Is this good,566uH ,3 ohm DC resistance ?

[ZILIVSS]

Again zilivss with bad english :-) ,
I build a coil for HH opt2 , 8" in diameter with 30 turns of 26AWG wire ,
my mesurements were 566uH , 3ohm , is this good , or I must change
the walue of R11 , 680 ohm or ???

[bbsailor]

The value of R11 is set to create a condition called "critical damping". Critical damping occurs when there is no visible ringing after the pulse dies down. All of the things in the circuit that add capacitance effectively in parallel with the coil (coax, MOSFET, coil shield, and capacitance between the coil windings) tend to store more energy that needs a smaller value of resistor to damp the oscillations from the stored energy. The value of 680 ohms will get you close but it is best found by using a scope to view the pulse at the output of the preamplifier and adjusting a temporary pot to optimize the damping for your particular design. Here is how to do it.

Put a 1 K ohm resistor in parallel with a 1K resistor in series with a 5K pot.
This will give you an adjustment range of 500 ohms to about 850 ohms. Use a .5 watt resistors and you will be able to effectively handle about .7 watts of energy. Attach the scope to the output of the preamp and adjust the pot for minimum ringing. Measure the value of the pot at that setting and find a single resistor or make up a parallel pair with the same effective resistance. If the resistors get hot at wide transmit pulse widths, increase the wattage to 1 watt.

Your coil size of 566 uH may be a little too high for low delay settings at or below about 15 uS. The optimum coil design depends on what you are looking for. For coin hunting you are probably OK using wider TX pulses and longer delays. For gold hunting you may need a few less turns on the coil. Here is how to do an experiment to see how fast you coil can respond.

While waving a gold ring over the coil reduce the main delay pot. If the detector stops responding to the ring, you have reached the minimum delay limit of that coil. One fix is to reduce capacitance by removing about 5 or 6 turns (new value of 300 to 350 uH) and re-adjusting the damping resistor. You can also increase the spacing between the coil shield and the coil to help reduce capacitance using plastic spiralwrap.

I hope this helps?

bbsailor

[ZILIVSS]

coin hunting !!!

I construct it primarly for coin hunting , ancient roman coins
made of bronze and copper , maybe some silver.There are so many locations for coin hunting in area where I live , you said that it will be probably OK for that purposes . The bad thing is that I don't have scope for measuring ,I must
find it or maybe some other solutions to solve this situation ,Carl , have you any suggestions ?