Spring 2018
Almost a Voltage Level Indicator
Find the current across a voltage source in a diode circuit
Learn It!
Pre-Requisite Knowledge
Almost a Voltage Level Indicator
What is the current, \(I\) supplied by the voltage source if \(V_{on} = 0.7V\)
What is the current, \(I\) supplied by the voltage source if \(V_{on} = 0.7V\)

\( \begin{equation} a. -10 mA \\ b. 0 mA \\ c. 30 mA \\ d. 70 mA \\ e. 200 mA \end{equation} \)
As with any problem with diodes and DC sources, we start by guessing. But I'm not sure what to guess, so I'm going to reshape the circuit a little.
Transform the circuit to make it easier to analyze
The circuit looks like this when fully transformed.
Part 2
Guess and Check
Now it is time to guess. The top one is next to the battery, which has a voltage of more than \(0.7 V\), so I'll guess that it is on.

The second diode is a tougher choice. I start off with \(1.0V\), and I guessed that the first diode was ON, so that uses up \(0.7\) of my \(1.0 V\). With only \(0.3\) left over, I'm going to guess that the second diode is OFF.
Step 1
Compute quantities based on guess
\[ \begin{align} 1-0.7-10 \cdot I &=& 0 \\ -10 I &=& -0.3 \\ I &=& 0.03 \end{align} \]
Write down KVL, subtract over the \(1 - 0.7\) and then divide over

This might be our final answer if the other diode is correctly assigned.
Step 2
Check guess using KVL
\(I = 0.03\)
Since our guess satisfied Kirchoff's laws, we are correct. The current we found before is the correct answer.