pi power

[Ferric Toes]

I am also not fan of lot of amperes in coil too. But in this particular case i did that because of the 1x1 meter coil which i made. It has 9 windings of 1mm litz, pvc coated cable. Overall it measures as i noted; 0.350mH and somewhat 1.5R with cable.
Such coil presents serious "drain" for only one fet, so as i noticed. Fet can get very hot on some adjustments. Especially on wider pulses. And that's what i want with that coil; wider pulses. I will search for medium sized and larger objects in soil with such coil.
So adding another fet was just sort of blind guess. But it was lucky guess, since now i have significantly better behavior.
I intend to inspect this further. Problem is in coil; it is too large to be tested indoor and now these days it is raining. Must wait.

Yours is a different problem and there is no need to pursue fast switch off and short delays. The way I would approach things is to push up the pulse current, widen the pulse, and reduce the pulses per sec. I would change the power fet for, say, an IRFP22N50A which will give you 500V avalanche and an ON resistance of 0.25 ohm. Better still an IRF250N which is 200v but has an ON resistance of 0.05 ohms. That will keep mosfet the heat down, but the coil will get hot when you are pushing 10 - 15A peak current. A coil I made for such a TX ran at 70C temperature in air. It was for an underwater application however, so in practice it was watercooled. TX pulses can be widened to 500uS or even 1mS, and a delay of 100 - 200uS works fine. Also widen the sample pulses to, say, 50uS. The TX mosfet can still get hot but I used a fan cooled heatsink as used in computers to keep the processor cool.

Just the project for cold rainy days.

Eric.

[ivconic]

Another approach is to halve the no. of turns in the Tx coil and double the current. The Ampere-turns remain the same, but the coil capacitance, inductance and resistance get divided by 4. Transient decays faster, allowing for earlier sampling. If the Tx period is also halved then the power consumption remains the same.

Bingo! That's what i had on my mind when making this coil! Despite the size; i wanted a bit earlier sampling and "hot" behavior. Got tired of "slow" and "inert" behavior as with standard Delta Pulse setup in the past.

[ivconic]

Yours is a different problem and there is no need to pursue fast switch off and short delays. The way I would approach things is to push up the pulse current, widen the pulse, and reduce the pulses per sec. I would change the power fet for, say, an IRFP22N50A which will give you 500V avalanche and an ON resistance of 0.25 ohm. Better still an IRF250N which is 200v but has an ON resistance of 0.05 ohms. That will keep mosfet the heat down, but the coil will get hot when you are pushing 10 - 15A peak current. A coil I made for such a TX ran at 70C temperature in air. It was for an underwater application however, so in practice it was watercooled. TX pulses can be widened to 500uS or even 1mS, and a delay of 100 - 200uS works fine. Also widen the sample pulses to, say, 50uS. The TX mosfet can still get hot but I used a fan cooled heatsink as used in computers to keep the processor cool.

Just the project for cold rainy days.

Eric.

Thanks a lot! Actually that's what i tried to "mimic" with two fets. Some fets are not easy to obtain in local shops, i don't have too much choice here. Will look up for IRF250N.

[Carl-NC]

I've started a fresh thread on the merits of TX flat-topping here:

http://www.geotech1.com/forums/showt...PI-TX-Settling

[6666]

Yet another way is to centre tap a standard coil and drive the tap and one end with the TX and have the full coil as RX. That way you get the benefit of 4 x less inductance etc for the TX but don't lose out on the RX side.

Hi Eric, did you do much field testing with this idea ? I guess the centre tap would allow the use of different types of wire in the two different sections of the coil.

[ivconic]

Hi Eric, did you do much field testing with this idea ? I guess the centre tap would allow the use of different types of wire in the two different sections of the coil.

That's very interesting idea for experimenting!
Now... i can think on at least four different cases:
Would be interesting to do a "study" and analyze each one of the cases; benefits and drawbacks.
Small and simpler PI design such as Barracuda is; would be suitable as experimenting platform for this.

[Ferric Toes]

Hi Eric, did you do much field testing with this idea ? I guess the centre tap would allow the use of different types of wire in the two different sections of the coil.

No field testing, just tests in the workshop as yet.

