Getting ready to build the MPP

[KRinAZ]

Hi KRinAZ, great work, thank you
Hey Chet, welcome!

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[Chet]

Hi KRinAZ, Attached is a file from green which is an example of your problem. I forgot that I had added this diode to my circuit and it works just like green shows. Here are the reference posts for this;

From Davor; post #396 of Replies-to-Minipulse-Plus
“@Qiaozhi, I may suggest a small improvement for a future release of the board. I placed a MUR460 diode in series with R3 resistor (my choice, not a suggestion), but it turned a bit of a struggle, as the diode's pins are very thick. I had to grind one pin so that it may enter a hole. Having larger holes there could be beneficial, as R3 has thick-ish pins as well.”


From green; post #21 of of N-Channel-vs-P-Channel-MOSFET
“Some real scope pictures of coil turn off. With no diode the circuit resonance drops from 1.64 Mhz to 395 Khz. Would need a lower value damping resistor, longer delay before first sample. Depending on the targets maybe not a problem.”

The circuit green used is somewhat different but shows the polarity of the diode. Most any high speed, high voltage diode can be used. Also forgot to ask if you had shielded your coil. Shielding of course will add more capacitance.

[KRinAZ]

Awesome, many thanks Chet - this seems like just what I'm experiencing. That diode apparently makes an enormous difference in the area we all want to achieve - up-ing the circuit resonance freq - seems like this mod should go into the next rev of the MPP - don't you think?

OK, the only electronic components I have are from gear that I have salvaged and robbed components from. I'll have to look and see what I can find in the way of a high speed high voltage diode. This can be a challenge as diodes can be hard to read the part number on - if they have one...

I also noticed green referenced holding the probe close to the diode to see the signal - and I had the probe connected to the circuit (you asked about that) so I will measure again by just holding the probe near the Tx area.

...& no shielding on the coil or wire leads & really don't want to shield - that's why I'm so keen on these basket weave coils - they are supposed to self-shield...we'll see...

[KRinAZ]

OK I'm stuck at step 5, dang it!.

Here's what I'm seeing - with the multimeter:
The build doc says there should be a:
+1VDC offset at TP3, I have -.36VDC
Small (quarter) target close to coil and level should drop maybe 10mV, I have a rise in voltage to around -.42VDC
Large target close to coil and level should drop to around 150mV, I have a rise in voltage to around -.6mVDC

So TP3's offset is negative instead of positive and behaves opposite of how it's supposed to.

On U5 (NE5532) pin1 (opamp A output) signal is identical to Pin7 (opamp B output), I'm thinking there should be signal gain or some change by the second opamp?

I have re-verified all steps up to this point - power supplies good, TX circuit working.
I re-verified that all components in step 5 are the correct ones (and measured the resistors with my meter to make sure) and in the correct place, no solder bridges, I can't see what's wrong.

What I see with the scope is the waveform of the low voltage version of the tx pulse and the flyback - & I can see the change in the flyback due to a target.

If anyone has some checks or ideas I would sure love to hear them!

By the way, I got rid on the noise at TP3 - it was all the lights & stuff around my work area.

[KRinAZ]

Hey Chet on your TP3 scope shot - it looks like TP3 is on CH2, set to 5V/Div & 10uS/Div - I set my scope the same and my waveform looks pretty much like yours - my scope shot:

So I'm wondering if my MPP is actually working correctly & I should proceed, even though this differs from the build doc, or ?

I'm going to stop here and see if anyone has feedback on this...

[Silver Dollar]

Look here for a lot of scope shots of my receintly completed Hammer Head; http://www.geotech1.com/forums/showt...s-best-Damping

[KRinAZ]

Aside from my Step5 prob's, something did go well this eve - behold! - I couldn't resist and made a basket weave 8x10 coil from the template previously posted.

Specs:
32 turns (6 layers plus 2 more turns) 26AWG stranded Teflon coated 600V wire
324uH
3.0 ohms
Capacitance unknown as I won't calc it until I have put a fast diode in series with the coil. Resonant freq without any diode was 454.5K, I experimented with (Chet suggested trying a high speed high voltage diode, which I will need to get) a 100V 10uS diode (too slow and too low reverse voltage but the magic smoke stayed in the diode) and the resonant freq went up to 833K, I'm guessing it will go up higher when a fast diode like the suggested MUR460 is used. Also it is still damped by the 470ohm R1, I need to find the correct damping resistor value once the diode is in place.

