Potentiometer Range - Page 1
Forum section: electronics / beginners

author: vidarr
author_rank: Frequent Contributor
timestamp: October 13, 2018, 07:13:42 pm
content: I have a recycled potentiometer that I want to use on very simple 3.3-5v projects. Right now, I am only getting a range from 2.9v to 3.10v. I saw some tutorials of people scraping the pots to decrease the resistance, so maybe I could get up to (or closer to) the full 5v? But, I don't see a way to easily do that with the one I have.Is there an easy way to do this another way? It can be very crude. I do not need precision here. I just need it for dimming LEDs, and for quickly prototyping simple stuff like that (real fast without using PWM).Thank you!

author: ataradov
author_rank: Super Contributor
timestamp: October 13, 2018, 07:52:52 pm
content: Potentiometers don't have voltage ranges. Show us how you use it.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 13, 2018, 08:40:38 pm
content: Like this pic.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 13, 2018, 08:43:00 pm
content: I saw another schematic with a resistor in parallel with the LED after the pot. I did not try that yet.

author: ataradov
author_rank: Super Contributor
timestamp: October 13, 2018, 08:43:13 pm
content: And what voltage do you measure? And what is the value of a fixed resistor? What is the battery voltage?Variable resistor typically go from 0 OHm to their nominal value. Shaving them can only increase the nominal value.

author: ataradov
author_rank: Super Contributor
timestamp: October 13, 2018, 08:45:41 pm
content: Also keep in mind that LED are driven by the current, not the voltage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 13, 2018, 08:50:21 pm
content: Quote from: vidarr on October 13, 2018, 08:43:00 pmTHAT circuit makes NO SENSE AT ALL.DO NOT use that circuit for ANYTHING.Unless you want to blow up the LED and/or the pot.You still have not shown us your circuit with "a range from 2.9v to 3.10v"?

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2018, 09:12:34 pm
content: The voltage across the LED will vary very little as the current is changed, as they have a fairly constant voltage, irrespective of current,

author: vidarr
author_rank: Frequent Contributor
timestamp: October 14, 2018, 12:51:00 am
content: I redid everything a couple more times. Whatever happened the first times, I could not repeat again exactly the same (I got +/- 0.7v max). I was having a fluctuation of about 0.20v, but no change in LED brightness. Pretty sure it was a short somewhere.I took everything completely apart and changed leads. Now, I am getting steady voltage while turning the pot. The LED goes from from "full" bright to almost off. The volts only go from 3.30 to 3.29. This is what I wanted.Somewhere along the way, I was reading some bad info (wrong words?) as to what is happening here (EDIT: along with a faulty setup). I knew what I wanted to do -- the results; I just need to learn all the technical English. I still don't understand the potentiometer fully. I get this analogy of the "wiper" I think. In my head, I just see a resistor that gets more or less resistant (bigger/smaller, stronger/weaker, etc). If you know some simple English explanation that would be great. I watched a bunch of videos on YT. I need the explanation that pops in my head.Thank you.edit: I changed my location setting too. I think the USA was confusing people. Now you know my issues... LOL

author: Nusa
author_rank: Super Contributor
timestamp: October 14, 2018, 02:13:07 am
content: https://en.wikipedia.org/wiki/PotentiometerThis may be about the right level for you:https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law/all

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 14, 2018, 02:15:18 am
content: Discussing an unknown circuit using words is almost useless. That is why we have schematic diagrams.Unless you can show us a schematic diagram of your test circuit, AND WHERE you are measuring the voltage, it doesn't mean much to us.There are plenty of resources about potentiometers available online.I spend a few quality milliseconds with Google and found hundreds.Here are a couple of the better examples I saw after a quick review...https://www.electricaltechnology.org/2015/01/resistor-types-resistors-fixed-variable-linear-non-linear.html#121_potentiometershttps://en.wikipedia.org/wiki/PotentiometerA potentiometer does NOT control voltage or current. It only controls resisistance.It COULD control voltage or current depending on how you connect it in a complete circuit.But when it comes down to Ohm's Law, a potentiometer is just like any other resistor.If you only connect between the "start" and "end" of the resistance, it is just like a fixed resistor.Except that you can change the resistance between each end and the wiper.And a potentiometer is effectively TWO resistors.One resistor between the "start" and the "wiper",and the other resistor between the "wiper" the "end" of the pot.

author: amyk
author_rank: Super Contributor
timestamp: October 14, 2018, 09:19:18 pm
content: Note that most potentiometers are not meant to pass large currents, and can be easily damaged if you do that --- ones which are, are usually called rheostats instead.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 14, 2018, 10:20:22 pm
content: Thanks guys! Both the EE Library and Sparkfun links are great. The library link is especially awesome for me.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 14, 2018, 10:27:49 pm
content: Is this a rheostat? I inherited it from my wife's dad.

