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**posted on**

- Charlie+

August 21, 2009, 8:15 am

The normal textbook formula for resistors in parallel is given as

for two Rt =(R1*R2) / (R1+R2)

or

for more Rt = 1 / (1/R1 +1/R2 + 1/R3 etc..)

I need the formula in the form R1= and to my shame I cant remember how to

do the transposition can someone help me out please!

I need a one off resistance of 53.6 ohms (as near as possible) to be made up

from salvage SMT standard values (which seem always slightly under the

nominated) and the Rt = formula isnt much help!

This resistor is for an 100x oscilloscope probe and needs to be as small in

size as possible, so two or 3 SMT resistors in parallel would be best.

Thanks

Re: Parallel resistors formula

Move anything containing R1 to the left and everything else to the

right of the equal sign, switching sign if moved:

1/R1 = 1/R2 - 1/Rt

Now, turn both sides of the equation "upside-down":

R1 = 1/(1/R2 - 1/Rt)

...or if you want to leave the math entirely to someone else, try

http://www.google.com/search?q=parallel+resistor+calculator

Then second hit is

http://www.electronics2000.co.uk/calc/series-parallel-resistor-calculator.php

--

RoRo

RoRo

Re: Parallel resistors formula

And in parallel resistors - the resultant is ALWAYS smaller

than the smallest.

One used to work with slide rules - simple rules were in the mind

to keep the result in bounds. 53 is less than 120 but 530 isn't.

Decimal point goes....

Martin

David Eather wrote:

http://www.electronics2000.co.uk/calc/series-parallel-resistor-calculator.php

Re: Parallel resistors formula

Finger wagging taken!

In the end I used nominally 56 // 1800 which with the value errors taken

into account got me within .05 ohm but I found bunging the formula in a

spreadsheet got me quick answers to any value change! And the oscilloscope

calibrates spot on with the 100x probe. I didnt find any 120 SM resistors

on any of my junk boards incidentally.

So thanks to all who gave it some thought and helped.

Charlie+

Re: Parallel resistors formula

Robert Roland wrote:

-1/R1 = 1/R2 - 1/Rt actually

Although it is easier and less error-prone to subtract 1/R2 from each side.

The procedure is also easier to define in any list of algebraic methods.

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