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

[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?

Homework Equations

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.

 

[CWatters]

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.

 

[ats3216]

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?

 

[CWatters]

Correct.

 

[rezeew]

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.