Getting ready to build the MPP

[KRinAZ]

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

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[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]

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, 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]

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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[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

[6666]

Tnks for pdf's

[KRinAZ]

Welcome!

[KRinAZ]

With the 180uH 8" basket weave coil - I calc'd the ring period and got the capacitance via LC resonant circuit calculator - here's what I got:

2.15uS ring peak to peak = 465.1KHz on 180uH coil = 650.1pF capacitance - not very good!

It would appear the wire I'm using is no good, or the coil build is no good, or both LOL

I will proceed with the MPP build and use this coil for now...& make a new coil when I get some different wire...

If anyone uses the templates in post #51 - I would like to find out what wire you used and how the coil came out

[Qiaozhi]

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?

Damping resistor adjustment tool:
10k pot + 200R series resistor, both in parallel with 1k2 resistor.
Adjustment range is between 171R and 1k1.

Critical damping occurs when the receive signal (measured at the preamp output) decays smoothly to zero volts without any ringing or overshoot.

[KRinAZ]

Excellent, thx much Qiaozhi! And this is with R1 470R removed I'm thinking? and preamp output being TP3?

[KRinAZ]

I wonder if someone could confirm this is what TP3 should look like after step 5 of the MPP build completed - or if this is not good if I could get a scope photo of your TP3 - thx!

I'm running the 180uH basket weave coil, just using the 470 damping resistor for now, no ringing in the flyback

The manual says should be about 1VDC offset with no target near coil, I'm seeing about (-.6VDC), with lots of noise making it hard to see the actual offset level, and the Rx pulses also.

For these photos my scope is set to 2V/Div with 0V at the center line & DC coupled, .2ms/Div time base:

No intended target near, but this is in a work area with metal objects everywhere, and lots of nearby electrical noise (I don't have a "cleaner" work area at the moment)

Heavy 4" looped rusted iron wire right against coil

[Chet]

Hi KRinAZ, Attached is a shot of TP3 with the flyback pulse on top. The frequency of your coil should be much higher. The capacitance is to high for it to be the coil only. The cable between the circuit board and the coil may be adding to much capacitance. Also your scope test lead may be paralleling it with capacitance. Try the frequency test again by just laying the probe near the coil circuit so there is no direct connection except the ground clip attached to the Minipulse ground. The 400v signal will couple to your probe through the air just fine.

The high frequency noise at TP3 looks like florescent light, TV or some other similar source. Try turning stuff off to see if you can locate the source.
Are you using a bench power supply? If so try batteries instead.

[Chet]

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

[ATTACH]31551[/ATTACH]
8" x 1.1" Ring

[ATTACH]31552[/ATTACH]
8" x 10" x 1.1" Ellipse

Hi KRinAZ, great work, thank you

[KRinAZ]

Hi KRinAZ, Attached is a shot of TP3 with the flyback pulse on top. The frequency of your coil should be much higher. That's whaThe capacitance is to high for it to be the coil only. The cable between the circuit board and the coil may be adding to much capacitance. Also your scope test lead may be paralleling it with capacitance. Try the frequency test again by just laying the probe near the coil circuit so there is no direct connection except the ground clip attached to the Minipulse ground. The 400v signal will couple to your probe through the air just fine.

The high frequency noise at TP3 looks like florescent light, TV or some other similar source. Try turning stuff off to see if you can locate the source.
Are you using a bench power supply? If so try batteries instead.
Thx Chet! I'm using a battery pack to power it, but the room is lit by fluorescent's, the soldering station is near, as is my CRT scope (which I would hope contains it's noise but maybe not), I'll try to isolate as much noise as possible. Thx for the scope shots. I made the coil with 4' of the wire as the leads back to the detector to eliminate using coax or a cable, and put the 3 turns per inch in the wires so I would hope the cable is as low capacitance as it can be. I'll do some more testing this eve and see if I can get a clean signal...

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