IB-PI coil

[Tinkerer]

TMPI coil #5

I know it is going to take a few coils for trial an error, to come up with the final coil. Therefore I wind and assemble the coils in a temporary manner, to be tested, before casting a final coil.
I also try to use ordinary wire and make the whole thing as simple as possible at the beginning. Once the right compromises have been found and the coil sensitivity is within acceptable limits, further refining can be done.

I think the coil #5 has reached acceptable limits. Here are the specs:

TX, OD 45cm, 16 turns of #20 AWG magnet wire.
RX, OD 22.5cm, 20 turns of #26 AWG magnet wire, center tapped.
Bucking coil, 4 turns of #20 AWG magnet wire, wound around the RX coil.

The cable is 2m long so that I can place the coil at some distance from the scope and breadboard. I use CAT 5 cable because I need at least 6 legs.
Since the CAT5 cable has 8 legs, I parallel the spare legs for the TX to reduce the resistance.
I end up with a total resistance (coils, cable and Mosfet) of about 2 Ohm for the TX.
With the pulse at 127uS and the TX voltage at 15, this gives a max TX pulse of about 4A.
For the testing I do not make a full shield, but I clamped the coil assembly between 2 sheets of Polypropylene "cardboard" that have been coated on one side with a graphite compound. A drain wire is embedded and connected to the ground.
The center tap is connected to the ground.
The wires are:
1+2, TX out
3+4, TX in
5 bucking coil current control
6 RX+
7 RX-
8 Ground

When I run the TX, and directly sample the input and then amplify it 15 times, I can see the response of the targets on the scope.
The smallest target that I can pick up right on the coil, is the 20mm diameter
on side copper PCB disk that I cut out. Unfortunately I don't know what the exact thickness of the copper is.
This minimum target gives me a 1.5mV signal at the center of the coil.
since this signal is amplified by 15, the actual signal response is 0.0001V or 100uV.

Is this good enough? Feedback and comments are welcome.

Considering, the basics of the coil are good enough, what can be improved?

The bare magnet wire produces quite a bit of inter-wire capacitance. Teflon coated wire puts a bit more space between the copper, so the capacitance is less.
Also I ended up with a about 2 Ohm of total resistance. Reducing the resistance would improve the efficiency and increase the TX pulse Amps.

Using several wires in parallel for the TX coil winding, for example 4 times #22AWG, would reduce the wire resistance by half and reduce the capacitance too. Again, Teflon would be very good, but as some of my earlier tests have shown, adding a Polypropylene string as spacer between the wires
reduces the inter-wire capacitance dramatically.

The coil cable: Seeing that the CAT5 cable is working quite OK, I would recommend shielded CAT6 cable that has AWG#22 wire instead of AWG#24
like CAT5.

The shielding of course should be done around each coil bundle, but with at least a 5mm spacer between the shield and the wire bundle.
The graphite compound seems to be doing a very good job, so I recommend it.
Tinkerer

[6666]

hi
good work
just wondering in post #8
what is the advantage of c9 and r8 ?
thanks

[Carl-NC]

The wires are:
1+2, TX out
3+4, TX in

You should consider

1+3, TX out
2+4, TX in

This will reduce magnetic field coupling to other wires. Also put RX+ and RX- on a twisted pair for best EMI rejection.

[authere]

coil making

Hi Tinkerer,
I am not in favour of bunch winding as it can produce hot and cold spots, we get an overall reading of inductance with an LCR meter but where the wires are bundled together closer there would be discrepincies.

I prefer to wind my coils around soft foam, something i can put a pin into and i pin the first row around the circle of req'd diam. of foam but then to reduce the capacitance i run a strip of plastic or cardboard over the first layer, super glue it into place and continue to lay the next layer with pins to keep it all neat and so on until i get the req'd inductance i am after.

No hotspots and if req'd a coat of varnish over it before you take it off the mould and in the end before the wire is taken off the mould i would use some fine cotton to tie the bundle together, not that huge big blue string that you are using as this just adds to the amount of plastic that the coil has to tend to. Just two half hitch's and a granny knot, cut and move onto the next knot.

