make it smaller

Keeping everything the same!

Garret made a 1" coil for there Detectors at one time.

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Gramps, keep all of the parameters you have measured the same. One additional factor needs to be considered. On the transmitter side, the resistance is important to keep your output transistors from damage. Match the resistance if you are able by using a different sized wire than the original. The diameter of the wire will be what determines the resistance. Use a multimeter to read your present coil. Record the resistance and when you wind your new coil, measure again. If it is too high or too low, rewind the transmitter coil. The receiver is not at all bothered by a differing resistance.

On every spool of magnet wire there is a resistance per foot rating which will help you with this. Since you know the diameter and can calculate the number of turns, figure out your total length of the wire. Multiply the resistance rating with the length and you will have a very close match. Inside of your machine there is a device which turns your TX on and off. It is known as a MOSFET - a type of transistor. The resistance is critical in two ways. The MOSFET has the ability to "sink" the current during the field collapse based on an internal factor of resistance (unknown to you as a coil winder). The coil resistance being off will cause it to overheat and eventually die an early death. Thus, this factor must be matched for the safety of your machine. Secondly, the field collapse time will be directly influenced by the resistance of the coil, the internal capacitance and the inductance. The combination of those three factors is called impedance. Not additive - logarithmic. Don't even try to calculate it. It'll warp your brain. Just know that the field must collapse on schedule (proper timing) or the receiver will be swamped and you will never sense a target.

Xavier will benefit by reading this because he is having some trouble with his VLF/TR coil design. As will you if you don't watch all of the important factors. Good luck with your project. I'm interested to see how things work out for you.

Posted on: March 10, 2013, 04:24:43 PM

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By the way, there are those who wind their coils based only on the resistance of the new coil matching the old one. While they are not getting the most efficient coil by a large factor, they are keeping their machines from going south on them.

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		Quote:			Posted by homefire
	Another Factor that's not mentioned is the fact the Receive and and Transmit Coils are NOT tuned to the same points in Freq.

He's using his original specs, measured. This is why I never mentioned frequency. It wasn't meant to be a full coil design tutorial where one builds a coil without a reference. The frequencies are irrelevant to the discussion.

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Your bucking coil will increase your frequency very slightly, remember that the bucking coil is an extension to the TX coil but in the reverse direction. You will need to play with the bucking coil in order to get the proper nulling, normally the bucking coil is TX/4 use this formula as a starting point. Good luck in your project and get lots of magnet wire. By the way I got mine working now, tested it with out shielding (I'll be using a graphite spray for the shield) I also have two transistors to replace from metal casing to plastic. I'll let you know how it performs once that is done Ho by the way you'll need a scope to null the coil properly.

All the best
Xavier       

Posted on: March 10, 2013, 11:08:03 PM

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You could use a voltmeter but I found it to jump around a bit

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Xavier

OK to do the nulling you will need to connect the TX to the detector, connect the RX to the scope now you have to add or remove some wire on the bucking coil until you have the smallest voltage (Oscillation) on the RX coil. Leave a loop for final adjustment, once the coil has been glued together use the loop for the final adjustment then glue the loop in place.  You have two options for shielding graphite spray or Mylar tape.     

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Xavier