neoshinzo said:no its not for homework, but i understand thanks.
hi,
For reference, although the 13.4V zener will start to conduct at slightly over 13.4V, the current thru R8 will not create a voltage drop large enough to forward bias the Q3 base, so it will not switch on.
As the current thru the 13.4v zener path increases due an increasing V1 more current will flow and the voltage across R8 will reach about 0.7V , at which point Q3 will start conducting.
Is this any clearer.?