By far the easiest method to make a coil is to use stranded tinned copper wire with a suitable insulation about 1mm sq and keep winding the wire no matter what the diameter until the req'd inductance is achieved. This is good for up to 16-18'' diameter.

This is just my take on how i go about my coils.

Regards Ron

[Tinkerer]

hi
good work
just wondering in post #8
what is the advantage of c9 and r8 ?
thanks

Hi 6666,

thanks for the feedback. You have a gift of acute observation. I also would like to congratulate you for being the first one to have noticed some of the gross errors on the cct of post 8.

You see, I have a little bit of dyslexia. This means that sometimes I don't know if a d is a b or a s is a z. My mind just twists things around a bit.
I have struggled nearly 70 years with that little problem always hiding the fact. Nowadays, the spell checkers are very good at fixing my spelling, but other things they don't fix.
Finally, after nearly 70 years I came to the point, where I can admit that I have this problem. So now I am not embarrassed with it anymore. After all, I have been very successful in spite of it.

So, on the cct of post 8, there are a few mistakes. You ask about R8. I don't see any R8. Are you maybe referring to R24, 1.8 Ohm? It is at the wrong place. It should go on the ground side.
I have replaced it with a choke. Lets call it L#1.
C9? C9 and L#1 prevent the ripple caused by the 4A TX pulse from reaching the main power. The mosfet switches very hard and this generates noise. So this filter reduces the noise.

Any suggestions for a better way of reducing the noise are welcome.

I am going to redraw the cct and post it again. However, there are other mistakes on it. Who can see them?

Tinkerer

[Tinkerer]

You should consider

1+3, TX out
2+4, TX in

This will reduce magnetic field coupling to other wires. Also put RX+ and RX- on a twisted pair for best EMI rejection.

Carl,

thank you for the input. Your suggestion makes a lot of sense.

Tinkerer

[Tinkerer]

Hi Tinkerer,
I am not in favour of bunch winding as it can produce hot and cold spots, we get an overall reading of inductance with an LCR meter but where the wires are bundled together closer there would be discrepincies.

I prefer to wind my coils around soft foam, something i can put a pin into and i pin the first row around the circle of req'd diam. of foam but then to reduce the capacitance i run a strip of plastic or cardboard over the first layer, super glue it into place and continue to lay the next layer with pins to keep it all neat and so on until i get the req'd inductance i am after.

No hotspots and if req'd a coat of varnish over it before you take it off the mould and in the end before the wire is taken off the mould i would use some fine cotton to tie the bundle together, not that huge big blue string that you are using as this just adds to the amount of plastic that the coil has to tend to. Just two half hitch's and a granny knot, cut and move onto the next knot.

By far the easiest method to make a coil is to use stranded tinned copper wire with a suitable insulation about 1mm sq and keep winding the wire no matter what the diameter until the req'd inductance is achieved. This is good for up to 16-18'' diameter.

This is just my take on how i go about my coils.

Regards Ron

Hi Ron,

your way of making the coil is definitely much better. Real PRO.

Can I ask you what you mean with: not that huge big blue string that you are using as this just adds to the amount of plastic that the coil has to tend to .
I like that blue string. It is made of polypropylene, that has a dielectric constant nearly as good as Teflon. In fact it is even better than Teflon, because it contains about 50% of air in it's diameter.
By picking the right diameter string, I can add a desired spacing between the wires or between the coil and the shield.
Tinkerer

[authere]

blue string

Hi Tinkerer,
Definetly no PRO,but a stayer and a need to get it right is more what i am about.

My theory of dialectric constant that you can use the lowest dial. constant material you can find but if you use too much of it you defeat the purpose.Along those lines, if you select a very thin tape to wrap your coil and you make sure it has a very low dialectric what would it matter if you used something with a higher dialectric because it is so thin anyway!!!!

Another example would be using spiral wrap to bind your coils, this stuff has a low dialectric but the amount of material that goes into a coil has to be detrimental to performance compared to say, bubble wrap which is 95% air.

