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Offline carlrodTopic starter
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« on: April 27, 2010, 05:57:41 am »
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 [hello]more OHM, COIL, reduces the depth? or increases? less OHM, reduces or increases, the depth of detection? has something to do, more or less ,OHM, with the stability in the detector, with sensitivity maxima? or not?
 and on: uh, if not exactly equal to the coil of origin makes a difference in, decrease or increase the depth of detection? uH+ or - uH?

 THANK-YOU,FRIEND,s

 CARLOS

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 P.S) COIL,  PULSE INDUCTION.
 ( 50x50cm)

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Offline avistor
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« Reply #1 on: May 09, 2010, 01:24:12 am »
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The greater the resistance of the coil, the greater the power wasted in it. For maximum field generation, depth /penetration (and coil Q, if resonance is used in reception), coil resistance should be kept as low as possible.

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Offline GoldDigger1950
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« Reply #2 on: May 09, 2010, 05:42:15 pm »
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Quote:Posted by avistor
The greater the resistance of the coil, the greater the power wasted in it. For maximum field generation, depth /penetration (and coil Q, if resonance is used in reception), coil resistance should be kept as low as possible.


This is absolutely correct. Resistance is the RESULT of the wire in the coil windings and not a design factor. You never, ever wind a coil to match resistance. You strive for the lowest resistance possible with the inductance being a match for the original coil. At some point, by changing wire diameter and the number of windings, you will cross into the area of the field being too weak to drive the coil. It then becomes a "dance of death" to which you must become a willing partner.

In other words, it's not as easy as some here would lead you to believe.

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« Reply #3 on: January 18, 2015, 04:06:41 am »
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I will be trying a variety of wire diameter and inductance once I get this pulse induction metal detector working.  I know there is more to it than lowest resistance otherwise I would just use 00 welding cables to wind a coil. It would proable weight a ton if more than one loop of 00 welding cable was used.

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Offline golddustcarl
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« Reply #4 on: January 18, 2015, 09:09:10 pm »
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Quote:Posted by johnsgtx
I will be trying a variety of wire diameter and inductance once I get this pulse induction metal detector working.  I know there is more to it than lowest resistance otherwise I would just use 00 welding cables to wind a coil. It would proable weight a ton if more than one loop of 00 welding cable was used.


#21 down to #26 are most common in DIY PI coils. The higher value you need in inductance the smaller the diameter of wire you will need, but then resistance goes up because you need more turns to get higher inductance. AGGGHHHHH!!! Like Avistar said, "dance of death". A value range of 350µH to 425µh is generally used as a pseudo standard. I use #21 or #22 in my coils. You are shooting for enough magnetic signal strength to reach down approximately 1-1/2 times coil diameter with out magnetizing the surrounding ground mineralization. This is where resistance comes into play, and there is no formula to calculate all of the variables that you might run into. However it is generally accepted that you not design too much current as then you will have to carry a car battery around with you to get any detecting time in. Vlf can get away with a nine volt battery because µprocessors oscillators and even Colpitts oscillators draw very little current. Pi needs much more current to get the same depth as VLF. Tradeoffs, tradeoffs, tradeoffs. Also watch out what you use for pulse repetition rate and duty cycle. 20 Hz to 400 Hz for repetition rate and duty cycles of 3% to 4%. Any more duty cycle than this, and the driver will have a hard time coming out of saturation and will overheat even if it is not drawing a lot of over current. Also make sure the coil is properly dampened with an appropriate resistor. This value is easily found by putting a 5k pot in series with a 220 ohm resistor and hook it in parallel with the coil while watching the waveform on an oscilloscope. Adjust the pot so that the "ringing" that trails the CEMF pulse turns to a flat line. If you fail to do this, the drive transistor could over heat and destroy itself as well as the detection circuits beyond it will not be able to see changes when the coil passes over metal targets. No no!
Hope this helps a little.  Wise

Dusty Carl

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