[SI-LIST] Re: LED with series bead
- From: olaney@xxxxxxxx
- To: john.zelno@xxxxxxxxxx
- Date: Fri, 9 Nov 2007 08:47:12 -0800
Paranoid indeed, as the referenced paper speaks to the case of RS-232
data directly modulating a status LED. Those rates are so slow that a
bead would do no more than slightly round the edges of the pulses, and is
useless. For serdes serial speeds, the LED response is too slow (around
50 nsec for garden variety indicator LEDs that I have bothered to check)
to respond to individual bits. In any instance, if you have control of
the design, then it is simple to drive the LED with something other than
raw data. The use of beads sounds like a fix dictated by a manager. Am
I right?
Orin
On Fri, 9 Nov 2007 07:07:22 -0800 "Zelno, John" <john.zelno@xxxxxxxxxx>
writes:
> We have a very paranoid customer, and have put beads on our LEDs to
> prevent serial data that may have coupled onto the LED signals from
> being modulated by the LED, and thus able to be picked up by a photo
> detector. Much of the paranoia was based on some studies done (for
> 1 example, see http://applied-math.org/optical_tempest.pdf).
>
> -John Zelno
> ________________________________
>
> From: si-list-bounce@xxxxxxxxxxxxx on behalf of Don Nelson
> Sent: Thu 11/8/2007 6:06 AM
> To: zhuyongfa@xxxxxxxxxx
> Cc: steve weir; vijayanand@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx;
> lihuafu@xxxxxxxxxx; duyumin@xxxxxxxxxx
> Subject: [SI-LIST] Re: LED with series bead
>
>
>
> I was once instucted to add ferrite beads to the front panel LEDs of
> a GigE linecard I was developing to prevent EMI (we already tested
> the card and passed EMI, so I was not pleased about this request).
> The LEDs in question were Link/Activity LEDs driven directly by the
> GigE PHY.
>
> When the cards came back from the factory and I began testing them,
> they were a complete disaster... Every once in a while, the PHY
> would go haywire and I'd need to reconfigure it. After a long night
> if pulling my hair out I realized that any time one of these LEDs
> switched from ON to OFF (such as pulling a fiber, causing the LINK
> LED to extinguish), the PHY got zapped; the ferrite caused a
> back-EMF into the PHY when the current tried to switch
> instantaneously from 15mA to 0.
>
> I replaced the ferrites with zerohms and the product worked
> perfectly -- and also passed EMC!
>
> -d
> --
> Don Nelson
> Ericsson
> Warrendale, (Pittsburgh) PA, USA
>
> "There are 10 kinds of people in this world: those who can count in
> binary and those who can't."
>
>
> On Wednesday, November 07, 2007, at 11:19PM, "z46147"
> <zhuyongfa@xxxxxxxxxx> wrote:
> >Content-type: text/plain; charset=iso-8859-1
> >Content-transfer-encoding: 7BIT
> >Steve,
> >
> >Agree with ESD susceptibility if LED applied to front panel.
> >
> >Thanks and regards.
> >
> >
> >Zhuyongfa
> >HUAWEI TECHNOLOGIES CO.,LTD.
> >
> >
> >Address: Huawei Industrial Base
> >Bantian Longgang
> >Shenzhen 518129, P.R.China
> >Tel:+86-755-89653025
> >Fax: +86-755-89650731
> >E-mail: zhuyongfa@xxxxxxxxxx
> >www.huawei.com
>
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> > ----- Original Message -----
> > From: steve weir
> > To: zhuyongfa@xxxxxxxxxx
> > Cc: vijayanand@xxxxxxxxxxxxx ; si-list@xxxxxxxxxxxxx ;
> lihuafu@xxxxxxxxxx ; duyumin@xxxxxxxxxx
> > Sent: Thursday, November 08, 2007 11:46 AM
> > Subject: Re: [SI-LIST] Re: LED with series bead
> >
> >
> > Beads sometimes get applied to front panel LEDs, switches and
> such in
> > the belief that this will limit EMI radiation and/or ESD
> > susceptibility. You can decide for yourself if you think, there
> is a
> > lot that a ferrite bead is going to do that a 100-300 Ohm current
> limit
> > resistor doesn't already do.
> >
> > Steve.
> >
> >
> > z46147 wrote:
> > > Content-type: text/plain; charset=gb2312
> > > Content-transfer-encoding: 7BIT
> > > Hi all,
> > >
> > > I also want to know root cause of this application.
> > >
> > > Any thought on it would be appreciated.
> > >
> > > Best regards.
> > >
> > > Zhuyongfa
> > > HUAWEI TECHNOLOGIES CO.,LTD.
> > >
> > >
> > > Address: Huawei Industrial Base
> > > Bantian Longgang
> > > Shenzhen 518129, P.R.China
> > > Tel:+86-755-89653025
> > > Fax: +86-755-89650731
> > > E-mail: zhuyongfa@xxxxxxxxxx
> > > www.huawei.com
> > >
>
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> > > ----- Original Message -----
> > > From: Vijay Anand
> > > To: si-list@xxxxxxxxxxxxx
> > > Sent: Tuesday, November 06, 2007 6:30 PM
> > > Subject: [SI-LIST] LED with series bead
> > >
> > >
> > > Dear All,
> > > In our design we connect a bead in series with LED while
> driving from an I/O pin. I dont know why this bead is connected (
> still series resistor is there for current limiting). Is there is
> any relation with EMI? Kindly do the needful.
> > > With Regards,
> > > Vijay Anand. G
> > >
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