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.