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PCB coil to replace hand wound coil.

PCB coil to replace hand wound coil.

PCB coil to replace hand wound coil.

(OP)
I have an application in which I am transferring power from a stationary device to a device that is rotating. The device that is stationary outputs an alternating magnetic field at 26kHz (sine wave) using a U shaped ferrite. The device that is rotating has a large, 7" diameter, coil wound in air with 24 turns of 26 gage wire. The device that is being powered by this coil has capacitors across the input such that the coil and capacitors form a resonant circuit. This aids in the power coupling across the gap. Power transfer works quite well. I have since designed a PCB with 4 layers of 6 turns each, one ounce copper, with traces .01" wide with .01" spacing between coils on the same layer. The board house tells me that the inter layer spacing is .04". The coil windings on the PCB are all in series, so it is also 24 turns. In the same way, capacitance is added to the device under power to bring the system to resonance. Problem is, this PCB coil does not pick up power nearly as efficiently as the hand wound coil does. It also doesn't seem to resonate nearly as well. I need to make changes to the PCB to improve this but changes are very expensive, so I need to understand what's going wrong. First off, the DC resistance of the hand wound coil is about 3 ohms, where as the DC resistance of the PCB coil is about 35 ohms. I tested the hand wound coil with a 30 ohm resistor in series with the coil, but still, ample performance was found. The resistor created loss of power transfer, but not nearly enough to cause failure. There are 2 other issues that I'm thinking of, but I can find very little about either on. First is skin effect. Second is the air gaps between the coil windings in the PCB are greater than those of the hand wound coil. I'm wondering if there is anyone that has a better understanding in this area that could shed light on what I should do to improve the coil design. Thanks.

RE: PCB coil to replace hand wound coil.

Are the original hand-wound coil and the new PCB coil both approximately the same dimensions and orientation?

If not, then perhaps a hand-wound version of the PCB coil might be a good experiment to distinguish between the geometry changes and material/coil-Q/resonance issues.

RE: PCB coil to replace hand wound coil.

(OP)
They are the same diameter. The hand wound coil is wound "tightly" with normal (round) 26 gage magnetic wire, so each winding is very close to the next. With the PCB, there are air gaps between the copper that are a fair bit bigger than the spacing between the copper of the hand wound coil. I'm not sure what to do with the PCB... pay extra to have the coils places extra close to each other? Pay extra to up the copper weight? Or, both, or something I haven't thought of?

RE: PCB coil to replace hand wound coil.

I don't understand why the DC resistances should be quite so different. ... but that's probably not the source of your problem, as indicated by the experiment with resistor.

I think the problem lies in the magnetic circuit.

Draw a cross section of the source coil, the ferrite cup, and the target coil, and I think you'll find that the printed target coil's windings are in the farther fringes of the target coil's magnetic circuit, because of the spacing.

It may be possible to add some passive soft iron to tighten up the magnetic circuit a bit, or maybe add a complementary ferrite.

Mike Halloran
Pembroke Pines, FL, USA

RE: PCB coil to replace hand wound coil.

(OP)
The DC resistance is high because the PCB traces don't have anywhere near the cross sectional area that the 26 gage wire has.

I can put the PCB coil in almost exactly the same location as the hand wound coil. Compared to the size of the u-shaped ferrite, the coil width/depth size isn't so large. An entire portion of the coil can sit between the 2 sides of the ferrite on the primary.

I wonder if the Q of the PCB coil is much lower than that of the hand wound coil, and if so, if this could be causing the problem.

RE: PCB coil to replace hand wound coil.

The 26 gage wire has just under twice the cross sectional area of the stated traces, so the resistance should differ by a factor of ~2, not ~10.

The measure of an electromagnet is ampere-turns.
Since the turns are the same and you are relying on resonance to push the amperes, the Q does matter.



Mike Halloran
Pembroke Pines, FL, USA

RE: PCB coil to replace hand wound coil.

(OP)
Diameter of .026 gage wire, .015". Cross sectional area, .0002sq".

Thickness of 1oz copper, .0014". Width of copper trace, .010". Cross sectional area, .000014sq".

.0002/.000014 = 14.3, which is about right.

RE: PCB coil to replace hand wound coil.

What power levels are talking about?

The diameter hints at at least ten watts. I cannot imagine that a ten-fold increase in resistance wouldn't be the difference between sucess and failure in such an arrangement. It is so obvious that I need to question how you verified operation of the coil+resistor. Same input and roughly same output?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: PCB coil to replace hand wound coil.

Thanks, rickford; I had the thickness wrong.

Something still ain't right.