Pics of the build and coil:



I drilled holes at each end of the wire cutout this time to make sure the slots are deep enough, then cut them with jigsaw (this is actually the method baum7154 suggested), then cleaned up the cuts with the 1/8" bit on the Dremel - this worked much better and faster for me.


I am hoping this coil works out well, can't wait find out it's capacitance, and try it out when the MPP is ready for field tests!

[Davor]

Neat.
My coil is not nearly as neat, and I'm not able to hook it up at this point, but I figured out a small detail that may save people a few long rows of cursing: I marked an outer lead with a water resistant marker. That particular wire is also used as a shield in this particular configuration, so it is kinda good to know which is which

[Qiaozhi]

If anyone has some checks or ideas I would sure love to hear them!

I seem remember commenting on this point some time ago. Just go ahead with the build. The voltage measurements I wrote in the Build Document for Step 5 were based on a single build, and I did [subsequently] find that these numberes varied between different units. The main point is to confirm that the voltage level changes when metal is placed close to the coil.

[Chet]

Hi KRinAZ, when I built mine I didn't even think to check the DC voltages on TP3. The scope is the way to go on most checks. Looks like you are well on the way to completing yours. The coils are very sensitive to any metal in the work area. Two solutions for this. Suspend the coil by cord from the ceiling away from the bench, or place it on a plastic wastebasket on the floor away from the bench.

I measured a 4' length of twisted # 22 Teflon wires with thinner insulation than yours. It measured 39pf on the capacitance meter.

Great job on the new coil. Looks like for others using your template that it should be a little greater in width and the slots should be a little deeper. This would help on the inside turns to keep them anchored better. I had a problem with this and it looked like baum7154 had a similar problem with his coil.

[Chet]

Hi KRinAZ, do you have a plan for a coil housing for your new oval coil? I haven't worked with Lexan. Do you think it would work for a housing?

[KRinAZ]

Look here for a lot of scope shots of my receintly completed Hammer Head; http://www.geotech1.com/forums/showt...s-best-Damping
Excellent, many thanks Silver Dollar!

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[KRinAZ]

Neat.
My coil is not nearly as neat, and I'm not able to hook it up at this point, but I figured out a small detail that may save people a few long rows of cursing: I marked an outer lead with a water resistant marker. That particular wire is also used as a shield in this particular configuration, so it is kinda good to know which is which
Thx! & yes I completely agree and I mark the outer wire also (forgot to mention that) and use it as the 0V connection - like you and others I've also found it's important to connect it that way - for the self shielding to work

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[KRinAZ]

I seem remember commenting on this point some time ago. Just go ahead with the build. The voltage measurements I wrote in the Build Document for Step 5 were based on a single build, and I did [subsequently] find that these numberes varied between different units. The main point is to confirm that the voltage level changes when metal is placed close to the coil.
Oh good, many thanks Qiaozhi, I will proceed with the build then.

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[KRinAZ]

Hi KRinAZ, when I built mine I didn't even think to check the DC voltages on TP3. The scope is the way to go on most checks. Looks like you are well on the way to completing yours. The coils are very sensitive to any metal in the work area. Two solutions for this. Suspend the coil by cord from the ceiling away from the bench, or place it on a plastic wastebasket on the floor away from the bench. Hi Chet, thx for that tip.

I measured a 4' length of twisted # 22 Teflon wires with thinner insulation than yours. It measured 39pf on the capacitance meter. Good to know - think there's any coax that would be lower in capacitance?

Great job on the new coil. Looks like for others using your template that it should be a little greater in width and the slots should be a little deeper. Yes, on the two coils I've built I've found the inner slots get crowded faster than the outer slots - cut the same width - the outers can take at least 2 more turns than the inners. I think the ring width may be OK but the inner slots could be cut .5" instead of.4". This would also move the coil out slightly. The Lexan is definitely strong enough to handle the deeper cuts. One reason my builds are getting crowded on the inner slots is because I'm using up this super fat wire that I won't get more of. Something like Silver Dollars .9mm Litz (my wire is approaching twice that diameter) would fit no problem, I believe.

This would help on the inside turns to keep them anchored better. I had a problem with this and it looked like baum7154 had a similar problem with his coil. Yes, I agree, I had this issue also, from my builds I would say if someone is using a wire with OD of 1mm or greater it would benefit to cut the inner slots to .5", or if more turns are desired and/or with 1mm or greater wire maybe cut both slots deeper, keeping the inner slot depth at least 2 wire OD widths deeper than the outer.

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