On 3/22/07, John Dammeyer wrote: > Hi Orin, > > > > > On 3/21/07, Vasile Surducan wrote: > > > Based on drawing ES-I is input and E_O is output and here the > > > protection could be OK. > > > Orin, a stabilizer circuit (even the three terminal one) has a > > > negative feedback, so the voltage can't grow to 11.4V even in bad > > > dreams. If the supply is based on a three terminal stabilizer the > > > reverse diode between Vin and Vout will become on and the spike will > > > be absorbed in the filtering cell. > > > > Well, that all depends on Vin to the regulator which is undefined... > > as you note, the supply schematics aren't given. > > > > Then the problem being protected against was a short to 12V, not just > > a spike. What if Vin to the regulator is the same 12V? > > > > Orin. > > It's currently a 7805 regulator. I just replaced a L4940V85 regulator that had literally split in two due to an indirect short on its output and those things are supposed to be protected against shorts and overheating! Still wondering how that happened, but it probably was a short on the output to greater than its Vin, but it also took out a circuit trace and a TCA2465 power op-amp (also supposedly protected against shorts and overheating). BTW, that's rather a long time constant with the 100k pullup and 0.1 uF cap. Since you don't seem to be low power, I'd go for a lower pullup. Careful where you put it or it ends up as a voltage divider with the series resistance. Orin. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist