Michael Rigby-Jones wrote:

>>-----Original Message-----
>>From: Bob Axtell [mailto:engineer@COTSE.NET]
>>Sent: 13 July 2004 09:55
>>To: PICLIST@MITVMA.MIT.EDU
>>Subject: Re: [PIC:] 18f452, problems with interference
>
>
>>Next, look at the relay circuits carefully. The collapsing
>>coils of a relay generate a noise spike of incredibly high
>>energy. You MUST use a Transorber or varistor- NOT a diode or
>>Zener diode- to stop the relay coil pulse, because diodes are
>>NOT fast enough to crush that strong signal. (I  know there
>>are many app notes indicating that diodes are recommended, but
>>these are WRONG by being outdated- they are from a time when
>>some noise could be tolerated- and those days are gone.
>
>
> Is this really true?  Whilst the generic 1N400x recitifiers are slow, the
> cheap 1N4148/1N914 type diodes are very fast, low capacitance switching
> diodes and are robust enough for clamping most relays.

Alas, while they are significantly faster than the venerable
400X-series, the delay in switching time is significant enough
to allow a lot of energy to get through. You can google for supporting
specs; look at the IR (Power MOSFET) website, if I recall, it had the
best documents on protecting power MOSFETs.

--Bob

>
> Regards
>
> Mike
>
> =======================================================================
> This e-mail is intended for the person it is addressed to only. The
> information contained in it may be confidential and/or protected by
> law. If you are not the intended recipient of this message, you must
> not make any use of this information, or copy or show it to any
> person. Please contact us immediately to tell us that you have
> received this e-mail, and return the original to us. Any use,
> forwarding, printing or copying of this message is strictly prohibited.
> No part of this message can be considered a request for goods or
> services.
> =======================================================================
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
> email listserv@mitvma.mit.edu with SET PICList DIGEST in the body
>
>


--

Replier: Most attachments rejected
       --------------
         Bob Axtell
PIC Hardware & Firmware Dev
   http://beam.to/baxtell
       1-520-219-2363

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body