> I noticed that Philip's voltage regulator zener diodes like the BZD27
series incorporate transient suppression so maybe these caps are unnecessary
if I pick an appropiate Zener with TVS?
>

The zener IS the transient suppressor BUT it will stop a transient going
"up" but not down. To maintain supply during "dropouts" or negative spikes
you need an energy reservoir and that's what the capacitor is for. The 470
uF you suggest is ample for most purposes - less is often OK.

> 2. Zener choice. Here's the Philips zener datasheet:
http://www.semiconductors.philips.com/acrobat/datasheets/BZD27_2.pdf
> This one looks good. I'm thinking of picking the BZD26-C5V6. This
selection would tie in with the thyristor triggering issues.

If you use an UNREGULATED supply the unloaded nominally 5 volt supply, the
unloaded  voltage will very likely cause the SCR to trip. You'll need to
check with the supply you intend to use. Sometimes a small load resistor is
enough to give enough regulation to allow a reasonably low crowbar voltage
to be used.

This circuit has its merits, but you can probably get most of the protection
you desire more cheaply by omitting the SCR, making R1a short circuit,
removing D1 and using a zener with substantially more power dissipation
capability (say 5 watts plus). If the voltage goes very high with normal
polarity the zener will conduct and blow the fuse. If the polarity is
reversed the zener becomes a standard diode (essentially) and blows the fuse
even more rapidly. This does NOT have the latching on and hard short
features of the SCR on excess forward voltage but will usually be adequate.
It also does a better job of clipping transisents (as R1  is now a short and
impedance of zener to ground is lower.)


    Russell McMahon

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