# Solving Diode Conduction Homework with KCL, KVL, V=IR

• ats3216
In summary: So for D2 to conduct, the voltage at that node needs to be greater than or equal to 0.65 volts. In summary, the question is asking for the values of R that would result in D2 conducting or not conducting based on the voltage at the node between the two diodes. This can be determined by applying KCL and KVL and using the equation V=IR to calculate the current flowing through D1 and D2.
ats3216

## Homework Statement

Use the approximation that the diodes conduct only at the indicated forward voltage. For what values of R is D2 Conducting and not conducting?

KCL
KVL
V=IR

## The Attempt at a Solution

I really am not sure how to approach this problem, as none of the ones we had done previously even remotely look like this. Could anyone direct me on where to start with this? Thank you.

#### Attachments

• diode.JPG
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See the node in between the two diodes where R connects. Work out by inspection what voltage that node has to be for the diode D2 to conduct...

For example if R was a short circuit that node would be pulled down to -5V which is below 0V so D2 would be reverse biased = off.

Now imagine R is increasing. It would stop pulling that node down to -5 and the voltage on that node would start to increase. At what voltage does D2 turn ON?

Then imagine you had magically discovered what value of R was required to maintain that exact voltage. Work out the current flowing through D1 and the different current through D2, then apply KCL to get the current flowing in R.

Last edited:
Ok, so by inspection (and don't laugh if I am speaking non sense as I'm still trying to learn how diodes work lol) I would assume that that the voltage at the node between the two diodes would need to equal at least 0.65 to activate the D2 correct?

Correct.

As a scientist, it is important to approach problems with a systematic and logical approach. In this case, we can use the principles of Kirchhoff's circuit laws (KCL and KVL) and Ohm's law (V=IR) to analyze the behavior of the diodes in the circuit.

First, we need to understand the concept of diode conduction. Diodes are semiconductor devices that allow current to flow in only one direction. When a diode is forward biased (positive voltage applied to the anode and negative voltage applied to the cathode), it conducts current and acts like a closed switch. On the other hand, when a diode is reverse biased (negative voltage applied to the anode and positive voltage applied to the cathode), it does not conduct current and acts like an open switch.

Using this understanding, we can analyze the circuit by assuming that the diodes conduct only at the indicated forward voltage. This means that we can simplify the circuit by replacing the diodes with closed switches when they are conducting and open switches when they are not conducting.

Next, we can apply Kirchhoff's circuit laws to the circuit. KCL states that the sum of currents entering and leaving a node must be equal to zero. KVL states that the sum of voltage drops in a closed loop must be equal to the sum of voltage sources in that loop. We can use these laws to write equations for each node and loop in the circuit.

Finally, we can use Ohm's law to calculate the current through the resistors in the circuit. By comparing the calculated current values with the forward voltage of the diodes, we can determine for what values of R the diodes are conducting and not conducting.

In summary, the approach to solving this problem involves understanding the concept of diode conduction, applying Kirchhoff's laws, and using Ohm's law to analyze the circuit. This systematic approach can be applied to many other circuit analysis problems and can help us understand the behavior of complex circuits.

## 1. How do I apply Kirchhoff's Current Law (KCL) to solve diode conduction problems?

KCL states that the sum of currents entering a node (or junction) must be equal to the sum of currents leaving the node. In diode conduction problems, this means that the current flowing through the diode must be equal to the current flowing through the rest of the circuit.

## 2. What is Kirchhoff's Voltage Law (KVL) and how is it used in solving diode conduction problems?

KVL states that the sum of voltage drops in a closed loop must be equal to the sum of voltage rises in that loop. In diode conduction problems, this means that the voltage drop across the diode must be equal to the voltage drop across the rest of the circuit.

## 3. Can Ohm's Law (V=IR) be used to solve diode conduction problems?

Yes, Ohm's Law can be used to determine the current through a diode when the voltage drop across it and the resistance of the circuit are known. However, it cannot be used to determine the voltage drop across the diode as diodes do not follow Ohm's Law.

## 4. What are some common mistakes when using KCL, KVL, and V=IR to solve diode conduction problems?

One common mistake is forgetting to take into account the voltage drop across the diode when applying KVL. Another mistake is assuming that the voltage drop across the diode is equal to the supply voltage, which is not always the case. It is also important to properly label the polarities of the diode and other components in the circuit when using KVL and KCL.

## 5. Are there any other methods besides KCL, KVL, and V=IR for solving diode conduction problems?

Yes, there are other methods such as using the diode equation and graphical analysis. These methods may be more accurate and efficient for certain types of diode circuits, but they require a deeper understanding of diode characteristics. KCL, KVL, and V=IR are commonly used because they are easier to apply and can provide a good approximation of the circuit behavior.

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