Whites Surf PI schematic

[Mustafa]

Schematic with part values.

Mustafa,
C15 looks reversed. Why do you leave out parts values?

[Mustafa]

...
But...this what i made is modified version, i dont have accurate schematic of it. Pretty occupied to trace pcb and draw schematic....hmmmm.
Is there right schematic for this modified Surfmaster?
Would be nice if someone post that here...

Hi,
This is my first drawing, next week I will try to build modified Surfmaster.

[hobbes_lives]

Aziz explained that the improvement comes from the fact that interelectrode capacitance of the diode is in series with the FET capacitance. This reduces the capacitance seen by the coil and reduces coil current decay time. You can then use a higher value damping resistor and your receiver will see a stronger signal.

Aziz said that? He was challenging the point with me when I said that before! I guess he changed his mind.

[porkluvr]

HI
So does anyone now how to get this machime more sensitive?
Regards
Nelson

Here's a trick you could try. Needless to say I have not done it myself because I don't own a Surfmaster. But if I ever get my PI finished it will have this feature. (Maybe if my design weren't so darn complicated... .)

You are probably aware of my ongoing recommendation to not try any extreme modifications without benefit of an oscilloscope to make sure everything is properly broken. (That's an extension of my military training. I loved the military.)

Now that I got that out of the way: take a look at Carl's Hammerhead II.

http://www.geotech1.com/forums/showthread.php?t=14940

Notice the silicon rectifier 1N4004 between the FET and coil. This should knock about two micoseconds off a coil's decay time. It depends on the coil. I would have reservation about using 1N4004 because coil current in the Surfmaster -well- exceeds the 1 amp rating of the 1N4004. Of course it's not continuous current, but I would still use a diode rated for 3amps. Minelabs uses a BYF28-200 in their SD2000 (with IRF640 FETs). That should work here. BYF28-200 is a 200PIV@3A fast recovery diode. There are plenty of others.

Aziz explained that the improvement comes from the fact that interelectrode capacitance of the diode is in series with the FET capacitance. This reduces the capacitance seen by the coil and reduces coil current decay time. You can then use a higher value damping resistor and your receiver will see a stronger signal.

You may also be able to take your primary sample sooner but there are risks involved in changing the timing. Hence the scope recommendation.
The scope would also be useful for finding the best damping resistor.

Here might be only two microseconds improvement, but even though I can't run experiments to prove this - I think that it would mean an improvement in target illumination even if you don't move the timing forward.
If the FET runs much warmer than before you might want to reduce the 3.3k 1% resistor a little.
Please don't blame me if something goes terribly bad, it's not my fault.

[nelson]

Hi porkluvr and bbsailor, and thanks for such clear explanation about damping resistor.
So first i ll go experimenting with it and after that if nothing happends i ll try to get IRF9640 instead of IRF9630 that i m using now.
Kind regards
Nelson

The damping resistor affects sensitivity because too low a value can load down the return signal. Strong returns get weakened and weak returns might get lost entirely.
Too high a value will cause overshoot and ringing. Coil flyback won't die down as quickly as with the correct value. Since you want the coil flyback to be essentially dead before you start sampling, this would mean you must sample later than otherwise.

If you are still using IRF9630 I would suggest that you try again to find IRF9640. The difference is not so great, but the higher IRF9630 drain resistance means less power to the coil. It also means that your coil decay curve will be longer. Both factors are detrimental to sensitivity.

[kuraja]

Can the Surf PI have a flat wound type of coil? And how would it benefit in depth field?

[bbsailor]

[quote=kuraja;85487]ApBerg...Today I think I've learned that different damping resistors affect the decay curve differently depending on the value of coil and cable capacitance. Since this capacitance is an unknown quantity, I am blowing smoke if I try to pin down a "best" damping R value.


[quote]

Yes, this is exactly true. Anything that contributes coil capacitance or capacitance in the TX circuit will lower the coil's self resonant frequency that is stimuated when the back EMF pulse is being damped by the damping resistor (Rd). The value of Rd will be lower with higher TX coil circuit capacitance. This lower value of Rd will change the coil discharge curve, extending sampling at the earliest time, causing lower sensitivity to some metals.

You have a good mental model of what is happening so you can appreciate that the fastest sampling occurs with coils that have low capacitance and TX circuits with low capacitance.

bbsailor

[porkluvr]

>....snip

So does anyone now how to get this machime more sensitive?
Does the dumping resistor help with sensitivity?
Regards
Nelson

The damping resistor affects sensitivity because too low a value can load down the return signal. Strong returns get weakened and weak returns might get lost entirely.
Too high a value will cause overshoot and ringing. Coil flyback won't die down as quickly as with the correct value. Since you want the coil flyback to be essentially dead before you start sampling, this would mean you must sample later than otherwise.

If you are still using IRF9630 I would suggest that you try again to find IRF9640. The difference is not so great, but the higher IRF9630 drain resistance means less power to the coil. It also means that your coil decay curve will be longer. Both factors are detrimental to sensitivity.

Use for coil wire 0.50 / set offset null (or in midposition) / adjust delay pot. for best sens.

[nelson]

HI
Last weekend i have retune my surfmaster and now is working very very stable. But sensitivity is still strong btw 14 and 15 cms for a 24 mm coin, and very weak btw 16 and 18 cms.
So does anyone now how to get this machime more sensitive?
Does the dumping resistor help with sensitivity?
Oh i forgot, i used two coils, one is 360 uH, 3 ohms and the other 407 uF, 3,6 ohms. Both are 21 cms in diameter.
I hope anyone can help me , cause my knolegments of electronics are very poor.
Regards
Nelson

My experience with TIP32 it is very very hot and surfmaster dont' work.
IRF9640 is good for this project.
No good 14-15 cm for 24 mm coins.
my surfmaster detect 20 cent euro at 23 cm in air and my mobile phone at 37 cm.

[cordius]

yes, pulse frequency

... if you want to change the pulse delay put a trimmer in place of R1

Ummh, not sure which schematic you're looking at, but I think changing R1 near the LM555-7 won't change delay time very much. It will change pulse frequency.

[kuraja]

Surf Pi is a nice project. I wonder if somehow you could put an analog meter just for estetic reasons...

[cordius]

I never believed in the simulator , the SURFMASTER is to be tested on the ground and not on the PC, look at the scheme, if you want to change the pulse delay put a trimmer in place of R1