On 5/3/07, Bob Blick wrote: > Peter P. wrote: > > In continuation of this thread, I have hinted before that I have a clamp circuit that uses generic parts and works well. I have posted it before on this list (eliciting no reaction). It is hardly ground-breaking, and it works very well. It can be implemented in several variants, and a zero power version is possible (by adding a MOSFET like a 2N7000 in series with R1, controlled by a voltage monitor or by a microprocessor output). > > Hi Peter, > > Another thing to point out about the circuit you posted, the transistors > are reverse biased B-E, so don't expect to use this circuit above 5 > volts because of the zener effect. Of course nobody would, but it's > sometimes an easy thing to forget. Two BE junction (mounted in antiparallel) of a fast switching transistors it was the best way for transient suppressing in all measuring equipment manufactured in '80-'90. It was safe, and it was fast as long the current is limited like you said and the input voltage was below 4V. I've used many times this trick instead of zenner diodes which has bigger parasytic capacitance and where quite lazy. Of course now things have changed, there are a few superfast schottky with very low forward drop voltage. Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist