Getting ready to build the MPP

[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]

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

[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.

[green]

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

[green]

Second try

[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

[KRinAZ]

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

& for 4) the form is 1.125 inches wide

[baum7154]

Thanks for the replies KR, good job!

[Davor]

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.

Nope, a diode kicks in only after the MOSFET recieves a full blow. It is after flyback that a diode gets reverse polarised and shields the coil from a MOSFET capacitance.

[KRinAZ]

Thanks for the replies KR, good job!

Thx Dan!

[KRinAZ]

Nope, a diode kicks in only after the MOSFET recieves a full blow. It is after flyback that a diode gets reverse polarised and shields the coil from a MOSFET capacitance.

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

[KRinAZ]

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

Hmmm, excellent info, many thx bbsailor!

[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 2500pps

Tx 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 35uS

Sample 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 148uS

Then 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 5000pps

EFE 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" / 8.89cm
1.99 gram 7" / 17.78cm
2.50 gram 7" / 17.78cm
4.44 gram 8.5" / 21.59cm
10.52 gram 9" / 22.86cm
Canadian 99% copper penny 8.5" / 21.59cm
The ubiquitous pull tab with straight tail 12.5" / 31.75cm

I'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 fwiw

But for now I am going out with this wonderful MPP to search for gold for a bit!

[6666]

Very interesting KR
when you say 7 basketweave turns
do you mean 7 turns then move forward one slot rather than wind the normal 5 turns then move forward one slot ?

[KRinAZ]

Very interesting KRwhen you say 7 basketweave turnsdo you mean 7 turns then move forward one slot rather than wind the normal 5 turns then move forward one slot ?

Thx 6666, & no I mean the normal 5 turns then move forward one slot - like in the pic you supplied so that 7 "basketweave" turns means 7 groups of 5 windings, for a total of 35 laps around the form. By the way, in a thread in the Coils forum it was decided to call this coil construction a spiderweave & refer to it as a 3D Self Shielding (I write 3DSS for short), so going forward that's how I'll refer to it - just a fyi