Strange damping on Surf PI 1.2 please help

[davy74]

Where do you add the snubber?

Ok so snubber (supressor?) goes across the coil obviously, my coil is not currenly sheilded either but based on info on last post by Sean, I'm gonna look at sheilding it before any proper field test.

[Ferric Toes]

Where do you add the snubber?

If you want an additional snubber, use the RDC type from macross's post. The diode Ds dumps the energy into the capacitor Cs, which is then discharged before the next pulse by Rs. Most PI's use the Mosfet avalanche to do the snubbing and additional circuitry is not needed.

Eric.

[davy74]

Thanks Eric,
For my first real MD project, I,ve built & posted a decent example of the basic surf PI 1.2 including external test point sockets delay pot, an adjustable damping resistor arrangement etc _ originally intended as a learning platform allowing me to experiment with different coils\damping setups etc whilst being rugged enough to actually be usable, coil is currently 25 turns of the braided wire taken from a computer monitor tube (I'm thinking this is litz wire?), tightly wound then ovalised to fit in crossfire housing.
I've just covered ground around an old derlict mill house which I,d previously covered extensively with a decent vlf machine & have dug a bagload of (mainly junk) but finds non_the_less_ interestingly at depths far exceeding what I had expected. I had studied electronic engineering at college working in electronics in the 90's but ended up down a more electrical\instrumentation route _ I'm enjoying the challenge of re-discovering practical electronics in a useful way as well. I might try the snubber circuit you refer to in a future project as an experiment to satisfy my curiosity but I now understand what is meant by the MOSFET avalanche. Just a few months ago I would'nt have imagined needing an oscilloscope in the house ! An LCR meter on the way also.
I,m learning fast here & I'm ever grateful for yours & anyone elses input.
Best regards, Davy.

[kosacid]

i done a few tests as well mine worked right away no need for adjustment, i used 0.9mm magnet wire 6" coil, 28 turns i tested it on the scope got a perfect form, i did shield it with foil tape and grounded it, but for some reason it was less sensitive so my coil is unshielded

[Ferric Toes]

i done a few tests as well mine worked right away no need for adjustment, i used 0.9mm magnet wire 6" coil, 28 turns i tested it on the scope got a perfect form, i did shield it with foil tape and grounded it, but for some reason it was less sensitive so my coil is unshielded

What foil tape did you use? Copper, aluminium, lead.......?

Eric.

[kosacid]

aluminum it was bad, at most i would use maby a wire shield on top of some sort of wrap not touching the coil, i was out testing my MD the other day power lines was the only thing messing with it other than that it worked fine found a few ship half penny's about 6" + deep

[Ferric Toes]

aluminum it was bad, at most i would use maby a wire shield on top of some sort of wrap not touching the coil, i was out testing my MD the other day power lines was the only thing messing with it other than that it worked fine found a few ship half penny's about 6" + deep

Here is another way of shielding a coil by using a fine pitch ribbon cable. At one end, all the conductors are joined together and brought out to a ground wire. The other end is all open (comb configuration). In both the coils shown, the ribbon cable is wound on top of a polythene helical wire wrap to space it off the coil. To make a neat job you have to experiment with the number of conductors to peel off, depending on the coil diameter. The one with a red conductor is the better of the two. It was an 11in winding which I have squeezed into a 10in shell. Hence the ingrowing loop.


Eric.

[antalaz]

Got news from the front !
I've ordered à brand new housing for my coil but i've encountered A problem: my spiral coil was to large

So i've rewired my coil in order to fit it in my housing and i've shielded it with scotch24.
It slowed it down (10uS added to my previous delay)

My new inductance is 250uH.. i was wondering, is it too low ?

Another thing: i've soldered à trimmer with my damping resistor (1k), and when i try to set my accurate value, my curve doesn't vary that much....

Any ideas??

[Ferric Toes]

Got news from the front !
I've ordered à brand new housing for my coil but i've encountered A problem: my spiral coil was to large

So i've rewired my coil in order to fit it in my housing and i've shielded it with scotch24.
It slowed it down (10uS added to my previous delay)

My new inductance is 250uH.. i was wondering, is it too low ?

Another thing: i've soldered à trimmer with my damping resistor (1k), and when i try to set my accurate value, my curve doesn't vary that much....

Any ideas??

It should work OK at 250uS as the inductance is not that critical.

Have you left a gap in the shield so that it is not a closed ring? The gap is usually at the position of the lead out wires. Solder the ground wire just to one end of the shield and take it to the coax braid at the point where one coil wire is connected to the braid.

Eric.

Eric.

[antalaz]

Yes i've left a small gap, maybe my trimmer is fried...

250 uS? The output of 5534 shows me a 15uS delay
maybe i didn't understand everything

Briefly, what is the influence of inductance ? A bigger inductance call more current ?
I would like to understand the balance between pulse width, freq, and coil caracteristics but there is many topics and clear information is difficult to get.

If you know à widely know topic where it's explained, i'm interessted but if not, i'll try to understand it from the basics of physics

Thank you

[hecbom]

Look into buying "Inside the Metal detector". It explains in detail what you are after.

[antalaz]

Yes, i'm thinkin about it

a good spend i guess

[antalaz]

I've sandwiched my spiral coil with some insulation material, my delay is now 10uS!

It's working but i think i can enhance it by adjusting delay in a clean environnement.
I'll do it tomorrow.

To be continued

[antalaz]

New coil ! Circular, insulated and shielded !

[Ferric Toes]

New coil ! Circular, insulated and shielded ![ATTACH]37472[/ATTACH][ATTACH]37472[/ATTACH]

There may be something amiss with the grounding of your preamp as you have a positive tail at the end of the recovery. This can happen if some of the recharge current to the TX reservoir capacitor flows in a part of the RX ground. A good way to avoid this is the ground the RX directly to the ground side of the TX capacitor.

Another cause of this type of overshoot is excessive capacitance in the coil and cable. Try as short a length of cable as possible and see if it improves. You could also increase the value of your damping resistor and see if the overshoot develops into a ringing waveform. If so, it is a coil capacitance problem. Sometimes you can get a faster response by allowing a cycle of positive and negative ringing, as long as the start of the sample pulse is just clear of this.

Eric.