# Need help with simple series and parallel circuit (ohms law) finding R2 size.

#### gandoN9

Oct 12, 2020
2

I know there is a serial and parallel circuit

I know R1 = 2k, R3 = 1k and i know Vout which is 1V. And i know 5V is coming from supply. I need to calculate the size of resistor R2.

Can somebody explain me with exact equations to calculate R2, and currents which run trough resistors (step by step please)

Help will be much appreciated!

Last edited:

#### AnalogKid

Jun 10, 2015
2,922
Homework?

R2 and R3 are in parallel, so together they have an equivalent resistance that is the lower leg of a voltage divider. With Ohm's Law you can determine that equivalent resistance, and with the equation for parallel resistors rearranged, you can determine R2.

ak

#### gandoN9

Oct 12, 2020
2
I can calculate :
1) IR1 = 5-1V/2000ohms = 2mA
2) UR1 = 0,002A*2000ohms=4V voltage drop on R1

How do i go on finding R2?

#### AnalogKid

Jun 10, 2015
2,922
Eliminate R3.

Calculate R2 based on the simple voltage divider equation. Call this R2x

When you add R3 back into the circuit, the parallel combination of R2||R3 must equal R2x.

ak

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
Eliminate R3.
To do this, replace R2 || R3 by a single resistor "Rp" as explained in post #2. You then have a basic voltage divider made from 2 resistors. You can then calculate the value for "Rp" from the value of R1 and the known voltages.
In a second step find R2 such that the paralell circuit of R2 and R3 equals "Rp".

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