Need Help In Potentiometer Equation

[renegix_01]

Hello all.I need a help regarding Potentiometer equation. From the schematic it is known that the equation for potentiometer is VL= ( (R2.RL) /  (R1.RL + R2.RL + R1.R2) ) .Vs but it can be simplified to this VL= ( R2/(R1+R2) ).Vs    May i know why it became like this? can u guys show me please the derivation and the reason why it can be simplified to tat formula.REALLY NEED UR GUYS HELP AND SORRY FOR ASKING A NOOOOBBBBISHHHH QUESTION.PLEASE!.

This is where i got the source http://en.wikipedia.org/wiki/Potentiometer

[pjwhite]

The explanation given on the wikipedia page seems pretty clear.  Just think of the pot as two resistors, with one of the resistors in parallel with the load resistance.

[PRR]

> May i know why it became like this?

If RL is large compared to the other resistances.....

If the pot is 1Meg, and RL is a tube grid, roughly 200Megs, then the grid "does not matter".

Or if you use the FULL equation, to get the "perfect" answer, it is only 0.1% different than the "simplified approximate" answer.

OTOH, a 10K pot feeding a 1K load RL, the load can't be simplified-away. The pot is set to "half", the simple formula says "50%", the full formula and actual result is 16%.

Same as in real life. If a room needs a new floor, and there are hardly-any obstructions, you just figure L*W. But if a floor has a vent here, a closet there, a bathtub and toilet.... then you want to figure more carefully so you don't buy more flooring than it needs.