# Help with ideal diode model question?

I've been asked to use the ideal diode model on the circuit below to determine the values of I and V.

To determine which diodes were on or off, I considered each possible case:

1. D1 and D2 are both off. This can't be the case because when both diodes are off, they each have positive voltages applied to them which would indicate that they should both be on. This contradicts the original assumption.
2. If D1 and D2 are both on, I get that V = 0V and I = -0.25mA.
3. If D1 is on and D2 is off, a positive voltage is applied to D2 meaning that it should actually be on.
4. If D1 is off and D2 is on, I get V = -1V and I = 0A.

Which one's correct? Case 2 or case 4?

• It is not clear what 'ideal' model you are using. Is it 1V forward voltage and zero resistance? If that is the case, it is impossible for D1 to be off, since the 3V source are enough to bias it. Commented Sep 11, 2016 at 13:56
• The ideal diode model says that if a negative voltage is applied to the diode, then it is reverse biased and will not conduct any current. If a positive current is applied to the diode, then it will conduct current and experience no voltage drop.
– user104243
Commented Sep 11, 2016 at 14:20
• And what happens if you apply a positive VOLTAGE (let`s say 0.1V) to the ideal diode? Infinite current? I doubt if your description is a working model. Perhaps you can assure yourself again about the ideal model you are obliged to use?
– LvW
Commented Dec 11, 2016 at 10:42