# Diode circuit problem

I've come across the following problem: I need to figure out what the voltage is over every element. I already know that only diode 3 is on and that diode 2 and 4 are off. But how do I determine the voltage? Do I need to use KVL? And how?

• Well if they are ideal diodes with no voltage drops and no allowance for reverse current, you could just remove D2 and D4 and turn D3 into a wire. Then it's easy from there. Aug 11, 2016 at 9:25
• This is an unusual homework question as OP asks how to find the solution, has thought about a possible way to solve it, but is not quite sure how to use the method.
– pipe
Aug 11, 2016 at 10:15

D3 conducts and creates a diode drop above ground (0.7v), the current will flow in series through R1+R2+R3 (total 430ohms). The total voltage across this resistor is 5V - 0.7V = 4.3V. Ohm's law: 4.3V/430ohms = 10mA

• Welcome to EE.SE, Rob. We generally don't solve homework questions but give pointers to the OPs to help them. In this case it may have been enough to suggest that the OP redraws the circuit with the reverse biased diodes removed. Aug 11, 2016 at 8:59

Since you have mentioned that D3 is ON and rest are OFF. What you can do is --

You can apply KVL across the loop V1-R1-R2-R3-D3 to obtain the current. Once you have the current, you can find the voltage in any part of the circuit.

For obtaining voltage across diodes (say D2), you can obtain voltages at both of its terminals separately using the obtained current from the above mentioned calculation and then subtract them.

i.e. V(D2+) = v1 -I*(R1+R2+R3) or you can just consider the drop across the diode D3 (i.e. 0.7V usually).

Similarly, V(D2-) = v1-I*R1.

Now, voltage across diode D2 would be V(D2) = V(D2+) - V(D2-)

Similarly, you can find the voltage across any component used in your circuit.

Current will flow as shown. Once you find the current path you can analyze it as if D2 and D4 were not there. Then you need to find the voltage drop across D3 and account for it. The resulting equation looks like this

I = (5- 0.7)/(100+180+150)


VR1= 100 I VR2= 180 I VR3 =150 I

• Please elaborate on why this is the case. Please use proper puncuation. Aug 11, 2016 at 16:06