Getting ready to build the MPP

[bbsailor]

green produced a table in #237. He takes into account the effect of input resistor - this is the big thing I failed to consider when I happily plugged figures into Miscel !
With the 1k input resistor in parallel with the damping resistor (bringing the value to 711 ohm), the figure at switch off does not look so alarming.

Hi KRinAZ, the short rapid turn off that your coil is capable of, is key for earlier sampling. We could also ask ourselves how much energy do we need to excite and ping desired targets? Especially smaller ones with a shorter TC.

bbsailor has some good posts on the cs6 pi. That model had loads of resistance in the tx circuit, and lower coil energy as a consequence. Yet its capable of eliciting a response from nickles and rings at a fair old depth. I guess it boils down to what we are looking for. At the beach, maybe its a competition for depth ? A low power cs6pi could be beat by many more powerful detectors. But searching for small targets at shallower depth ? Maybe this power thing is only important for people concerned with conserving energy/battery.

Eric Foster's CS6-PI design also changes the TX pulse width along with changing the PPS frequency and the delay. This design keeps the power in the flyback energy approximately the same over a wide range of frequency adjustments. Eric's high frequency PI machines (3000 PPS) only adjust the frequency range about 10 percent to stay away from other PI machines operating on the same frequency or be near a harmonic of the power line frequency. This small range of TX PPS adjustment keeps the energy in the damping resistor near the same level for optimum delay sampling.

The practical aspect of this observation is to keep the energy level in the damping resistor the same over the full range of pulse width adjustments or have a way to fine tune the damping resistor for an optimum value for the earliest possible sampling. The coil discharge time constant really matters when stimulating a target. This is calulated by dividing the coil inductance by the damping resistor value. Optimum transfer of energy to a target is to turn off the TX pulse 5 times faster than the TC of the target sought. This way you can switch into the RX mode as fast as possible and still have some time to detect the quickly decaying eddy currents in that small target. Balanced DD coils typically sample a little faster than mono coils as there is less voltage on the RX side that needs to wait to die down. On DD coil circuits the first amplifier stage can come out of saturation a little faster than a mono circuit and the input resistor is no longer in parallel with the damping resistor down to 0.6 volts. Because there is less energy in the RX coil in DD coils the damping resistor on the RX side is typically a higher value than the TX damping value. If the TX and RX damping values are near the same value they probably put more turns on the RX coil to make it more sensitive and thus use a little lower RX damping resistor value to damp the extra RX turns.

This all gets down to making trade-offs between target size (time constant), search coil swing speed, battery power consumed (hunting time), TX coil current, coil type, coil size and ground conditions. Most good beach hunting coils are not good for seeking small gold nuggets. Select the right tools for the primary targets sought and ground conditions.

Joseph J. Rogowski

[green]

Second try

[green]

Think I made an error when calculating avalanche energy. Trying again. Maybe someone else could try to compare results.

[greylourie]

Input R parallel to Damping R.

green produced a table in #237. He takes into account the effect of input resistor - this is the big thing I failed to consider when I happily plugged figures into Miscel !
With the 1k input resistor in parallel with the damping resistor (bringing the value to 711 ohm), the figure at switch off does not look so alarming.

Hi KRinAZ, the short rapid turn off that your coil is capable of, is key for earlier sampling. We could also ask ourselves how much energy do we need to excite and ping desired targets? Especially smaller ones with a shorter TC.

bbsailor has some good posts on the cs6 pi. That model had loads of resistance in the tx circuit, and lower coil energy as a consequence. Yet its capable of eliciting a response from nickles and rings at a fair old depth. I guess it boils down to what we are looking for. At the beach, maybe its a competition for depth ? A low power cs6pi could be beat by many more powerful detectors. But searching for small targets at shallower depth ? Maybe this power thing is only important for people concerned with conserving energy/battery.

[KRinAZ]

``````````````````````````````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

Hi Dan; 1) it's rated at 600 volts, 2 (I didn't calc the freq so I don't know, I just used the damping tool per Qiaozhi's instructions & my o'scope to find critical damping, then measured the tool's resistance, placed a resistor network of the same value in the circuit, & verified damping looked the same as with the tool, 3) yes, a total of 35 laps around the form, 4) I will look at my notes & let you know hopefully in a day or two

[KRinAZ]

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[ATTACH]33314[/ATTACH][ATTACH]33315[/ATTACH]

Good question greylourie - my understanding is the flyback spike voltage level is proportional to the efficiency of the coil and to the speed of the collapse of the magnetic field - the higher the current flow the faster the collapse & higher the voltage spike. In my case I believe the wire resistance (3 ohms) is limiting the current & therefore the collapse speed.

[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.

[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.

[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?

[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?

[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]

``````````````````````````````

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

[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

[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

[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?

[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...