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Potentiometer With

Resistors Divider Ratio

Schematic for a potentiometer with two resistors to limit the division ratio.

    Using a pot as a voltage divider where very fine adjustment is needed can be problematic because the output is too ‘jumpy’. When you need a fine adjustment range on a pot, you can incorporate two resistors that minimize the adjustment range. This solver finds the needed values. Rather than specifying an input and output voltage, this solver works in terms of the ratio between the output and input (Vout/Vin). This is particularly useful when you are using the divider to scale an input that is used to drive an ADC or amplifier and dealing with absolute voltage levels is less useful than dealing with the attenuation factor.

    After specifying the pot value, you can either specify Ratio upper and Ratio lower or R1 and R2. If you specify Ratio upper and Ratio lower, you can optionally specify R1 or R2. If over specified (R1, R2, Ratio upper, and Ratio lower are entered), the values of R1 and R2 are ignored. Note that in all cases, the pot value must be specified.

    Give the the tolerance associated with typical potentiometers, the results do not include values for 0.1% resistors as their use in this circuit is nonsensical. Additionally, the results reflect the voltage range with R1, R2, and VR at their nominal values assuming the adjustment range has been specified with some leeway in it.

    As is the case with all resistive voltage dividers, placing a load on Vout will change the output level. Unless Vout is very lightly loaded (for instance the input of a non inverting op-amp or other high impedance load) you may need to buffer its output first. If you wish to lower the resistances found to minimize loading effects, one convenient strategy is to divide the value of R1, R2, and VR by ten. The resulting values will still be standard values for R1 and R2 in the original tolerance and the pot will likely be available in that value as well.

    This solver uses the normal voltage divider equation (shown below) to compute its values. The values of R1 and R2 are iterated through with the value of the pot included as needed to reach a solution.

copyright © 2021 John Miskimins


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