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Anonymous
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I have enjoyed the discussion on ground elimination. I thought I might share a most interesting article that I found on the internet.
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Anonymous
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Brilliant Dave, now can you explain the bit after
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Anonymous
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Anonymous
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Re: the article. . . CANDY USES A LINEAR ON RAMP INSTEAD OF A RECTANGULAR PULSE!!! * * * * * METAL AND FERRITE HAVE DIFFERENT TIME DECAYS FOR DIFFERENT RAMP ON TIMES. That my friends would seem to be the answer. What a great article for sure, Dave. * * *
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Anonymous
Guest
Hi Dave, you hit the black, but there are no rectangular pulses, the exponential rise depends on the coil L-R constant. It is necessary to specify the coil resistance and to know more about permeability saturaion, is n
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Anonymous
Guest
Sid, We should be able to figure it out using the equation provided in the article. The Minelab coil construction should also provide a few clues as to what is going on, Dave. * * *
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Anonymous
Guest
Hi Dave,
With cable, the Minelab coil is about 0.5 ohms and 300uH. Time constant is therefore 600uS. For the current to grow to its maximum value, defined by the resistance, the pulse duration would have to be 3600uS. A 60uS TX pulse (Minelab short pulse) will give 0.1 of the resistive current at its peak, and a 300uS long pulse will give 0.4 of the resistive current at its peak. Both pulses will be almost sawtooth shape due to the inductive current growth, the wide one having the greater curvature.
That's a great paper on magnetic viscosity Dave. I assume it is fairly recent, but could not find a date. I am getting some more information together and will do a post in due course.
I did some more curves late this afternoon with different coil resistances, currents and different rocks and soil. Seems like magnetic relaxation can be all things to all people. Most papers on the subject say that the decay changes with pulse width. However......, more on that later.
Eric,
With cable, the Minelab coil is about 0.5 ohms and 300uH. Time constant is therefore 600uS. For the current to grow to its maximum value, defined by the resistance, the pulse duration would have to be 3600uS. A 60uS TX pulse (Minelab short pulse) will give 0.1 of the resistive current at its peak, and a 300uS long pulse will give 0.4 of the resistive current at its peak. Both pulses will be almost sawtooth shape due to the inductive current growth, the wide one having the greater curvature.
That's a great paper on magnetic viscosity Dave. I assume it is fairly recent, but could not find a date. I am getting some more information together and will do a post in due course.
I did some more curves late this afternoon with different coil resistances, currents and different rocks and soil. Seems like magnetic relaxation can be all things to all people. Most papers on the subject say that the decay changes with pulse width. However......, more on that later.
Eric,
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Anonymous
Guest
A few more clues.
Appears that the drive to improve de-mining techniques will generate lots of scientific research into the earth
Appears that the drive to improve de-mining techniques will generate lots of scientific research into the earth
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Anonymous
Guest
Hi Eric,
I believe all SDs use Litz wire. This would indicate that this coil uses about 50 strands of 36 awg, am I in the ballpark?
Ta
Kev.
I believe all SDs use Litz wire. This would indicate that this coil uses about 50 strands of 36 awg, am I in the ballpark?
Ta
Kev.
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Anonymous
Guest
Hi Kev,
Yes, it is Litz. The overall wire diameter is 1.5mm but I don't know how many strands there are.
Eric.
Yes, it is Litz. The overall wire diameter is 1.5mm but I don't know how many strands there are.
Eric.
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Anonymous
Guest
Hi Kev,
I have been making mono coils for the SD2000 for a few years so may be able to help with you question. If you feel so inclined you can make your own Litz wire quite simply.
I'll use the 13 1/2 diameter mono coil I make as an example. This has an inductance of 300 micro Henries and a Q of 4.5 ( this figure includes the resistance of the cable) Capacitance of the coil is 76 micro Farads. No of turns approx 17. I usually measure with an LCR meter and adjust turns as required..
To give the required final Q, the Q of the coil will need to be approx 4.9. When you connect the cable it will drop down to 4.5.
The Litz wire is made using 30 to 32 strands of 0.2 mm diameter enamelled wire. Each strand is approx 22 meters in length. I bunch up the wires and apply a twist to the total wire bunch. This will take some time but if done carefully will give good results. The diameter after the twisting will be about 1.5 mm.
