Re: Potentiometer curve modification by the use of resistors.

Martin Evans <mcenews@xxxxxxxxxxxxxx> writes:

Some considerable time ago (maybe two or three computers ago and
possibly back in the time of Netscape 2!) I came across an online
calculator that plotted the effect caused by the addition of a couple
of resistors between the wiper and the "ends" of a potentiometer.

(Similar configuration to http://sound.westhost.com/pots-f10.gif )

Of course my bookmarks from that era are long gone and Google nor the
more usual electronic sites I frequent don't seem to turn anything up.

Does anyone know of such an online resource?

Calling his 3.3k resistor R1, his 4.7k resistor R2, the total resistance
of the pot R, the wiper at a resistance x, and an input voltage of 1.0,
the output voltage is going to be

(1/(1/x + 1/R1))/(1/(1/(R - x) + 1/R2) + 1/(1/x + 1/R1))
which simplifies to
(R1*(R + R2 - x)*x)/(R*R1*R2 + R*(R1 + R2)*x - (R1 + R2)*x^2)

googling for
free online plot function
turns up
http://www.coolmath.com/graphit/index.html
which seems to want to throw a lot of cookies at me, but if I
reject all advertising and host cookies it still seems to work.

If I choose R1=3300, R2=4700, R=10000 and simplify gives me
(3300*(14700 - x)*x)/(-8000*x^2 + 10000*(4700*x + 3300*(4700 + x)))
bang the result into the little expression box, tell it
Xmin=0, Xmax=10000, Ymin=0, Ymax=1
and I get a curve that looks almost like your web page plot,

RATS! I didn't add a variable for voltage input and just used 1volt,
add another variable V times all this to fix that for you.

Googling and algebra can give you other equivalent ways of doing this.

If you don't have to have it web based you could try the free 30 day
trial of Derive from www.derive.com. It can do this and a few thousand
other things. I don't get paid to say that, I've just been a happy
customer for the last fifteen years. If you need help using it just
toss me mail, I'm always happy to find new ways to torment it.

But finding and really learning one good mathematics tool will reward you.
.