Getting ready to build the MPP

[6666]

7 "basketweave" turns means 7 groups of 5 windings

Got it, thanks.

[Achillion]

Good work KR. Sincerely hope you will pick up a nugget. Would be the sugar on the cake. Good luck mate!

[Davor]

Hmmm, ok, so you're saying the flyback voltage passes back thru the diode in the direction the diode normally blocks current flow & voltage & makes it to the mosfet? I'm not understanding that, seems like the diode then isn't doing what diodes do, or am I just completely not getting it? - thx

So a MOSFET conducts, and a diode conducts together with it. Then the MOSFET stops conducting, and a coil tries hard pushing the current through it, resulting in a flyback, while a diode continues conducting shortly until the Mosfet capacitance is brim full with charge at high voltage (that's why we use fast high voltage diodes there), and only after a coil voltage drops below its peak voltage the diode goes to reverse polarisation, keeping the Mosfet charge from discharging through coil.

[green]

[All timings are read from where the Tx pulse at TP1 just transitions to it's downward slew at the end of the pulse]

Hi KR, I've been using the mosfet gate turnoff as time zero. I think start of flyback is normally used for zero time. Again I'm liking you experimenting. Trying to compare your results with my on the bench testing and the PI simulator. Timing is important. I'm not understanding why increasing pulse rate makes it better unless it's a smother signal, less noise. Just what Davor said in reply 258 plus mosfet avalanche current if enough coil energy.

Thanks green

[KRinAZ]

So a MOSFET conducts, and a diode conducts together with it. Then the MOSFET stops conducting, and a coil tries hard pushing the current through it, resulting in a flyback, while a diode continues conducting shortly until the Mosfet capacitance is brim full with charge at high voltage (that's why we use fast high voltage diodes there), and only after a coil voltage drops below its peak voltage the diode goes to reverse polarisation, keeping the Mosfet charge from discharging through coil.

Ok, many thx Davor for that explanation, sounds like the diode does not "turn off" fast enough to block the flyback from getting briefly to the mosfet. This is one of those things I think I will grasp better when I see it happening on my o'scope. Next time I get a chance I will take a look at the signals at the mosfet. Again thx, I learned something new here...

[KRinAZ]

[All timings are read from where the Tx pulse at TP1 just transitions to it's downward slew at the end of the pulse]Hi KR, I've been using the mosfet gate turnoff as time zero. I think start of flyback is normally used for zero time. Again I'm liking you experimenting. Trying to compare your results with my on the bench testing and the PI simulator. Timing is important. I'm not understanding why increasing pulse rate makes it better unless it's a smother signal, less noise. Just what Davor said in reply 258 plus mosfet avalanche current if enough coil energy.Thanks green

Hi green, thx! - I'm having fun with the experimenting. As far as the depth increase with higher pulse rate - I've actually made a lot of mods to my MPP & it may be that those mods are needed to get a benefit from the increase to the TX 5K PPS. One important change was shortening the TX pulse width to just where the coil charge voltage flattens out (30uS in my case), also shortening the sample widths & the EFE delay. I have also added a Sensitivity & a SAT Speed control, & changed how the (Audio)Threshold pot/resistor network are balanced so that the full turn of the (Audio)Threshold control is useful. In it's original form only about 1/5 of the pot's sweep was useful & it was hard to fine adjust the Threshold. These IMO are needed on the MPP just for general use - and as a bonus - with these controls & mods I have no problem using my MPP on highly iron mineralized ground - with no GEB. GE is there, but doesn't interfere with target responses & if I need to - by decreasing Sensitivity, decreasing (Audio)Threshold, & increasing SAT speed (actually increasing SAT speed A Lot) I can tune out the GE, with a little loss in depth/sensitivity but not bad - it's as George & Carl say - there's no free lunch - haha.

[KRinAZ]

Good work KR. Sincerely hope you will pick up a nugget. Would be the sugar on the cake. Good luck mate!

Thx Achillion, sure do hope I nab a nugget! If I do you all will sure know about it haha.

[6666]

I can tune out the GE

Hi KR
more interesting results, do you have access to a whites V3I, would be interested in a ground probe reading on your highly iron mineralized ground
and what happens when you throw a Neodymium magnet on the ground does the surface material clump around the magnet ?

[KRinAZ]

Hi KR
more interesting results, do you have access to a whites V3I, would be interested in a ground probe reading on your highly iron mineralized ground
and what happens when you throw a Neodymium magnet on the ground does the surface material clump around the magnet ?

