﻿ Current in a Diode Circuit
Spring 2019
Almost a Voltage Level Indicator
Find the current across a voltage source in a diode circuit
Learn It!
Pre-Requisite Knowledge
Goal
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 $V_{on} = 0.7V$

$$$a. -10 mA \\ b. 0 mA \\ c. 30 mA \\ d. 70 mA \\ e. 200 mA$$$
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.
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1A
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.
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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.
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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.