Help needed with resistor spreadsheet

[Norwegian_68]

Hi all, my first post

Heres the case: I have 48 different values of resistors, each i
0,25watt, I need a specific value that is 2watt, that means I have t
use 8 resistors in parallel. So how to find the 8 resistors that get
me closest to the target 2watt resistor?

I tried making a spreadsheet that calculated all possible combinations
but it got so huge that I ran out of resources.

I think that my experience with excel is to limited to find a solution
so all help is welcome

 

[Guest]

Hopefully, you would have 8 of each value of resistor on hand so that the
current and therefore the power dissapation through each would be balanced in
the array.

To that end, put =$A$1 in cells A2:A8 and put this formula in B1...

=1/(1/A1+1/A2+1/A3+1/A4+1/A5+1/A6+1/A7+1/A8)

Then, whatever value of resistor you have on hand, you can type in A1, and
B1 will display the total resistance of the array of 8 in parallel.

You can, of course type individual values in A1:A8.....juggling them until
the B1 answer is what you seek.

hth
Vaya con Dios,
Chuck, CABGx3

 

[Bruno Campanini]

Hopefully, you would have 8 of each value of resistor on hand so that the
current and therefore the power dissapation through each would be balanced
in
the array.

The current (and the power dissipation) in the array will
be balanced only if all the 8 resistors have the same resistance in Ohm.
This leads to very simple calculation: you need X ohm at 2 watt?
Set up a parallel of 8 times 8*X-ohm at .25-watt resistors.

Don't you have 8*X-ohm resistors and can't you assemble a
number of EQUAL resistors to do X ohm?
Then you can't have current (and power) equally distributed
among the resistors and the calculation becomes much more complex.
As you can parallel a number of resistors for making the required
resistance value but the total power dissipation is NOT the sum
of individual power dissipation.

Bruno