Mike Halloran
Pembroke Pines, FL, USA

RE: PCB coil to replace hand wound coil.

(OP)
I have a small circuit that runs on the secondary coil. It rectifies the power from the coil and filters it to get a clean DC power supply. The power supply working range is anywhere from 6.5V to 25V. With the hand wound coil, I can tune the power circuit up to provide as much as 25VDC (at about 30mA) to the circuit. Putting the series resistor in drops the voltage at the same tuning to about 17VDC. The load runs off of a linear regulator, so the current draw is roughly constant over the input voltage. Putting the PCB in place, the highest I can tune the supply to is about 7V. It works, but it's way too marginal. I would like to see from 12V to 15V available from the supply so I can have confidence in it.

RE: PCB coil to replace hand wound coil.

(OP)
No, I didn't think there would be any losses there. It doesn't seem to be magnetic in any way, and it's not conductive, so I don't have to worry about eddy current losses.

RE: PCB coil to replace hand wound coil.

(OP)
BTW, the relative permeability of FR4 is 1, so I think this supports the idea that FR4 is not creating any more losses than the air wound coil.

RE: PCB coil to replace hand wound coil.

The two big issues are the resistance of the traces and the gaps between them. Magnetic flux that cuts between the traces is lost from exciting the other coils. It's a multiplicative loss.

I'd suggest re-doing the board with the traces stacked up as much as possible so if it were all clear you wouldn't be able to shine a light past adjacent coils. I'd also have the coils about 8 mils apart (just before a board fab premium kicks in) if you can pull that off. Set them up with all trace spacings symmetrically covered by a centered trace on a different layer.


For the heck of it I'd also do a 2 layer board with the same overlap scheme with the same total winds with "4 mill copper" which really means 2 mill copper with 2 mills of tin on them. This would dump the high resistance and reduce the cost on the board by being only 2 layer.

Keith Cress
kcress - http://www.flaminsystems.com

RE: PCB coil to replace hand wound coil.

Quote (itsmoked)

"4 mill copper" which really means 2 mill copper with 2 mills of tin on them.
I certainly hope not (though I'm assuming you mean 4 [b]oz[/u] copper, not mil). Layer thickness should not include anything other than bare copper... it could be copper electro-dsposited onto a thinner starter layer, but in the end the 4 oz should be all copper.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: PCB coil to replace hand wound coil.

(OP)
Interesting that you suggest staggering the coils like that. That's exactly what I don't have. Right now, all 4 layers lay right on top of one another so that you can see right through the gaps.

What happens to the magnetic flux as it passes through one of the copper layers? Is it redirected? If so, how does it then interact with layers behind it? I would think it has to be redirected to some degree because the copper trace will have a current induced and that will produce a magnetic field of its own. This stuff fries my brain. If the windings of a coil are far apart, the magnetic field produced by any one winding circles the winding itself, and not the group. When the windings are close together, the bulk of the magnetic field engulfs the entire group of wires... So what happens to the magnetic field that induced the current? Yup, brain in frying.

RE: PCB coil to replace hand wound coil.

I am reminded of that Govt. warning.....Your resistance may vary. You may not have enough skin in the game. Hate to calculate so this what I use:

http://www.powerstream.com/Wire_Size.htm

Look down and at 20 gauge there begins a penalty over 27khz. Given the thickness of the trace, your resistance could be a lot higher than 30 ohms.

RE: PCB coil to replace hand wound coil.

(OP)
I found a calculation of AC resistance due to skin effect, and it was minimal, assuming I did it right. Only a couple ohms.

RE: PCB coil to replace hand wound coil.

It might be something that we can't see over the Internet. Triple check that the PCB turns are wired in series and in the correct phase. Triple check that you haven't got a shorted turn (eating power). Triple check the absolute basics, because the higher order potential problems aren't leading anywhere so far.

RE: PCB coil to replace hand wound coil.

Yeah MacGyverS2000 I slipped up on the units there, I meant "oz" not mills.

I think you may find a common way board houses increase the "oz" it's purely by running the board thru a wave solderer before solder masking.


Quote (rickford)

What happens to the magnetic flux as it passes through one of the copper layers?
Doesn't pass thru the copper - around it.

Keith Cress
kcress - http://www.flaminsystems.com

RE: PCB coil to replace hand wound coil.

Any thing that you can do to reduce air gaps will help proportionately. When calculating amp turns per inch, one inch of air is equal to over 10,000 inches of good transformer grade iron. Amp turns per inch are important and when the air gap is relatively large, calculations may be based on the length of the air gap alone, neglecting the path through iron, without serious error.
The spacing between the coils may be reducing the self inductance of the windings. This will affect your inductance and your tuning. Try different values of capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PCB coil to replace hand wound coil.

