Hello everyone, im new to electronics and though I am making some progress (slow) i need to find a little help with this question:

-In this figure, what value of R2 will make the voltage at the junction of R1 and R2 3V?

I have guessed the correct answer of 100k Ohms but want to know how the calculation / Formula for this should work.

The circuit is a series circuit with two resistors, one has the given value of 300k Ohms (R1) the other is the required value (R2) supply voltage is 12V

Many thanks for anyone that can help with an explanation.

Bob

Edit: I may have got something wrong with adding the image of the photo, I notice that if I right click on the above icon and select open in another tab it will open the image of the circuit that is in a Picasa public web album

 

Hello,

You can upload the pictures right to the forum itself.
Here are the instructions on how to do it:
Attachments and Images

Bertus

 

Thanks for the info here is the circuit

 

Hello,

Have a look at this page of the eBook:
http://www.allaboutcircuits.com/vol_1/chpt_5/2.html

Bertus

 

There are a number of ways of tackling this, but consider this one:

Q1) If you have the desired voltage at the junction of the two resistors, what is the voltage that is dropped across R1?

Q2) Given the answer to Q1 and the value of R1, what is the current flowing in R1?

Q3) Given the answer to Q2, what is the current in R2?

Q4) What is the voltage dropped across R2?

Q5) Given the answers to Q3 and Q4, what is the resistance of R2?

Another better route, because it results in a more general answer, would be the following:

Using this same circuit topology, given unknown values of R1 and R2 (i.e., they are just two variable at this point) that are connected across an unknown voltage source Vo (i.e., another variable). What is the current, Io, that is flowing in the circuit? Given that current, what is the voltage, Vout, at the junction of the two resistors?

At this point, you will have an equation that relates Vout to Vo, R1 and R2 (the Io should have dropped out). If you know any three of the four (such as Vout=3V, Vo=12V, and R1=300kΩ), then you can solve for the fourth (such as R2).

 

There are a number of ways of tackling this, but consider this one:

Q1) If you have the desired voltage at the junction of the two resistors, what is the voltage that is dropped across R1?

Q2) Given the answer to Q1 and the value of R1, what is the current flowing in R1?

Q3) Given the answer to Q2, what is the current in R2?

Q4) What is the voltage dropped across R2?

Q5) Given the answers to Q3 and Q4, what is the resistance of R2?

Another better route, because it results in a more general answer, would be the following:

Using this same circuit topology, given unknown values of R1 and R2 (i.e., they are just two variable at this point) that are connected across an unknown voltage source Vo (i.e., another variable). What is the current, Io, that is flowing in the circuit? Given that current, what is the voltage, Vout, at the junction of the two resistors?

At this point, you will have an equation that relates Vout to Vo, R1 and R2 (the Io should have dropped out). If you know any three of the four (such as Vout=3V, Vo=12V, and R1=300kΩ), then you can solve for the fourth (such as R2).

Hi and many thanks for taking the time to answer. this is a multiple choice question with the possibilities of R2 being:

100k
200k
300k
400k

So using the values of only 4 possibilities I just went through each one to get the correct answer as A: 100k Ohm.

I just wasnt sure if there was an equation available to work it out in one step rather than my "simple" elimination process.

Again many thanks. I think this is a great forum though at this stage almost everything I see here is just way over my head. Still I have the interest and hope I am able to progress.

Cheers

 

If you have the skill to check each of the possible answers and determine if it is correct, then you have the skill to construct the equation you seek. If you want to progress, then you have to struggle to make each step. The step in front of you is to understand how to construct this equation for this problem.

So, given Vo (the supply voltage) and R1 and R2, what is the equation for the current in the circuit? Make you best attempt and we will make corrections as needed.