# Thevenins theorem + two diodes and a resistor using graphical solution

I've got a homework to solve this circuit by a thevenin's theorem and get a current and tension of the diodes using graph. I've looked all around the Internet and couldn't find one single video that does the same thing. Can anyone help me please ? Things that I know : U=10V,R1=10Ω,R2=20Ω, R3=30Ω • I've looked all around the Internet and couldn't find one single video that does the same thing - how about books? Lecture notes? Do you know how to apply Thevenin theorem to a circuit? Dec 10 '18 at 19:38
• Yes I think so :-). My teacher doesn't really know how to explain stuff or I'm just bad at understanding it, So I had to figure it out. But if my calculations are correct after applying thevenins on the circuit (I dont know if our marking is the same) the Ui is 6.67 and Ri is 6.67 but I dont know how to implement it into the graph.And how to get the currents and tensions from it. Dec 10 '18 at 19:46
• How are the diodes to be treated? Are you supposed to use the Shockley diode equation? Or? I ask because you write: "and tension of the diodes using graph." In short, are you supposed to use a V-vs-I diode curve?
– jonk
Dec 10 '18 at 20:11
• Ye it should be V - I graph. And I'm supposed to get current and tension on the diodes... But I'm not sure how to do it. Dec 10 '18 at 20:22 simulate this circuit – Schematic created using CircuitLab

My approach to this would be as follows

1. Create VI curve for D2 in parallel with R3 since we are given a VI curve for D2 and we now the current in R3 from ohms law

2. Add that to the VI curve for D1 to get the VI curve for D1, D2 and R3 combination.

3. Work out the Thevenin equivalent of R1, R2 and U1.

4. Plot a load line with this and our combined VI curve to find the voltage at the anode of D1 and the current in D1.

5. From current in D1 we can find its volt drop and from there the voltage across D2 and the current in both D2 and R3.

The voltage across each diode should be constant, probably about 0.7V but look it up in your assignment. Give it a little thought, and this should already give you the current and voltage on R3.

Now convert R1/V into a Norton equivalent, and add R2 in parallel...you know the voltage and combined resistance, right? You can figure out the current in that resistor combo.

Any current that doesn't go through R1/R2 will go through D1. Whatever part of that that doesn't go through R3 goes through...?

• Okay, But how am I supposed to get those values inside the graph ? I'm lost with drawing the graph. Its hard for me because we did one example with only one diode so drawing graph in that example was easy. Because you only had to connect the two points(one on I and one on V) and than draw diode characteristic into that and where it went through the line there was the tension and current of that diode. But In this example its harder because I have to somehow get R3 Into the graph. + there is one more diode in the problem. Dec 10 '18 at 20:41
• It's going to be more work if you're using curves instead of ideal assumptions. I assume you mean you have a graphic representation of the diode's I/V curves. Draw a line from the origin to the point where it crosses the I/V curve at the operating point...this is the effective resistance at that point in its operation. A complete solution may require some iteration...I'm welcome to suggestions if anyone knows a closed form approach with nonlinear devices and only graphical information. Or if I'm misunderstanding the issue. Dec 10 '18 at 21:24
• No, thats exactly what I'm trying to achieve. Dec 10 '18 at 21:31