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Offline Peti80Topic starter
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« on: January 30, 2010, 06:32:57 am »
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Hi Guys,

Do you have experience on the difference between single and dual coil PI detectors? I mean that type of design which uses different coils for transmitting and receiving.
The dual design has the advantage, that it doesn't need a galvanically isolated voltage pump for the opamps. Are there any disadvantages besides having to build another coil? Is the sensitvity worse? By chance, doesn't magnetic targets couple the 2 coils better than non-magnetics (giving better sens to magnetic targets)?

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Offline GoldDigger1950
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« Reply #1 on: January 31, 2010, 08:52:45 am »
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The second coil has a damping effect on the overall performance and can limit power to the ground. The single coil is the most common design and would be my choice since that receiver coil acts like a big antenna right in the same field and alignment as the transmit coil. Unlike a VLF/TR machine, the dual coils are usually very close to the same size in PI units.

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Offline DOOLEY1972
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« Reply #2 on: May 30, 2011, 02:40:50 pm »
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i like the idea of dual coil pi's , only require single supply (when one ind of rx conected to zero volts), as return signal returns down to zero volts instead of going high and reurning down to + rail, and a step up transformer effect can be used , more turns on rx = more voltage deviation for a given target.

tried this my self , experimentaly , using 3-4 times as many turns on the rx , please dont scope the rx coil without a divide by 10 probe as voltage can peak at over 2000 volts , if scope good for 400v a divide by 10 get you upto 4000v.

accidentaly touching the rx wires will result in jumping a little.

1n4148's only good for 100v so higher resistance coupling resistor needed , about 10k ish
damping resistor required on rx is alot "lighter" 1k to 3k ish depending on coil.
have experienced increace in sensitivity and apparently no loss in effective power / field.

still thinking this is the way to go for sensitivity and simple receive design.

Posted on: May 30, 2011, 02:38:18 PM
forgot to add one thing , with over 100 turns on the rx it will need shielding , otherwise you have a real nice radio aerial.

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Offline dr.jare
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« Reply #3 on: May 31, 2011, 12:45:46 pm »
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Dual Coil PI detectors is good for "bad ground" but the "average ground " has less depth of detection of single coil PI detectors.

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Offline GoldDigger1950
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« Reply #4 on: May 31, 2011, 02:52:09 pm »
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Dooley, from a design perspective it is much safer for the circuitry to have a dual coil setup and Faraday shielding is an absolute necessity. The dual coil eliminates the need to blank off the receiver during the transmit pulse, which is something I do in my designs. In my current project, I am using a dual coil design. The TX coil field decay time and the huge gain in the RX circuits are making things difficult and turning the machine into a very slow one. One has to avoid the TX signal being mistaken for a target signal. Know what I mean?

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Offline DOOLEY1972
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« Reply #5 on: June 01, 2011, 02:08:26 am »
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hello GD . sort of know what you mean , with dual coils the decay takes longer.
with a single coil i was playing with last night , and it was only tight wrap copper ennamel , i was below 0.7v only 3us after switch off ,  where as with a dual coil , tx30t , rx100t , i'm getting 50us ish before it drops below 0.7v.
even with a mono of 100t i'm getting 35us decay , and thats with insulated wire , which is suposed to be faster due to lower inter winding capacitance.

conclusion is the more turns , the longer the wait , which will "miss" low conductivity targets.
but more turns = more deviation / sensitivity , it's all a compromise.

thought about 3 coils , a low 30 turn tx and 2x50 turn rx coils wired to give + and - then op-amp the differance , bet if i tried this the 2x50 turns would still add up to the same long decay.

thought of a poss silly idea the other day after someone mentioned that aposing coils kills flyback , but they also kill field strength ,so , 2 opposing coils , 1st is tX and the second kicks in when the tx is at flyback peak , decay should come down quick and being after tx peak , the field strength should not be reduced.

this should get a short decay regardless of how many turns.

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