Differences in coil Inductance?

[Unregistered]

There is much to read here on this forum so thought this also would be a good place to find further info regarding inductance differences for coils produced for Minelab Pis and following the recent discussion "need help reading inductance."

I had read of differences regarding lower inductance readings between newer and older coils produced by CT and NF for use on ML detectors. The uH readings on the newer mono coils now range from a low of 226uH to 256uH while the older coils came in at 300-310uH.

Some of what has been posted here on this forum says the Minelab GP series detector coils need to have the ohms right at 0.4 with an inductance of 300 uH (microhenries) The inductance is fairly critical as ML machine don't like if the value deviates too much from 300uh and if trying to make a coil for the ML the key is to get both the resistance and inductance as close to the manufacturer's coil as possible. You will find that for a specific size coil there is only one wire size that will get you to that resistance and inductance. ML uses a front end that differs noticeably from other pi detectors so you will have to carefully “fit” the coil to the design.


I own both CT and NF coils for use on both my SD2000 and GP3000 and after reading the above as well as the inductance differences between the newer and older coils which seem to deviate from 300uh by a fair amount I therefore wish to ask the following question.

What should I expect to encounter using a newer lower inductance coil on either of my ML detectors.?

Thanks for your answers,
Gary

[garyq]

Sorry members I forgot and meant to register before I posted my question above.

Thanks again,
Gary.

[IBGold]

Hi Gary as a relativly new coil constructor I have found that for Minlab coils the inductance should be between250uH and 350 uH try for 300 uH, my Minelab coils and Coiltek vary from 238 uH to 330 Uh the trick is the coil Q im told for SD2200 and gp,s aim for completed Q of 3.8 and for the SD2000 4.2 and you will have the best coils going I am building for both SD2000 and GP3500 diferent coils for each to achive the ultimate I am very impresed with the eight coils that I have completed all are extremly sensitive and show improved depth capability using latter coils on the SD2000 wont achieve maximum sensitivity Regards IBGold.

[garyq]

Hey thanks for your quick reply IBGold and good on you for building your own coils. From your reply it reads as though you have got your head around what is involved to successfuly build and match your homemade coils to operate on both your older and later model ML detectors. I can recall reading on Eric's forum many moons ago what Gary Robinson mentioned and his success building his own coils to match his SD2000,who I believe now builds coils under the Wescoil brand.

I have found when running my old CT18"DD it seems not to run as smooth on my 3000 than it does on my 2000. I have yet to try my NF18" mono on the 2000 to see if the opposite might occur with a newer coil. However my ML8"mono runs well enough on both machines.

I am thinking of trying to get hold of some LCR readings on my coils and see the differences and by how much, but would not have a clue where to connect the leads of a LCR meter to the coil plug connector if I could get my hands on such a meter. Therefore most likely would have to take my coils into a electronics shop for some help and readings if they knew what they had to do.

RobbyH has made mention that lower inductance may help to cope with ground noise. Also I believe doug on the P'inOz forum had hoped to test an experimental elip mono coil with 2 ohm resistance, which is suppose to help with ground noise, and compare it on the same noisy ground to a standard resistance NF mono coil (298uH,0.35ohmns and a Q = 5.2)

Also interesting to read in your reply of the trick in the coils Q.

Not sure what resistance might do depth wise?

Gary

[ivconic]

Sounds like new coils are suited mostly for new GP4xxx serie??? I am not sure, just guessing...
Good way to find out is to put GP3500 and GP4xxx on test with those coils and see eventual differences...
I dont think that those differences plays important role...Whole adjustment is made at device itself...
Refgards!

[IBGold]

Gary get a multi-meter **** Smith Q-1559 about $90. will do your resistance and inductance measurements get a coil socket and bridge pins 1 and 5 together and solder a wire to 5 this becomes the common wire solder wires to pins 4 and 2 pin4 is the TX and pin 2 the RX now strip about 10 mm on wire ends solder seperatly now you have a test rig push the soldered ends into the inductance socket as required to test both mono and DD coils of coarse momo only use 4 and5/1 DD use TX 4 and 5/1 and RX 2and 5/1 hope that all makes sense. REgards Ian.

