1. The problem statement, all variables and given/known data I have looked in all my textbooks but i can't find anything about this diode or what it does? Here is the symbol of that diode (from a City & Guilds past exam paper): [URL]http://img5.immage.de/2204diode.jpg[/URL] 2. Relevant equations It's just an unknown diode symbol to me, and there are no equations that i know of associated with it. 3. The attempt at a solution At first look, it appears to be a Zener diode, but it might not be, unless it's an old version of the Zener diode??