Remanence and core material - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 06, 2021, 10:18:19 pm
content: Summary need to make a small diameter 4mT electromagnet to test a binary hall effect sensor, details here:https://www.eevblog.com/forum/projects/electromagnet-to-test-hall-effect-switch/msg3378558/#msg3378558So I wound a couple of test coils on some 1/8" steel dowels I had laying around and I'm was able to activate the hall effect sensor.One of the cores now appears to be permanently magnetized. I tried demagnetizing it with an old Radio Shack tape deck head demagnetizer and it helps, however there is enough residual magnetism, that it will trigger the hall effect sensor with no power applied and also move the needle on a compass. I also tried applying about 1A ac and then slowly reduce current to 0, same thing?The other core EM is working, I can activate the sensor by simply applying about 1A or so of current, and turning the current off deactivates the sensor. My concern is that I do observe a small amount of residual magnetism in the working EM, not enough to false trigger the sensor, but enough to weakly drag the compass needle around. Perhaps it will get worse over time?I'm wondering if I should be using something other than a steel pin for the core, any suggestions?

author: Benta
author_rank: Super Contributor
timestamp: January 06, 2021, 10:51:16 pm
content: Look for electrical steel or silicon steel, which is used in transformers and inductors. It's normally available in sheet form, which mighy not fit your application.Ferrite rod could be another option.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2021, 11:07:13 pm
content: Steel dowels, like dowel pins? Those are hard steel, they'll take a set, yeah.Can try annealed mild steel or cast iron, if you don't have anything "softer".Iron is annealed by heating to ~orange heat, where coincidentally it becomes nonmagnetic. If you have a torch, this is probably doable. Still, I'm not sure that hard steel loses much remenance when annealed.4mT isn't very much, you might just as well use an air core coil. For example, a thin loop of 2cm dia., carrying 100 At, has a field at the center of 6.3mT. 100 turns of 26 AWG wire would be able to do that, and wouldn't exactly be "thin", so the field strength will be a bit weaker than calculated -- but still more than enough. It could also be smaller, concentrating the field even more; or use more amp-turns.Tim

author: Benta
author_rank: Super Contributor
timestamp: January 06, 2021, 11:45:16 pm
content: Annealing does not change the magnetic properties of steel significantly.Yes, you can remove remanence by heating, but the next time you magnetise it, it'll be the same.Are you suggesting that after every test, the core should be heated to orange-red?Austenitic stainless steels are non-magnetic, but that's a bit of a special case (with very high alloy percentages).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2021, 12:19:33 am
content: Are you sure? Hard alloys are, well, magnetically as well as mechanically, hard. Even just work hardening causes magnetic domains to harden (flux pinning). Annealing relieves the work hardening, also softening the magnet. Ye Olde iron magnets, were simple bars, horseshoes, etc. of hard iron (until Alnico and friends were invented).Afraid I don't have any data handy. But who would tabulate magnetic properties of otherwise magnetically-irrelevant alloys like 1020 or 1080 anyway... (Though I would love to see how strong the effect actually is.)If annealing didn't do anything, there would be no reason for mu-metal shields to be annealed, yet this is standard practice (in a hydrogen atmosphere -- don't do this at home!).Interestingly enough, austenitic stainless is largely non-magnetic, but picks up some magnetism upon work hardening.Tim

author: Benta
author_rank: Super Contributor
timestamp: January 07, 2021, 01:00:49 am
content: You're confusing mechanically hard with magnetically hard.Mechanically hard is obviousMagnetically hard refers to the B/H curve and hysteresis as well. The old Siemens and Philips data books were titled "Soft Ferrites", although ferrite is pretty hard, even brittle mechanically.Mu-metal consists to around 80% of nickel, so not really relevant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2021, 07:22:15 am
content: Neat, some related information is surprisingly available. And from our friends at CERN, which isn't really all that surprising considering how much damn magnetism they work with!https://cds.cern.ch/record/1334290/files/39.pdfPage 11 for example shows that coercivity due to cold working is not confined to iron, but includes mu metal as well.Tim