TEFLON MULTI STRAND WIRE QUESTION

[baum7154]

If you would like to see construction pictures of this coil and feed look at the CHANCE PI COIL thread Post #30.

Dan

[bbsailor]

Photo of two sizes of beach baskets, 12 inch diameter and 8 inch diameter.

[daverave]

Photo of two sizes of beach baskets, 12 inch diameter and 8 inch diameter.[ATTACH]29859[/ATTACH]

thanks for the photos

[Hannu]

This was very helpfull. Thanks.

[bbsailor]

ive been reading your reply with great interest and trying to get these principles into my head....do you think its good to use a high a high pulse frequency ??? on my first detector i used 1500hz which is very good for some targets but not fantastic on like 18 & 22ct gold rings....but thin 9ct seems much better detection depth....my second detector i used 2.6khz but not as deep as my first detector but pinpointing much better....i noticed most of the commercial pulse detectors run at about 700 to 800hz and still get good depth...im a little vague about which frequency is best.

Daverave and all interested,

Here is something to help you form a better mental model of what is happening with all the variables in your question. There is a relationship between the ramp-up of the TX turn-on and the step slope of the turn-off of the TX pulse. All current rises by the time constant of the coil as governed by the coil's resistance and inductance. After three time constants, the current in the TX pulse reaches about 97 percent of the maximum current and any more TX on-time will not stimulate small targets (coin sized and smaller) very much more. The size of the damping resistor governs the TX turn-off time constant which should be about 5 times faster than the time constant of the particular target you are seeking. Given this, you can optimize your coil size, resistance, inductance and TX pulse width and TX pulse speed for a particular range of targets. Longer TX pulse widths create a higher flyback pulse thus you need a lower value damping resistor resulting in a less steep turn-off slope affecting the how small of a target TC you can effectively detect.

Now on the RX side, using an integration circuit allows you to improve the sensitivity of the RX circuit by collecting many samples while the target is in the magnetic field as governed by the coil size, sweep speed and the TX PPS (pulses per second) rate. See some of Eric Foster's early posts as he designed some very good PI machines in the 3000 PPS range. Some of Eric's coils are in the 5 to 10 ohm range. See his Goldquest schematics where he even puts a resistor in series with his coil to obtain the TX pulse time constant that he wants and can quickly damp to allow earlier sampling. As Eric said, it is a balancing act between coil size, target size and battery power consumed.

I hope this answers your question?

Joseph Rogowski

[daverave]

Daverave and all interested,

Here is something to help you form a better mental model of what is happening with all the variables in your question. There is a relationship between the ramp-up of the TX turn-on and the step slope of the turn-off of the TX pulse. All current rises by the time constant of the coil as governed by the coil's resistance and inductance. After three time constants, the current in the TX pulse reaches about 97 percent of the maximum current and any more TX on-time will not stimulate small targets (coin sized and smaller) very much more. The size of the damping resistor governs the TX turn-off time constant which should be about 5 times faster than the time constant of the particular target you are seeking. Given this, you can optimize your coil size, resistance, inductance and TX pulse width and TX pulse speed for a particular range of targets. Longer TX pulse widths create a higher flyback pulse thus you need a lower value damping resistor resulting in a less steep turn-off slope affecting the how small of a target TC you can effectively detect.

Now on the RX side, using an integration circuit allows you to improve the sensitivity of the RX circuit by collecting many samples while the target is in the magnetic field as governed by the coil size, sweep speed and the TX PPS (pulses per second) rate. See some of Eric Foster's early posts as he designed some very good PI machines in the 3000 PPS range. Some of Eric's coils are in the 5 to 10 ohm range. See his Goldquest schematics where he even puts a resistor in series with his coil to obtain the TX pulse time constant that he wants and can quickly damp to allow earlier sampling. As Eric said, it is a balancing act between coil size, target size and battery power consumed.

I hope this answers your question?

Joseph Rogowski

Hi Joseph
i can see now by what you have written that there are many parameters in making a fast pi detector....im trying to learn more each day as i find making pulse metal detectors so fascinating and it really gives me something to think about...i really want to make a detector that is a good all rounder on 9/18/22ct gold and i was thinking of experimenting with the sample width and see if i can improve on this !! you mentioned about eric foster using coils in the 5 ohm range....my first detector coil is around 5 ohms and gives me better depth than my 2 ohm coil....i guess its getting all these parameters correct and low noise in making a great detector...i really appreciate your advice and help.