Parallel resistors formula

parallel conductances are much easier.... do that.

 write it like this:

On 21 Aug 2009 09:37:05 GMT, Jasen Betts wrote as

Already done that - thats easy ! Doesnt get anywhere though - I need R1 = as the formula so I can look through a range of values leading to the total I need to get to - thought that was clear!

Move anything containing R1 to the left and everything else to the right of the equal sign, switching sign if moved:

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

Then second hit is

On Fri, 21 Aug 2009 12:48:50 +0200, Robert Roland wrote as:

Thanks very much Robert - just what I needed!! (some basic reminding!! ) and also for the pointer... Charlie+

a little bit of successive approximation would have easily given you this answer in less than two minutes.

(head shaking and finger wagging at your lack of self application)

R1 = - (1/((1/R2) + (1/R3) - (1/Rt)))

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....

Mart> Charlie+ wrote:

On Fri, 21 Aug 2009 16:50:58 -0400, Hammy wrote

Thanks for your equasion - much easier than the one I home brewed in the end: R1=(RtR2R3)/(R2R3)-(RtR3)-(RtR2) Both got to the correct answers in the application! Charlie+

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+

-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.