Surfpi high inductance coils

[qute1]

i am not a noob but have up until recently been building the recommended coils for the PI's available but it has occurred to me that i have some very nice fast high uh coils 500-700 uh and 2-3 ohms.
What would be the best way to set up these coils with the surfpi 1.2 or similar that normally go 230uh .9ohm ish.
Changes to damping resistor etc (feel free to share your own experiences and set ups) or mosfet firing?

[daverave]

ive made several surf pi detectors and ive found the best inductance is around 400 uH....using 30 or 32 SWG enamelled copper wire.

[kt315]

if you want setting coils in surf pi you can set the overdumping resistor IN EVERY COIL, not on the pcb.
then you will get good compatibility.

i also reminder to people that surf pi is diver detector for underwater seeking and does not
have possibility to set coils. it uses one coil, preferebly DF one.
no socket, no connector, deadly sealed on time and on alllife.

surf pi is simplest project for quicker realisation and good platform for learning PI principle only.

[daverave]

i made a DF coil some years ago from cat 6 cable and it works well and good on small gold items....i think the surf pi in its basic form is very limited but needs lots of mods to get better proformance.

[qute1]

Thanks guys yes i have a surfpi that is in testbed mode and will be having lots of mods VCO delay etc but limited feedback from experiments says big coil larger inductance then add the rest when i have it damped in (yes kt i will be adding the dumper at the coil).

[rickb]

The l/r time constant for that coil is much longer than a normal coil. To get the same rise time will require higher voltage. Same principle as using 25-35 volts to make 2.5 volt stepper motors rated at 200 steps/second to run at 2000 steps/sec. for cnc routers. The resulting high voltage when the field collapses will be "interesting" to deal with. May be time to look at silicon carbide fets. What is the SRF?

[qute1]

srf looks to be around 8.4khz give or take cant seem to get the delay just right though its fine on the 364uh coil bit above 500uh it gets real choppy meh!

[waltr]

What does the Pre-amp output look like on an O'scope???
Post the scope picture.

My GUESS is this Large inductance coil has a long decay time therefore the first sample must wait longer than the other coils.

[rickb]

That srf looks completely wrong. It would take much more capacitance for it to resonate that low. 8.4 KHz srf is not fast. Something smells fishy. (like 3 days in the sun fishy)

[qute1]

rickb i apologise for the typos should have been 840khz was on the tablet... it is a simple question and has nothing to do with the preamp but thanks for the input waltr i will see if i can do that at some point this weekend, it was a simple enquiry about coil loading/damping and yes it appears that the bundle has opened up somewhat (tape has softened in the heat here) so i will recheck it tomorrow before i measure it i suspected it was higher as label says 1.24mhz thought it was higher but haven't had time to play Oscopes and only have a single channel pocket job to play with anyhow.

[waltr]

I suggested looking with scope after preamp since it is this signal that is sampled into the integrator. At this point you can see IF the coil is not decaying fast enough and measure the actual Minimum Delay usable and if the coil is properly Damped. Nothing to do with the actual preamp circuit.

[qute1]

Thanks waltr i will look at that i have been looking at flat topping the impulse recently and jumbled your suggestion up in my head many apple logies...

[green]

rickb i apologise for the typos should have been 840khz was on the tablet... it is a simple question and has nothing to do with the preamp but thanks for the input waltr i will see if i can do that at some point this weekend, it was a simple enquiry about coil loading/damping and yes it appears that the bundle has opened up somewhat (tape has softened in the heat here) so i will recheck it tomorrow before i measure it i suspected it was higher as label says 1.24mhz thought it was higher but haven't had time to play Oscopes and only have a single channel pocket job to play with anyhow.

Surf PI doesn't have a fast diode between coil and mosfet. Circuit resonance probably closer to .3MHz. Rd=pi*L*circuit resonance. Rd=3.14*500*.3=470 ohms for a 500uH coil.

[qute1]

Thanks Green THAT is exactly what i was looking for i will get too some measurements tomorrow i hope need to measure the actual capacitance of the coils (as didn't put that on the labels only uh and ohms) as i have now re tied the bundles it has probably shifted a bit anyhow so will recheck tomorrow when cured.

[green]

Thanks Green THAT is exactly what i was looking for i will get too some measurements tomorrow i hope need to measure the actual capacitance of the coils (as didn't put that on the labels only uh and ohms) as i have now re tied the bundles it has probably shifted a bit anyhow so will recheck tomorrow when cured.

Rd=3.14*L*circuit resonance(not coil resonance). Disconnect Rd and input resistor, connect scope to insulation on coil hot lead. Measure circuit resonance. Including some scope pictures I did awhile back with no diode and some different diodes, looks like I connected the scope to the hot lead. Think you would be better to connect scope lead to insulation of coil hot lead. With no diode oscillation doesn't start until coil volts has decayed to less than .6v(mosfet back diode drop). If I use a pot to trim Rd, slightly
underdamped seems to be best with my circuit, resistance close to calculated.

Not familiar with your scope, if you think you might damage something don't try it.