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Minelab double D 11" DD coil

A

Anonymous

Guest
Hi,
I have one double D 11"DD coil from minelab and I conect It in my PI circuit It didn't work.
I set my PI for mono coil and conected only Tx coil, in this case It worked well, but when I set my PI for double D and conect Tx and Rx It dont work well.
My questions:
Do I need use R dump in parallel with Tx and Rx coil ?
The Tx coil has 280uH and Rx has 430uH, How can I calculate the ideal R dump to this coil ?
How can I adjust the value of the R dump with the oscilloscope ?
Best Regrds,
Gilmar
 
Hi Gilmar,
The Minelab coils that I have used already have an RX damping resistor inside the coil housing. That way, you don't need an additional resistor on the board. Minelab coils with the above inductances work fine with standard PI circuits down to about 15uS delay. Keep in mind that the TX coil resistance is very low; about 0.4 ohms. This has an impact on current consumption and TX transistor temperature.
Eric.
 
Hi Eric,
Thank you for your reply,
I didn't know that Rx coil has R damping inside.
And about the Tx damping, Does It has inside too ?
If no, what is the ideal R damping for Tx coil?
I would like to learn how to manufactory a DD coil, How I can do the correct adjust between the Tx and Rx coil and how I can visualize the signal in osciloscope for better adjust ?
I want to use the BELDEN double coax cable SVHS for DD coil and coax cable 50R for mono coil.
Is It a good chose ?
Thanks.
Gilmar
 
Hi Gilmar,
There is no TX damping inside the coil. I use a 470 ohms 2W metal film, and that gets quite hot.
Belden S-VHS cable looks good. The 1808A is the one I would choose as it has a round cross section.
There may be some detailed information on the Geotech Forum about making and balancing DD's, but basically they are two equal sized windings in a bloated D shape with one mirror imaged to fit in a circular shell. For a PI, an accurate balance is not needed, so you can feed, say, a 1kHz signal into the TX and with a scope on the RX, adjust the overlap for minimum coupling. As in the Minelab coil, the RX can have a higher inductance, and be much finer wire, as it is not carrying any current.
Eric.
 
Hi Eric,
Thank you for your reply.
I'll try to use your tips and soon return the results
Gilmar.
 
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