# Calculating voltage drop across diodes

Hey guys im completely new to this site and electronics so forgive me. Ive uploaded a photo of the question. Im trying to work out at what values does

just D1 conduct
D1 and D2 conduct
then finally all of them.

Im really stuck here. i know that the diodes start conducting when the voltage across them is equal to .7 volts.

i know that for only d1 conducting

Vout = 5/6 Vin

but i cant work out at what voltage does d2 being conducting. i know it has something to do with the voltage across the 5k resistor but im not sure how to maniuplate it.

any advice or help would be so very grateful..
thanks

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Redbelly98
Staff Emeritus
Homework Helper
but i cant work out at what voltage does d2 being conducting. i know it has something to do with the voltage across the 5k resistor but im not sure how to maniuplate it.
What are the current and voltage of D2, when it is just at the threshold of becoming conducting?

yep d2 starts conducting when the voltage across it is .7. so yeah the question is. what is the minimum voltage out for d2 to start conducting.

so i reasoned that the voltage across the 5k resistor would also be .7. so the current would be 1.4 along that wire.

from there i did took the equivalent resistance so i was left with a 1k resistor andd the 15k resistor in parralel with the new 4.28 resistor. Since i could calculate the current approaching the node above i said that that current by the 4.28k resistor would equal the voltage out. so that equals 1.4..

but im not sure if that works at alll.. i really have no idea

Redbelly98
Staff Emeritus
Homework Helper
yep d2 starts conducting when the voltage across it is .7. so yeah the question is. what is the minimum voltage out for d2 to start conducting.

so i reasoned that the voltage across the 5k resistor would also be .7. so the current would be 1.4 along that wire.
How about 0.14 mA ... does that sound better? And therefore Vo would be ____?

from there i did took the equivalent resistance so i was left with a 1k resistor andd the 15k resistor in parralel with the new 4.28 resistor. Since i could calculate the current approaching the node above i said that that current by the 4.28k resistor would equal the voltage out. so that equals 1.4..

but im not sure if that works at alll.. i really have no idea
I don't know that combining resistors in parallel will work here, because of the diodes. But if you can get Vo, you can calculate:
• the current through each parallel path, and then
• the current through R1, and then
• the input voltage Vin = VR1 + Vo

Then you'll have Vin and Vo at the point where D2 begins to conduct

Firstly i meant to say .14mA not 1.4 V.
so that still leaves me with a final answer of Vo = 1.4 V

It makes alot of sense what you saying.. i just need to work out Vo first. and thats where i have difficulty as im not sure if my answer is correct.

Im positive however, that you can combine resistors as when a diode is conducting you can remove it and place a resistor.. but since there is already resistors you can just remove them. that allows you to take the parralell resistance.

Thank you for help. Its greatly appreciated!