V = r2 / r1 + r2 * v
But could I use this instead (as it's easier for me to remember)
V = V / R1+R2 * R2
They both give 6.6v and 3.3v (from 10v and 2 ... 10ohm and 20ohm resistors) but will I run into problems using this formula (yeah my math skils suck hard)
Multiply and divide bind tighter than addition and subtraction, so you must use
parentheses thus:
V = r2 /( r1 + r2) * v
V = V / (R1+R2) * R2
(Both for humans and machines to read)
Note that this equation is simply a consequence of the current being the same in
the two resistors (so you mustn't load a resistive divider significantly, note).
I = V1/R1 = V2/R2,
V = V1+V2,
V1/V = V1/(V1+V2) = I R1 / (I R1 + I R2) = R1 / (R1+R2)
Unfortunately, although Mark has resolved the addition ambiguity, there is still a potential problem with the *R2 term
You'd be better (in my opinion) to re-arrange the equation to read V = v *R2 / (R1 + R2)
That way it's quite clear that v and R2 are multiplied together before being divided by the sum of R1 and R2
Your original equations also mixed up V and v , which is a common mistake when using upper and lower case letters to mean different things. If you must use upper and lower case labels in the same equation you'd also be better to use the convention of upper case for the primary voltage and lower case for the divider voltage ie v=V*R2/(R1+R2)
It is semantics but considering that the divider voltage will always be less than the supply voltage it is clear that v < V
How could it be just "V"?
In our texts it was always a matter of "VR1" and "VR2"
I've always written it this way,
Vout = Vin *R2/(R1 + R2)
or maybe
Vo = Vi*R2/(R1+R2)
to be a little shorter
cjdelphi:
.... 10ohm and 20ohm resistors
5Volt 30 Ohm that is ..... 167 mA ... quite a lot.. better use 10K and 20K resistors
Another common mistake with voltage dividers is not taking the load resistance into consideration.
Yep, subscripts every time then it's relatively clear what the term relates to.