Potentiometer with Ohm range 2.9 - 22.5 - 2.9?? - Page 1
Forum section: electronics / beginners

author: burningkrome
author_rank: Newbie
timestamp: August 21, 2020, 09:47:08 am
content: I have a 5 wire Potentiometer (three wire with a switch) and I'mtrying to find if it's linear or logarithmic. If it helps, it does have an "off" click when I rotate the knob all the way counterclockwise.When I set mymultimeter to 200K Ohm, and test either the middle and left, or middle and right wires - I get kind of aquadratic curvewhere at the lowest "on" setting of the knob,I read 2.9. As I turn the knob clockwiseit goes up to ~22.5 near the halfway point, and thenas Icontinue increasingthe knob clockwise, it drops back down to 2.9.Of course, when this knob is controlling a motor, the motor continues to get faster as the knob is turned clockwise. The motor speed does not go up and then start dropping once you've turned past the middle point.There's no markings on the POT to help me search.What kind of potentiometer is this?Thanks!

author: Damianos
author_rank: Frequent Contributor
timestamp: August 22, 2020, 10:15:12 am
content: Is the potentiometer "alone" or is connected in a circuit?Anyway the 2.9k minimum seems a little strange, for a total of (I'm guessing) around 50k.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: August 23, 2020, 05:44:05 am
content: Quote from: burningkrome on August 21, 2020, 09:47:08 amThis can make sense if the pot is a linear 100K pot.First of all, I assume that the 2.9 figure is Ohms NOT KiloOhms.When you measure theun-powered pot in-circuityou may be measuring the Thevenin equivalent of the pots resistance as if it were a voltage source. In that case the meter is seeing the resistance of the left-middle part in parallel with the right-middle part. At half way rotation this would present the maximum resistance seen since only at the middle position would that parallel combination be maximized. Since your reading is 22.5K then then each half must be 45K and the total would be 90K from left to right end. 90K is reasonably close to the standard value of 100K for pots within the tolerances usually seen.If the pot were a log/audio taper pot then the maximum point would be much nearer one end or the other so it must be linear.And, yes, two of the wires are for a switch.

author: burningkrome
author_rank: Newbie
timestamp: August 23, 2020, 01:27:00 pm
content: Thanks for the reply @Damianos,It came attached with this board:https://www.amazon.co.uk/gp/product/B07VRWM7VM/ref=ppx_yo_dt_b_asin_title_o06_s00

author: JackJones
author_rank: Regular Contributor
timestamp: August 23, 2020, 01:49:50 pm
content: It's marked "B100k" in the pictures, usually B stands for linear. Marking "A" would stand for logarithmic.If you're still unsure, you can easily disconnect the connector and measure it out of circuit.

author: burningkrome
author_rank: Newbie
timestamp: August 23, 2020, 01:52:19 pm
content: Thanks for the response @basinstreetdesignAt risk of revealing just how ignorant I really am, I'll describe what I see.1. So, first - I retested it on a more stable surface. It responded more-or-less the same, except I was able to register values below 2.9 on the starting (far counter-clockwise) side. It registered 0, then 0.2, then 1.6, then jumped to 2.9. Getting a reading of anything below 2.9 on this side required a feather-light touch. The slightest turn clockwise caused it to jump to 2.9.The measure at the center (22.5) stayed the same, and at the far right stopping position (far clockwise setting) it also still read a solid 2.9 (which no "feather-light touch" caused it to go below. It was at the dial stop.)2. As for 2.9 Ohms, versus 2.9 kOhm ... I don't know.What I can say is, the multimeter had the "200k" setting selected on the dial - and there was a "200" appearing below the decimal point the entire time.The display did not seem to change anything but the numbers between 0, 2.9, and 22.5. I.e. it did not say 22.5k, or "22500" or "22.5e3" or anything, nor did the "200" below the decimal go off for any of the readings, nor was there any other visual indicator I could see that implied we have changed scales between the 2.9 and the 22.5.It just said "2.9" on the low ends, and "22.5" in the middle.

author: burningkrome
author_rank: Newbie
timestamp: August 23, 2020, 01:55:39 pm
content: Achso! Thanks for catching that. I hadn't seen that!The actual potentiometer sitting in my hand does not have those markings, but I'm guessing it's probably safe to assume it's the same or similar.Thanks again!