Chance PI coil...

[baum7154]

Actually, the diode will not get anywhere near 0V during Rx sampling period, and it is because drain capacitance keeps ample charge for quite some time. Trouble is in discharging that happens in exponential fashion, and thus it interferes with targets reception. It can be fixed by clamping a drain to a fixed potential via a resistor or otherwise.
Few MOSFETs have capacitances modelled correctly, so this mechanism seldom appears in simulation.

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I don't want to hijack this thread with a lengthly discussion on this topic of Mosfet capacitance but after reading your post I looked closely at the schematic of the CHANCE PI detector and there is a second Mosfet Q9 whose purpose is to shunt the high side of the coil back to system ground on the individual multipulse flyback. This event happens about 16 times each pulse string cycle before the coil is allowed to fully go into flyback. I have pictures of the coil flyback in Post 53 of the CHANCE PI BUILD thread in the Tech Forum.

I'm wondering if we can quantify in microseconds the amount of time that the transmit mosfet is discharging though a hypothetical 1k resistor. It would seem that an IRF9640 mosfet with a 370pf COSS and a 1K resistor would have a T/C of about 2.32us X 3 TC = 7us. Since I can't sample sooner than 8us I should not see the effect of this phenomenon if the 1k is present. Have I missed something?

Regards,

Dan

[bernte_one]

i made this crazy thing too, but in another shape, seems to work

[Davor]

I'm wondering if we can quantify in microseconds the amount of time that the transmit mosfet is discharging though a hypothetical 1k resistor. It would seem that an IRF9640 mosfet with a 370pf COSS and a 1K resistor would have a T/C of about 2.32us X 3 TC = 7us. Since I can't sample sooner than 8us I should not see the effect of this phenomenon if the 1k is present. Have I missed something?

Dan, I'm sorry for not noticing your message before.
Yes, you are missing an elephant in the room. 1k can not provide critical damping of the coil with bare MOSFET, hence the settling time is longer, and therefore a diode with capacitance under 1/10 Coss helps. As I noted before, the diode introduces a new set of problems that may be solved by clamping circuitry.

[baum7154]

Thanks for the response Davor. Actually I was thinking of placing the 1k resistor between the mosfet and the diode in the output of my CHANCE PI and would not be using it for the damping resistor. The flyback causes the diode to be in reverse bias and the 1k bleeds off the capacitive charge of the mosfet. I have not tried this to see if performance is improved.

Regards,

Dan

[Davor]

That would be the clamping action. Yes, it can work, but you need to connect the 1k bleeding resistor to a potential that will keep the diode in reverse polarisation to keep its low capacitance and all the benefits.

[baum7154]

Just wound a new oval 'Racetrack' spider coil 4" x 12.5" with 8.5" parallel sides using 24 AWG 600 volt PTFE/Teflon wire on a .093" thick LEXAN form. It measures 325uh with the coil and 35" feedline attached SRF is 1.070 mHz. On my CHANCE PI with 2 stage preamp it detects the following targets (in air) repeatably at the distances shown:


TARGET---------------------SOLID TONE DISTANCE---------------------1ST DETECTION DISTANCE

Aluminum pop can---------18" end on 20" broadside-------------------23"

1" X 1" AL soda can -------11"-------------------------------------------12.75"

8.6 grain dental gold-------2.75"-----------------------------------------4.5"

1/4" X 1/4" AL soda can---1.75"------------------------------------------3.0"

Gold placer flakes----------0.75"-----------------------------------------1.125" Note: Gold flakes in glass vial - largest piece is .95 grain, 2 pieces - .19 grain, 1 piece - .12 grain, 1 piece - .11 grain. Total weight of all dust and flakes 2.92 grains. With the flakes loose in a glass vial it is hard to present the target consistently to the coil as the pieces can shift or be on edge.

Next step is to encase the coil in fiberglass and I will post these construction details/photos as it progresses.

Regards,

Dan

[6666]

What type of wind are you calling a 'Racetrack' spider coil ????

[baum7154]

"Racetrack" refers only to the shape of two parallel sides with two half circle ends. It is the generally the same image as in post 134 of this thread but with .5" inner slot depth and made of LEXAN. The wind is the same self shielding 'spider coil' wind you and I have done in circular coils and by the way this coil does not see my hand fully grasping it.

Cheers,

Dan

[6666]

OK racetrack got it. tnks. Also the test with the Al can is a good indicator.

[baum7154]

With this racetrack design the objective is to build a fast 3:1 self shielding spider wound coil with a blade type of field to be used to quickly cover ground in search of small gold nuggets. The length of the 'blade' is about 8" and it appears to have good depth and sensitivity. Initially I intended to build an elliptical coil (see 'Elliptical Coil Question' thread)for the same purpose but then I found some threads Eric posted a while back about the performance of coils with 3:1 & 4:1 profiles. After doing some prototypes it appears that the 3:1 coil is best for my purposes. I am anxious to get this coil out in the mountains once the snow melts.

Regards,

Dan

[kellogs]

Hi baum 7154 how have you been as you have probably seen i have been out of action and have missed so much i finished my basket weave round coil for my surf pi and detect a dollar coin at 150cm and my wedding ring at 100 cm i like this new coil of yours i an going to have a go at it great to back have a great day Kindest Regards kellogs.

[mickstv]

Hi baum 7154 how have you been as you have probably seen i have been out of action and have missed so much i finished my basket weave round coil for my surf pi and detect a dollar coin at 150cm and my wedding ring at 100 cm i like this new coil of yours i an going to have a go at it great to back have a great day Kindest Regards kellogs.

150cm on a dollar coin, so how big is your coil ?

[baum7154]

Hi baum 7154 how have you been as you have probably seen i have been out of action and have missed so much i finished my basket weave round coil for my surf pi and detect a dollar coin at 150cm and my wedding ring at 100 cm i like this new coil of yours i an going to have a go at it great to back have a great day Kindest Regards kellogs.

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It is great to hear from you once again Kellog. Glad to see you are back on the forum and you are on the mend. Amazing results on your detector/coil project. What diameter is your coil? Are you pleased with it's self shielding performance? After years of work these are the only type of coils I now build. Good luck with your build on the 3:1 racetrack coil. If you have any questions on it let me know and I'll try to assist.

Cheers!
Dan

[kellogs]

I don't know if remember you and chet helped me with the 8'' coil the only thing i did different iafter every 5 turns i wound plumbers teflon tape pink or 0.1 mm thick.wound 40 turns and then removed till the test with alloy paper 1/2x1/2x 1/8 thick ended with 354 turns once again mate thannks for your help kellogs

[kellogs]

kellogs again did read my post correction 35 turns.