[SI-LIST] Re: LED with series bead

  • From: "Zelno, John" <john.zelno@xxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 9 Nov 2007 07:07:22 -0800

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
>  > 
> -------------------------------------------------------------------------------------------------------------------------------------
>  > This e-mail and its attachments contain confidential information from 
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> Any use of the
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>  > disclosure, reproduction, or dissemination) by persons other than the 
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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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