Calculate the voltage across a resistor using Thevenin's Theorem

[Andrei0408]

Homework Statement

The diode in the next figure is considered ideal (i.e. works as a simple switch, being turned ON when U_D >= 0 and OFF when U_D < 0). What is the voltage U_S across R_S?

Relevant Equations

Thevenin's Theorem

I'm thinking I should apply Thevenin's Theorem to find the voltage, but I need to find I_D in order to be able to calculate U_S.

 

[BvU]

I need to find I_D in order to be able to calculate U_S.

You have been told that the diode works as a simple switch. So $U_D = U_S$ if the switch is ON -- never mind $I_D$. Can you decide it is OFF or ON ?

$\$

 

[DaveE]

One way to approach this is to substitute your diode model (a switch and a battery) into the circuit. This model has two switch states that have associated restrictions; no forward voltage across the switch when it's open and no reverse current when its shorted. Then you can just pick one state and analyze the circuit in that configuration to determine when the associated restriction is violated. That is the operating point at which the diode will switch states. You can do it with either "on" or "off" assumption, sometimes one is easier to solve than the other.

This approach is a good way to get "unstuck" when you have complex circuits and you don't know where to start. Just make an assumption and get started, but with the knowledge that you may have guessed wrong, then find out if, or under what conditions, you are right or wrong.

 

[Andrei0408]

You have been told that the diode works as a simple switch. So $U_D = U_S$ if the switch is ON -- never mind $I_D$. Can you decide it is OFF or ON ?

$\$

I'm not sure how to do that

 

[BvU]

Pick one and see if it leads to an inconsistency