Is wire-wrap spacing and consistency important in electromagnets?

Question

I'm creating an electromagnet (several, actually) to test a hall effect switch. I understand that more wire wraps (turns) increases the magnetic B-field, as does more electric current. Adding more turns of course increases the resistance because there is more wire. I can specify the voltage and change wire gauge to accommodate whatever current is needed, so I have some flexibility.

My questions are, for a given length of core (ferrite rod):

1. Do the wraps need to be fairly uniform and consistent, or are small gaps of little consequence?
2. How do 2+ layers of wire affect performance? I assume that as layers are added and radius from core center increases, the effectiveness of those layers diminishes. Is this correct, and is there a "rule of thumb" as to how many layers I should consider a maximum?

Answer 1

Do the wraps need to be fairly uniform and consistent, or are small gaps of little consequence?

If you are winding on a ferrite rod, it might be important to remember that they have fairly low magnetic permeability compared to ferrite transformer cores because they are intended for decent operation in the MHz regions. This means that you cannot expect winding turns at one end to couple magnetically with turns at the other end and therefore you progressively lose flux density as gaps between wires get bigger.

How do 2+ layers of wire affect performance? I assume that as layers are added and radius from core center increases, the effectiveness of those layers diminishes. Is this correct, and is there a "rule of thumb" as to how many layers I should consider a maximum?

Adding layers is usually a beneficial thing to do because you don't lose flux density as much as when using one long winding (as per my words above). I'm not sure about a rule of thumb but, stacking layers will be better than having longer and fewer layers. I would aim to make the height of windings (stacked up on top of each other) no greater than the length of all the windings along the ferrite rod/core.

Answer 2

With any ferrite core you should be able to easily saturate the core material, which will have a maximum flux density of about 400 gauss. Additional current and/or turns will give you only marginal improvement in field strength (roughly the same increase as with an air-core coil). In addition, the field from a ferrite rod diminishes quickly as you get farther from the end, because the flux direction quickly fans out. Slight variances in distance will cause large variances in field strength.

You would be better off making a coil using a gapped c-core like the one shown below. Laminated steel will provide higher flux density in the core by a factor of five or so. More importantly, the field strength will be much mode consistent in the gap area, making you less prone to position-induced variations.

If you are stuck with ferrite rods, try using two rods in line with end-to-end (N to S) with a gap between for your hall device.

Good luck!