Author Topic: Junk Box Toroids: Is It Even Worth It?  (Read 2213 times)

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Offline ikraseTopic starter

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Junk Box Toroids: Is It Even Worth It?
« on: May 03, 2018, 05:00:30 am »
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?
 

Offline james_s

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #1 on: May 03, 2018, 05:38:30 am »
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.
 

Offline T3sl4co1l

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #2 on: May 03, 2018, 05:48:28 am »
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^2

Ferrites 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
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Bringing a project to life?  Send me a message!
 
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Offline BravoV

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #3 on: May 03, 2018, 05:55:31 am »
..... 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, regarding these unknown low mu cores, is it correct that they're more suitable for power inductor ?

Offline T3sl4co1l

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #4 on: May 03, 2018, 02:33:07 pm »
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. ;D

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline CopperCone

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #5 on: May 03, 2018, 04:37:25 pm »
What is A_L

Is 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 turns

Also whats a good setup to do pulsetesting?
« Last Edit: May 03, 2018, 04:42:01 pm by CopperCone »
 

Offline BravoV

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #6 on: May 03, 2018, 05:00:34 pm »
Also whats a good setup to do pulsetesting?

If 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




Online David Hess

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #7 on: May 04, 2018, 06:20:32 pm »
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!).

Besides 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.
 

Online David Hess

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Re: Junk Box Toroids: Is It Even Worth It?
« Reply #8 on: May 04, 2018, 06:25:08 pm »
If 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

My 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.
 


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