Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I purchased a 25Ω linear potentiometer, and found that the minimum resistance is 1.3Ω 0.9Ω. I expected that the minimum would be closer to 0Ω. Is this typical? Should I return the potentiometer as faulty? Is there a spec for this?
EDIT: I originally said 1.3Ω, but several people correctly guessed that I did not account for the meter's resistance of 0.4Ω.
It's not that unreasonable, but be sure that you're actually measuring the resistance through the rheostat. Short the meter probes and subtract that reading from the minimum resistance reading.
The parameter is called "end resistance" or ER, which includes wiper resistance.
For example, this Indian made pot has a maximum ER of the greater of 0.1% or 2\$\Omega\$, so it would meet spec.
In the distant past, pots sold by Radio Shack were made by a major Taiwan-based company and were actually pretty decent quality-- you might be able to find more detailed specs online.
The problem with pots like that is they're among the cheapest of the cheapest pot designs (and then sold by retailers like Ratshack at despicable mark-up), and tolerances are not good to begin with. Also, at higher (and dare I say it, more typical full-range ohm values, i.e. 500, 1k, 10k, 100k), an ohm or two at the bottom end doesn't really make much difference. If you tear one of those things apart, you might be able to appreciate what an... 'agricultural' approach they take to being a pot, compared to more moderns designs.
That Radioshack page has a "spec" (tho not a datasheet in the traditional sense), but even something like this ( http://www.bitechnologies.com/pdfs/p160.pdf ) doesn't make any claims about how closely it can get to 0-ohms, either.
You could go and waste time taking it back & hoping for the best that another will be better, but GTD says to go with something a little more modern in its construction technique, IF that actually getting closer to 0-ohms is a necessity for your application.
When measuring resistance under a few ohms, you must allow for the resistance of your meter leads and connection to the device you are measuring.
Many meters will read 0.5 ohms or so with the leads shorted, so you have to subract this value from the reading to get the actual resistance of the device you are measuring.
