Grenade coil type systems

[Itsu]

I made a coil of 0.8mm magnet wire around the left part of the soft cores with the 4 magnetron magnets stuck to the right part of the soft cores.
There is 530 turns in this coil.

When increasing the current from 0 to 2.6A, the compass needle is turning about 45 degrees not further also if i increase the current.

Seems the compass is stuck mechanically or magnetically then.

Video here:  https://youtu.be/nEW6PDMlHcw



Itsu

[Apecore]

Seems the compass is stuck mechanically or magnetically then.

Always looking good your setups, nice clean work.
The compas is not the idealist measurement tool, Better would be Fe powder.
No flatscreen type of field visualizers available or smart options?

[Verpies]

When increasing the current from 0 to 2.6A, the compass needle is turning about 45 degrees not further also if i increase the current.

What if you reverse the DC polarity - will the compass turn the other way then ?
Also, if you move the compass left-right (in the orientation as it was filmed) can you find a place at which it is   at 90° ?  ...even over the cores or coil

[Verpies]

Always looking good your setups, nice clean work.

Yeah, he could have made a slip-shod rat-nest coil, ....but noooo, he had to make a neat one.

The compass is not the ideal measurement tool, Better would be Fe powder.

Yes, but most people don't have it on hand, unless they have kids and a broken etch-a-sketch.

[Apecore]

Yeah, he could have made a slip-shod rat-nest coil, ....but noooo, he had to make a neat one.

He makes your calculations easier to do.

[Grumage]

Well, perhaps we need an even lower tech solution?

What about suspending a magnetised needle? The Vikings certainly found it useful.... 

Cheers Grum.

[Verpies]

What about suspending a magnetised needle?

A suspended needle has more than the three rotational degrees of freedom.  It is likely the magnets will attract it linearly

[Itsu]

What if you reverse the DC polarity - will the compass turn the other way then ?
Also, if you move the compass left-right (in the orientation as it was filmed) can you find a place at which it is   at 90° ?  ...even over the cores or coil

If i reverse polarity then the needle goes the other way, but only like 35 degrees.

What i noticed then is that the current was limited to 2.9A due to the low voltage setting, so after increasing the voltage setting, i now can really increase the current, and it will turn the needle 90 degrees at 5.5A.
I do have to move the compass up and down to get the right position for it to move smoothly.

Itsu

[Verpies]

...after increasing the voltage setting, i now can really increase the current, and it will turn the needle 90 degrees at 5.5A...

You probably know this, but I must warn you that disconnecting that coil, while current is flowing through it, could damage your power supply.

What are the currents needed to move the compass needle parallel to the device's axis (0°) and perpedicular to that axis (90°) anywhere ?  ...especially where the Al ring was (if you can disassemble it easily)

[Itsu]

Yeah, i know, therefor i use the current limiter to slowly turn off / on the current.


When the current gets too high (like 7A), then the little compass is attracted and is jumping over to that coil as it seems that its center stud is made of metal.

The way i had the compass in the picture / video, the needle is pointing at an angle of 45° toward the nearest soft core, and when energizing the coil with a strong enough current (5.5A), then it points to the nearest soft core in the coil also at 45°, so it is never parallel to the device's axis.

When i put the compass on top of the alu pipe, then in rest the needle is parallel to the device's axis, but at 4A in the coil, it attracts the whole compass toward the coil, and if i hold it in place and wiggle it somewhat it jumps over 180°, so no smooth transition from North to south, so no 90° 

[Itsu]

I found a suitable metal box to house the electronics for the pulser, so here a glimpse of what i am trying to do with the coax in a copper tube coil:

As the current drawn is very low, i will power it from 4x 9V batteries parallel.

[Verpies]

... the little compass is attracted and is jumping over to that coil as it seems that its center stud is made of metal.

I can see how this would be a problem.
I thought that your compass is made out of glass like mine ...except the needle.

[Verpies]

The way i had the compass in the picture / video, the needle is pointing at an angle of 45° toward the nearest soft core, and when energizing the coil with a strong enough current (5.5A), then it points to the nearest soft core in the coil also at 45°, so it is never parallel to the device's axis.

That's because it is too far away from the axis. 
( aiding fields diagram not included )

See the off-axis 45° angles depicted in red below:

[Verpies]

I found a suitable metal box to house the electronics for the pulser,...

Is it solderable after sanding ?
Did you notice that the lid doesn't make an electric contact with the rest of the box because it is painted ?

...so here a glimpse of what i am trying to do with the coax in a copper tube coil:

Will you straighten the section of the tube which I marked in red color and solder it into the hole which I marked with blue arrow ?

[Verpies]

When i put the compass on top of the alu pipe,...

We are really interested in the direction of the flux in the gap (the space occupied by the Al ring).

..., then in rest the needle is parallel to the device's axis,

That flux should be parallel to the axis somewhere in the gap when the two fields are not bucking and even more so when they are aiding.

but at 4A in the coil, it attracts the whole compass toward the coil, and if i hold it in place and wiggle it somewhat it jumps over 180°, so no smooth transition from North to south, so no 90°

That 180° jump could be the compass getting remagnetized ...which is not good.
That 90° flux of the neutral line must be there somewhere. Maybe not in the gap or over the gap, but it must be there somewhere, with +DC or with -DC.

[Itsu]

Is it solderable after sanding ?

Yes, its solderable then

Did you notice that the lid doesn't make an electric contact with the rest of the box because it is painted ?

yes, it will need some sanding to have it sealed.

Will you straighten the section of the tube which I marked in red color and solder it into the hole which I marked with blue arrow ?

