Thanks John for picking up the cudgel on my behalf, but those sort of folks don't really put me off. However, should I happen to walk by while they are puzzling about the black goo coming out of their exhaust I will just smile and walk on by, if you know what I mean.

Danny03, you are right in that generally speaking diodes do allow current flow in only one direction. However, one of the neat things that a typical silicon rectifier diode does is that when it is forward biased, conducting current, it develps a voltage drop across it of approximately 0.62 volts and this voltage will remain fairly constant over a wide range of current flow. This characteristic makes for a very simple, inexpensive, and reliable voltage regulator. At least to an old electronics hacker like me.

You can usually obtain a spec sheet from a manufacturer which will tell you the voltage range they specify for their products.

Good Luck All
Bill