Junk Box Toroids: Is It Even Worth It? - Page 1
Forum section: electronics / rf-microwave

author: ikrase
author_rank: Regular Contributor
timestamp: May 03, 2018, 05:00:30 am
content: It seems like my junk box has a number of toroids in it, and there are also a lot to be had for next-to-nothing at the local electronics surplus store.However, these are usually unidentified and often unmarked entirely. Micrometals iron powder toroids can be IDed by color code, but you are out of luck with ferrites and any other cores.Meanwhile, the larger sizes of cores are expensive.I have an oscilloscope and a function generator. Is it worth doing measurements to ID these things, or should I just buy them new?

author: james_s
author_rank: Super Contributor
timestamp: May 03, 2018, 05:38:30 am
content: It depends on what you need. I keep a small box of random ones around for times when I need something now and don't want to wait to order it, but I wouldn't purchase a pile of unknown assorted cores. When possible I try to sort them between types intended as cores for inductors and the sort used for noise suppression.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2018, 05:48:28 am
content: Sure!Put down N turns, to measure A_L.Measure it however you like: RLC meter, set up a resonant circuit and ring it out, GDO, etc.A_L = L / N^2Ferrites will typically be well over 0.2 uH/t^2, and powder will be less.Ferrites are useful for ferrite beads, pulse transformers, gate drive transformers, power transformers, etc. Ferrites don't store much energy, which is just what you need for transformer action (an ideal transformer has infinite magnetizing inductance --> zero energy storage).As for IDing powder type when it's not a Micrometals color, who knows? I've seen Kool-mu in black, and I think blue (or was that a different powder type?). I think I've seen MPP in gray. To some extent, you can identify the material based on mu(freq), i.e. test at several frequencies and see. Also see if you can measure Q or AC resistance.Which, is the best option of all -- if you can simply measure it at the frequency of interest, there you go. Q too low? It's wrong for the application.Can also do pulse saturation test, to see how much magnetization it takes. Typically, high mu powders (#26, 52, and the denser grades of Kool-mu, MPP, etc.) are fairly easy to saturate (~100At), medium mu is harder to saturate (e.g., #8 around 200-400At), and low mu is hard to saturate (#2 over 600At, it's hard to get enough wire on the core to even see it before DCR limits current!).Tim

author: BravoV
author_rank: Super Contributor
timestamp: May 03, 2018, 05:55:31 am
content: Quote from: T3sl4co1l on May 03, 2018, 05:48:28 amTim, regarding these unknown low mu cores, is it correct that they're more suitable for power inductor ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2018, 02:33:07 pm
content: Not much difference I think, power needs acceptable losses at low frequencies, which translates to good Q at medium frequencies (until mu' falls over at some higher frequency and Q goes to pot). #52-like materials fall off at 200kHz already, so are hardly useful at 50kHz switching -- think of them as the magnetic equivalent to electrolytic capacitors, you need way more than the minimum value just so the thing can survive its own losses. Medium and low mu materials are good enough to run at medium to high ripple, and at higher frequencies.Assuming of course, the materials aren't simply worse altogether. You can always make a material worse, say by wrapping a shorted turn on it. If you discover such a pathological material, you might want to throw it out instead.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: May 03, 2018, 04:37:25 pm
content: What is A_LIs there some kind of process to extrapolate multiple turn behavior of cores from vna curves to aid in catagorization of random shit to get more accurateparameters? From a test with like 3 turnsAlso whats a good setup to do pulsetesting?

author: BravoV
author_rank: Super Contributor
timestamp: May 03, 2018, 05:00:34 pm
content: Quote from: CopperCone on May 03, 2018, 04:37:25 pmIf what you mean is pulse core saturation testing, Jay_Diddy_B forum member here, designed one and built by many others here too, details ->Inductor saturation tester

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 06:20:32 pm
content: Quote from: T3sl4co1l on May 03, 2018, 05:48:28 amBesides the AL measurement which I did using my impedance bridge, this is what I did long ago. I did the math for calculating the saturation current based on two measurement points from the current versus time results to remove the effects of the winding resistance. Today if I did it at all, I would do something different but it worked.Later I got a pile of scrap 3C8 pot and E cores where I could adjust the air gap for whatever I needed.

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 06:25:08 pm
content: Quote from: BravoV on May 03, 2018, 05:00:34 pmMy home made one looked a little like that but used an LM318 and power MOSFET to apply a fast precision constant voltage to the inductor under test. I had to do some complicated math on the results to remove the effect of winding resistance.