I normally wind the coil using a former and lace up the final coil to hold all the coils in place.
Apply a varnish and wait for it to dry. I have found that varnishing the coil stops spurious noise in the coil when you hit a hard object such as a rock.
For a 16 1/2 diameter coil the number of runs is about 24 with coil length about 25 meters.
If you require a lower Q remove a run or two. Requires a little trial and error on first coil.
Regards,
Stef
I have been making mono coils for the SD2000 for a few years so may be able to help with you question. If you feel so inclined you can make your own Litz wire quite simply.
I'll use the 13 1/2 diameter mono coil I make as an example. This has an inductance of 300 micro Henries and a Q of 4.5 ( this figure includes the resistance of the cable) Capacitance of the coil is 76 micro Farads. No of turns approx 17. I usually measure with an LCR meter and adjust turns as required..
To give the required final Q, the Q of the coil will need to be approx 4.9. When you connect the cable it will drop down to 4.5.
The Litz wire is made using 30 to 32 strands of 0.2 mm diameter enamelled wire. Each strand is approx 22 meters in length. I bunch up the wires and apply a twist to the total wire bunch. This will take some time but if done carefully will give good results. The diameter after the twisting will be about 1.5 mm.
I normally wind the coil using a former and lace up the final coil to hold all the coils in place.
Apply a varnish and wait for it to dry. I have found that varnishing the coil stops spurious noise in the coil when you hit a hard object such as a rock.
For a 16 1/2 diameter coil the number of runs is about 24 with coil length about 25 meters.
If you require a lower Q remove a run or two. Requires a little trial and error on first coil.
Regards,
Stef
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Anonymous
Guest
Thanks for that Stef, I'd never thought of making my own Litz. Your using 0.2mm puts you in about the 10kHz frequency range. I wonder if this is related to preferred target frequency response, as it can't be the pulse repetition rate?
Cheers
Kev.
60 HZ - 1 KHZ
28 AWG = 0.32 mm
29 AWG = 0.287 mm
1 KHZ - 10 KHZ
30 AWG = 0.254 mm
31 AWG = 0.226 mm
32 AWG = 0.203 mm
10 KHZ - 20 KHZ
33 AWG = 0.180 mm
34 AWG = 0.160 mm
35 AWG = 0.142 mm
20 KHZ - 50 KHZ
36 AWG = 0.127 mm
Cheers
Kev.
60 HZ - 1 KHZ
28 AWG = 0.32 mm
29 AWG = 0.287 mm
1 KHZ - 10 KHZ
30 AWG = 0.254 mm
31 AWG = 0.226 mm
32 AWG = 0.203 mm
10 KHZ - 20 KHZ
33 AWG = 0.180 mm
34 AWG = 0.160 mm
35 AWG = 0.142 mm
20 KHZ - 50 KHZ
36 AWG = 0.127 mm
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Anonymous
Guest
Hi Kev,
Actually it's not all that complicated. I also built coils using 0.17 mm wire. This required 42 to 44 strands to give a Q of 4.50. When I compared the coils I could not find any real difference in performance. So I still use 0.2 mm wire as its easier to use than the thinner wire and I can get it readily. Later I found that Minlab in their 18 1/2 inch coil also used about 0.2 mm wire.
I found something on the net which basically gives the frequency range where the " skin effect" predominates. Although I use Litz wire I tend to agree with Eric that there may not be significant benefits in using Litz over stranded wire within the frequency range used for PI's.
Regards,
Stef
Actually it's not all that complicated. I also built coils using 0.17 mm wire. This required 42 to 44 strands to give a Q of 4.50. When I compared the coils I could not find any real difference in performance. So I still use 0.2 mm wire as its easier to use than the thinner wire and I can get it readily. Later I found that Minlab in their 18 1/2 inch coil also used about 0.2 mm wire.
I found something on the net which basically gives the frequency range where the " skin effect" predominates. Although I use Litz wire I tend to agree with Eric that there may not be significant benefits in using Litz over stranded wire within the frequency range used for PI's.
Regards,
Stef