At 09:53 AM 1/23/2004 -0500, you wrote: >-- Original Message -- >From: Jinx >THe bottom of the 5K pot is connected to circuit Ground. >The top is connected to some voltage source, which is considered to be a = >Low impedance (if it IS a voltage source). =20 >The pot wiper is centered. =20 > >Now assume you are the A/D input looking OUT toward the pot. >You see 2.5K to Ground (bottom half) and 2.5K to the Voltage Source (top = >half). >Because the V.S. (top) is Low-Z it can also be considered to be connected= > to ground (from the viewpoint of the A/D input). > >So you have 2.5K in parallel with 2.5K =3D 1.25K. This assumes that the voltage source (Vdd, below) is regulated. If the voltage on the pot is established by a voltage divider it's a bit more complex (see below). For the situation Roy describes: Vdd Vdd o o | | .-. .-. | | R*(1-a) Rotation 0..1 | |<--o | | | | '-' R '-' | | +-------o o | 0V .-. | | R*a | | '-' | o 0V Note that the impedance for R=5K varies from 0 to 1.25K (ideal pot), with the maximum when the wiper is centered. Rp = a*R || (1-a)*R where R is the element resistance, a is the shaft electrical rotation from 0..1 = R * (a - a^2) You can find the min/max by setting dRp/da = 0, so 0 = 1 - 2*a => a = 0.50. If the voltage on the pot is established by a voltage divider the maximum impedance will always be higher, and the maximum will occur at a pot rotation closer to maximum rotation. Limiting case (a current source) will give an impedance maximum of R, not R/4, occurring at the maximum position of the pot rotation, not halfway. Or, suppose we have a 5K pot with 5K in series from the top to Vdd (bad design, but let that slide..) Then the maximum will occur at 100% rotation (as it will for any series resistance >= R) and will be Rp = 2.5K. Or if there was 1K in series with the top of the element, the maximum would occur at 0.60 rotation and would be 1.5K. Etc. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics