[SI-LIST] Re: LED with series bead
- From: Don Nelson <dhwn@xxxxxxx>
- To: zhuyongfa@xxxxxxxxxx
- Date: Thu, 08 Nov 2007 06:06:18 -0800
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
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>Shenzhen 518129, P.R.China
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>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
> >
> -------------------------------------------------------------------------------------------------------------------------------------
> > This e-mail and its attachments contain confidential information from
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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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