Current role in a coil...

[1843]

Hi everybody,
In a coil, current has a more role or number of turns in detection depth?
Regards

[bbsailor]

1843,

Go to the following web site and download a program called "miscel".

http://www.miscel.dk/MiscEl/miscel.html

This program will help with many aspects of designing and calculating PI circuit values. There are two features that apply to your answer about the effects of current or inductance: (1) inductor charge and (2) inductor discharge. I'll explain in detail below.

On the inductor charge drop down menu enter the battery voltage and enter the total coil resistance but include MOSFET on-resistance and any series resistance (if you use it). You will see a graph that shows the inductor charge time that is determined by the coil inductance divided by the total coil circuit resistance (include MOSFET on-resistance). The TX pulse width will determine how high the current gets before the pulse turns off. After 3 coil Time Constants (TCs) the current will raise to about 95 percent of max current near the top of the curve when the current approaches the flat portion of the curve. Write down the current at the point of turn off as determined by the pulse width of your PI circuit. This will be a input value for
the discharge curve.

Now, use the inductor discharge drop down menu and enter the coil inductance, current, and discharge resistance. Note the discharge resistance is equal to the value of the damping resistor in parallel with the operational amplifier input resistor value down to 0.6V or the voltage value of your clamping diodes. What you will see is that the ampliture of the flyback voltage is the start point of the discharge curve. More coil turns, meaning a higher coil inductance, will create a higher flyback voltage as will increasing the current will also make a higher flyback voltage.

Below 0.6V the damping resistor value is on its own and will have a different discharge slope. Just note the current at 0.6V and enter that as a new start current and enter the value of the damping resistor alone as the discharge resistance value. Then you can see how long it takes to get the flyback discharge down to near where the operational amplifier comes out of saturation. For those looking to sample earlier with the shortest delay, this visualization of the coil current, inductance and coil TC will be very educational and help optimize the PI circuit and coil designs.

Now, everyone who makes a coil can see the effect of changing coil inductance, resistance and coil current (through adjusting TX pulse width) on the charge and discharge curves of the PI coil.

According to Eric Foster, the most effective portion of the coil charge turn- off is near the top of the curve, 3 or more coil TCs. Now to answer your question about what is more important coil current or inductance... it is both and it is based on the practical matter of battery discharge current, size and weight. At low voltages in the 12V range, the only way to obtain more current is to reduce resistance but that puts you lower on the coil TC curve so you need a longer TX pulse width to turn-off the pulse near the top of the inductor charge curve. While you will get a target response from turning off the TX pulse lower on the curve, the maximum target response depends on the nature of the desired target, metal type, size and distance from the coil. Saturating the target, which energises the full amount of eddy currents both on the surface as well as the deep eddy currents, will ensure the optimum detection distance from that particular target.

As you can see, there is no easy answer here, coil current, coil inductance, target size, metallic composition and desired detection distance all interact. Use the "miscel" program to see for yourself how all of these factors interact on your particular PI circuit and coil design.

bbsailor

[Carl-NC]

There are 2 parts to your question. One, what determines the strength of the transmitted magnetic field? Two, what determines the sensitivity to the target field?

The transmitted field strength at the center of the coil is B = uNI/2R, where N is the # of turns and I is the current. So you can see they carry equal weight for transmit.

For receive sensitivity, the transmit current is turned off so it is not a factor. We are now trying to generate a receive signal from the target's field, so you might be tempted to rewrite the same equation as I = 2BR/uN and say that 1 turn is best. However, we are not dealing with a shorted coil, so this is incorrect. Instead, we use the induced EMF from Faraday's law, which says v(t) = -N*dB/dt. This v(t) is what gets applied to an opamp or whatever. So for receive, we want lots of turns.

The lots of turns, however, results in more inductance, which slows down the turn-on time during TX, and more parasitic capacitance, which affects decay speed. So you have to balance all of this.

Generally, for a large-coil deepseeking PI, I think you can get away with more turns (higher inductance) because you don't need a blazing fast coil. Just run the pulse rate slower, and the pulse width longer.

- Carl

[okantex]

Hi Carl,

I could not understand your words.
"
Generally, for a large-coil deepseeking PI, I think you can get away with more turns (higher inductance) because you don't need a blazing fast coil. Just run the pulse rate slower, and the pulse width longer.
"
must I increase N at big coils?
we generally decrease it. 1*1m coil 16-18 turns. D20cm coil T30.
I am confused.
and I want your offers for big coils.
what must be inductance ,resisitance of coil ,wire thickness etc.

[okantex]

and something else.
what about shielding at PI coils.
do we need it.in PIs there is 50us waiting for eliminating ground effect.do we need extra protection .does not shielding bad effect on TX .blocking magnetic field?

