Ferrite rings do anything or just marketing frill?

The little lost angel wrote: (snip)

This approach works well to contain the energy that is intended to pass along the cable, if the signals are in the form of a balanced differential pair (one signal going positive as the other goes negative). This causes their external fields to approach zero when the cabling is balanced (twisted pairs or shielded twisted pairs).

But it still doesn't solve the problem of the whole cable resonating as a single conductor and acting as an antenna for any noise on the signal common that matches its resonant frequency. A cable shield with nothing inside it will radiate this way.

The ferrite bead around it, absorbs energy that damps that resonance, as charge bounces back and forth along the shield.

It's marketing truth.

So since the wires in a PSU simply carry energy and not data, i.e. there's no existence of balanced differential pair, doing this would effectively be pointless right?

So the ferrite bead is effective even tho it's only at one end of the wires? How would it compare to say a metal mesh shield wrapped around the wires carrying power?

--
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On a sunny day (Fri, 02 Jun 2006 19:27:27 GMT) it happened a?n?g?e? snipped-for-privacy@lovergirl.lrigrevol.moc.com (The little lost angel) wrote in :

The ferrite bead, look at it as a series inductor It forms a core around the wire, making its inductance bigger. (Like if the wire was wound around some iron core). Higher inductance means higher impedance for higher frequencies. Z = j.w.L (w is omega, 2 x pi x f

If there is any capacitance at the end of the wire, it wil lform a low pass filter.

The current in the wire will decrease more for higher frequecies i = U / j.w.L

Think of j as 90 degrees phase shift, impedance and current is a complex number.

Also that bead has some losses, likely also increasing with frequency.

Zank you

If the supply produced pure DC voltage (no high frequency ripple or spikes) and the load consumed pure DC current (no logic loads switching), then yes. Otherwise, there is some unwanted high frequency content on those lines that might escape as radiation., so a ferrite bead might be placed to suppress it.

That is the place they usually work best, but the actual best spot may need to be found by trial and error, making measurements of the radiated noise spectrum for each trial. Think of the wire as a whip antenna sticking out of a ground plane. The peak current in the whip occurs at the ground plane end, with peak voltage at the free end.

A metal shield around the wires contains the electric fields but the magnetic fields just move to the outside of the shield. Ferrite beads do nothing for electric fields, except to choke off the charge movement that feeds them.

No, you're not understanding the point. For one thing, the outputs of the PSU are balanced to the extent that all of the current that flows out of it flows back in. Going through the ring, these magnetic fields cancel. But if there's some electrical noise, the conductors act like an antenna. Switching noise on the lines is probably single ended, too; these are the things that the ferrite ring filters out.

If it is grounded, it will protect against capacitive feedback, but for the kind of coupling that a ferrite ring is used for, a shield without the ring would just make it worse, since it's a single conductor that can pick up induced fields and reradiate them.

Hope This Helps! Rich

On Wed, 31 May 2006 04:15:05 GMT, a?n?g?e? snipped-for-privacy@lovergirl.lrigrevol.moc.com (The little lost angel) put finger to keyboard and composed:

Aluminium electrolytic caps don't filter high frequencies very well. That's why a lot of designs add smaller caps (eg tantalums or ceramics) in parallel with them.

- Franc Zabkar

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On Fri, 02 Jun 2006 15:25:18 GMT, a?n?g?e? snipped-for-privacy@lovergirl.lrigrevol.moc.com (The little lost angel) put finger to keyboard and composed:

I have some cables like that. I'm not sure where they came from, though.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.

Would they help with induction voltages? We run very long cables -

2000' - along 480 VAC power lines and the induction voltages can put 70 V on a cable pair that's not connected to anything (nutted on one end, voltmeter on the other)....

I wonder if something so simple would work.

(And no, I don't have an EE degree either. :-) )

--Yan

If you mean 60 Hz voltages magnetically induced from power lines into your signal cables, no, the ferrites won't help. They have useful impedance only at high, megahertz, frequencies. I have seen larger cores, toroids and laminations, used to add common-mode impedance at line frequencies, as in audio and video systems suffering from induced hum.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

The normal thing to do is use a small transformer to get rid of the common mode, or alternatively, blocking caps. What is the cable carrying? Ethernet uses transformers I believe. So there is no real reason to worry about the 70 volts of AC common mode, except it will give you a nasty jolt. Shielded cable?

del cecchi

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