Getting ready to build the MPP

[KRinAZ]

I made templates for cutting the Lexan forms for the basket weave coils I'm learning to build - made PDF's of them - scalable - so you can reduce or enlarge them as desired for your coil size. They are intended for 8" x 10" x .091" Lexan sheets, but you can scale it to the size of your project - the wire slots should be cut 1/8" wide and rounded at the inner end - see the Coils/Chance PI Coil... thread for much more info - enjoy

8 inch Coil Form v1.pdf
8" x 1.1" Ring

8x10 Coil Form v4.pdf
8" x 10" x 1.1" Ellipse

[KRinAZ]

Hi KRinAZ, You are correct on adjusting the damping resistor.


Critical damping is achieved by adjusting the resistance to highest possible value that will still prevent oscillations. Oscillation or even partial oscillation destroys or distorts the target eddy currents. Higher resistance allows the highest useful voltage and current to circulate within the coil. The voltage and current in the damping resistor is wasted energy dissipated as heat. That is why the damping resistor normally has 1 or 2 watt rating. Higher resistance value equals less wasted energy.
Very good, thx Chet, now I know how to select the resistor, l'll post when I've determined the value for this coil.

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[Chet]

Hi KRinAZ, You are correct on adjusting the damping resistor.


Critical damping is achieved by adjusting the resistance to highest possible value that will still prevent oscillations. Oscillation or even partial oscillation destroys or distorts the target eddy currents. Higher resistance allows the highest useful voltage and current to circulate within the coil. The voltage and current in the damping resistor is wasted energy dissipated as heat. That is why the damping resistor normally has 1 or 2 watt rating. Higher resistance value equals less wasted energy.

[KRinAZ]

Wow, many thanks Chet, that gives me a much better understanding of how the search coil & Tx circuits interact. I will read it over more times to solidify the concepts in my mind.

Glad to know I don't need to worry about the flyback voltage or the low inductance - from an electronics reliability perspective - and good idea - I will hang on to this coil for now to compare against future builds - & post of course.

I was wondering if I should be adding a heat sink to the MOSFET so that issue is answered - I won't worry about it.

Yes I always [try anyway] err on the side of safety and power down equipment when hooking up test equipment to high voltages - to me that means anything above 48V.

I still have the 470 ohm damping resistor in the circuit - have been looking forward to the step where I actually establish the correct value - and it's of course going to change with the next coil build so I may just keep a pot in place for a bit - haven't put a pot in yet.
I just looked again at the build manual to see which step I establish the correct damping resistor value and realized it doesn't discuss how to establish the correct value. So I got out my copy of Inside the Metal Detector and see where the PI section refers to the coil being "critically damped" but doesn't discuss how to establish it - and I don't know what that is exactly or how to establish it. I'm guessing it involves removing any ringing in the flyback without having the resister any lower in value than needed...
Could you tell me if that is what I do? - just find the highest resistance that damps any ringing? & if you know could you tell me just what "critically damped" means?

Thx for that way to establish the coil capacitance! I'm thinking it would be handy to have a little circuit around just to apply to a coil to observe it's freq.

For now I plan to use smaller coils - the 8" mono, and the biggest currently being an 8x10 oval/elliptical basket weave (as mentioned above) I'd like to try out. For me that would be on the big side.

Thx again Chet!

[Chet]

Hi KRinAZ, Great looking coil. A few simplified equations for you to ponder over; I am not an engineer, mathematician or scientist so the following has short cuts, rounding and violates many purist rules. It is from a hobbyist point of view to get some understanding of pulse induction and coil interaction.


Time constant of a coil is the time required to reach 63% of maximum current;
Maximum current (which is considered to be near 5 time constants) is I=E/R;
Max current = 12v divided by a MOSFET on resistance of 0.55 ohms + coil resistance of 2.1 ohms
Max current = 12/2.65 = 4.5 amps
The time constant of your coil and MOSFET circuit is;
TC=L/R; TC=180uh/MOSFET on resistance 0.55 ohms + coil resistance 2.1 ohms
TC=180/2.65 = 68 us
At the end of 68 us your coil will be increasing up to 63% of 4.5 amps = 2.8 amps


Approximately 7 additional turns would bring your coil to 300uh with 2.7 ohms resistance.
Max current = 12/0.55+2.7 = 3.7 amps
TC=300/3.25 = 92 us
At 92 us the coil will have increased current up to 63% of 3.7 amps = 2.3 amps


The Minipulse Plus transmit pulse is approximately 50us wide. The charge current slope at this time is somewhat linear. With your coil drawing 2.8 amps at a time constant of 68us; then at 50us it will draw approximately 50/68 % of the 68us current; 2.8 x .735 = 2.1 amps. The 300uh coil at 50us will draw approximately 50/92 % of the 92us current; 2.3 x .543 = 1.2 amps.


The IFR740 MOSFET is rated at 10amps continuous or 40amps when pulsed at a 2% duty cycle with proper heat sinking which isn’t needed with the currents developed in the Minipulse. The two coil examples above will be up to maximum currents of 2.1 and 1.2 amps after charging for 50us. The average current drawn over the entire 50us is far less. So the bottom line is that your coil will work just fine without any concern of stressing the circuity.


