Re: What size fixed resistor to use in voltage divider with variable resistor
As a general answer, you're looking at a tradeoff between range and precision.
If you put a 10k pot in series with a 100R fixed resistor, you'll be able to tune it across 99% of the voltage range. Thing is, you'll do well to get within 2% of any specific voltage, assuming you have a single-turn pot with a zero-to-full range of 270 degrees.. a 1% adjustment in voltage requires a 2.7 degree turn.. and given the mechanics of an actual pot wiper, there will be some sticking, slipping, and sticking as you move the wiper.
If you flip the values around and put a 100R pot in series with a 10k fixed resistor, turning the wiper 27 degrees changes the output by about a tenth of a percent.. much higher precision, much smaller range.
The general idea is to decide how finely you want to adjust the voltage, use fixed resistors to get close to the target value, then use the pot to tweak the details.
If you want wide range and fine adjustment, try building a divider from two 10k pots with a 1k pot between the wipers. The idea is to set the general voltage with the 10k pots, then do the fine adjustment with the 1k pot. The trick is to keep the resistance across the whole thing close to 10k at all times.. if you want to adjust around 90%, set the top 10k to about 500 ohms and the bottom one near 8.5k, then take values between 85% and 95% from the wiper of the 1k.
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