A friend pointed out that maxim have an overvoltage protection controller: http://pdfserv.maxim-ic.com/en/ds/MAX4838-MAX4841.pdf has anyone used the 4840? their pricing looks reasonable at $.70 for 2500 and hopefully someone stocks discreet parts. https://shop.maxim-ic.com/cgi-bin/Maxim.storefront/60800530/Product/View/MAX4840EXT&2DT might it be worth it to use this with a single FET and a single 1uF cap. Looking at the datasheet, everything about it seems to look good. I'm not too knowledgeable about it so maybe someone else might spot any problems with it, if they exist. Thanks, Roines Russell McMahon wrote: > 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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. --------------------------------- Do you Yahoo!? Yahoo! Movies - Buy advance tickets for 'Shrek 2' -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.