And so it begins...

[moorejl57]

The noise level looks ok. If you change the orientation of the coil you should see the noise level change, indicating that it's externally generated.

Great, thanks for the reassurance. I was also having a senior moment and realized I hadn't connected the shield to the coil yet and I am in a noisy environment. Other than dropping parts on the floor and then not able to find them, the build has been going smoothly.

[moorejl57]

On stage 6, I am getting a short EFE pulse width (63us) at TP8. Is this an issue or within normal variation? TP9 and TP10 seem in the ballpark at 7.2us each.

[Qiaozhi]

On stage 6, I am getting a short EFE pulse width (63us) at TP8. Is this an issue or within normal variation? TP9 and TP10 seem in the ballpark at 7.2us each.

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I assume you're building the REV-E version. In which case, TP9 and TP10 should be close to 10us (or maybe slightly higher due to the two diodes), and the EFE delay should be 100us.

[moorejl57]

I assume you're building the REV-E version. In which case, TP9 and TP10 should be close to 10us (or maybe slightly higher due to the two diodes), and the EFE delay should be 100us.

OK, thanks, I will check the schematic vs. parts loaded and see if I can find anything out of place. I tested every component with a LCR meter before loading, but I didn't test any of the diodes. I didn't find any components that were noticeably out of spec.

[moorejl57]

OK, found root cause on my TP8, 9, 10 timing issues. The parts I recently received in the MPP Beach kit for U7 and U10 are MM74HC4538N. Looking at the data sheet, the time pulse period IS NOT RxCx. Instead it is spec'd as 0.7RxCx. As such for the EFE delay of 100us, R32 needs to be more like 10K/0.7 ~= 14.3K. In the same vein, R35 needs to be 4.7K/0.7 ~= 6.7K. Hope this helps other folks building this kit.

[Qiaozhi]

OK, found root cause on my TP8, 9, 10 timing issues. The parts I recently received in the MPP Beach kit for U7 and U10 are MM74HC4538N. Looking at the data sheet, the time pulse period IS NOT RxCx. Instead it is spec'd as 0.7RxCx. As such for the EFE delay of 100us, R32 needs to be more like 10K/0.7 ~= 14.3K. In the same vein, R35 needs to be 4.7K/0.7 ~= 6.7K. Hope this helps other folks building this kit.

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Nice detective work.
The datasheet states that the MM74HC4538 is pin compatible with the CD4538. However that doesn't mean that it's compatible in other ways.
Both the CD4538 and the MM14538BCP have a pulse width output of T = R*C.

[moorejl57]

Nice detective work.
The datasheet states that the MM74HC4538 is pin compatible with the CD4538. However that doesn't mean that it's compatible in other ways.
Both the CD4538 and the MM14538BCP have a pulse width output of T = R*C.

Lol, true that. I put in the new resistor values (where I work has a "makers space" with some parts bins and I found what I needed in 1%) and got almost exactly 100us at TP8 and 9.8us at TP9 and TP10. I also changed the dampening resistor to 680 ohm for my 170mm coil, set frequency to around 500 pps and TX pulse width to 185us. Now on to designing the enclosure and coil shells.

[6666]

Seems like this could work as well, thanks.

You get a direct ratio of the voltage IE: a 700 volt peak will give you 7 volts on the scope, so you dont blow your scope

[moorejl57]

You get a direct ratio of the voltage IE: a 700 volt peak will give you 7 volts on the scope, so you dont blow your scope

I am definitely going to give this method a try, I never did actually measure the flyback voltage since my scope maxes at 50V/div and was well more than full screen. Based on what I have read here in the forum, I should not let the MOSFET avalanche since this will slow down the return to zero and is probably hard on the MOSFET as well. I assume the remedy is to add serial resistance to the coil?

[moorejl57]

In my first pass testing of the completed board with a 7" bundle wound coil of 328uH and 2ohm resistance, damping resistor at 680ohm, SRS around 800kHz, I think I am not getting the best depth with it. It can air detect a US quarter at about 7" and a US penny at about 8-9". What are typical results for a well tuned MPP Rev. E so I know what to expect?