author: ataradov
author_rank: Super Contributor
timestamp: October 14, 2018, 10:29:05 pm
content: This is air capacitor.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 14, 2018, 10:59:35 pm
content: Interesting... I use it on my desk to keep the air from blowing my papers away.

author: AngraMelo
author_rank: Regular Contributor
timestamp: October 15, 2018, 02:27:41 am
content: Hello my friend,I think it would be rude of me to answer you in Portuguese given that we are in a english forum.Im also a beginner but I believe I can help you.If you need assistance with that experiment email me at: thiagomelogomes@yahoo.com.br

author: tooki
author_rank: Super Contributor
timestamp: October 15, 2018, 12:28:14 pm
content: Quote from: vidarr on October 13, 2018, 07:13:42 pmBy the way, scraping the conductive trace in a potentiometer would increase its resistance, not decrease it.

author: rstofer
author_rank: Super Contributor
timestamp: October 15, 2018, 02:16:48 pm
content: Quote from: vidarr on October 14, 2018, 10:59:35 pmDon't damage it and don't lose it! Those air capacitors are rare and valuable. They were used for changing the frequency of radios and such.

author: ataradov
author_rank: Super Contributor
timestamp: October 15, 2018, 04:25:24 pm
content: Quote from: rstofer on October 15, 2018, 02:16:48 pmI would argue the valuable part.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 15, 2018, 05:40:01 pm
content: Quote from: ataradov on October 15, 2018, 04:25:24 pmValuable perhaps for people working on vintage circuits (i.e. using firebottles)But yes, modern solid-state circuits don't depend on such vintage parts like those capacitors.

author: Nusa
author_rank: Super Contributor
timestamp: October 15, 2018, 05:49:04 pm
content: Quote from: ataradov on October 15, 2018, 04:25:24 pmAgreed. The ebay value would probably be under $10. They generally outlive the devices they were used in, so besides takeouts, you can even find some parts stock from over 50 years ago, still in boxes.They're simple enough that an amateur can specify and make their own, if they really want to. The how-to information is out there, if you look.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 15, 2018, 11:03:22 pm
content: I am going to hide that air capacitor now -- that is R$40 here. Just kidding. But seriously, you could get shot for less than $10 here. Actually, you can get shot for free.No, but seriously, I was waiting to think of something cool to do with it. Just nothing "steam punk". I broke my only nixie tubes and do not have any aviation goggles to wear.I got a bunch of old stuff her from her dad. He was an electrical engineer. The problem is, my wife and her mom think it is all junk. They are throwing it away slowly. He has rooms and rooms full of stuff. There is nothing I can do. They won't let me touch anything. I looked into the rooms one day when I had access to the keys and I almost peed my pants how much cool stuff was there. I try not to think about it.I got a vintage Syder amplifier that was the best stereo amp in all Brazil during the military dictatorship (ok, not best, only amp). There is more, but I want to post here some stuff another time about some project ideas I have from some of this stuff I was able to save.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 15, 2018, 11:17:47 pm
content: Here is the Spyder. I took apart to clean and troubleshoot. Her dad cut the transformer out. Everything I have here that doesn't work has a broken transformer.Notice the green and yellow wires and wrap. Green and yellow were the dictator's favorite colors. Everything had to have green and yellow or it was banned, confiscated and could not be sold. They told us green and yellow are the best and only colors in the world. I don't know why they told us in school that, it is totally obvious. That is why whenever you see Brazilains, everything is green and yellow.

author: vidarr
author_rank: Frequent Contributor
timestamp: October 15, 2018, 11:21:23 pm
content: Bigger.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: October 21, 2018, 04:22:56 pm
content: Quote from: Richard Crowley on October 13, 2018, 08:50:21 pmWhy not. If the fixed resistor limits the current to the maximum amount the led can handle, the pot can dim the led. You would want a value that would give you the most usable range before the led drops outSent from my SM-G900V using Tapatalk

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 21, 2018, 05:35:03 pm
content: Quote from: 6PTsocket on October 21, 2018, 04:22:56 pmYou must not be looking at the circuit @vidarr was talking about. Let's review the schematic:First, the resistor in parallel with the LED (R2) is completely superfluous and serves no useful purpose.Second, the potentiometer will allow more and more (uncontrolled) current to flow as you approach zero ohms to the point where the LED will burn itself out unless the current is limited somewhere beyond the schematic we are seeing.Third, the potentiometer may itself burn out as more and more current is shoved through it as it approaches zero ohms. Depending on the potentiometer, of course.

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2018, 07:03:38 pm
content: They're valuable if you happen to need one, good variable capacitors are a lot harder to find than they used to be, so while they're not generally worth a lot of money, it can be handy to have one around if you want to experiment with radio circuits.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: October 22, 2018, 04:31:55 pm
content: Quote from: Richard Crowley on October 21, 2018, 05:35:03 pmI was looking at the other schematic with the fixed resistor, the pot ( actually a rheostat) and the led all in series. I am easily confused LOL!!Sent from my SM-G900V using Tapatalk