SURF PI HELP

[kt315]

daverave, please read this pages. I hope George and Carl would not be angry on me if I give a quotation for the book. but it is a determining of SAMPLE DELAY.
it is in english and it is VERY CLEAR reading for you, not for me...

[daverave]

daverave, please read this pages. I hope George and Carl would not be angry on me if I give a quotation for the book. but it is a determining of SAMPLE DELAY.
it is in english and it is VERY CLEAR reading for you, not for me...

[ATTACH]26687[/ATTACH]

thanks for that...as a matter of interest can this book be bought from a shop or where would i buy this ??????

[greylourie]

Amazon UK. Very decent delivery.

[daverave]

ok thanks

[kt315]

may be from George 'Quoazhi' directly... I received from him.

[Qiaozhi]

ok thanks

Or you can buy direct from the authors at www.geotech1.com/itmd
All UK and European orders are shipped from the UK.
It's also available on eBay UK.

[kt315]

if I give a quotation for the book

ooh sorry... to read "FROM the book".

[kt315]

in text at the page:

Gold has a lower conductivity than silver or copper

here that is meaning EDDY CURRENTS, NOT OHM'S current. we call eddy currents in russian 'vikhrevyje toki', something like vortex-whirl currents direct translated.
for Ohm's galvanic current gold has a high conductivity (not the lower like for eddy currents).

[daverave]

[QUOTE=kt315;178819]in text at the page:

here that is meaning EDDY CURRENTS, NOT OHM'S current. we call eddy currents in russian 'vikhrevyje toki', something like vortex-whirl currents direct translated.
for Ohm's galvanic current gold has a high conductivity (not the lower like for eddy currents).[/QUOTE

i appreciate all your help to me...im slowly learning all this information....that book looks really interesting...and i will buy when i can the book...there is so much i want to learn...at one time in the past i gave up electronics and sold all my equipment and now ive got into metal detectors im having slowly to buy secondhand equipment and parts...and my knowledge is slowly coming back which i forgot !!!
i find it all exciting now making these detector projects...gives me something to think about !!!!

[porkluvr]

A 400uH coil with 4.4Ω (or 5Ω as you said) has a much longer time constant (tc) than the 'stock' coil does (but please don't ask me about a stock coil).

All other things being the same, what you do by increasing tc is to cause the FET to be cutoff while the coil is way far from being fully charged. Switching off the FET before the coil current is allowed to approach its maximum value (at which time the field stabilizes) causes target eddy currents to be generated, but with opposite polarity of eddy currents caused by the main field's collapse. Sensitivity suffers as a result.

What to do? You can calculate the approximate maximum possible coil current value by ohms law; dividing battery voltage by the sum of coil resistance plus series resistance.

Now, I do not know how to determine a minimum acceptable value, or even, how to quantify the effect, but I am supposing that 95% of the maximum current would is good. (Eric Foster's writing about some -other- machine is where this idea comes from. Not my idea.)

My take on this is that you could take a two pronged approach to make the 95% number happen.
1. Increase transmit pulsewidth.
2. Add series resistance.

Problem with #1 is that your battery life suffers unless you perhaps reduce PPS; not something to be taken lightly.

Problem with #2 is that efficiency suffers (but, so what... really) .

If you insist on using a 400uH coil then I would recommend increasing pulse-width and also add some series resistance between coil and FET. Beware: that series resistor could be subject to some serious power dissipation - depending on value and PW. A 2W or 3W resistor might be wise...

How you trade off battery drain and sensitivity would be up to you. I might be able to supply some LTspice simulation pics to help illustrate this concept, later.

[daverave]

my coil is already potted in resin so im stuck at 400 uH...i will next time make a coil with less inductance...i did try a series resistor of 5 ohms but i could not see any improvement and the 0.25 w resistor ran cold...im a little concerned cause im using the 390 ohm damping resistor as in kit spec...and there was no ringing with my coil....i wonder if the damping is wrong....i may try and experiment increasing the tx power using a spare pcb i have...i just feel that using a 15 x 12 coil i should be getting a bit more depth so something must be incorrect.

[daverave]

when i lay my coil flat on the floor i cannot detect metal...even if i pass a coin over the coil...i think there is large metal below my floor in my flat...is this normal for the surf pi to do this ??? im still wondering if my damping is wrong using the normal 390 ohm resistor...i have not altered the damping as i have no ringing with this 5 ohm 400 uH coil....maybe making the sample pulse in the wrong postion even when delay control is set fully anti clockwise....can anyone pls advise.

[kt315]

maybe your home has a metal armature everywhere around, in wall in floor in roof. if your home like is same a village home done from wood it is good for testing inside at floor.

[porkluvr]

If you write that you detect no ringing then I have to assume that you have the use of an oscilloscope?

Are you using IRF9640 or the TIP32C?

You did not describe your coil with any abundance of detail. Is your coil wound with enamel wire, or is it solid or stranded PVC, or what? How thick is the insulation (or what is the voltage rating)? (Important details when trying to analyze performance.)

(Did you subtract the meter leads' resistance from what was displayed when you measured the coil resistance?)

I believe your damping resistor is pretty close to optimum but you do have reason to be concerned about your first sample timing. That would apply ESPECIALLY at the ccw settings of the delay pot.

I believe you also should be concerned about what I described in my previous post; not allowing coil current to stabilize before causing the magnetic field to collapse, and how that affects sensitivity. I have some ideas but I want more info.

Can you answer some of the questions above?

[daverave]

im using the IRF9640...my wire is standard 30 swg enamel copper wire solid type...i used 6mm spiral wrap single layer with scotch 24 shielding....my coil is 400 uH at 5 ohms resistance...i didnt change the damping resistor 390 ohm as in spec..but when i scoped the back emf and sample pulse i noticed that at the full anti clockwise setting of the delay pot the leading edge of the delay pulse is getting rather close to the falling edge of the tx pulse as if the sample pulse is sampling too early.