TDI Coil (Problem)

[Geo]

Hi.
I made a coil with 60 cm diameter for the TDI.
It reads about 340uH (as the original 12") and has 12 turns 22 AWG Silver Teflon Wire with 19 stran. Shield is from scotch 24 tape.
The problem that i have is with the discrimination. When i use the 12" coil, the GB to 5.... 6 and the delay to 15us, it has the ability to reject the iron objects as a Low conductivity (high tone).
With the handmade 60cm coil and the same adjustments, it can't detect any metal as a low conductivity. It detect all objects as high conductivity.
Any idea???

Regards

[Mrand]

I think, that the signal in output frontend is not clean (oscillations). Probably it is necessary to select damping resistor. Mrand

[Geo]

I think, that the signal in output frontend is not clean (oscillations). Probably it is necessary to select damping resistor. Mrand

Hi.
I checked it and it is OK!!!!

[Mrand]

Compare the resistance of both coils. The time constant L/R is very essential. The detector should "see" coils as absolutely identical. Seek the difference and correct her.
Mrand

[Geo]

Compare the resistance of both coils. The time constant L/R is very essential. The detector should "see" coils as absolutely identical. Seek the difference and correct her.
Mrand

Hi.
The homemade coil has 1Ohm less resistance than the prototype.
I connected a 1Ohm resistance in series with the coil without results.

[mario]

When I use a calculator it gives me around 25 turns for that size wire for 340uH....something doesn't sound right about 12 turns. Are you measuring inductance at 1kz?

[Geo]

When I use a calculator it gives me around 25 turns for that size wire for 340uH....something doesn't sound right about 12 turns. Are you measuring inductance at 1kz?

Something is wrong!!!
My Calculator gives me 12..13 turns, the 2 inductance meters the same!!!
Use radious 12", length 0.4" turns 12, L=???

Regards

[Geo]

Use Qiaozhi inductance calculator

[mario]

If I go by my Minelab specs, that coil would never work for my machine.

1) 22 AWG is too thin. Too much resistance when the target is .5 OHM. You need at least 19AWG size wire to bring the resistance down. 22 AWG size was barely adequate for my 7.5 Inch coil. Why use .5 OHM target? if you want fast sample settings, you need minimal resistance.

2) No matter how you look at it, not enough windings. I just did a 11.5 Inch coil and it required me 25 turns! Originally, I was getting 400+ uH with just 16 turns. Then I noticed my LCR meter was measuring inductance at 200 kH frequency. When I used the appropriate 1 kH frequency, the induction climbed down dramatically to 120+ uH. You need to set your LCR meter to sample at 1kH.

[mario]

Calculators I used:
coolcircuits
Gary's

[Geo]

If I go by my Minelab specs, that coil would never work for my machine.

1) 22 AWG is too thin. Too much resistance when the target is .5 OHM. You need at least 19AWG size wire to bring the resistance down. 22 AWG size was barely adequate for my 7.5 Inch coil. Why use .5 OHM target? if you want fast sample settings, you need minimal resistance.

2) No matter how you look at it, not enough windings. I just did a 11.5 Inch coil and it required me 25 turns! Originally, I was getting 400+ uH with just 16 turns. Then I noticed my LCR meter was measuring inductance at 200 kH frequency. When I used the appropriate 1 kH frequency, the induction climbed down dramatically to 120+ uH. You need to set your LCR meter to sample at 1kH.

As Carl and Reg said, 22AWG is OK for TDI.
The coil has the same inductance with the original 12" and the same DC resistance. If the meter calculates the inductance at 100Khz (for example), it do the same and for the original coil.
So the problem is at other point!!

[WM6]

As Carl and Reg said, 22AWG is OK for TDI.
The coil has the same inductance with the original 12" and the same DC resistance. If the meter calculates the inductance at 100Khz (for example), it do the same and for the original coil.
So the problem is at other point!!

Geo compare impedance of both coil at possibly near to PI pulse rate frequency, say 1000Hz (not at 100kHz). Coil act in tank circuit as impedance.

[Mrand]

Try to make homemade coil identical as original TDI (the diameter and other measurements). Check whether work identically as the original. Check selfresonance frequency. Measure the capacity of the cable. The problem can be other than you think. You must find this small secret. When you will make the good copy of the original coil, only then try to modify. Try also to ask Eric Foster. Mrand

[kt315]

So the problem is at other point!!

oh boy... take Vision, return back the TDI...

[Geo]

Geo compare impedance of both coil at possibly near to PI pulse rate frequency, say 1000Hz (not at 100kHz). Coil act in tank circuit as impedance.

Hi.
I have 3 inductance meter. All indicates about the same!!!. I have not the ability to change the frequency of the Inductance meter.