PI help

[green]

(I was checking some coils that I have and saw something I didn't understand. Why a big difference in the decay time? Including a plot. Channel 1 is the coil volts, channel 2 is (U1 output mono coil amplifier reply#7), Rd is R5, Mhz is decay freq. with Rd disconnected. No target, no shield, 24awg is solid.)

(Making a Fast Pulse Induction Mono Coil) says to use stranded for AWG 22 to 26. I don't have any 24 gage stranded. I tried some 28 awg teflon coated stranded to compare with the 28 gage magnet wire. A small difference. Should the coil response with 24 gage stranded look similar to the 28 gage magnet wire if coil specs are similar except for resistance?

[Carl-NC]

Generally, stranded is slightly better than solid, thicker insulation (Teflon being the best) has lower C but also lower L than magnet wire shellac, and smaller gauge wire has lower C but higher R than heavy gauge. It's a Big Trade-off Matrix.

Your 24awg coil has a lower C (which should help) but also a much lower R which probably results in a higher turn-on current, which takes longer to dissipate. Try adding a series 1.6-ohm R to the 24awg coil and see if it gets closer to the 28awg response. This R may get quite warm.

[green]

(Your 24awg coil has a lower C (which should help) but also a much lower R which probably results in a higher turn-on current, which takes longer to dissipate. Try adding a series 1.6-ohm R to the 24awg coil and see if it gets closer to the 28awg response. This R may get quite warm.)

Thanks for the reply. I was using a 30usec on time to match the LTspice example I was using. I tried varying the on time from 30 to 100 usec and added a 2 ohm resistor in series at 30usec with the 24awg. I saw a time change of the 450 volt fet clamp. I didn't see a change in the curve after it came out of clamp,
.

[Waikiki_Sweep]

Also solid cooper wire instead of stranded wire holds eddy currents inside making decay little longer.
You can cut a piece of solid wire and same size and AWG of stranded wire and try to detect it with your detector.
You will see a BIG difference.
It is good idea to make your coil from parts you can not detect to prevent it from detecting itself.
1. Sranded wire. (not solid)
2. Graphite shield. (no foil) Connect shield with stranded silver plated wires.
3. Coaxial with stranded wire core (not solid) and stranded or mylar shield (no foil).
Perform same test with pieces of coaxial cables you are going to use.
Sure if you can detect it from long distance it is bad cable.

[KingJL]

Great little tool!

I was reading (Making a Fast Pulse Induction Mono Coil) by bbsailor again the other day. For test equipment I have multimeter and a DSO that can measure frequency. If I disconnect the damping resistor from the PI coil driver I can measure the frequency of the damped wave. Probably less than the coil resonance. I don't have a signal generator so I made a circuit from spare parts to measure resonance and with an output to check amplifier response. I measured three coils. 253uh 2Mhz, 162uh 4Mhz, and 200uh 2.19Mhz. The last coil measured 2.10Mhz when connected to my MD coil driver, less change than I was expecting. Should I expect much of a difference if I measure coil resonance with a signal generator? Any suggestions to improve the circuit or add capability would be appreciated. Coil inductance is calculated using coil resonance with a 10nf capacitor in parallel with the coil. Chart included.

Great job... Your little project grabbed my interest... and my desire to tweak things got the better of me!
A little background... I recently aquired a Diligent Analog Discovery. It is a handy little device... an O'scope, Waveform generator, Voltage supply, Digital analizer, and Digital pattern generator. But right now I am just using the analog side (O'scope, Waveform generator, and voltage supply). I am doing some experimenting with FPGA coding toward building digital PI metal detector and plan on exersizing the Analog Discovery heavily as a test instrument in this endeavor.
Back to present... I thought your little project would also make a great little tool for my endeavors... View the output with the Analog Discovery O'scope and use the FFT function to accurately determine frequency. I made a few changes to the original circuit to enhance stability and improve the signal quality for the measurement.
First I wanted to drive the circuit from the Analog discovery Waveform Generator. Next I reduced the total circuit losses by moving to a small MOSFET. This had the added benefit of a better controlled ON pulse due to the elimination current tail that is inherrent when driving a current load with pn junction transistors. During testing, I found that I needed more capacitance coupling for the low frequency ( 10 nf across coil ) test to determine inductance. As I was playing around with jumpers to change the coupling capacitors, I stumbeld upon a scheme to change the coupling with one jumper that also equalized the output between the two frequency modes.
I set up the waveform genetator to a square wave with 20% positive @ 10 kHz (2 us @ 10 kPPS). I use the low frequency mode of the coil driver (with 10n coil cap w 10p coupling cap) and measure the frequency with the o'scopes FFT function. I use this to determine the coil inductance using 10.012n for the capacitance. I then remove the jumper that connects the 10n coil cap and 10p coupling cap. I then measure this frequency with the o'scope FFT function. I use this frequency and coil inductance obtained in the previous step to determine capacitance.
I have performed numerous tests on coils I have laying around some with coax attached. As near as I can tell, this methodology results in about 1% accuracy. I verified this by measuring the same coils with and without coax attached. The capacitance increases appropriately for the length of coax.
A good site to use for LC resonance calculations is http://www.midnightscience.com/formu...lculators.html

I have powered this little beast with -5Volts provided by the Analog Discovery and by a 9 volt battery. It has worked perfectly both ways. With the 9V battery the high frequency test signal is a little stronger and consistent (not much though). I think I am going to stick with the 9V battery.
All in all this makes a handy little tool... now to pretty it up a little and make it a breeze to connect the 4 Analog Discovery leads (scope + & -, AWG signal, and GND).
Attached is the final schematic of the little beast.
BTW you can connect an in-circuit coil and tweak your damping R. (just make sure the detector is not powered on!!!

[Carl-NC]

Long time no see, King.

[KingJL]

Been here off and on... Just nothing worthwhile to say!!! Glad to see you missed me!!

[6666]

Reduce your dumping resistor on 50-100 Ohm and coil will work in any position.
Last time I saw the same pictures by submerging my perfectly dumped coil into salt water.
In my case surrounded by water coil has a little more capacitance and requires lower value of dumping resistor.
So now I keep a gallon of ocean water at home to test coils and adjust dumping to real conditions instead to do it in air.
Now I am scratching my head and thinking why I found so many gold and platinum items if my coil was underdamped all the time

Is there much difference in the resistor value from when the coil is in the ocean water and out of it ?

[Waikiki_Sweep]

If coil is unshielded the difference is significant because ocean water creates capacitor all around coil so I have reduced dumping on 200 Ohm or so in comparison with on air conditions. Not yet checked with shielded coil but I expecting to see some difference too.
Also I have observed influence of depth of water. Integrator goes to saturation if I go chest deep into salt water so I have corrected delay, dumping and also added a bipolar capacitor 10uf to the integrator input to
let it self-adjust for the rising bias between + and - samples. It works kind an automated ground balance.
Probably my problems occur because I have increased power of pulse in few times and integrator gain in comparison with official version of SurfPI. But sure, everyone wants to add those couple inches of depth

[Waikiki_Sweep]

Also use of underdumping coil was an interesting experience. Sure underdumped coil is noisy and unstable but probably there is a some space for invention of sensitive detector.
If you look on oscilloscope you will see that bump or curve of ringing. Even small target can change shape of that curve significantly. And change occurs in short delay area - exact gold range!!!
So if detector with manageable underdumping will be created it have to be extra sensitive to fast decay targets like gold.

[6666]

Thanks Waikiki_Sweep, good info

On my list of things to try is a variable damping resistor useing a fet like Moodz used in one of his designs.
Interesting about the 10uF bipolar capacitor to the integrator input trick.

[Waikiki_Sweep]

Integrator saturation in SurfPI sometimes is a problem.
Because decay curve is exponential you will always have a bias between samples. One sample always will be lower then other.
If difference between samples is big enough you will see OpAmp goes into saturation like +3.75 V is set on the integrator output and your detector stops reacting on targets.

Factors that may rise bias between samples and shift integrator into saturation:
1. Target is too big.
2. Target is too close to the coil.
3. Detecting in bad conditions like salt water or minerals.
4. Delay set is to short. (To find small gold)
5. Dumping resistor is too small. (Slope is faster and samples are more different in voltage).
6. Impulse is too long (providing more power to increase depth.)
7. Gain in integrator is too high. (Like it is in PI PRO schematic where gain was increased two times to increase depth)
8. Coil is to big generating stronger signals.
9. Capacitance of coil is to large requiring longer delay settings.

You can easily subtract that bias between samples and amplify only changes
by adding capacitor in line on integrator negative input.
Let say 10 uF bipolar electrolytic or two polar electrolytic that oriented + to + each other.



The capacitor will slowly corrects difference between + and - inputs of integrator.
As result you will never have a situation when your detector go blind over big target or bad ground, e.t.c.
because integrator starts to adjust to signal like motion detectors always do but for much wider dynamic range.

Also additional diode helps to discharge integrator fast when you move coil away from target to let you to detect target again as fast as possible.

Now you will have ability so search over bad ground because detector
will compensate that ground signal much better and show you only difference that target creates over the ground.

I saw some depth increase because instead of working on the edge in +3.70V - +3.75V range output
Integrator works in the middle range in much comfortable 0 - +0.5V output.

Practically I have measured voltage on the capacitor and see it changing somewhere between 0.3-1.6V
depending on conditions. You can see that 1.6V between + and - inputs of OpAmp will send OmAmp into saturation for sure.

Also that capacitor makes SAT reaction slightly faster. It is good for me because
I like to go fast but if you preferring slow searching you can increase capacitance or make a corrections to
SAT setting resistor-capacitor at the output of integrator.

Also you will see that integrator will settle output to 0 V after few seconds once you hit a target or some bad ground. So you need move your coil to detect as it always was.

Threshold setting has to be adjusted to work with that 0 V level instead of some constant bias up to +3.75V on integrator output as it was previously.

Good Luck!!!

[Waikiki_Sweep]

Please discard attachment in previous message and use the link on image inside message. That outdated attachment pop-ups as result of my attempt to edit message on the forum and I can't delete it.
My later schematic contains an additional diode that helps too much for good timing and fast detection.

[green]

(I was checking some coils that I have and saw something I didn't understand. Why a big difference in the decay time? Including a plot. Channel 1 is the coil volts, channel 2 is (U1 output mono coil amplifier reply#7), Rd is R5, Mhz is decay freq. with Rd disconnected. No target, no shield, 24awg is solid.)

(Making a Fast Pulse Induction Mono Coil) says to use stranded for AWG 22 to 26. I don't have any 24 gage stranded. I tried some 28 awg teflon coated stranded to compare with the 28 gage magnet wire. A small difference. Should the coil response with 24 gage stranded look similar to the 28 gage magnet wire if coil specs are similar except for resistance?

I got some stranded awg24 pvc coated to try. Looks like the solid wire is acting as a target. Everyone says to use stranded. I didn't expect to see that much difference in the trace. Looks like the awg28 is a target also. I have been winding small coils to experiment with. Need to try a 10 inch coil closer to 300uh. Thanks for the suggestions.

[Waikiki_Sweep]

Thank you for interesting experiments!

Picture 1 (awg24 solid) You can see that small spike after TX impulse end. Looks like coil just little bit under dumped.
If dumping resistor is same for all setups than "awg24 solid" has a little bit more internal capacitance.
As you can see lower self resonance frequency 2.3 Mhz proving presence of such capacitance.

Also interesting that awg28 is thin enough to reduce inter wire capacitance significantly.
Self resonance frequency go high even if it has more inductance because wires packed more closely.

Looks like switching on thin wires is a way to make a fast coil.
But thin wire has higher resistance and to keep same current we need longer impulses or high voltage power supply.
Easy solution is to use DC/DC step up voltage converter on LM2577 or XL6009 ($2 on ebay) to kick that high resistance coil with 36-50 Volts.


So thin wire coils have to decay faster because of lower inter wire capacitance and lower coil to shield capacitance. It is great to find small gold!!!!
Also such coils should be light weight: less wires, smaller shield, less plastic to hold construction.
And my favorite quality !!!!! - less drag in water

Can you try awg31 or even smaller but using higher voltage power supply? awg31 has five times higher resistance than awg24.
http://en.wikipedia.org/wiki/American_wire_gauge