# Confusion about how KCL and KVL are used for diode circuits

• timnswede
In summary, the conversation is about a solved example problem in a book where the solution involves assuming a very negative Vin and a turned on diode. The question is raised about why there is current going through the R2 branch if the diode is a short circuit. The conversation also discusses the use of KCL/KVL and the constant voltage model for the diode.
timnswede

## Homework Statement

This is just one of the example problems in my book which is already solved for me, but I don't really understand their solution which I'll post here:
They start out with assuming Vin is very negative, which makes D1 turn on and makes Vout=VD,on+Vin. THen they solve for the currents.

KCL/KVL

## The Attempt at a Solution

What I am confused on is that if the diode is turned on, then the branch with the diode is a short circuit, so why is there current going in the R2 branch?
The book solves all the problems like this one the same way, but I don't really get it. Are we assuming the diode is actually on the brink of turning on/off, so it allows current through it, but it's actually not a short circuit yet?

timnswede said:
What I am confused on is that if the diode is turned on, then the branch with the diode is a short circuit, so why is there current going in the R2 branch?
What does VD.on represent? Is the diode treated as ideal or non-ideal in these problems?

timnswede
timnswede said:
if the diode is turned on, then the branch with the diode is a short circuit,
No. You are asked to use the constant voltage model for the diode.

timnswede
Oh wow, not sure why I didn't realize it until you guys said it, but now it makes sense, thanks!

## 1. What is KCL and how is it used in diode circuits?

KCL, or Kirchhoff's Current Law, states that the sum of currents entering and exiting a node in a circuit must equal zero. In diode circuits, this means that the current flowing through the diode must be equal to the current flowing in the opposite direction through the rest of the circuit.

## 2. How does KVL apply to diode circuits?

KVL, or Kirchhoff's Voltage Law, states that the sum of voltages around a closed loop in a circuit must equal zero. In diode circuits, this means that the voltage drop across the diode must be equal to the voltage source in the opposite direction.

## 3. Can KCL and KVL be used interchangeably in diode circuits?

No, KCL and KVL are two separate laws that are used to analyze different aspects of a circuit. KCL is used to analyze current flow, while KVL is used to analyze voltage drops. Both laws are necessary for accurately analyzing diode circuits.

## 4. What is the purpose of using KCL and KVL in diode circuits?

KCL and KVL are used to ensure that the circuit is operating correctly and to determine the current and voltage values in the circuit. By applying these laws, we can ensure that the circuit is functioning as expected and troubleshoot any issues that may arise.

## 5. How do KCL and KVL help us understand diode behavior?

KCL and KVL help us understand diode behavior by providing a framework for analyzing the flow of current and voltage in the circuit. By applying these laws, we can determine the direction and magnitude of current and voltage in the diode and the rest of the circuit, which helps us understand how the diode is functioning within the circuit.

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