# resistors in parrallel

#### boobear

Joined Jul 1, 2005
1
I'm struggling with the following problem.

I need to find the value of a resistor (R1) that is in parallel with a 60 ohm resistor (R2), the equivaltent value of both resistors is 15ohms

I have used a online calculator to find the value of R1 to be 20 ohms, but i would like to know the formula to solve the problem.

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by boobear@Jul 1 2005, 12:15 PM
I'm struggling with the following problem.

I need to find the value of a resistor (R1) that is in parallel with a 60 ohm resistor (R2), the equivaltent value of both resistors is 15ohms

I have used a online calculator to find the value of R1 to be 20 ohms, but i would like to know the formula to solve the problem.
[post=8865]Quoted post[/post]​
Calculating the effective resistance of parallel resistors is an important basic skill in circuit analysis. Since you posted this question in the "homework" forum, I have to imagine that you have access to a textbook that contains the formula. If you don't have access to a textbook you should be able to look around the Internet for the formula.

hgmjr

#### n9xv

Joined Jan 18, 2005
329
I love this kinda stuff!

There is a couple of ways to write this formula so as to make it logical. I like to put it this way;

To find the "unknown" parallel resistance, one known value in parallel with another unknown value producing a known equivalent resistance.

Unknown value of parallel resistance = Unknown R

Value of existing resistance = Existing R

Equivalent total esistance = Equivalent R

(Existing R * Equivalent R) / (Existing R - Equivalent R)

So,

60 * 15 = 900

60 - 15 = 45

900 / 45 = 20

To be sure of your result, work again with the normal parallel formula and make sure the equivalent resistance is always < the value of the smallest resistance.