Ferrite rings do anything or just marketing frill?

[The little lost angel]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

TIA!

 

[Pooh Bear]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

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

 

[Tim Wescott]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

TIA!

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[John Popelish]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

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:
http://www.fair-rite.com/
http://www.steward.com/

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.

 

[Wes Stewart]

[snip]

But of course not having any formal training in EE means I could be
the one getting it all wrong.

You think?

So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

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?"

 

[John Larkin]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

TIA!

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

 

[Pooh Bear]

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.

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

 

[Jan Panteltje]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

TIA!

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.

 

[rphenry]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

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

 

[John Larkin]

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.

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

John

 

[Joel Kolstad]

If you connect an oscilloscope to for exampel the +5V line, you will
see it is very noisy indeed, with short spike too.

....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.

Normal cases are closed so for RFI to the outside world it should not help.

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

 

[zekor]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

But of course not having any formal training in EE means I could be
the one getting it all wrong. So the question to all the experts is
that: Are these on an ATX PSU leads there for a real engineering
purpose or just marketing as I believe?

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

 

[chrisv]

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.

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

 

[daytripper]

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

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

 

[krw]

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

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).

 

[Joel Kolstad]

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

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

 

[Pooh Bear]

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").

That probably helps sales too !

Graham

 

[chrisv]

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

Gads. What a world.

 

[Guest]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI. Then there are all those caps on the cards and motherboards
that would probably deal with any ripple/noise etc already.

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.

 

[larry moe 'n curly]

I've been noticing in some ATX PSU, they have been putting ferrite
rings (that's the round thing I see on the end of video cables right?)
claiming that it reduces EMI and improves quality of the video output
as well as system stability.

My intuition says that's bullshit because on those leads, the PSU is
delivering power at 12V level, unlikely to be affected significantly
by EMI.

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?

http://static.flickr.com/66/158482345_d4351fe79c_o.jpg

(300W Delta DPS-300BB)