The 350v high voltage stated in the build manual is misleading. If you limit the transmit current to achieve 350v you will lower the performance of the detector. The IFR740 MOSFET has an internal diode that protects the device from voltages above 400v. Flyback voltages from either of two examples here will exceed 400v. The voltage at flyback time is approximately E = IR; E = 1.2 amps x the 470 ohm damping resistor = 564 volts; or for your present coil 2.1 amps x 470 ohms = 987 volts. Neither of these will overheat and cause a failure of the MOSFET. By the way the 400v limit varies from device to device. I have seen some go as high as 500v. That is why you observed the high voltage limited at 450v.


I did not see in your post that you had changed the value of the 470 ohm damping resistor. You should unsolder one end of the 470 damping resistor and insert a 500 or higher pot in series with it. Then adjust the pot for the best damping (no oscillation), and then replace the 470 ohm with a resistor equal to 470 + the pot value.


!!!!Caution always connect your meter or scope when powered down. It is very easy to cause part failures with momentary slips of the probes. And there are hundreds of volts present in the MOSFET areas!!!!


While you have the damping resistor disconnected you can fire up the circuit and observe the frequency of the oscillations with your oscilloscope. Measure the time between sine wave peaks and convert to frequency by dividing the time into 1. Now you can look on the internet for the equation for LC resonance to compute the capacitance of your coil.


As for performance; an important coil transmit factor is amps and turns. Ampere-turns = number of turns times the amps. It determines the strength of the magnetic field transmitted. In these two coils we have; 25 turns x 2.1amps = 52.5 ampere-turns; and 32 turns x 1.2 amps = 38.4 ampere-turns. This is approximately a 37% increase in transmitted energy. Through more involved factors the ampere-turns in an 8” coil produces a magnetic density of millijoules per square inch. Approximate values in these two examples would be 0.012 and 0.008 millijoules per square inch.


Keep this in mind when going to larger diameter coils. For the same 300uh inductance there are fewer ampere-turns in the coil. The resulting weaker magnetic field is spread over a much larger area. The signal is received by fewer turns in the coil. Thus larger coils require much larger targets.


In the receive area there is a gain in that your coil is faster which is good for small gold nuggets. But there is some loss of signal strength; there are ways to calculate the two way paths, angles, losses, etc. but for simplicity lets approximate that a ratio of receive coil turns between the two coils is 25 to 32 = a 21% loss in the receiver signal path with your present coil.


Overall your present coil will work fine. It would be interesting to see a performance comparison between this coil and your next coil.

[KRinAZ]

Here's the start of the Apollo Pi Basket Litz 90;
Cool coil project![ATTACH]31550[/ATTACH]

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[KRinAZ]

Hmmm, interesting on the center tap coil, & thanks for all the info again - gaining knowledge from all you seasoned coil builders...I'll use a resistor to bring the flyback down to 350V until I get another coil built.

OK this wire mic's to .060 - just under 1/16" - a whopping 1.524mm. When I use this wire up I'll get "normal" thinner wire, but I will say this stuff is super durable...& maybe with the super thick teflon the capacitance will be very very low...

One thing I'm thinking with these basket weaves - is that it can maybe just be stuffed into a tight fitting clam shell - this one I made would be quite durable - both the lexan and the wire are quite tough - might be a simple way to enclose the coil...

The ring is 1 1/16 inch wide - which is counted on both the left and the right sides to make the difference between inner and outer dia's.

And...wait...I've got an idea! - how cool would it be to have a 8x10 oval/ellipse version of this basket weave? I've found I can use Quaozhi's coil calculator on the basket weave and it works out that the basket weave is 95% of the calc'd inductance.

This evening I will be working on making a PDF of a complete cut ready 8x10 oval/ellipse template that can be printed and scaled to different sizes...once I get that done I will make a round version that can also be scaled (ring width would also scale) - should work on your 12"

[Silver Dollar]

Here's the start of the Apollo Pi Basket Litz 90;

[Silver Dollar]

Well according to the description on ebay the 176/46 Litz is 0.90mm.

A higher value resistor indicates a faster coil. Regular coils use say 300 - 500 ohms.
Fast coils are 800 - 1200 ohms.

So the ring is approximately 2" wide? I'll have to scale it up to use 12"...
I'm a bit worried about flopping of the form and attaching the stick. I have
found filling the coil with epoxy does slow it down so I'll try filling the inside
with foam and fiberglassing the outside. I thought of using some spokes but
will have to figure how to glass around them..

I use a series resistor on all my coils to control the current. Somewhere around
6 - 10 ohms will keep you in the ball park. I may change that later when I try to go
real deep but for now it works.

A detector is designed to have a pulse width range to work with it's coil inductance
without using too much power. So you can use a low inductance coil and just make
some changes to control the current. I have a center tapped coil that I will use
as 80 uh for the transmit for fast high current and as 300 uh for the RX to see a
bigger signal

[KRinAZ]

OK that makes sense Silver Dollar, so this coil at it's low inductance isn't of much use since I don't want to run higher than needed current & I'll re-use the wire for something else. I think I'll add a resistor to lower the current and use it to finish the MPP for now while I'm making a new coil.

