Brian Aase wrote:
> 
> > <tjaart@WASP.CO.ZA> writes:
> (snip)
> > We've tested a
> > range of diodes, and only a few could conduct quickly enough to short
> > these spikes. The neg side of the relay can go quite negative.
> 
> Would you be willing to share with us which specific diodes you
> found to be suitable?

We found a combination of things to be desirable. I built 
a super-zapper that we used to test how long our input 
protection would last. We tried tried the trusty 1n4007, 
1N4148 etc. They seemed to work ok, but the fast rising 
times of the spikes still cause very sharp 400V spikes 
to get through. We tried neon bulbs which worked damned 
well for dissipating large transients (into light), but 
we were still left with those horrible things. Next up 
we tried a 1k5E tranzorb that worked amazingly well. 
Zeners don't even see half the spikes, so if you want 
to use zeners, make sure you have a series resisitor 
and a parallel cap (100nF).

If your budget allows, you can use fast recovery diodes, but 
I think it would be cheaper to use four different size caps 
over the supply with standard diodes. 100uF, 100nF, 1nF, 10pF. 
A 1R series resistor between the supply and te PIC can also do 
wonders for noise rejection.

You can also trick the transients into seeing the caps 10 times 
bigger than what they really are without the ESR penalty by 
"hashing" your groundplanes. This "hashing" effectively slows 
the transients down up to around 10 times, increasing the effective 
protection.

On a PIC input, I like to use a the following :

        1k      5V1 100nF 220k   15k
PIC<--/\/\/\/----*----*----*----/\/\/\/---- Horrible outside world
                 |    |    |
                ---  ---   \
                / \  ---   /
                 |    |    \
                 |    |    /
                 |    |    |
   --------------*----*----*--------------

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