(OP)

Quote (itsmoked)

Doesn't pass thru the copper - around it.
I have a hard time getting my head around the minute details of this. If the field doesn't pass through the copper, then how does the copper know it's there? Magnetics was not my strongest class back in school. It's as if the wire has an invisible lever arm sticking out and the magnetic flux passing by pushes on this lever. I wonder if I think about it this way, if it would help. The wire is like a screw with a nut on it. The nut has several paddles on it, and as this "magic" field passes by, it strikes the paddles, rotating the nut, and propelling the screw in one direction or another. Of course, the direction of the screw movement would be the current. If the screws are too far apart, the spinning nuts don't work in concert with one another and much of the field cuts through the middle, producing an opposite spin on some of the nuts, or, at least subtracting from the force of rotation.

As for the layout, the wraps are all in the same direction. The only issue, which I have now discounted, is that the board house could have reversed a layer. After looking at the layout again, it wouldn't have any continuity at all if that happened.

RE: PCB coil to replace hand wound coil.

I have a hard time getting my head around the minute details of this. If the field doesn't pass through the copper, then how does the copper know it's there?
It passes around it and verrrrrrry close to it, thereby affecting the free electrons in it.

Magnetics was not my strongest class back in school. It's as if the wire has an invisible lever arm sticking out and the magnetic flux passing by pushes on this lever.
Naw. I find it way easier to think about the actual effects on individual electrons. An electron in free space would react the same way (vector right-hand rule). In a wire it's constrained like it's in a pipe.

I wonder if I think about it this way, if it would help. The wire is like a screw with a nut on it. The nut has several paddles on it, and as this "magic" field passes by, it strikes the paddles, rotating the nut, and propelling the screw in one direction or another. Of course, the direction of the screw movement would be the current. If the screws are too far apart, the spinning nuts don't work in concert with one another and much of the field cuts through the middle, producing an opposite spin on some of the nuts, or, at least subtracting from the force of rotation.
Well I guess you could think about it that way. I prefer to think of it more like if the wires are bundled tightly the magnetic field that has to divert around each wire would instead divert around the entire bundle and therefore affect the eletrons equally in all of the wires. Whereas gaps will cut out various wires from being included in the mass effect.

As for the layout, the wraps are all in the same direction. The only issue, which I have now discounted, is that the board house could have reversed a layer. After looking at the layout again, it wouldn't have any continuity at all if that happened.

Keith Cress
kcress - http://www.flaminsystems.com

RE: PCB coil to replace hand wound coil.

"the wraps are all in the same direction"
Does this mean that the current circulates in the same direction in each layer?
It should, but the wording makes me wonder.

Have you measured the inductance of the two coils?

Was the 30 Ohm resistor really part of the resonance circuit in your test? That is, one end of the capacitor was connected to the inductor and the other end to the resistor?

RE: PCB coil to replace hand wound coil.

(OP)

Quote (ijl)

"the wraps are all in the same direction"
Does this mean that the current circulates in the same direction in each layer?
It should, but the wording makes me wonder.
Yes, the current should be running in the same direction, as the loops all go around the circle in the same direction.

Quote (ijl)

Have you measured the inductance of the two coils?
I don't have an inductance meter, but I did put each into a tank circuit and using a function generator and a scope, I measured the resonance frequency and solved for the inductance. They are not the same. The PCB circuit inductance is much lower.

Quote (ijl)

Was the 30 Ohm resistor really part of the resonance circuit in your test? That is, one end of the capacitor was connected to the inductor and the other end to the resistor?
Yes, the resistor absolutely was in the circuit. Note that it also changed the results.

itsmoked, I like your explanation of the magnetics. It helps.

It sounds like my windings may have too much gap between them. Sounds like I need to tighten up the spacing, not only of the gaps between the traces on any given layer, but I need to specify a narrower gap between the layers.

RE: PCB coil to replace hand wound coil.

The current in each wire develops a magnetic field which affects the surrounding wires. This is mutual induction.
I hope that the outside of each layer is connected to the inside of the next layer. If identical layers are connected inside to inside and outside to outside then the coils will tend to cancel each other.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: PCB coil to replace hand wound coil.

Why not reproduce the PCB coil geometry using the wire and see if the problem can be reproduced? This would confirm or refute the geometry theories in about an hour.

RE: PCB coil to replace hand wound coil.