Yes, there will be a sealed connection around the coax from coil tube to metal box.

[Itsu]

We are really interested in the direction of the flux in the gap (the space occupied by the Al ring).

Yeah, but thats a problem as where there is an Al ring, there can be no compass so we will keep an offset.

That flux should be parallel to the axis somewhere in the gap when the two fields are not bucking and even more so when they are aiding.

Indeed, and i see that when reversing the current (aiding flux), the compass needle will move from 45° pointing to the magnets to more like parallel to the axis.

That 180° jump could be the compass getting remagnetized ...which is not good.

Right, its remagnetized now as without any magnets, it points with south to the northpole.

That 90° flux of the neutral line must be there somewhere. Maybe not in the gap or over the gap, but it must be there somewhere, with +DC or with -DC.

You mean the 90° flux at the positions shown in this picture:

[Grumage]

So in layman's terms, are we looking for a place that the two magnetic fields are at a null point?

How about using a length of Piano wire, magnetised by stroking and placing the end between the gap. The pivot could then be placed much further away unaffected by the strong magnetic field?

Cheers Grum.

[Verpies]

So in layman's terms, are we looking for a place that the two magnetic fields are at a null point?

Yes, like Wesley's neutral line, which you should be very familiar with.

How about using a length of Piano wire, magnetised by stroking and placing the end between the gap. The pivot could then be placed much further away unaffected by the strong magnetic field?

The pivot in the decent compass should be glass or jeweled in the first place.

[Verpies]

You mean the 90° flux at the positions shown in this picture:

Yes, but that diagram shows the flux lines of two equal coils.  Your magnets and ferrite sausage provide a little different field geometry.

Keep in mind that even with aiding flux in the gap, not all of it is parallel to the axis. As a matter of fact, the wider the gap is, the more the flux bulges out.



@Grum: That's why narrower gaps are better and 6mm is better than 30mm.

[Itsu]

Ok, so i might use a smaller Al ring (6mm is better than 30mm) to see if that 90° flux of the neutral line becomes visible.




Concerning the shielding of magnetic field of my nano-pulser, i found this interesting video:   https://www.youtube.com/watch?v=TSwohZSjSdg

So it turns out that the magnetic field (H field) is very hard to shield of especially at high frequencies (guess that the nano-pulse can be considered high frequency).

So is it even doable to shield it?

[lfarrand]

There is no way to shield a magnetic field. You can direct it using magnetically permeable material like Mu-metal or Terfenol-D.

As you say, it won't work very well for high frequency magnetic fields because it will cause the shield to heat up due to eddy currents. That's how an induction cooker works.

Shielding the electric field is easier because you can put something in a sealed metal box and ground it.

[Verpies]

So it turns out that the magnetic field (H field) is very hard to shield of especially at high frequencies

No, jest the opposite.  Watch, Dave's video again.
25kHz is considered low frequency by us.

(guess that the nano-pulse can be considered high frequency).

Yes

So is it even doable to shield it?

Yes, but the conductivity and continuity of the shield must be assured.

[Itsu]

No, jest the opposite.  Watch, Dave's video again.
25kHz is considered low frequency by us.

Right, so the better magnetic shielding is at higher frequency thus the nano-pulse

[Verpies]

There is no way to shield a magnetic field.

This is almost^* true in reference to constant magnetic fields.

You can direct it using magnetically permeable material like Mu-metal or Terfenol-D.

Yes, this type of materials would be very effective at diverting constant magnetic fields somewhere else, but there is a class of materials that can beet them hands down.

As you say, it won't work very well for high frequency magnetic fields because it will cause the shield to heat up due to eddy currents. That's how an induction cooker works.

Just the opposite.
You are correct that the shield heats up due to eddy currents and that's how an induction cooker works, but also notice that this is an energy dissipation mechanism (a conversion of alternating magnetic fields to heat).
Furthermore, this dissipation mechanism is more pronounced at higher frequencies, so higher frequencies are dissipated MORE.

I think you forgot to differentiate that constant magnetic fields are shielded by a different mechanizm than varying magnetic fields.
The former is done just like you wrote - by diverting the flux away from the shielded space.
The latter is done by our good old friend, the Lenz's Law (or eddy currents as its corollary). 

The current induced in a conductive shield due to the Lenz Law, generates magnetic field which is the opposite to the field which caused the current to be generated.
The more conductive the shield is, the higher the induced current and in the limiting case (a perfectly conducting shield) the induced current gets so large that it generates magnetic field which is equal and opposite to the magnetic field that caused that induction in the first place.  This creates perfect shielding of varying magnetic fields without diversion.

If the shield is not perfectly conductive, then the eddy currents induced in it decay with time and this means that as time goes on, less and less opposing flux is generated and the shielding effect fades.  This is especially true at low frequencies because their cycle periods are so long that these eddy currents have the time to decay significantly,  but they don't have enough time to decay at short periods (high frequencies), which explains the frequency dependence of this shielding process.

So conductive shields act as virtual diamagentics (with negative dynamic magnetic susceptibility) for varying/alternating magnetic fields.  The best ones for shielding varying magnetic fields are superconductors^*, followed by silver, copper and aluminium.
That is why the shield of high performance coaxial cables and waveguides is made out of silver or copper.  The skin effect guarantees that even thin conductive shields are effective at high frequencies, but low frequencies require thick shields.

Shielding the electric field is easier because you can put something in a sealed metal box and ground it.

Yes

^* _{Superconductors are as good at shielding constant magnetic fields as the varying/alternating ones, because the eddy currents induced in them never decay.  They beat mu-metal hands down, but they do it by a different mechanism.  See this}