Thanks Kai, for filling in the finer details that I glossed over. It seems to me that a lot of the participants of this forum lack the basics needed to understand electronics as a complete field, leaving us with the options of a basic explaination plus rules that will prevent obvious errors. Or a detailed short course on the specific subject . I favor the former approach as it presumes the least background knowledge. I am always reminded of the dictum " a little knowledge is a dangerous thing" however I relearn a lot from the detailed explainations given here and do appreciate them Thanks Terry