Getting ready to build the MPP

[KRinAZ]

Ok, I spent some quality time with my MPP today & tweaked & experimented with the TX pulse width & rate. I've been wanting to experiment with these so I finally replaced the resistors with pots (more below). Since I have wound yet another experimental 8" round BW coil, I also wanted to do some measuring. This coil is 7 BW turns, 301uH, critical damping at 1620 ohms, passive resistance is 3.0 ohms (I want to get the ohms lower, using 26AWG now, will have to get larger wire). I got some great results by changing both the TX pulse rate & width. But first I had to fix some instability issues with fast SAT & high sensitivity so I determined by swapping & rechecking that the minimum series resistor for the sensitivity pot is 1K, & the minimum series resistor for the SAT pot is 37k. Well now my MPP threshold is nice & stable even with delay at minimum, fastest SAT, & highest sensitivity. Now to tweak the TX pulse. I swapped R7 for a 4K7 trimmer pot (it adjusts the TX pulse width) & R8 with a 22K trimmer pot (adjusts the TX pulse rate, a 30K would be better but this is what I had on hand), with the 22K pot I can't go down to the default 1K TX pulse rate, but can go much higher - which is what I wanted to do. I left the sample pulse widths at 15uS & the main & secondary delays unchanged. I tried many combinations of TX pulse widths & rates & expected to increase sensitivity & depth with longer pulses - but - I got much better sensitivity & depth [on small low conductive targets] when I ran the combination of 2500 TX pulses per second, & 40uS TX pulse width! That's a 33% drop in TX pulse width/power & I got no benefit when increasing the TX pulse width all the way up to 85uS. This was quite unexpected & there's more - I verified the EFE is still nulled out so that's good, I found there's no target response at all to a large ferrite core (very nice), so I experimented with how it handles AZ's high iron mineralized ground - & with this tweak it is MUCH smoother on bad ground - so much better I think I could use it without ground balance on a lot of the ground here! Wow! I now need to get this thing on a pole setup & get out in the goldfields with it. When I get a chance I'll do some reasonably scientific sensitivity & depth comparisons between the two settings. I wouldn't be surprised if this is part of the timings Minelab runs for their "Fine Gold" setting...

By the way, by "[on small low conductive targets]" what I mean is very small gold and lead bits (since lead and gold behave so similar to a detector) - this added for the sake of clarity...

[6666]

He also has one or two posts that might be of help to anyone looking at building DD coils for coping with more difficult ground.

Hi GL where have you been looking at for that info ?
thanks

[greylourie]

Hi 6666,

I will see if I can pm you the link.( As its about DD coils. Don't want to sully KR's good thread.)

[KRinAZ]

OK, I've build yet another 8" round SSBW (self-shielding basketweave) coil and I'm going to stick with it as I'm finally happy with the coil - I finally got it as tight as I wanted - to improve the magnetic field's strength, quality, and consistency. It uses exactly the same length of wire as the last one so has the same resistance, just wound tighter, and gave me a wonderful 20% increase in inductance (& presumably magnetic field strength) - without sacrificing even 1uS in speed, very nice. Previous coil was 300uH.
Here are it's spec's:
7 basketweave turns
360 uH
3.0 ohms (due to the small gauge 26AWG wire)
2455 ohms critical damping
Flyback spike recovered at 4uS after end of TX pulse - by monitoring the coil, and also at TP3.
Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet, anyone have suggestions on a good higher voltage replacement? 800 volts would be nice...

Also since this coil has has relatively high resistance & about the same ohms as current limiting R3 (3.3 ohms) I removed & jumpered R3.

This coil should be very good for trying out the timings I'm going to experiment with. I am almost done putting the MPP on a pole setup, so more soon...

[Achillion]

You may consider the Infineon CoolMOS series Mosfets. Watch the Coss when selecting. Even with a diode in series you still want to keep the capacitance as low as possible to get the most out of your coil. Higher voltage seems to give output capacitance, which makes sense. For the diode I found the Vishay VS-5EWH06FNHM3 had good specs. Would you mind publishing a photo of your coil. Interested to see how you went about it

[KRinAZ]

You may consider the Infineon CoolMOS series Mosfets. Watch the Coss when selecting. Even with a diode in series you still want to keep the capacitance as low as possible to get the most out of your coil. Higher voltage seems to give output capacitance, which makes sense. For the diode I found the Vishay VS-5EWH06FNHM3 had good specs. Would you mind publishing a photo of your coil. Interested to see how you went about it

Hmmm, thx Achillion, I'll check the mosfet & diode out, & I added a coil pic earlier today at the bottom of the post about the coil build, let me know if you have trouble accessing it - look for coil.jpg

[KRinAZ]

MPP Rocks!

Ok I got the MPP on a home made pole - nothing fancy but highly functional (pics soon) & took it out to a spot with plenty of brass & ferrous trash & very bad iron mineralization & the MPP performed very well! I am still running the 30uS TX pulse width & 15uS sample widths - that I don't plan to change for now - & the 2500 PPS Tx freq & 100uS secondary sample delay - which I definitely will be changing. I find that with a little tuning of Sensitivity, Threshold, & SAT Speed: 1) on the less severely mineralized ground (by Arizona standards, which would be high minerals in most other locales) that ground balance isn't needed, barely perceptible ground effect. 2) on really badly mineralization the ground effect is obvious but ironically not a big issue. If I pump the search coil over really bad minerals (like I'm adjusting GEB on a VLF) there is a target sound, & on a VLF machine I would adjust the GEB. But, on the MPP the sound is clearly different from a target sound. The ground effect sound is quite wide and slow and lazy, and right within it even a somewhat faint target is crisp and audible. Right now I'm questioning whether ground balance is worth the added complexity, and the highly potential response hole. I still have quite a bit of experimenting to do with both the TX freq & secondaty sample delay, & depth tests as well. As it is right now my MPP isn't as sensitive to really small gold & lead as a Goldmaster is, but I do now get target response on small gold/lead that gave no target at all in the MPP original form (even with 10us sample delay) so that alone is a great improvement & it gives a target response where the Whites Classic with "Mr Bills Mods" still couldn't. But comparing the MPP directly to the Goldmaster isn't really appropriate because the Goldmaster (or any super sensitive VLF) drives me nuts in hot & cold rock littered fields & heavy black sand (where lots of Arizona gold is), but where the MPP was a dream to run today...every target sound today really was a target, not minerals... to be continued soon...

[green]

OK, I've build yet another 8" round SSBW (self-shielding basketweave) coil and I'm going to stick with it as I'm finally happy with the coil - I finally got it as tight as I wanted - to improve the magnetic field's strength, quality, and consistency. It uses exactly the same length of wire as the last one so has the same resistance, just wound tighter, and gave me a wonderful 20% increase in inductance (& presumably magnetic field strength) - without sacrificing even 1uS in speed, very nice. Previous coil was 300uH.
Here are it's spec's:
7 basketweave turns
360 uH
3.0 ohms (due to the small gauge 26AWG wire)
2455 ohms critical damping
Flyback spike recovered at 4uS after end of TX pulse - by monitoring the coil, and also at TP3.
Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet, anyone have suggestions on a good higher voltage replacement? 800 volts would be nice...

Also since this coil has has relatively high resistance & about the same ohms as current limiting R3 (3.3 ohms) I removed & jumpered R3.

This coil should be very good for trying out the timings I'm going to experiment with. I am almost done putting the MPP on a pole setup, so more soon...

[ATTACH]33301[/ATTACH]

I'm liking your experiment. [[Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet]] (W=LI^2/2) I calculate about 1 amp peak with 30usec on time. 370uH*1^2/2=.000185 joules. .000185 joules*2500 pulses/sec=.463 watts. Am I looking at it wrong or is .5 watts a problem?

[greylourie]

Charge and Discharge.

What limits the flyback to 500 volts. Is it the mosfet diode ? Or mosfet breakdown ? If I try to use Miscel to see the inductor charge at 30µseconds, and then plug the figures into see the inductor discharge curve... it shows the initial spike in the 1.7kV range. And if I use figures from a 60µsecond On time, the initial turn off spike is sitting on just over 3kV.

I used this post as my guide to using Miscel. http://www.geotech1.com/forums/showthread.php?17360-Coil-field-collapse-peak-current&p=122629#post122629

[Qiaozhi]

In general, it is not a good idea to let the MOSFET go into avalanche mode. This causes the junction temperature to increase, and can cause premature failure of the device. Some [rugged] MOSFETS have an avalanche rating in the datasheet, but you need to be careful of this rating, as its meaning is dependent on how the manufacturer performed the avalanche tests.

Here's a detailed explanation -> http://www.vishay.com/docs/90160/an1005.pdf

[baum7154]

OK, I've build yet another 8" round SSBW (self-shielding basketweave) coil and I'm going to stick with it as I'm finally happy with the coil - I finally got it as tight as I wanted - to improve the magnetic field's strength, quality, and consistency. It uses exactly the same length of wire as the last one so has the same resistance, just wound tighter, and gave me a wonderful 20% increase in inductance (& presumably magnetic field strength) - without sacrificing even 1uS in speed, very nice. Previous coil was 300uH.
Here are it's spec's:
7 basketweave turns
360 uH
3.0 ohms (due to the small gauge 26AWG wire)
2455 ohms critical damping
Flyback spike recovered at 4uS after end of TX pulse - by monitoring the coil, and also at TP3.
Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet, anyone have suggestions on a good higher voltage replacement? 800 volts would be nice...

Also since this coil has has relatively high resistance & about the same ohms as current limiting R3 (3.3 ohms) I removed & jumpered R3.

This coil should be very good for trying out the timings I'm going to experiment with. I am almost done putting the MPP on a pole setup, so more soon...

[ATTACH]33301[/ATTACH]

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Four questions on the coil.

What is the voltage rating of the PTFE wire insulation?

What is the Self Resonant Frequency of the coil?

Is 7 Basketweave turns the same as 35 turns around the coil form?

What is the width of the coil form?

Thanks,

Dan

[green]

In general, it is not a good idea to let the MOSFET go into avalanche mode. This causes the junction temperature to increase, and can cause premature failure of the device. Some [rugged] MOSFETS have an avalanche rating in the datasheet, but you need to be careful of this rating, as its meaning is dependent on how the manufacturer performed the avalanche tests.

Here's a detailed explanation -> http://www.vishay.com/docs/90160/an1005.pdf

Thanks for the explanation. I did a spreadsheet and included some IRF740 avalanche specs. Using KR's coil specs and 1 amp peak current the avalanche energy is less than 1% of the IRF740 repetitive value. I'm not suggesting operating the mosfet in avalanche. Just trying to determine at what energy level there might be a heat problem. With 1 amp peak current KR would need a snubber or a 800 volt mosfet to not avalanche. Would .110 mJ, cause the mosfet to over heat? Without a snubber, the mosfet avalanches at about 450 volts with all of my coils. Seems like any critically damped fast coil is going to exceed 450 volts on turn off with 1 amp or higher peak current. Does a snubber circuit have to dissipate the same amount of energy?

[green]

[ Does a snubber circuit have to dissipate the same amount of energy?]

After thinking about it I think the energy has to be dissipated somewhere. R input, R damping, mosfet avalanche or a snubber. Another question, The amplifiers I've been using and the NE5532 have a pair of back to back diodes across the inputs. The spec sheet says to limit the current thru the diodes to 10ma. The MPP has the + input to common so the diodes are in parallel with the two 1N4148 from the - input to common. With 450 volts across the input resistor, 450ma has to go somewhere. With a 800 volt mosfet 800ma has to go somewhere. Is that a problem or is the time so short that it doesn't matter?

[KRinAZ]

I'm liking your experiment. [[Flyback spike now at 500 volts due to the fast coil & I'm getting a bit worried I'll fry the mosfet]] (W=LI^2/2) I calculate about 1 amp peak with 30usec on time. 370uH*1^2/2=.000185 joules. .000185 joules*2500 pulses/sec=.463 watts. Am I looking at it wrong or is .5 watts a problem?

Haha, thx green, I believe it was I who was looking at it wrong - a late night post & I forgot to consider (if I'm correct on this) that the MUR460 diode was isolating the flyback spike from the mosfet. I think the mosfet will be fine as it's got life pretty easy.

[KRinAZ]

In general, it is not a good idea to let the MOSFET go into avalanche mode. This causes the junction temperature to increase, and can cause premature failure of the device. Some [rugged] MOSFETS have an avalanche rating in the datasheet, but you need to be careful of this rating, as its meaning is dependent on how the manufacturer performed the avalanche tests.Here's a detailed explanation -> http://www.vishay.com/docs/90160/an1005.pdf

Thx Qiaozhi, good information to keep in mind.