Vary equivalent resistor linearly between two values

Question

I'd like to use a potentiometer (with a value like 1kOhm) in a resistor network to achieve a close to linear variation between 35 and 250 Ohm. I've tried several configurations, but I'm having a hard time finding a possible configuration.

I've tried the following circuit:

R2 needs to be around 220 Ohm to allow this variation. Not only there aren't potentiometers with this values, as the response curve is very non-linear:

The response doesn't need to be super-linear, but needs to allow the user to vary it continuously.

Answer 1

So you need a 215R POT. If you can buy a 250R pot it may be close enough with a 1.54K in parallel.

^{simulate this circuit – Schematic created using CircuitLab}

NOTE: You can get pots closer to 215 than 250, to use with a larger resistor and get closer linearity, but they are uncommon and cost more.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Using a stereo potentiometer to double the standard resistance gets you fairly close.

Figure 2. 2-gang or stereo potentiometer.