Nice job on the coil! Pics! It sounds like from what you said that the lower the damping resistor the faster the coil? Good to know...

The dimensions on the form on this coil:
8" outer diameter of lexan ring
5 7/8" inner diameter of lexan ring
Wire OD 1.9mm I believe, will verify later today
37 slots cut inside & outside, 3/8" deep

Note - for this particular (very thick OD) wire and to make the 7 layers/35 turns I plan to use next time - the form will need 1/2" deep slots

I will get my mic out today and get an accurate wire OD measurement, & curious what the OD is of your Litz

[Silver Dollar]

That basket coil is fast! It damps at 800 ohms.

Higher flyback voltage indicates more current flowing.
Lower inductance will allow more current.

The problem is low inductance = less receive signal so the
idea is to balance transmit power and receive signal.

It seems 300 uh has been found to be a good compromise
though sometimes RX is set to 450 uh in a DD to gain a little.

I thought I had enough space too but ended up adding an
extra row of holes to fit all my wire. I think I'm going to try
a basket with the 176/46 Litz maybe 12" using a plastic
tray I found. Can you post some dimensions ? I'm wondering
how wide is the material? Maybe litz will allow more wire to
fit in the slots?

[KRinAZ]

OK!, Behold!, my very first coil wind is a basket weave and it works! No errors on the lacing (thanks completely to all the pointers I got from you all), but I did make an error on the form that wasn't apparent until the end of the lacing - I somehow cut the inner wire slots more shallow than the intended .375" and there wasn't enough room for the 6th layer, so it's got 5 layers for 25 windings. I guess it gives me an opportunity to see how a low inductance coil behaves. I will be much more careful about measuring and squaring cuts when I build the next form. Pics of course:



So the wire turned out to be stranded instead of solid core - was a surprise to me & hope it works out better than solid core would have, it has a very thick insulation jacket, overall diameter is about 1.9mm.

It's specs:
180uH
2.1 ohms
My capacitance meter wasn't able to give a reading so don't know the static capacitance.

Out of curiosity I plugged the coil's parameters into Qiaozhi's Coil Calculator and they came out pretty close - it said should be 193uH - I'm guessing the difference is the basket weave instead of a standard tight wound coil - and the difference seems close to the difference between a standard and a D shaped coil (from what I read in the Coil threads).

Hooked it up and it does work. The flyback voltage is about 450 volts (my scope w/10x probe only goes to 400V so I'm guesstimating by how much I had to turn the position knob to find the peak of the pulse) - is that OK? The manual says it should be about 350 volts, that's a lot higher, could the 100V higher flyback be related to the low inductance?

Along those lines, what are the implications of lower inductance coils - my understanding is that the range for the MPP is 300uH to 500uH and a coil closer to 300uH makes for a faster coil, but why is 300uH the minimum?, what happens when the inductance is lower? I guess I need to learn more about how coil parameters interact with the electronics...

[KRinAZ]

Well I will test my basket coil tonight. It has AWG 24 teflon wire and a low resistance.
The biggest benefit (if it works) is self shielding. My New Litz came in today. I might build
a big foot DD with it though you have me thinking a basket might be nice.

I got 110' of 176 / 46 for $22 on ebay. This guy has it and ships fast!

http://www.ebay.com/itm/160567800290?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Last time I got 100' of 60/46 and was able to make 2 coils out of it. The resistance was on the higher side so I went
with 176 strands this time...

OK, good to know, thx Silver Dollar. Next time I get wire I'll look that eBay person up. I did find a bonus in my wire I didn't know about though - it is stranded instead of solid core - when I stripped the ends to solder I discovered this - so this wire may work OK also...we'll see...curious how you're testing went...

Question on the resistance - how many ohms is considered low, and considered high?

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[Silver Dollar]

Well I will test my basket coil tonight. It has AWG 24 teflon wire and a low resistance.
The biggest benefit (if it works) is self shielding. My New Litz came in today. I might build
a big foot DD with it though you have me thinking a basket might be nice.

I got 110' of 176 / 46 for $22 on ebay. This guy has it and ships fast!

http://www.ebay.com/itm/160567800290?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Last time I got 100' of 60/46 and was able to make 2 coils out of it. The resistance was on the higher side so I went
with 176 strands this time...

[KRinAZ]

I used some Lits on a quick and dirty coil here; http://www.geotech1.com/forums/showt...over-Litz-Coil

Preliminary testsing shows a faster response than other wires maybe it's just the case that it cannot see itself?
Thx Silver Dollar, I would definitely prefer to build with Litz as you recommend - but alas I have no Litz :^< and do have about 250' of this teflon wire already :^> so I figure it should hopefully be OK for my 1st coil build experiment. Will give me something to compare against - as I make better coil forms and use Litz wire I can measure and post the actual differences. For now I just want to get a working coil going so I can proceed with the MPP build...

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