AVR-Chat

Thanks, Jim.  You have been a good help.  I do have a bunch of RF ferrite
chokes that I put here and there.  They didn't do anything to solve the SSB
radio turning on the horn (!), but a shielded cable did.  

The tip about there being no one solution helps actually.  I guess I was
looking for that, and you've made it clear that's impossible.

So I'm going to do more or less what you suggested and use the 3.3 V
Transguards with a series resistor, at least on low frequency signal uC
ports.  However, I'm still struggling with the numbers.  

Poking around a bit on AVRFreaks seems to indicate the internal protection
diodes can handle up to 1 mA rather than the 0.1 mA you quoted.  Is that
what you meant by your "I think" caveat?   Taking this figure and the
clamping voltage of 12 V for the 3.3 V working voltage Transguards, I
compute (12-3.3)/1 = 8.7 K for the series resistor.  Actually, I suppose one
could substitute 3.8 V, the abs. max. voltage the port can take, and reduce
this to 8.2 K.  At any rate, being a bit conservative, choose 10 K.  Do
these numbers make sense?

For a max. frequency, the particular Transguard I chose (and happen to have
100 or so lying around) has a whopping high capacitance of 5000 pF.  If I
continue to use these, I compute a max. usable frequency of about 10 KHz.
Do you concur?  This would be ok for many signals I have.

However, I do have some higher frequency signals.  I looked at the
UltraGuard series, which are for "high speed clock...application".  But the
lowest capacitance for, say, a 5 V one, is only 175 pF, not a heck of an
improvement by the above numbers.  Plus there is no breakdown or clamping
voltage in the data sheet.  So I continue to be confused.  Do these work
differently or something?

Steve

| -----Original Message-----
| From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
| Behalf Of Jim Wagner
| Sent: Friday, January 07, 2011 9:21 PM
| To: AVR-Chat@yahoogroups.com
| Subject: Re: [AVR-Chat] AVX Transguard
| 
| I missed the part about radios, radar, etc. So, in addition to
| transient protectors, liberal use of lossy ferrites (beads, etc) are
| strongly called for.
| 
| There really is no one technology that will handle the full spectrum
| of ESD, lighting, general conducted EMI, and RF through microwave. I
| am sure that I'm not the only one on the list who has been through
| this commercially. Its NOT a trivial task and its particularly
| difficult to do with confidence in the absence of testing. Lacking
| that, you are going to have to rely on your own interpretation of
| other's experiences and hope that you understand what is offered.
| 
| Jim Wagner
| Oregon Research Electronics
| 
| On Jan 6, 2011, at 9:15 AM, Cat C wrote:
| 
 

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