Potentiometer voltage range?

[grosh]

We are attaching a potentiometer to an axle to indicate arm height. We are using a 100ohm / 10 turn pot. We are getting values of 4.5V to 5.0V over the entire range of 10 turns. We are worried that that is not enough of a range of values because...the pot will only be turning about 2 turns(not the full 10 turns it is designed for). We anticipate only having a range of 4.5V to 4.6V to use for feedback.

Will it work even with the small range of values?

Will a different potentiometer give us more of a range?

[flameout]

How is it wired up?

If it is a 3-pin potentiometer, then one should be connected to ground, one to 5v, and the sweeper to the analog input.

A potentiometer acts as a voltage divider -- it needs a ground and 5v to function. It sounds as if you might not have ground connected.

If ground is connected, take out a multimeter and check the resistances. Two leads should have a 100-ohm resistance between them (no matter where its input is). It is also possible that you wired 5v to the sweeper arm, 0v to one of the other pins, and the analog input to the last.

Also, once you've verified that it's wired right, check it with the voltmeter. Maybe it's a software issue (i.e. reading from the input next to where it's connected).

I hope this helps.

[Mark McLeod]

After you find and fix your wiring problem, it also sounds like you should be using a 3-turn pot to get better resolution.

Better resolution means more accurate positioning of your arm.

[billbo911]

Originally Posted by Mark McLeod

After you find and fix your wiring problem, it also sounds like you should be using a 3-turn pot to get better resolution.

Better resolution means more accurate positioning of your arm.

The first thing to do is get a 5k to 10k 3 turn pot. 100ohms will likely load the analog board too much and yield poor results.

Then, for absolute best resolution of position, try to match the number of turns as closely as possible. Always make sure the pot can travel a bit more than the actual turns needed, but not by too much. Most pots don't handle being turned past their end point. Although, continuous pots don't care, but they seem to come in single turn only.

[sam_who]

You could power a 10-turn pot with 12 volts. That will give you 1.2 volts per turn.

CAUTION! If you choose this method, you must ensure not to go past ~4 turns or you risk burning the input (for starters). Also, you have to use the lower end of the pot (0-4.8 volts) rather than the upper (7.2-12 volts).

Finally, take the advice offered earlier and get a higher value pot.

[Al Skierkiewicz]

grosh,
From your description, it sounds like you either failed to wire one side of the pot to power common or the wire is broken. The voltage range with no power common connection is determined by the pot and the input impedance of the analog board. At 100 ohms, you are dissipating 1/4 watt in the pot. If it is not rated for that power, you should change values to something higher. 10k would be a better choice. What teams forget is that you can add a gear to the shaft of the pot and a gear to the device you are trying to measure. Just select the reduction that gives you about 200-240 degrees of rotation and you can use a standard one turn pot (nominally 270 degrees of rotation).