Coil uH

[Tibuck19]

hey guys, could you all help me understand coil Uh more, say we have a 10 inch coil , what difference would a 300uH be compared to a 500uH will a 500Uh detect deeper?? or less?? or faster or slower? or will it just drain batteries faster? I know its a loaded question but not knowing has been eating at me for months.

[homefire]

The Fly Back on Lower Inductance Coils Settles down faster allowing you to sample faster better for Low Magnetic Conductance targets like Gold. Higher Inductance Coils take longer Not so good for Gold but can detect smaller targets better. You need to figure what you want. GOLD or SMALL ... Small Gold takes more then just a fast coil. http://www.geotech1.com/pages/metdet...s/FastCoil.pdf

[Tibuck19]

The Fly Back on Lower Inductance Coils Settles down faster allowing you to sample faster better for Low Magnetic Conductance targets like Gold. Higher Inductance Coils take longer Not so good for Gold but can detect smaller targets better. You need to figure what you want. GOLD or SMALL ... Small Gold takes more then just a fast coil. http://www.geotech1.com/pages/metdet...s/FastCoil.pdf

Good evening homefire, Thanks that helps a lot. So if I get what your saying right the inductance has nothing to do with the depth the coil can go that is the size and shape, fast coil smaller 28 gauge wire, with less inductance is better for gold and more inductance will pick up small nails hahaha I mean coins ( so far just this summer um up to 56 pounds of nails bottle caps and other metals ) loll, Hey btw whooohooo first silver Canadian dime with the MMP at about 7 inch.

[homefire]

Less turns could do the lower Inductance.

[SVEN1]

Less turns could do the lower Inductance.

As well as how tite you coil the wire bundle. Adding spiral wrap and foil shielding also increases inductance. Again how tite you wrap the spiral will determine how much higher the inductance will be.
Example: using same amount of winds
tite wire bundle 285
loose wire bundle 310
spiral wrap can add 5-15uh
foil 5uh

You want small gold, circuitry with 10-13uh pulse width
pulse delay 10-12us, for starters.

[Tibuck19]

As well as how tite you coil the wire bundle. Adding spiral wrap and foil shielding also increases inductance. Again how tite you wrap the spiral will determine how much higher the inductance will be.
Example: using same amount of winds
tite wire bundle 285
loose wire bundle 310
spiral wrap can add 5-15uh
foil 5uh

You want small gold, circuitry with 10-13uh pulse width
pulse delay 10-12us, for starters.

Thank you, I was more wondering about difference of uH for ground penetration, As homefire said, smaller uH decays faster for lower Magnetic Conductance targets, but i'm more wondering which will go deeper in ground 300uh vs a 500uH, I remember reading somewhere that to go deeper you need to add more power to the coil.

[Qiaozhi]

Thank you, I was more wondering about difference of uH for ground penetration, As homefire said, smaller uH decays faster for lower Magnetic Conductance targets, but i'm more wondering which will go deeper in ground 300uh vs a 500uH, I remember reading somewhere that to go deeper you need to add more power to the coil.

It's not quite as simple as that.

Let's say you have two coils, one with 300uH inductance, and the other with 500uH. You then restrict the maximum current in each case so that they are identical. For argument's sake, let's set the maximum current to 2A. In the case of the 300uH coil, the energy stored, at just before TX-off, will be 600uJ. For the 500uH coil, the energy stored will be 1000uJ. It should be fairly obvious that the 500uH coil will be able to detect targets at a greater distance than the 300uH coil. However, it does depend on what target you are searching for. For small gold there may well be a decrease in distance due to other factors, such as being forced to use a longer sample delay because of the increased coil capacitance.

Now, if we arrange things such that the energy stored in each coil is the same, with the 300uH storing 600uJ, and the 500uH also storing 600uJ, we may still have an issue with small gold targets due to the higher coil capacitance in the 500uH coil. But, ignoring that for the moment and assuming we're looking for coins and that the sample delay is the same in both instances, the 500uH coil will still outperform the 300uH coil, even though the stored energy is identical. This is due to the fact that a mono coil is used for both TX and RX, and (in the case of the 500uH) the RX coil has a higher inductance, and therefore a higher received signal.

It is also important not to neglect the problem of flat-topping. If the TX-on current is still rising at the point of TX-off, this will have a negative impact on the target signal.

In conclusion, the answer to your question is a complex one, as there are many other factors that prevent the answer being yes or no. IMHO you should select the highest coil inductance that is practical for your intended application. A good example is the Vallon detector, which uses a very high inductance coil. However, in that particular design, the slowness of the coil is overcome by driving it with a high voltage to reduce the TX-on time, and switched damping to reduce the sample delay time during TX-off. But this is a special case.

[green]

Thank you, I was more wondering about difference of uH for ground penetration, As homefire said, smaller uH decays faster for lower Magnetic Conductance targets, but i'm more wondering which will go deeper in ground 300uh vs a 500uH, I remember reading somewhere that to go deeper you need to add more power to the coil.

