Algebra

[iqoruvuc]

Sadly my maths education was spent drawing pictures of people, writing down the names of my favourite tracks, and going home.  I did not take much in at after the age of about 11.  I am having a problem with trying to work something out.

I am building a glitch desk from an old dj mixer case and want various controls such as pots.  Because different machines require different value pots, I will need to be able to switch between the various resistances such as 1 meg, 470K, 100K etc.  I wired up a one meg pot and was planning to reduce the overall resistance by using voltage dividers that can be selected by a rotary switch.  So a 1 meg pot with a 1 meg resistor in paralell with have an overal voltage of 500K.  What I would like to do is work out the other values by using an equation rather than trial and error.  I know the formula for working out the overall resistance of resistors in parallel but I cannot for the life of me work out the individual resistance of a resistor when I know the overall value.  So for instance if the overal value is 100k, what resistance would Resistor A (the voltage divider) be when Resistor B (the pot) is 1 meg?

Any pointers would be much appreciated.

P.S. anyone off to The Glade Festival this weekend?

[iqoruvuc]

Hi didn't make much sense there.  What i meant was if the Overal Resistance was 100k, what value would Resistor A be if Resistor B was 1 meg? 

[Signal:Noise]

I know that the calculation for combining resistors in parallel is 1/Rtotal = 1/Ra +1/Rb, Ra and Rb being your different values of resistors, so all you'd need to do to work out Rb if you knew Rtotal and Ra would be to rearange the eqaution so it read 1/Rtotal - 1/Ra = 1/Rb.

Hope that helps.

[iqoruvuc]

Hi that does help, I have just chucked a few values in where I know the answer and it's working.  It seems was reading it wrong.  I tried to rearrange the formula before and I kept getting stuck but I can see what I was doing wrong.  That's really been a help, thanks!