No access to a V3I but a GMT shows 82 (mild for this area) to 99 & Overload message at times & a pic says a thousand words - my plunger neodymium magnet is covered with black sand & magnetite after going over about 8 inches (20cm) while held about 1/2 inch (1.25cm) above the ground. This 1st pic is the tool clean, the second with the black fuzz...

[KRinAZ]

OK, this new coil wind is proving sensitive, it appears to be able to rival the Minelab Commander 8" mono coil (their small gold coil) & exceed the current design of the MPP. I made some timing changes & did air tests today on very low TC targets.But first, hooked up my o'scope & read all the timings again to record where it is right now, those in a minute. All timings are read from where the Tx pulse at TP1 just transitions to it's downward slew at the end of the pulse.Also, I should now mention I'm using a 10 pak of AA NiMH's to have a power supply similar to a SLA battery - the pack is full charged at 14 volts and full discharged at 10 volts, 2400mah batts. This compared to a AA Alkaline 10 pak where it's fresh (full) charge is 15 volts & full discharge at 10 volts. An AGM SLA full charge is 13.6 volts and full discharge 10.6 volts. This matters, as I found that as I vary the power supply from NiMH full charge to full discharge I lose about an inch in air test detection distance. I also found (as I expected) that the coil voltage spike drops as the power voltage drops - it acts like (actually is I believe) a step up transformer. The batt pack at the beginning of air tests was at 13.44 volts.The timings I read today before doing anything else:Tx pulse frequency 2500ppsTx pulse width 30uS (the minimum time to fully saturate this particular coil's magnetic field)Coil flyback stabilization where it just flattens after the spike recovery knee 2.5uS (wow!)Primary sample delay 6uS (if I go to 5.75uS the signal flatlines so this as we know is MP14538 chip's limitation - better than spec anyway, I would love to start sampling at the 2.5uS the coil is capable of, I'll be working on that) & range via the delay control if I remember correctly (forgot to write it down) 6uS to 35uSSample width's 22uS (longer than I thought, I could probably replace the resistor with a pot and adjust this down to around 12uS to 15uS, I need to understand better the implication of changing the sample widths, maybe we'll discuss this...)EFE (secondary) sample delay 148uSThen I did air tests on detection depth on very small to moderate sized pieces of lead (.05 gram to 10.52 gram, a copper Canadian penny, and a pulltab with tail intact and straight, more below...Then I changed timings, anything not mentioned below stayed unchanged as above:Tx pulse frequency 5000ppsEFE delay 140 (basically adjusted it so that the end of the EFE sample is complete about 20uS before the next Tx pulse, this much time may not actually be needed...)I got a very consistent extra inch / 2.54cm of detection depth increase across the board on targets I could detect.At 5000pps I got a good target signal on a .33 gram lead piece at 3 inches / 7.62cm, and all targets smaller got absolutely no signal, so I think the response cliff the MPP falls off is related to TC, not receive gain, especially since the smallest target depth is 3 inches (& not less) and if I could sample earlier I believe I could detect smaller targets.The lead piece target responses:.05 gram no response.12 gram no response.19 gram no response.33 gram 3" / 7.62 cm.45 gram 3.5" / 8.89cm1.99 gram 7" / 17.78cm2.50 gram 7" / 17.78cm4.44 gram 8.5" / 21.59cm10.52 gram 9" / 22.86cmCanadian 99% copper penny 8.5" / 21.59cmThe ubiquitous pull tab with straight tail 12.5" / 31.75cmI'm pleased that the MPP can detect a .33 gram nugget at 3", and that a larger nugget's (4.44 gram & up) detection depth surpasses the coil width.Then, just to test it's cache hunting ability (for such a small coil) (think Forest Fenn's Treasure!) - my Jeep as a test target - 5 feet / 152.4cm - wow, didn't expect that kind of distance for any target...I think the coil is performing well & I will now concentrate on the electronics. I am a big fan of keeping the target signal path analog, seems to me to have more of a "direct connect with and feel for" Mother Earth, but I think better control of the timings - especially via a fast PIC (it's been a while now but I can probably still program in C or Assembly pretty well) - and the capability of additional samples at various points in the complete cycle - would greatly benefit the MPP and make it a challenger of some commercial products. my2c fwiwBut for now I am going out with this wonderful MPP to search for gold for a bit!

Oops, I posted a wrong value - the statement "Coil flyback stabilization where it just flattens after the spike recovery knee 2.5uS (wow!)" - it's actually just past 5uS, not 2.5uS - my sincere apologies! I have my minimum primary delay set to 5.5 currently.

