All About Circuits Forum Finding unknown resistor in parallel
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#1
05-07-2009, 04:31 AM
 newtoelectronics New Member Join Date: Jan 2009 Posts: 2
Finding unknown resistor in parallel

I'm sure this is a very simple problem for everyone here, but I'm unsure on how to solve this particular problem in my text.

The problem is asking me if RT is 400 Ohms, R1 and R2 are 2k ohms each, what is R3 if these resistors are connected in parallel?

I know to calculate RT if the resistors are known using the reciprocal of 1/r for more than 2 different value resistors, r1*r2/r1+r2 for two resistors in //, if I have say 3 equal resistors of 3 ohms each for example, I can divide that by the # of resistors in // and get 1 ohm as Req.

So how can I solve for an unknown resistor in //? What I have found to be a problem is when I am asked to solve for something that wasn't covered in the text. I am unsure what formula to use or do not know how to derive a formula. Tips and help is greatly appreciated in advance!
#2
05-07-2009, 04:45 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,949

Use this

1/RT=1/R1 + 1/R2 +1/R3

Re-arranging

1/R3 = 1/Rt - 1/R1 - 1/R2

and so on.

For N resistors in parallel

1/RT= 1/R1 + 1/R2 + .....+1/RN

 Tags finding, parallel, resistor, unknown

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