The fact that you use it inside the windings is a sound idea but 1/2 hitching all the way around the coil, apart from not looking PRO adds more dialectric constants the coil has to deal with.Thats why i mentioned that using a cotton to tie off the coil to keep the bundle neat with two half hitches together and a granny knot.

It's the old story, each to their own but i thought i would share my interpretations on the matter.

I am following the progress the both of you are having and looking forward to be able to making one myself if you's prove the process.

Regards Ron

[6666]

Hi Tinkerer
ok on the fet making noise and c9/r24 now a choke being a filter

dont know why I wrote r8 should have been r24
noise is always a problem

whats r27 for

[Tinkerer]

Hi Tinkerer
ok on the fet making noise and c9/r24 now a choke being a filter

dont know why I wrote r8 should have been r24
noise is always a problem

whats r27 for

R27 is the damping resistor that is common in traditional PI. Without damping, the Flyback generates wild oscillations.

Tinkerer

[Tinkerer]

Hi Tinkerer,
Definetly no PRO,but a stayer and a need to get it right is more what i am about.

My theory of dialectric constant that you can use the lowest dial. constant material you can find but if you use too much of it you defeat the purpose.Along those lines, if you select a very thin tape to wrap your coil and you make sure it has a very low dialectric what would it matter if you used something with a higher dialectric because it is so thin anyway!!!!

Another example would be using spiral wrap to bind your coils, this stuff has a low dialectric but the amount of material that goes into a coil has to be detrimental to performance compared to say, bubble wrap which is 95% air.

The fact that you use it inside the windings is a sound idea but 1/2 hitching all the way around the coil, apart from not looking PRO adds more dialectric constants the coil has to deal with.Thats why i mentioned that using a cotton to tie off the coil to keep the bundle neat with two half hitches together and a granny knot.

It's the old story, each to their own but i thought i would share my interpretations on the matter.

I am following the progress the both of you are having and looking forward to be able to making one myself if you's prove the process.

Regards Ron

Hi Ron,

your comments are much appreciated. This is what forums are about. A place to exchange ideas, views and methods.

http://www.mogami-wire.co.jp/e/cad/electrical.html

At the address above is a calculator that helps me a lot in understanding the inter wire capacitance and the coil to shield capacitance.

In few words: high dielectric = high capacitance
more space between the wires = less capacitance

Not all dielectrics are equal. The dielectric constant of PVC for example changes a lot with temperature.
This change will also change the capacitance.
The increase in capacitance increases the Flyback decay time.
The increase of the Flyback decay time makes the coil less sensitive to small targets.
How many degrees in coil temperature increase, when the coil is exposed to the hot Australian sun for a few hours?

The other thing that I try to pat attention to, is that the coil assembly is solid. Solid in the sense that there is no movement within the coil assembly when the coil knocks against a piece of wood or a stone, so that it does not produce a false signal.

The more I learn about PI detectors, the more new questions seem to be turning up. I am glad that there are others, like you Ron, who collect information and are willing to share this information on the forum. Thanks.

Tinkerer

[6666]

R27 is the damping resistor that is common in traditional PI. Without damping, the Flyback generates wild oscillations.

Tinkerer

ok on r27 being the damping resistor
I am used to seeing the loop and damping "R" drawn in parallel then off to the diodes
battery through the single loop in series with the main loop and another battery feed to the fet threw me off a bit .
I'll leave you in peace now

[Tinkerer]

ok on r27 being the damping resistor
I am used to seeing the loop and damping "R" drawn in parallel then off to the diodes
battery through the single loop in series with the main loop and another battery feed to the fet threw me off a bit .
I'll leave you in peace now

Thanks for the feedback.
anyway, it has been a good lessen. Post your mistakes on the internet and they will come back and bite you forever.

Since you have been very helpful, here is a tip. The Mosfet Q10 is a P type. It works better if you turn it around so that the source is on the 15V rail.

All the best

Tinkerer