Comments on: Parts: Ferrite beads

By: paul

paul — Wed, 14 Jul 2010 20:40:44 +0000

ferrite increases the inductance of a conductor many times, the increase depends on the permeability of the the ferrite . There are ferrites which have values 8000 , this means , just sticking a bead over a leg of say a D connector WILL PREVENT high frequency signals ie noise entering or leaving the enclosure .ON the other hand DC will go through as if there was no resistance. Simply put ,they stop AC but not DC. They are used a lot in RF engineering for all types of filters,rf transformers.

By: sigtermer

sigtermer — Mon, 11 Jan 2010 04:01:26 +0000

thanks hackaday

By: jason

jason — Fri, 17 Jul 2009 04:12:11 +0000

I’d like to see a post on triacs. I’m working on reverse engineering a project that uses them for switching AC loads without relays.

Thanks.

By: Rlyeh_drifter

Rlyeh_drifter — Thu, 09 Jul 2009 08:04:56 +0000

@dan
Thank you!
The key is the permeability! I thought it would be somewhat constant, but through frequency-dependance comes a complex permeability, so thats where the rising resistance comes from!

This document served me well:
http://www.steward.com/pdfs/emi/technical/Use%20of%20Ferrites%20in%20EMI.pdf

By: cath0de

cath0de — Wed, 08 Jul 2009 04:23:14 +0000

you said butt beads

By: Dan

Dan — Wed, 08 Jul 2009 00:52:58 +0000

@Rlyeh_drifter

Check out the article by Murata, which is linked to by the wikipedia article: http://www.murata.com/emc/knowhow/pdfs/te04ea-1/23to25e.pdf

The graph shown depicts a frequency-dependent resistance. They don’t plot the imaginary part of the impedance but they show the magnitude approaching the real component for high frequency which seems to imply that the imaginary component approaches zero. The real part starts below 1 ohm and rises to over 100 ohms. A second graph contrasts this to an air-core inductor in which the imaginary impedance increases linearly until capacitance comes into play, with the real part of impedance (the resistance) being insignificant throughout.

The wikipedia article and the murata article both make it a point to say that these ferrite beads are different from ordinary inductors, and energy dissipation in the form of heat seems to be a significant feature of their operation.

By: Wwhat

Wwhat — Tue, 07 Jul 2009 17:39:19 +0000

AFAIK their function is to create eddies inside them that resist/counter high frequencies, they are meant to block unwanted frequencies and therefore need to be selected on that function too, but of course while being compatible with the circuit they’re in.
So yes to say you should only look at the voltage might be going too far even for the must uninterested, even when they can be considered having a broad effect, like gloves can protect your hands but if you go skiing you might still want to not use welders gloves even when they are better than no gloves and do insulate you from the cold too.

By: peteredworthy

peteredworthy — Tue, 07 Jul 2009 16:43:37 +0000

When reclaiming parts I’ve always just ignored SMT ferrite beads. This is mostly because of the difficulty in determining enough about them to be able to re-use them. If pretty much all of them are suitable as high frequency filters they suddenly become much more interesting.

Unfortunately current rating seems like it’s the hardest parameter to non-destructively determine.

Are there any rules of thumb, such as minimum power ratings for various sizes?

By: anon

anon — Tue, 07 Jul 2009 10:40:05 +0000

does any one have a suggestion for a reasonable meter that will do L and C in addition to R, V, and T?

By: Rlyeh_drifter

Rlyeh_drifter — Tue, 07 Jul 2009 09:25:36 +0000

@noobius
thanks for correcting, typing faster than thinking is a problem :-)

By: Noobius

Noobius — Tue, 07 Jul 2009 09:04:20 +0000

grrr i meant to write X = j*w*l, z = r + j*w*l. this blog needs an edit button ^^

By: Noobius

Noobius — Tue, 07 Jul 2009 09:02:07 +0000

@Rlyeh_drifter

The complex impedance for an inductor is z = j * w * L and for a capacitor it’s z = 1 / (j * w * C), where j = sqrt(-1) and w = 2 * pi * f.

By: Rlyeh_drifter

Rlyeh_drifter — Tue, 07 Jul 2009 08:44:40 +0000

@dan
parasitic resistance is everywhere, every solder joint has some, also every conductor has parasitic capacitance and inductivity. But the real part of complex resistance (impedance) is frequency independent in opposition to its reactance. Some real resistance is needed to dampen resonance and also limits the maximum current.
I cannot agree “the impedance being dominated by the resistive component”, it is shunted by parasitic capacitance and increased by inductance. What you get is a few ohms at dc and 1kOhm at 10MHz for example, the real part of the resistance is constant, so it can only be the complex part.

By: Code

Code — Tue, 07 Jul 2009 08:42:06 +0000

And now after reading all comments I think i know just a bit more about Ferrite beads.

Thx HackaDay and al the commenter’s. Positive or negative, I dint knew what those ferrite parts where used for, now I now a bit more I can do some Research myself.

Hackaday is a collective ? Borg? :P

By: Dan

Dan — Tue, 07 Jul 2009 06:46:54 +0000

From the Wikipedia article http://en.wikipedia.org/wiki/Ferrite_bead it appears that the distinguishing characteristic of ferrite beads is not just that they can operate at high frequencies but that they actually have parasitic resistance at these frequencies. So it is not the case that the imaginary component of the impedance dominates, but rather the real component. This is contrary to what is wanted in a transformer, which I imagine may be made of a different form of ferrite. The link at the bottom of the wikipedia article titled “Understanding Ferrite Bead Inductors” gives a plot showing the impedance being dominated by the resistive component. They claim that a ferrite bead can be thought of as an inductor in series with a frequency-dependant resistor.