hacking pots question

[Randall]

I need a 5k linear pot for a small DC power supply I am building. I have a 10K linear pot. I jumpered a 10K resistor to the outside legs to make a 5K linear pot. But did I? While it does adjust from about zero to about 5K, it doesn't appear to be linear. In other words, the 2.5K adjustment is not in the center of the range of motion, it's more like at 1/3 position. Can someone explain this please?

[The Dude]

If you draw the circuit, you will see that there is a series/parallel combination created when you do this. I can't, for the life of me, figure out proper wording to explain it, but if you draw yourself a picture, and imagine current path, you'll most likely see what the "problem" is.

[Tom Phillips]

See the article at The Secret Life of Pots
The tapering section explains how pot performance is altered by added parallel resistance. Although the resistor across the outer legs example, as you used, isn't shown, you can work out the effect by drawing the equivalent circuit with the pot represented by two individual resistors. When you do the Ohms law calculations you will find that the 10k R across a 10k linear pot is not the same as a 5k linear pot. In other words, theory agrees with your observation.

[Randall]

Thanks guys. It's about what I thought it was. Always good to get an explanation from the higher ups! A B5k it shall be.

[The Dude]

I don't consider myself any "higher up" than the next guy. Without seeing the circuit, I don't see any harm in using it as you have it, unless knob position is a bother. It certainly shouldn't hurt anything- just won't be a linear adjustment.

[Tom Phillips]

...just won't be a linear adjustment.

Agreed. Could turn out to be just fine or you may discover that you like it better than stock linear.

[Randall]

It is to be the adjustment pot for a LM317 based power supply, so I think I would like it to be linear.

[Dave H]

It is to be the adjustment pot for a LM317 based power supply, so I think I would like it to be linear.

To use the 10k pot in place of the usual 5k you could double the value of the fixed resistor between the 'Adj' pin and 'Out' pin of the LM317 eg. from 240R to 470R

[Mike Sulzer]

To use the 10k pot in place of the usual 5k you could double the value of the fixed resistor between the 'Adj' pin and 'Out' pin of the LM317 eg. from 240R to 470R

Yes, values as high as 1.2K are found in this http://www.ti.com/lit/ds/symlink/lm317.pdf and other data sheets.