I would like to do some experiments to improve the detection depth for coins and rings etc... and would like some advice on what are the top Pareto items to change first.
a. Improve critical damping (I haven't been able to really dial this in yet due to jitter from TX pulse period messing with scope trigger)
b. Modify coil current with serial resistance to avoid MOSFET avalanche (do I just change the existing 3R3 2W resistor on the board?)
c. Use a different coil design (basket, spiral etc...)
d. Change current shielding from wrapped copper tape to something else?

[green]

In my first pass testing of the completed board with a 7" bundle wound coil of 328uH and 2ohm resistance, damping resistor at 680ohm, SRS around 800kHz, I think I am not getting the best depth with it. It can air detect a US quarter at about 7" and a US penny at about 8-9". What are typical results for a well tuned MPP Rev. E so I know what to expect?

I would like to do some experiments to improve the detection depth for coins and rings etc... and would like some advice on what are the top Pareto items to change first.
a. Improve critical damping (I haven't been able to really dial this in yet due to jitter from TX pulse period messing with scope trigger)
b. Modify coil current with serial resistance to avoid MOSFET avalanche (do I just change the existing 3R3 2W resistor on the board?)
c. Use a different coil design (basket, spiral etc...)
d. Change current shielding from wrapped copper tape to something else?

a. To see coil decay shape. Clip probe to coil hot lead insulation, can't read volts but shape should be similar as hot lead. Can see if mosfet avalanches, flat at first part of decay. Including picture of my Tx circuit with hot lead and hot lead insulation(not MPP). I like to look at amplifier out and hot lead insulation on the scope when adjusting damping.

b. If you are not looking for short TC targets with a fast coil(first delay time less than 10us)I would jumper MUR460 in series with coil. Adds some capacitance, requires a lower damping resistor, allows a higher peak coil current before avalanche.

c. Use a spiral or basket for a fast coil with the MUR460 not jumpered, fast coils require less peak current for the mosfet to not avalanche. Less peak current less signal if Tx shape stays the same. Bundle wound might be better if not looking for short TC targets?

d. Might try a different shield if the copper tape is acting as a target. Post a scope picture of amplifier out and coil hot lead insulation with no target.

[moorejl57]

a. To see coil decay shape. Clip probe to coil hot lead insulation, can't read volts but shape should be similar as hot lead. Can see if mosfet avalanches, flat at first part of decay. Including picture of my Tx circuit with hot lead and hot lead insulation(not MPP). I like to look at amplifier out and hot lead insulation on the scope when adjusting damping.

b. If you are not looking for short TC targets with a fast coil(first delay time less than 10us)I would jumper MUR460 in series with coil. Adds some capacitance, requires a lower damping resistor, allows a higher peak coil current before avalanche.

c. Use a spiral or basket for a fast coil with the MUR460 not jumpered, fast coils require less peak current for the mosfet to not avalanche. Less peak current less signal if Tx shape stays the same. Bundle wound might be better if not looking for short TC targets?

d. Might try a different shield if the copper tape is acting as a target. Post a scope picture of amplifier out and coil hot lead insulation with no target.

Thanks green. Here is CH1 on coil hot lead insulation and CH2 on TP3 pre-amp out.

[mushaba]

I think that's ringing on tp3..

[green]

Thanks green. Here is CH1 on coil hot lead insulation and CH2 on TP3 pre-amp out.

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Don't have a MPP so I'm guessing, looks like copper tape might be acting as a target. Not avalanching, maybe overdamped a little? Does the ringing damp out? How many micro seconds to damp out?

[green]

Tried a spice simulation. middle trace(fast coil, critical damped, avalanched) bottom trace(fast coil, overdamped, reduced Rd to eliminate avalanche) top trace(added capacitance to make slow coil, same Rd as bottom trace, critical damped, no avalanche) slow coil critical damped decays faster than fast coil overdamped with same Rd.