# Is wire-wrap spacing and consistency important in electromagnets?

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?

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.

• Andy, hugely helpful to know this. I appreciate the insight as to how layers affect performance. Feb 25, 2019 at 16:48
• Changing to a low-carbon steel core, increasing wire gauge, and thus amp*turns has resulted in a functional electromagnet. This answer was key in pointing out the permeability issue with ferrite. Mar 1, 2019 at 17:38

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!

• John, thanks for the invaluable information! This is a custom-made part so I can definitely switch from ferrite rod to laminated steel. Can you elaborate on how the magnetic field propagates from the C-core so that I can make a case for using it (it will affect how the bracket/holder for said magnet gets fabbed). If I understand correctly, the HE sensor will need to be placed in the air gap. However, one design constraint is that the DUT with the HE sensor is placed by hand into a test fixture; ideally we wanted the magnet to be under the DUT. Feb 25, 2019 at 16:44
• With the c-core, you will have a square or rectangular cross section. Try to keep the gap as small as you can practically to fit your device in the gap. The field will be consistent across the gap in the center of the cross section; out near the edge the flux lines spread out, known as "fringing," which you can Google. You can increase the area of consistent field strength by increasing the cross sectional area as well, at a cost of more ampere-turns for the same field strength. For a reasonable ratio of gap length an area, you can directly calculate the field strength. Feb 25, 2019 at 17:01