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.
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.
RE: PCB coil to replace hand wound coil.
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.
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 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.
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.
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.
Something still ain't right.
Mike Halloran
Pembroke Pines, FL, USA
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
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.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: PCB coil to replace hand wound coil.
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.
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.
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
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.
Doesn't pass thru the copper - around it.
Keith Cress
kcress - http://www.flaminsystems.com
RE: PCB coil to replace hand wound coil.
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.
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.
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.
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.
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.
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.
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.
RE: PCB coil to replace hand wound coil.
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.
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.
TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
RE: PCB coil to replace hand wound coil.
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.
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.
RE: PCB coil to replace hand wound coil.
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.
Well, thank you all that contributed. I'm about ready to commit to a layout so I guess here goes...