[baum7154]

Hi KRinAZ,

Can you post a screen shot of the coil fly back? I'm confused about the start point of the coil decay measurement. In my Chance PI Coil measurement I used the end of the coil charge pulse as the Start of the Coil decay measurement and got 6us on an 8" 335uh coil. Is that what you are doing? There is a significant amount of time, perhaps 3us, where the coil dwells at the max fly back voltage before falling back to it's low state. I posted a screen shot of this on page 5 of the Chance PI Coil thread a few years ago.

Thanks,

Dan

[Qiaozhi]

I'm confused about the start point of the coil decay measurement. In my Chance PI Coil measurement I used the end of the coil charge pulse as the Start of the Coil decay measurement and got 6us on an 8" 335uh coil.

The accepted definition for the sample delay is from the time the MOSFET is turned off to the start of the sample pulse (as you've described).

For reference:

1. Inside the METAL DETECTOR - Edition 2 : page 153, Fig.11-17 - "Switched Coil Waveforms"
2. Inside the METAL DETECTOR - Edition 1 : page 150, Fig,11-12 - "Sampling Waveforms"
3. Hammerhead article - HHv15.pdf : page 4, Fig.6 - "Timing"
4. MiniPulse Plus (MPP) Build Document - REV-D : page 15, Fig.10 = "Sample Timing"

[KRinAZ]

The accepted definition for the sample delay is from the time the MOSFET is turned off to the start of the sample pulse (as you've described).

For reference:

1. Inside the METAL DETECTOR - Edition 2 : page 153, Fig.11-17 - "Switched Coil Waveforms"
2. Inside the METAL DETECTOR - Edition 1 : page 150, Fig,11-12 - "Sampling Waveforms"
3. Hammerhead article - HHv15.pdf : page 4, Fig.6 - "Timing"
4. MiniPulse Plus (MPP) Build Document - REV-D : page 15, Fig.10 = "Sample Timing"

Excellent, many thanks Qiaozhi for the clarification and references

[KRinAZ]

Hi KRinAZ,

Can you post a screen shot of the coil fly back? I'm confused about the start point of the coil decay measurement. In my Chance PI Coil measurement I used the end of the coil charge pulse as the Start of the Coil decay measurement and got 6us on an 8" 335uh coil. Is that what you are doing? There is a significant amount of time, perhaps 3us, where the coil dwells at the max fly back voltage before falling back to it's low state. I posted a screen shot of this on page 5 of the Chance PI Coil thread a few years ago.

Thanks,

Dan

Dan, thanks for asking these questions. A couple things came to light - one is that it is apparently not a good idea for me to observe signals on the 'scope and then later document my findings, my memory apparently isn't good enough to do that. For the future I will document as I am observing and take pics where it would benefit.

So the short answer is yes I will post pics (more on that below) and that I was again wrong, at TP3 I have the flyback signal recovered at about 7uS. I am sampling at 5.5uS though, just past where the signal comes out of clamping.

I'm not seeing the signal dwell before falling, there is a sharp peak and immediate drop in signal, hmmm.

In viewing the signals today with the 'scope I finally realized what I had suspected and what was causing my mix of signal recovery numbers. I was looking at a lot of signals at various points of the preamps...got them confused in memory I guess...and...

I found that there is a significant and progressive amount of delay in the preamp stages! More than the recovery time of the coil!

I think this merits a new thread so I will now make a thread to discuss this. 'Scope pics will be attached. I think this new thread will more fully answer your questions. I'll call it "MPP Coil And Preamp Speed".

Regards,
Kyle

[baum7154]

Thanks Kyle,

In the picture I posted a while back on the Chance PI Build the measurement point was at the amp input just after the back to back input diodes. Thinking back on that I'm pretty sure that the capacitance of my scope probe affected the measurement negatively. When I did the 2 stage amp design/mod on the Chance PI I was able to shave 9us from the decay signal fed to the A/D by keeping the amps out of saturation. That 2 stage mod changed the performance for the better on small gold. The MPP having a 2 stage amp should make it better for small gold than Barracuda or Surf PI. I did find that stage gains in excess of 35 did begin to add a bit of delay to the decay curve.

I would ask that you add another target to your MPP testing. A simple square piece of the side of an aluminum can cut 1/4" X 1/4" and presented flat to your coil is a pretty small target. I am interested in what detection distances you might get with the criteria being 3 consecutive passes and 3 consecutive detections for a given distance. In my testing I often get a detection at about 3" or more but on subsequent passes at that distance there may be no or sporadic detection. In order to get consistency I move in closer to the coil till I get three detections in three passes and assume that to be repeatable in the future. Anyway I appreciate you including this very small target in your future testing.

Best regards,

Dan