# Homework Help: Electronic Circuit with 2 diodes

1. Jun 3, 2017

### masterchiefo

1. The problem statement, all variables and given/known data

I have to find Vo in every diode state possible.
We know diode Vf = 0.7

2. Relevant equations
Va-Vc < 0.7 = diode OFF
Va-VC > 0.7 = diode ON
3. The attempt at a solution
State 1:
if D1 OFF and D2 OFF
Vo will be equal to Vi

State 2:
if D1 OFF and D2 ON
Vo will be equal to Vi

State 3:
if D1 ON and D2 OFF
Vo will be equal to 0.7 since the diode is limiting component it at 0.7

State 4:
if D1 ON and D2 ON
Vo= impossible since D2 will not receive enough volt to activate, it will receive less than 0.7 as the resistance will take some.

2. Jun 3, 2017

### SammyS

Staff Emeritus

To what extent are the diodes to be considered to be ideal ?

3. Jun 3, 2017

### Staff: Mentor

Only if there is no load (see state 3).
Why?
I don't understand that argument, and the conclusion is wrong. This state is only possible if there is a load at V0.
I don't understand that argument and the conclusion is wrong.

4. Jun 3, 2017

### masterchiefo

Each state is independent from each other, its like 4 different problems.
In state 1, both are off so its like there is no diode at all and the second resistor is out as well.
which means Vo = Vi

for state 2, I honestly have no idea how to do it. It confuse me a lot. Can you give me some tips ?

For state 3, I came up with a different solution : Vo = Vi - 0.7

For state 4, I came up with the same solution as state 3, Vi -0.7 = Vo

5. Jun 3, 2017

### masterchiefo

They are perfectly ideal, with vf = 0.7
question is to find Vo in every state as stated in the first post.

Each state is an independent problem.

6. Jun 3, 2017

### Staff: Mentor

Only if there is no load at V0. But if there is no load, you never have current through D1!
If diode 2 is conducting current, where does the current go? What does that mean for the voltages? There are two things you can say.
Only with a load. Otherwise: Where does the current go?
You can make an additional statement in this case, based on the conducting diode 2.

Can you have a load attached yes or no?

7. Jun 4, 2017

### masterchiefo

Not really sure how to answer the load attached question, this is all I know about the problem. I believe there is load.
I forgot to mention that it is also possible that some state are impossible.

For state 1, that would mean Vo = R*I

For state 2, I would say current would go to Vo, as D1 cant receive current in that direction. only from Anode to cathode.
so Vo would be -0.7 - R*I = Vo

I wanna clear state 1 and 2 before trying again with 3 and 4, one step at a time so I understand the problem correctly

8. Jun 4, 2017

### Staff: Mentor

That is the voltage drop at the resistor, the difference between two potentials - it is not Vi or V0
But the resistor can.
Right, assuming I is the current flowing through the lower part.

9. Jun 7, 2017

### rude man

Staes 1, 3, 4 OK
State 2 wrong.