[joecoin]

Gary get a multi-meter **** Smith Q-1559 about $90. will do your resistance and inductance measurements get a coil socket and bridge pins 1 and 5 together and solder a wire to 5 this becomes the common wire solder wires to pins 4 and 2 pin4 is the TX and pin 2 the RX now strip about 10 mm on wire ends solder seperatly now you have a test rig push the soldered ends into the inductance socket as required to test both mono and DD coils of coarse momo only use 4 and5/1 DD use TX 4 and 5/1 and RX 2and 5/1 hope that all makes sense. REgards Ian.

Are these pin outs common to all current production Minelab detectors?

Thanks,

joe

[Woody.au]

On the weekend we tested some coils at the test site and found that a NF 20 inch coil would not work at all on a SD2000 clocked at 1.25 Mhz but sort of worked at 2Mhz but it was on the noisy side. The best coil on the SD2000 was 14 inch experimental NF coil with a Q of 5.2 and inductance of 340uH at 0.35 ohm. This coil was smooth and sensitive on the SD2000 but on a 3000 or 4000 it was so noisy it could not be used. Tested out the 3ft coil and can ping an aluminium test target at 4ft. One thing I noticed is that Litze wire makes a coil noisy in some designs, tinned multi always makes a quiet coil in most designs. To control the Q of the coil try different Faraday shield spacings.

We should not worry about coil Q too much as a 5 in air is most likely a 1 when sweeping the ground.

[JC1]

Coil Q

On the weekend we tested some coils at the test site and found that a NF 20 inch coil would not work at all on a SD2000 clocked at 1.25 Mhz but sort of worked at 2Mhz but it was on the noisy side. The best coil on the SD2000 was 14 inch experimental NF coil with a Q of 5.2 and inductance of 340uH at 0.35 ohm. This coil was smooth and sensitive on the SD2000 but on a 3000 or 4000 it was so noisy it could not be used. Tested out the 3ft coil and can ping an aluminium test target at 4ft. One thing I noticed is that Litze wire makes a coil noisy in some designs, tinned multi always makes a quiet coil in most designs. To control the Q of the coil try different Faraday shield spacings.

We should not worry about coil Q too much as a 5 in air is most likely a 1 when sweeping the ground.

So if the Q of a coil is inductance/dc resistance then which

changed by a factor of 5 when sweeping the ground?

Or a little of both?

Not sure how you reached this conclusion.

Can you explain?

thanx

[JC1]

...

Actually 2*pi*f*L/Ri = Q

but I am assuming the frequency is not changing

because of sweeping the ground. Also the internal

resistance is probably not changing. So is the

inductance changing by a factor of 5 lower.

i.e. 300 uH coil becomes 60 uH?

Why?

[TheWizard]

2*pi*f*L/Ri = Q
Why?

Pulse Induction..... f = ???

[1843]

Hi,

Could you explain more about coil Q?

Thanks,
1843

[Qiaozhi]

Hi,

Could you explain more about coil Q?

Thanks,
1843

In a metal detector coil the resistance is usually low in comparison to the inductive reactance. This is because the resistance is basically just the resistance of the wire from which the coil is wound. is therefore greater than R and the voltage developed across the coil can be greater than the power supply. This occurs because at resonance the impedance of the circuit is simply R, and so the current is . The voltage across the inductor is , and therefore . The factor (where ) is known as the magnification factor, and is given the symbol Q because this is a measure of the quality of the coil. In an ideal inductor the resistance would be zero, and hence Q would be infinite.

For example, if you have a 6mH coil with a DC resistance of , and the operating frequency is 10kHz, then:
and .

Phew! - this LaTex stuff is a bit tricky to get right.

[TheWizard]

Right but for sinus signal....
For PULSE Induction you need to look Inductive Energy for pulsed period
some like http://maxwell.byu.edu/~spencerr/web...00000000000000

[Qiaozhi]

Right but for sinus signal....
For PULSE Induction you need to look Inductive Energy for pulsed period
some like http://maxwell.byu.edu/~spencerr/web...00000000000000

The Q of the coil is still the same. It is the inductive reactance divided by the DC resistance of the coil. In the case of a pulsed signal, you must look at the rise and fall times of the pulse, and not the period. The Q is directly proportional to the frequency, and the higher the frequency, the higher the Q. So the faster the edges of the pulse, the higher the Q.