You could use different types of wire but my tests were done with a low pulse current and high repetition detector. The TX could be thin wire too and I used Teflon insulated single strand 0.25mm wire for the whole coil, with the tap brought out as a twisted loop. If you wanted a high pulse current with thick wire for the TX, then it would probably be simpler to have two separate coils. One sitting on the other or immediately side by side in the shell. Say 200uH for the TX and 800uH for the RX. Bear in mind that this acts as a step up transformer so your flyback voltage will be multplied. (Ignition coil principle). Some commercial PI's have coaxial coplanar coils, or even DD coils, where the RX coil inductance is considerably higher than the TX coil and the wire is thinner.

Eric.

[green]

I tried the center tapped coil and it looked like loosely coupled would be better than close coupled. Another thing to test.

[6666]

No field testing, just tests in the workshop as yet.

Thanks Eric, lots of food for thought.

[Altair]

That's very interesting idea for experimenting!
Now... i can think on at least four different cases:
Would be interesting to do a "study" and analyze each one of the cases; benefits and drawbacks.
Small and simpler PI design such as Barracuda is; would be suitable as experimenting platform for this.

From what Ferric Toes has written I understood another case, different from A, B, C and D:

+----- RX
L1
+----- TX
L2
+----- Common

this would be an "E" case!

[ivconic]

From what Ferric Toes has written I understood another case, different from A, B, C and D:

+----- RX
L1
+----- TX
L2
+----- Common

this would be an "E" case!

Yes you are right!
I wonder; would my "cases" makes any sense?
Now on second thought; i think it would mainly depends on how TX and RX front end are designed.
With common approach there should not be any problem to try either of cases.
In TX period; RX is clamped with diodes anyway.
When TX is shut off; RX will pick up only the RX "side" of coil, regardless what's happening or was left in TX "side" of coil.
Hmmm... does this makes any sense?

[ivconic]

"...When TX is shut off; RX will pick up only the RX "side" of coil, regardless what's happening or was left in TX "side" of coil..."

On second "second" thought; this is not true!
Since TX and RX are close parts of the same coil; RX will be also inductively coupled with TX!
Cases "A" and "B" are presenting coils wounded in same direction, cases "C" and "D" in opposite.
So... in cases "A" and "B" we will have directly picked RX signal with additional portion of induced signal with same polarity.
In cases "C" and "D" we will have directly picked RX signal with additional portion of induced signal with opposite polarity.
Story complicates even more in cases "A" and "C" where RX coil is having more turns than TX coil...
Sheeesh! I am lost!
I am switching now to ninja mode for a couple days!

[ivconic]

No field testing, just tests in the workshop as yet.

You could use different types of wire but my tests were done with a low pulse current and high repetition detector. The TX could be thin wire too and I used Teflon insulated single strand 0.25mm wire for the whole coil, with the tap brought out as a twisted loop. If you wanted a high pulse current with thick wire for the TX, then it would probably be simpler to have two separate coils. One sitting on the other or immediately side by side in the shell. Say 200uH for the TX and 800uH for the RX. Bear in mind that this acts as a step up transformer so your flyback voltage will be multplied. (Ignition coil principle). Some commercial PI's have coaxial coplanar coils, or even DD coils, where the RX coil inductance is considerably higher than the TX coil and the wire is thinner.

Eric.

[Altair]

"...When TX is shut off; RX will pick up only the RX "side" of coil, regardless what's happening or was left in TX "side" of coil..."

On second "second" thought; this is not true!
Since TX and RX are close parts of the same coil; RX will be also inductively coupled with TX!
Cases "A" and "B" are presenting coils wounded in same direction, cases "C" and "D" in opposite.
So... in cases "A" and "B" we will have directly picked RX signal with additional portion of induced signal with same polarity.
In cases "C" and "D" we will have directly picked RX signal with additional portion of induced signal with opposite polarity.
Story complicates even more in cases "A" and "C" where RX coil is having more turns than TX coil...
Sheeesh! I am lost!
I am switching now to ninja mode for a couple days!

Just a thinking to have less TX inductance but more on RX...

[ivconic]

Just a thinking to have less TX inductance but more on RX...

That makes sense.
From your schematic is obvious that L1 and L2 must be identical.
Would be interesting to see such coil in "OO" arrangement!
Remember the "8" coil benefits!

(ah i can't stay in ninja mode for long!)