Help please Carl with Surf pi coil

[Micheal]

Hi carl please help ,I have the surfmaster pi with dual field coil...This coil is way too big and it detects way too deep and is very tiring to pinpoint the target which is very hard work even with a very large scoop,when you have a bad back.I need help to make a smaller 10inch coil ( I know warranty will be void but I dont care)....what parameters should I use to make the coil how many turns and what is the coil inductance measurement,many poeple are complaining of the same issue please help us.Or can you suggest a coil that will work on this machine....
I am thinking desolder the existing coil and measure the inductance then make a smaller diameter coil with the same values ,would this be the right way to go about it,this way I shouldnt have to touch the circuit in any way like damping resistor...please help

[Carl-NC]

We used to build these with a 950 loop (~9") and even made them weighted for neutral buoyancy. Contact the factory (service dept) and see if they have a used loop they will send you. They're easy to swap.

If you really want to build one, a simple mono coil with 240uH will work fine. Use 20-22 awg wire.

- Carl

[Micheal]

Thanks Carl graetley appreciated,I will attemp making a coil,with the 22awg wire how many turns is for a 9inch coil... 240uH ?

I remember somwhere on the site there is a coil calculator but I just cant seem to find it.

[Qiaozhi]

Thanks Carl graetley appreciated,I will attemp making a coil,with the 22awg wire how many turns is for a 9inch coil... 240uH ?

I remember somwhere on the site there is a coil calculator but I just cant seem to find it.

See attachment.

[homefire]

I cant get any of the files to open!

[Micheal]

I have measure the 12 inch dual field coil and the resitance is 2.8ohms...
Before I make a 9 inch coil with 2.8ohms,
1---must I match the inductance aswell?is it important in any way?
2---and finally is the resistance mesurement of 2.8ohms enough of measurement and is it ok to just wind the wire to this measurement?
3---Is the wire thickness important and in what way example 20awg and 25 awg is enamel coated wire better than the pvc plastic or teflon coated wire....

[Micheal]

Today I bought some new wire to experiment with ...
22AWG 30 strand oxygen free copper wire ...$20 for 20 meters..

I have tested the resistance and the wire is 1.6 ohms for the whole roll of 20 metres...( I will have to get a about 40 metres length)
but the original dual field coil is 2.8 ohms ...
This looks like very efficient wire...what do guys think

I will add pictures later...

[chemelec]

Today I bought some new wire to experiment with ...
22AWG 30 strand oxygen free copper wire ...$20 for 20 meters..

I have tested the resistance and the wire is 1.6 ohms for the whole roll of 20 metres...( I will have to get a about 40 metres length)
but the original dual field coil is 2.8 ohms ...
This looks like very efficient wire...what do guys think

I will add pictures later...

Oxygen Free? It Sounds like its just "LITZ" WIRE.
And at a Very Expensive Price!

Resistance is LESS Important than the INDUCTANCE of the Coil!

[Micheal]

you say resistance is not as important as the inductance.then can I ask you this.If I change the resistance of the coil which is 2.8 ohms to a different value then I will have to change the damping resistor and make modification to the circuit of the detector and I dont want to do this.
So if I match the resistance of 2.8 ohms the circuit should still be safe shouldnt it,I dont want to burn out the circuit.

[kt315]

PI Coil Lesson, Eric Foster

Resistance is LESS Important than the INDUCTANCE of the Coil!