If the inductance is much lower, then the Q-value of the circuit is much lower, and you get a (much?) lower voltage. Simply increase the number of turns, so that the inductance is as large as or larger than the inductance of the other coil. If possible, use thicker copper and wider traces in order to get the resistance down. And to not worry too much about the gaps!

Please forgive me, but I still do not understand that "the loops go in the same direction". As I understand, there is a planar spiral in each layer, and the spirals are connected vertically. Correct? If you connect them in the shortest way, the spirals should go "inwards" and "outwards" in aternate layers.

RE: PCB coil to replace hand wound coil.

(OP)

Quote (waross)

I hope that the outside of each layer is connected to the inside of the next layer. If identical layers are connected inside to inside and outside to outside then the coils will tend to cancel each other.
If this is true, then you may have hit on the problem... though I have trouble seeing why it would be true. The layout is this... looking down on the board, a imagine a connection point at 12:00 position. The connection point is 2 terminals. From the right connection, a trace travels upward in the 12:00 direction about 3/4" to meet the inside of the coil. It then makes a right turn and goes clockwise around the coil, with each wrap getting slightly larger than the last. After 6 turns, the trace enters a via where it drops to the next layer. In the next layer, it continues to travel clockwise starting at the outside, with each wrap getting slightly smaller than the last. After 6 turns, the trace enters another via to go to the next layer, and the coil winds back out again. On the next layer, it winds back in again until finally meeting where the coil originated, then turning down and into the left terminal of the connection.

How does returning to the inside of the coil at the start of each layer improve the issue? Isn't the current always traveling in the same direction around all the loops? When I wind a coil by hand, the windings go across instead of in and out radially, but still, they go across one way, then back the other, then continue back and forth as the coil grows. What's the difference?

RE: PCB coil to replace hand wound coil.

(OP)
This is about the best I can do with the program I have. See attached pdf file.

RE: PCB coil to replace hand wound coil.

(OP)
Well, I guess that didn't work. Looking for another way...

RE: PCB coil to replace hand wound coil.

(OP)
Note on the pdf that the 4 inner layers are used for the coil. There is no copper on the top and bottom layers except for the via's and the connection pads. This is done this way to protect the coil in case it momentarily rubs while spinning during setup.

RE: PCB coil to replace hand wound coil.

It looks like you have room to make the traces _much_ wider.
That would reduce the amount of copper you have to remove from the board, reducing your waste stream volume, reduce the coil resistance, and possibly make it less sensitive to alignment errors or runout.

Mike Halloran
Pembroke Pines, FL, USA

RE: PCB coil to replace hand wound coil.

(OP)
I have done some more experimenting with resonance and a function generator. This is what I have so far...

The PCB coil:
Tuned resonant frequency with .15uF cap in parallel, 31kHz. Calculated inductance based on resonance and known capacitor in parallel, 176uH. Q of tank circuit, about .55.

Hand wound coil with 26 gage magnet wire:
Tuned resonant frequency with .1uF cap in parallel, 33.5kHz. Calculated inductance, 226uH. Q of tank circuit, about .9.

Hand wound coil with 26 gage wire with teflon insulation (about .042" OD) producing about .027" spacing between copper windings:
Tuned resonant frequency with .15uF cap in parallel, 31kHz. Calculated inductance, 176uH. Q of tank circuit, about 1.1.

The caps are the same type, and so their Q should be similar. I believe this would mean that the difference in Q of the entire circuit is largely due to the difference in Q of the coil... and since Q is a measure of losses in the coil... of which resistance is a major factor in the PCB... it all seems to point back to resistance. There's only so much I can do to make adjustments on the PCB, so I'm leaning toward just making the traces as wide as possible, making the traces as thick as is reasonable, and the gaps as narrow as is reasonable and being done with it.

RE: PCB coil to replace hand wound coil.

(OP)

Quote (MikeHalloran)

reducing your waste stream volume
What is waste stream volume?

RE: PCB coil to replace hand wound coil.

There are other ways, but most PC boards are made by a subtractive process.
You buy a sheet of FR4 clad entirely with copper.
You use a photographic process to mask the copper that will remain.
You use an etchant to dissolve the remainder of the copper.
The etchant plus copper becomes a toxic waste, which you must pay to have recycled or discarded, and you must keep detailed records tracking the stuff.
You don't have to pay for disposal/recovery of copper left on the board.
So the board _may_ get cheaper, and you get environmental creds, by maximizing the copper area on the finished board.

Mike Halloran
Pembroke Pines, FL, USA

RE: PCB coil to replace hand wound coil.

(OP)
Ok, that makes sense.

Well, thank you all that contributed. I'm about ready to commit to a layout so I guess here goes...

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