I’m looking for some pointers on winding coils. On a second layer of winding, should the direction reverse when reaching the end of the core, or return to the starting side and wind in the same direction? It seems like winding in the reverse direction would cancel out the field. Does it matter which direction?
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Assuming that the coil is being wound on a bobbin or other form, the coil is wound beginning at one end, laying down the wire turn by turn until the opposite end of the bobbin is reached. At this point, winding continues in the same direction, but proceeding turn by turn back to the point of beginning, and so on. For an inductor, it doesn't matter which way you wind the turns so long as you're consistent throughout.
Others have raised this question; see, for example, https://electronics.stackexchange.com/questions/369884/how-to-wind-an-inductor.
A good document on building inductors was found on the ARRL site:
It's the direction the current flow rotates around the former that matters. This does not change when your winding goes back along the core.
It is OK to wind a bobin from one side to the other and back. That is the way it is normally done. If you were to go back to the same side every time, you would end up with a nasty bump in the coil layers where the wire returns. And no, that does not cancel the flux field. Looking from the end of the coil, the windings are still in the same direction, either clockwise, or counter clockwise. If you were to change direction, say part of the coil is clockwise, and you now reverse that direction to counter clockwise on the next part, than those windings would cancel out the same number of earlier windings (in an idealized scenario). Those windings would become a simple resistor from an electrical perspective.