1. The problem statement, all variables and given/known data The diodes are ideal, so their internal resistance is zero. The zener diode has a 4V activation voltage. There is only one variable element in the circuit is V0. The question is what is Vab for every value of V0, and what are the directions of the current. 2. Relevant equations Kirchoff laws The directed sum of the electrical potential differences around any closed circuit must be zero. And the properties of the diodes. 3. The attempt at a solution Well, I have the solution but I don't understand some questions, that is the reason of this thread. The solution is this: -V0<0 implies that the current goes for the mesh of the mesh of the zener diode. The other diode cut the current in his mesh. -6V> V0 > 0 , is the same situation but with less current in the same mesh. -10V> V0> 6 V , there is no current in any mesh -14V> V0> 10 volts, zener polarized but we don't have sufficient voltage to polarize the other mesh of the diode. -V0>14 volts, both diodes are polarized and they carry the current. I don't understand something, in this analysis we consider the rises and drops of the voltage in the diodes and in the voltage sources to know if a mesh will carry current or not, but it seems to forget the resistances . But evidently if the resistances carry some current they will produce some potential drop. So ¿why do you need only V0=14 volts to polarize the zener diode?, ¿Should not be higher due to the presence of the resistance r0?. ¿What is the reasoning to arrive at that 14 volts?. And finally ¿what is the potential difference in ab due to the 8V source?¿How does the diode affect it? THanks in advance.