http://www.findmall.com/read.php?34,...014#msg-127014

Hi All,
Below is a repost of an article I did a while back on my Tech forum.
"There are many factors that influence the design of a PI coil. However, let's decide on two starting parameters i.e. 100uS pulse width and 2kHz pulse rate. One very important factor is what the earliest sampling time is required to be, as this does have a major effect on the maximum useable coil inductance. The greater the inductance and the higher the current, the longer it takes to reduce the field to zero and the later the sample delay has to be. If you are designing a detector for beach and shallow water hunting a good minimum sample delay to aim for is 15uS. With the right components in the transmitter circuit you can then use a coil inductance of 300uH. I have found it best to wind the coil with 10/0.1 PVC insulated stranded wire (10 strand, 0.1mm strand diameter) as this minimises eddy currents generated in the wire cross section. For a 10in coil the winding resistance is about 4 ohms and with an additional 4.7 ohm 4W resistor in series (in the drive circuit) gives a coil time constant (L/R) of about 30uS. For a 100uS TX pulse width (3.3 times the coil TC) the current will have reached over 95% of its maximum value at switch off, which is a satisfactory situation.
Another important factor to decide at the outset is the type of battery and the required battery life. At 2kHz the current consumption with the above values will be about 250mA at 12V which really requires the use of 10 C cells as a minimum. You can of course design for less current and smaller batteries by using a shorter TX pulse, a higher inductance coil with higher series resistance but there will be some performance trade off. Some PI detectors, particularly earlier designs, do not use a series resistor in the coil circuit to limit the current. Is this case the pulse current is often limited by the coil inductance i.e. the current is switched off well before it has reached its final value which would be determined by the coil resistance. Alternatively, the pulse repetition rate can be lowered until the current is of an acceptable value. There are disadvantages to both of these techniques:- Inductance limiting results in the magnetic field still changing rapidly in a positive direction at the point of switch off. This can cause some signal loss due to switch on eddy currents cancelling those generated at switch off. In pulse frequency limiting, the response time and noise averaging of the detector may not be as good.
The shunt, or damping, resistor, will be determined by the resonant frequency of the coil plus stray and cable capacitance. Not being able to display formulae yet (hope to soon as scanner arrives on Monday) you could look up in a radio or electronic text book the formula for a critical damping resistor for an LC with a given resonant frequency. Or you could just try it experimentally. Start with 1k ohms and look at the receiver output on a scope. The receiver (again it depends what front end I.C. you use) should recover cleanly with no ringing before the point of sampling. Add more resistors in parallel until this is achieved. You should end up with maybe 500 ohms across the coil as described above. Use a good quality 0.5W metal film as the peak emf at switch off can reach several hundred volts and I have found that smaller wattage carbon film resistors can break down and go noisy. I have tried a cermet preset in series with a fixed resistor to give an easily variable adjustment but again the high voltage can cause burning at the wiper contact and eventual failure.
Eric.

[mikebg]

Test posting for formula writing

TX current at time t after switch S closing is}\end{Maple Normal}

\mapleinline{inert}{2d}{i = U*(1-e^(-t/T))/r}{$\displaystyle i=U \left( 1-{e}^{-{\frac {t}{T}}} \right) {r}^{-1}$}
U is supply voltage,}\end{Maple Normal}

r is sum of all resistances in coil branch,}\end{Maple Normal}

\mapleinline{inert}{2d}{T = L/r}{$\displaystyle T={\frac {L}{r}}$}
is timeconstant of coil branch.}\end{Maple Normal}

End of test

[Micheal]

I have bought a whites coil ,the one from a whites Surfmaster Pi pro ...well this is the one the whites dealer recomended.I have measured the resistance of the new coil and it is 1.8ohms and the original coil is 2.8ohms ...
So is it a direct fit or do I need to put a 1ohm resistor to bring it to 2.8 ohms...

[Micheal]

here is the coil pictures

[Micheal]

Just fitted the 950 coil from the surfmaster pi pro onto my surfmaster dual field.
I did some hunting with it today to test it out...
The dual field coil is better to use than the 950 coil,and is deeper than the 950 coil.
The 950 coil requires some fiddling around with the pulse settings to get it running smoothly but it does work and is fairly sensitive for those interested in trying it out.
IT is sensitive to hot rocks and black sand ,and is fairly chattery in the water so the pulse setting has to be adjusted to around 9 o'clock position to stop these problems.
Thank you Carl for assistance

[Cesar]

Hi Michael, do you still have the dual field coil ? do you want to sell it ? or where can i get one like these ?

thanks