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Offline w5cdtTopic starter
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« on: December 09, 2009, 01:02:41 pm »
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I'd like to design a new coil for my 1225X for a special application.  What I need now is the pinout for the search coil connector.  I'm an experienced electrical engineer and would appreciate any 1225x schematic or tech documentation that you might have or know of.

Thanks,
CT

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Offline GoldDigger1950
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« Reply #1 on: December 09, 2009, 03:55:21 pm »
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As an engineer you should be able to do a pinout based on resistance readings. The PC board inside your unit may also provide clues. As a rule, transmit coils are smaller than receive coils. The sensitivity of the receiver coil circuitry has to be pretty high. It's reading the tiny fields that build in and around the metal objects in the transmitted field. The transmit coil needs to be as small as possible and also ensure field collapse is rapid. Thus, you will probably find a 'snubber' resister across that coil in the unit's main board. The transmitted field must collapse rapidly so the receiver coil is not swamped by the unit's own transmitted signal.

As you can see, the coil is a complex passive device that must appear to be active to ensure proper working. Most coils have 4 wires but yours might have 5, one acting as a shield or ground wire to keep cable noise to a minimum. You're in for a fun but bumpy ride, mate. Enjoy!

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Offline w5cdtTopic starter
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« Reply #2 on: December 09, 2009, 05:18:54 pm »
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 Smiley

Thanks for the info.  I just traced out the TX and RX coil pinouts.  When you mention rapid field collapse and "snubbers" it sounds like you are describing a PI detector.  The Fisher 1225X is an induction balance metal detector that runs at a sinusoidal 5 KHz. It may have a resonating cap but the TX coil is part of a sinewave oscillator circuit. 

More later as I make a few coil inductance measurements.

Cheers,
CT

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« Reply #3 on: December 09, 2009, 08:04:26 pm »
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Quote:Posted by w5cdt
Smiley

Thanks for the info.  I just traced out the TX and RX coil pinouts.  When you mention rapid field collapse and "snubbers" it sounds like you are describing a PI detector.  The Fisher 1225X is an induction balance metal detector that runs at a sinusoidal 5 KHz. It may have a resonating cap but the TX coil is part of a sinewave oscillator circuit. 

More later as I make a few coil inductance measurements.

Cheers,
CT


Hello w5cdt . You received many good answers . The 1225x runs at 5.5 khz . The inductance that is required on the 8 inch fisher coil must be: TX = 2.4 millihenry and RX = 10.8 millihenry .
I am checking for number of turns / wire size .
Regards.........Eugene

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« Last Edit: December 09, 2009, 08:06:31 pm by Eugene52 » 		Logged
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Offline w5cdtTopic starter
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« Reply #4 on: December 09, 2009, 08:36:18 pm »
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Eugene,

Thanks for the help.  On the Fisher 1225-X search coil connector can you tell me the pin numbers for TX and RX coils?  I assume the higher resistance is the RX coil since it has the highest inductance.

Thanks,
CT

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« Reply #5 on: December 09, 2009, 09:05:53 pm »
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Quote:Posted by w5cdt
Eugene,

Thanks for the help.  On the Fisher 1225-X search coil connector can you tell me the pin numbers for TX and RX coils?  I assume the higher resistance is the RX coil since it has the highest inductance.

Thanks,
CT
Correct , the lower resistance TX provides more current ie [transmit power] and the RX higher resistance is more wire surface free electrons to pick up charged targets in the ground . I still have to search for more coil information . Sorry I did not answer you sooner but my time online is limited due to being super busy . What size coil do you want to make or use ? I will look for color/pin configuation .
Regards.............Eugene

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Offline w5cdtTopic starter
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« Reply #6 on: December 09, 2009, 10:23:17 pm »
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Eugene,

I'd like to make a smaller coil for hunting trashy areas.
Now that I know the ballpark numbers for the TX and RX inductance
I should be able to figure out the number of turns for each coil.
I already know how to null the bucking coil.

Thanks,
CT


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Offline GoldDigger1950
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« Reply #7 on: December 10, 2009, 12:55:44 am »
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Quote:Posted by w5cdt
Smiley

Thanks for the info.  I just traced out the TX and RX coil pinouts.  When you mention rapid field collapse and "snubbers" it sounds like you are describing a PI detector.  The Fisher 1225X is an induction balance metal detector that runs at a sinusoidal 5 KHz. It may have a resonating cap but the TX coil is part of a sinewave oscillator circuit. 

More later as I make a few coil inductance measurements.

Cheers,
CT


The Fisher is a VLF/TR machine according to this.

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You still need a resistor to collapse the transmit field as quickly as possible to allow for target detection by the receiver coil.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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Offline Eugene52
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« Reply #8 on: December 10, 2009, 08:49:07 am »
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Hello w5cdt .
Here is a great pdf article on coplaner search coil construction , Or you can make a DD configuration . I am trying to verify that the 10.5 inch Fisher and [Detech] DD is 2.2 millihenry @4.9 d.c. ohms for the TX and the RX is 10.4 millihenry @ 39.3 d.c. ohms for 5.5 khz [1225x] .
HH.......Eugene

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