[1843]

Thanks for the advice bbsailor & Carl.

[bbsailor]

Coil Shield

Okantex

At delays above about 30uS ground signals are not detected very much. This means that coil shields are not normally necessary as the ground response to the pulse will have died out by the time sampling is occuring. However, as the delays are made shorter, below 30uS the ground becomes more easily detected and shielding becomes more critical as the delay is lowered.

Typically, when hunting in remote areas you will have very little noise problems, but if you are using an unshielded coil in a noisy area, even though you are using a delay above 30uS, a shield may help reduce the noise pickup.

Shields tend to introduce additional capacitance to the coil. I use Scotch 24 which is a wire mesh and only causes a coil-to-shield capacitance of about 135pf for a 10.5" inside diameter mono coil. Only about 20 percent of this 135pf or 27 pf gets imposed on the coils self-resonance to lower the coil resonance. This additional capacitance affects the value of the damping resistor, making it smaller, and the potential speed of the coil. Shielding a coil is very critical for those looking for small, low conductivity targets like gold or jewelry on the beach at delays 15uS and below.

Larger coils seeking larger targets operating at low frequencies in the 100 to 500 pps range don't need shielded coils when operating at longer delays above about 30uS.

bbsailor

[okantex]

hi
I build a coil today .tested it.
coil is rectangular. 60cm*100cm 26awg adsl cable.cat5.I unshielded it.it is 8*2=16 turns. according to gary's calculator ,it is 860uH.I did not measure it's ohm value.
in air it can detect sewage cover(something like 70-80cm diameter) from 180cm.
any big metal from 180cm.

but it could not detect aliminium jug with handle.it is buried at 140cm .it is in upside down position.bottom diameter is almost 40cm.
my delta could not detect.
lorenz with 45cm coil and homemade two coil detector detected it before with success.
sawage cover is close to this jug.I mean 20 meters.my coil is effected from cover.but can not detect jug.
lorenz and two coil detector did not have problem with cover.
what is wrong with my coil.
I will try ivconics 42 cm diameter coil with 35T and 0.7mm wire.As I remmeber it can detect 1 lt paint can from 140cm.
any offers

[Geo]

I dont know the wire diameter of the 26awg but for this coil (60x100 cm) you must use wire with about 1mm diameter.

[okantex]

26awg = 0.45-0.50mm

if I ıncrease wire diameter to 1mm from formula R= qL/a . 1/4 of current coil. means 4 times much current on coil.
do you think it will detect the target from 140cm than.
also it is aliminium .can it be problem.
is it the problem
and any offer for big 30-40cm diameter targets at 150-250cm dept.
can you give coil parameters to build.
I am looking for gold , copper ,bronze silver targets in general.
do I have to change frequency of of my DP.
I omitted PR1(strapped) ,so my frequency is something like 90-100hz.

[okantex]

D20cm T30 coil test result for gold.
gold ring from 10-12cm
small gold coin same with ring.
8 bracelets form almost 60 cm.
4 small gold coins flattened from 27-30cm

[1843]

Hi Carl,

While the MOSFET is on, the produced field calculate by the formula B=uNI/2R;

And when it turns off, a feild (with opposite direction) become produced which is correspond to dI/dt.

Now my question is:
which one is used to detect objects? First or second field?

I am confused... Coils with much turns produce a good field of first above, but their opposit field is weaker than low inductance coils (due to their interwinding capacitance) and...

Best regards

[Carl-NC]

While the MOSFET is on, the produced field calculate by the formula B=uNI/2R;

Right... this is a static field.

And when it turns off, a feild (with opposite direction) become produced which is correspond to dI/dt.

Right... this is a dynamic field that produces the target eddy currents.

Now my question is:
which one is used to detect objects? First or second field?

Both.

Target eddies are produced by the collapsing magnetic field dB/dt. The stronger the dB/dt, the stronger the eddy currents, and the stronger the resulting target magnetic field.

There are two ways to make dB/dt stronger: a larger B (initial magnetic field), or a shorter t (faster turn-off). The field turn-off is the same as the coil current turn-off. So a higher B field, and a faster dI/dt, result in a higher dB/dt.

And you are correct... trying to get both a higher B field and a faster dI/dt are difficult. That's why engineers get paid so much.

- Carl

[1843]

Thanks!

Thank you very much,

My another question is:
Why the both surfaces of a coil, detects the objects?

Does a coil produce two kind static field( in opposit directions ) and also two secondary field?

The battery poles are constant, so the static field must be in one direction and also
the secondary field.

Best regards

[Carl-NC]

Yes, for a planar loop the field above is the same as the field below, except for opposite polarity.

- Carl