Newbie question about trim pots

[atakacs]

Hello everyone, I've been googling trying to find an answer to this question but couldn't find what I was looking for. I'm about to begin my first builds and a few of the pedals that I am researching use trim pots. My question is this- If the vero layout shows a 100k trim pot, does that mean that the pot itself is a 100k trim pot, or is it any trim pot that can be adjusted to 100k (i.e. a 200k pot with the wiper dialed in to 100k, or something of that sort). I have a decent but of experience wiring electrical panels for programmable machines but analog circuitry still holds some mystery for me!

[jez79]

Depends on how the pot is used
If it's used as a variable resistor (wiper connected to one end of pot), you can most likely get away with a larger value
But if each leg of the pot is connected to something different, you should use the value listed.
In general, to be safe, use the value listed.

[Tony Forestiere]

Agreed. Also keep in mind that trimmers are linear taper.

[PRR]

Buy the number indicated.

I *do* have a book which shows "69K" trims, meaning an approximate trimmed value to get the circuit +/- happy. But that is very unusual, for teaching, not for building.

[Jdansti]

Welcome! Usually the value listed on a schematic, layout, or bill of materials is the value of the trim pot, i.e., the maximum resistance when rotated to its fullest point.  This is the same as a "regular" pot. As others have mentioned though, you can often use a different value within certain limits.

Trim pots are usually used as "set and forget" adjustable components. Optimally, you don't want the range of the resistance to be too large relative to the set point because it becomes more difficult to dial in the desired setting as the maximum resistance of the pot increases above the set point. In other words, you don't want the set point to be in the lower 10% of the pot's sweep because tiny movements of the wiper would cause large changes in the circuit. Having the set point around 20-80% of the sweep is probably good. The designer of the circuit should specify a value for the trimmer that allows you to easily dial in the set point.

Also, as jez79 mentioned, if all three legs of the trimmer are connected to the circuit, the further you get away from the specified value, the more likely you'll have problems.