[SI-LIST] Re: EMI Ring in PCB
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 16 Feb 2007 09:16:57 -0500
Hi Doug,
Thanks for reminding us how ESD protection has evolved since Henry Ott
published the second edition of his book almost 20 years ago. What was
good practice then has been greatly improved upon over the years,
especially with regard to the section on board layout!
Is it still good practice to provide a very high impedance (>1MegOhm)
discharge path between circuit ground and the chassis, to bleed off any
accumulated charge before the PCB is inserted into the backplane? Or,
if the PCB no longer connects the metal faceplate to the card edges,
which contact the metal card guides (also attached to the chassis), is
it better to allow circuit ground to extend to the card edge so as to
contact the metal card guides, both dissipating any charge and reducing EMI?
This also raises questions about the best way to handle items on the
faceplate: I/O cabling, switches, and LEDs. LEDs can be isolated
through the use of light pipes, but what about cabling and switches?
Should one still follow the other ideas put forth by Ott in chapter 12?
As you said, it would take pages to adequately explain how to handle ESD
currents. Perhaps you will have some time to give us your thoughts on
current best practices when you return from teaching.
Best regards,
-Bill
=================================================================
Doug Smith wrote:
> Hi Bill and the group,
>
> In general, an ESD ring causes as many problems as it attempts to fix.
> I would not recommend using them and certainly not connecting them to
> chassis ground through a backplane connector. That just inserts lots
> of ESD noise into the sensitive signals in the backplane.
>
> It would take a couple of pages of typing to adequately discuss how to
> handle ESD currents, here is a couple of major points (individual
> exceptions may apply):
>
> -Isolating circuit boards from the chassis except through one point,
> generally increases ESD problems and forms a great tuned circuit
> (parallel plate capacitor from board and chassis and inductance of the
> signal connection). There are a few articles on my site at
> http://www.dsmith.org on this.
>
> -Keep the ESD off the board entirely either through insulation or a
> metal faceplate robustly connected to chassis.
>
> -ESD can generate hundreds of Volts/inch as it flows on a wire or
> circuit board trace. Even a tiny fraction of the coupled into nearby
> paths can spell disaster.
>
> Lots more, but out of time. I have to finish loading up the equipment
> into the minivan for my teaching next week .
>
> Doug
>
> Bill Wurst wrote:
>
>
>>Jongbae,
>>
>>I believe you may have confused ESD with EMI. If that is the case, I
>>would refer you to Chapter 12 in "Noise Reduction Techniques in
>>Electronic Systems" (2nd edition) by Henry W. Ott, and in particular,
>>Figure 12-17 on p.343.
>>
>>Best regards,
>> -Bill
>>
>>
>> /************************************
>> / billw@xxxxxxxxxxx /
>> / /
>> / Advanced Electronic Concepts, LLC /
>> / www.aec-lab.com /
>> ************************************
>>=======================================================
>>jbtera77@xxxxxxxxx wrote:
>>
>>
>>>Hi, all
>>>Do anybody know about EMI ring and its effect?
>>>
>>>If someone recomment any document or paper which demonstrates EMI ring in
>>>PCB, it will be very helpful.
>>>
>>>Thanks in advance,
>>>Jongbae Park.
>>>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
