Ferrite rings do anything or just marketing frill?

They're exclusively to meet regulatory requiremnts for conducted RFI most likely. If they could get away without then you can be sure they would.

Graham

The power supply is a switcher, which means that it generates plenty of high-frequency crap internally. The ferrite rings are there to try to keep the crap inside.

They produce a little lossy inductance ( 1 or a few micro henries) for any current trying to pass through the hole. This keeps standing waves from resonating efficiently on the cable, which would make it an efficient antenna for the resonant frequency. If any current goes through one way on one wore, and returns back through on another wire, the bead is electrically invisible.

Here are links to a couple of the many manufacturers:

Digikey sells three grades (low frequency high permeability, medium frequency medium permeability, and high frequency low permeability) of the Steward line. They are also make flattened ones for ribbon cables.

You think?

The manufacturers just have a lot of these left over from other projects so they throw them on in case Joe Sixpack, who is browsing computers at Costco, asks, "Are they using ferrite rings on the PSU in this model?"

They're serious. They can reduce radiated EMI from switching power supplies and digital electronics, enough to make the difference between passing and failing FCC and CE regs.

Ferrite beads can also reduce suceptability by blocking rf entry and nuking cable resonances. I reduced the RF sensitivity of one of my temperature controllers by over 20:1 by passing the thermocouple leads through a figure-8 (dual hole) ferrite bead, apparently by taming a few prominent high-Q resonances.

John

Those figure 8s are handy. I used them on a product's intenal ac mains leads together with a couple of Y caps to get it inside the conducted limits. It was previously just a couple of dB over. A heck of a lot cheaper thana full blown line filter.

Graham

On a sunny day (Wed, 31 May 2006 04:15:05 GMT) it happened a?n?g?e? snipped-for-privacy@lovergirl.lrigrevol.moc.com (The little lost angel) wrote in :

If you connect an oscilloscope to for exampel the +5V line, you will see it is very noisy indeed, with short spike too. I am not sure if putting ferrite will prevent noise from teh mobo to the suppy, where it will be shorted buy the filter caps, or noise from the supply to the mobo, where RF spikes may interfere. Normal cases are closed so for RFI to the outside world it should not help. It will provide a high impedance path and reduce high frequency currents. A good video card has some regulation perhaps and 4 sure filtering to the DA converter (VHA) if it is not already digital (DVI, HDMI), in the digital case it will not help. It may help any analog digitizers... TV cards... So depends on the design of the plugin cards I suppose (or on board video). So, YES, it may help.

It's not the DC you need to worry about, but the non-DC going along with it.

We use them on switching supply outputs, too. They give both common-mode and differential loss.

John

...and even a power rail that looks exceedingly "clean" on an oscilloscope can still have plenty of noise riding on it if you start looking closely with, e.g., a spectrum analyzer!

The manufacturer claiming that the ferrite beads will "improve (the) quality of the video output as well as system stability" is undoubtedly overly optimisitic (as others have stated, the primary for the ferrite beads is for meeting regulatory emission requirements), although I did once use a PC that had such a noisy power rail that there was *visible* interface on the video card's output and *some* ISA cards simply wouldn't operate at all. It was an utterly generic no-name box.

Actually, unless you're getting a case from the likes of Dell or HP or someone else with a name to protect, many "normal" cases leak like a sieve. And of course the current fad in the modding community of putting a large piece of plexiglass in the side of the case to show off the internals doesn't block anything below the audio range of EMI. :-)

---Joel

I used them all over my boat. CB, Marine, AM/FM, stereo speaker leads. When you transmit, the other equipment does not like it. Ferrites on mic cords, coax, 12v, grounds, etc I used it to surpress a parasitic geting into the marine radio, causing it to lock when scanning channels..

greg

I was wondering about those... But don't they have to meet FCC specs?

An empty case will almost always sail through FCC testing ;-)

someone

Cases? No. ...or at least they only have to pass as sold (who knows what you're going to put in them). The electronics need to pass part 15 rules (6dB relaxed for "open box" test).

I believe the way the case manufacturers get around that problem is by telling the purchaser that all they're doing is providing a mechanical structure, and it's up to the person who starts putting electronics inside to meet FCC regulation. I have even seen stickers on some windowed cases stating something similar ("only for use in a research environment").

---Joel

That probably helps sales too !

Graham

someone

Gads. What a world.

Generally, any feature prominently mentioned in advertising aimed at non-rational retail consumers, such as computer decorators (excuse me, "modders") is BS. But even power supplies sold mostly to the industrial and OEM markets have chokes on their output lines, even internally, where they obviously must serve a real purpose.

If they didn't help, would this one have been installed -- inside, where it can't be seen, by a company that doesn't bling up its PSUs?

(300W Delta DPS-300BB)