400 to 680 ohm , definitely starts ringing above 400 , by 600 ohms it is ruined.

To calculate inducatnce I connect a 1uF capacitor in parallel and measure the frequency of oscillations on a scope. By connecting and then disconnecting 3v, and catching the oscillations on the scope.

ta

[DOOLEY]

hello tec ,

i'm no expert , but that picture looks wrong , sample is "normaly" taken further to the left on your picture , after the bend where it starts to straighten out.

[Tec]

hello tec ,

i'm no expert , but that picture looks wrong , sample is "normaly" taken further to the left on your picture , after the bend where it starts to straighten out.

Yes we are on the straight.



See attached

[golfnut]

Hi, TEC.

If you want an accurate L value - I can measure it on my works HP digital LCR bridge.

What continent R U

Steve

[mikebg]

Hi, TEC.

If you want an accurate L value - I can measure it on my works HP digital LCR bridge.

What continent R U

Steve

No need to know accurate L value. Most important is what happens with resonance frequency of network where the coil is connected. Simply disconnect the damping resistor and measure parameters of ringing process.