Tx strength proportional to ampere turns, Rx strength proportional to turns. Pi with mono coil. Signal strength proportional to ampere turns squared. Inductance is proportional to turns squared. Signal strength proportional to ampere*inductance. Everything else staying the same(current profile, sample times, coil diameter, etc.)doubling peak current or inductance should increase detection depth from 8 to 9 inches with a 8 inch diameter coil.

[Tibuck19]

It's not quite as simple as that.

Let's say you have two coils, one with 300uH inductance, and the other with 500uH. You then restrict the maximum current in each case so that they are identical. For argument's sake, let's set the maximum current to 2A. In the case of the 300uH coil, the energy stored, at just before TX-off, will be 600uJ. For the 500uH coil, the energy stored will be 1000uJ. It should be fairly obvious that the 500uH coil will be able to detect targets at a greater distance than the 300uH coil. However, it does depend on what target you are searching for. For small gold there may well be a decrease in distance due to other factors, such as being forced to use a longer sample delay because of the increased coil capacitance.

Now, if we arrange things such that the energy stored in each coil is the same, with the 300uH storing 600uJ, and the 500uH also storing 600uJ, we may still have an issue with small gold targets due to the higher coil capacitance in the 500uH coil. But, ignoring that for the moment and assuming we're looking for coins and that the sample delay is the same in both instances, the 500uH coil will still outperform the 300uH coil, even though the stored energy is identical. This is due to the fact that a mono coil is used for both TX and RX, and (in the case of the 500uH) the RX coil has a higher inductance, and therefore a higher received signal.

It is also important not to neglect the problem of flat-topping. If the TX-on current is still rising at the point of TX-off, this will have a negative impact on the target signal.

In conclusion, the answer to your question is a complex one, as there are many other factors that prevent the answer being yes or no. IMHO you should select the highest coil inductance that is practical for your intended application. A good example is the Vallon detector, which uses a very high inductance coil. However, in that particular design, the slowness of the coil is overcome by driving it with a high voltage to reduce the TX-on time, and switched damping to reduce the sample delay time during TX-off. But this is a special case.

Thank you, I understand its a very complex question that is dependent on more then one factor nonetheless, your answer does help shed some light,( was this explained in the book and missed the answer?) For this particular instance, I was thinking for coin coil as I had notice, the only silver coin I had found thus far, was a good 7 inch deep and lead me to wonder how many more did I miss due to lack of ground penetration, So for I have not made a coil pass the 360uH, and was wonder if making a 500Uh ( for the MMP) was worth it, but mostly is to gain knowledge from you guys experience. So from what you say if I was to make a DD coil unsteadied of a mono coil, the TX would be around 300Uh for fast fly back decay and a RX of 500uH to pick up the return better and deeper, Right?

[Qiaozhi]

So from what you say if I was to make a DD coil unsteadied of a mono coil, the TX would be around 300Uh for fast fly back decay and a RX of 500uH to pick up the return better and deeper, Right?

That's the advantage of using an induction balanced coil. i.e. you can increase the numbers of turns on the RX coil to improve sensitivity.
But ... bear in mind that the loops of a DD coil will be smaller than the same diameter mono coil.

[Tibuck19]

That's the advantage of using an induction balanced coil. i.e. you can increase the numbers of turns on the RX coil to improve sensitivity.
But ... bear in mind that the loops of a DD coil will be smaller than the same diameter mono coil.

I just made a 500uH mono loop coil for the mini pulse, Do I need to also adjust R-3 as-well?

[Qiaozhi]

I just made a 500uH mono loop coil for the mini pulse, Do I need to also adjust R-3 as-well?

According to my calculations (and assuming a TX pulse width of 58us) I estimate that the coil current will peak at 2.26A for the 500uH coil, and 1.6A for the 500uH coil. Since the energy stored in the coil is proportional to the coil current, you will most likely see no improvement in detection distance if you leave everything else the same. Rather than change the value of R3, I would suggest increasing the TX pulse width for the 500uH coil until the average current drawn by the circuit is the same. Alternatively you could measure the maximum coil current in each case using an oscilloscope connected across R3, and calculate it using Ohm's Law. Also, don't forget to adjust the damping resistor.

[WM6]

the coil current will peak at 2.26A for the 500uH coil,

Probably 300uH?

[Qiaozhi]

Probably 300uH?

Sorry ... that was a typo. Yes, it should have read 300uH.

[Tibuck19]

According to my calculations (and assuming a TX pulse width of 58us) I estimate that the coil current will peak at 2.26A for the 500uH coil, and 1.6A for the 500uH coil. Since the energy stored in the coil is proportional to the coil current, you will most likely see no improvement in detection distance if you leave everything else the same. Rather than change the value of R3, I would suggest increasing the TX pulse width for the 500uH coil until the average current drawn by the circuit is the same. Alternatively you could measure the maximum coil current in each case using an oscilloscope connected across R3, and calculate it using Ohm's Law. Also, don't forget to adjust the damping resistor.

To increase the tx pulse width, I change the Value of R7 while monitoring on TP1 ?