is it acceptable to use a potentiometer as a rhetostat? - Page 1
Forum section: electronics / beginners

author: ftransform
author_rank: Frequent Contributor
timestamp: March 24, 2013, 12:38:53 am
content: So I would like to get the most stability and precision out of my potentiometer, which I am currently using as a rheostat.However I got this funny feeling and remembered reading somewhere in some book that using a potentiometer as a rheostat degrades its stability due to wiper resistance or something.I cannot find this book again, but I remember it saying that there was a SIGNIFICANT disadvantage to doing so, at least with cheap parts (like an order of magnitude). I don't really remember the reason it gave but it made it out to be a big problem.Is there any truth to this, especially when using a good quality pot like bourns (not a bo-rang) ?Does anyone know what I am jabbering about?

author: Rufus
author_rank: Super Contributor
timestamp: March 24, 2013, 12:58:07 am
content: The voltage from a potentiometer varies little with wiper contact resistance because there is usually little current flowing through the wiper contact.The voltage from a potentiometer doesn't vary at all with variation in the resistance of the potentiometer element providing that variation is uniform across the whole element.So the voltage coming out of a potentiometer is much more stable than the resistance between one end and the wiper.

author: houdini
author_rank: Regular Contributor
timestamp: March 24, 2013, 02:15:01 am
content: Book is analog SEEKRETs i think theres a copy somewere on this site.

author: mamalala
author_rank: Unknown
timestamp: March 24, 2013, 02:24:33 am
content: Unless you are working with very small currents, forget about it.Imagine a 10k pot set to th middle. The source impedance for the following circuits will be 5k. Any load applied will change the output voltage.Of course ther are heavy-duty wire.wound resisitors/pots used for stuff like "ancient" stage lighting, etc. But they still suffer from the same problem. The only way to mitigate this would be to use a pot with a much lower impedance/resistance than the load. But of course this also means that a lot of the consumed power is wasted as heat...So, no. As rheostat, forget it, if you want it to be practical.Greetings,Chris

author: ftransform
author_rank: Frequent Contributor
timestamp: March 24, 2013, 02:31:28 am
content: Quote from: mamalala on March 24, 2013, 02:24:33 amIt is acting as a adjust resistor for an op-amp, so the currents are very small.I will attempt to wire it as a potentiometer instead.

author: mamalala
author_rank: Unknown
timestamp: March 24, 2013, 03:26:21 am
content: OK, not much of a problem there then. However, be aware that a rheostat usually means an adjustable pot for "high" power. The basic principle is the same for a simple pot and rheostat, just that the latter is meant to handle a _lot_ more current/power...Greetings,Chris

author: Rick Law
author_rank: Super Contributor
timestamp: March 24, 2013, 04:40:12 am
content: If you are thinking about using the single-turn potentiometer, it is hard to get precision because small movement makes a big change. On the other hand, if you are using 25 turns trimpot type potentiometer, you get much better precision but the big problem is durability. Even good trimpots have a life time of only about 200 turns.Rick

author: amspire
author_rank: Super Contributor
timestamp: March 24, 2013, 05:05:38 am
content: Quote from: ftransform on March 24, 2013, 02:31:28 amThat is a common way potentiometers are used.QuoteThe only advantage of using all 3 pins on a potentiometer when used as an adjustable resistor is that if the wiper goes open circuit, the potentiometer will go to its maximum value rather then open circuit.A potentiometer is actually a type rheostat, but it is really only fairly recent usage that has used rheostat for high powered wirewound adjustable resistors, and potentiometers for low power adjustable resistors. "Potentiometer" used to mean a precision instrumental resistive divider often used to compare an unknown DC voltage to a standard cell. What we now call a potentiometer used to be called a rheostat.Don't get caught up on the names - the thing is if you want to use a modern resistive-film based potentiometer, the key is to keep the current - particularly through the wiper - as low as possible.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 24, 2013, 08:20:37 am
content: This is what the book says, now that I found it.It is indeed "analog secrets"

author: vk6zgo
author_rank: Super Contributor
timestamp: March 25, 2013, 04:21:10 am
content: Quote from: amspire on March 24, 2013, 05:05:38 amI think you're talking Geological time here,the term "potentiometer" or "pot" for short,used to refer to the low power devices used as,for instance,radio volume controls has been current for 80 years or more.The precision instrument called a "Potentiometer" was never common outside Metrology Labs & Physics textbooks.Various manufacturers,such as Marconi,AWA,Pye,Philips,STC,HP,etc have successfully used pots as variable resistors over the years.There are many cases where extreme accuracy of the sort referred to in the linked information is not necessary.The point is,that good designers use their own judgement as to where & when a particular technique is appropriate,rather than slavishly following the dictates of some "Guru".

author: BravoV
author_rank: Super Contributor
timestamp: March 25, 2013, 04:38:31 am
content: An idea how a precision pot looks like, a 15 turns 15 watts with0.02%linearitybeast.To give you the idea of it's size compared to soda can and other "smaller" precision and two el-cheapo pots.

author: amspire
author_rank: Super Contributor
timestamp: March 25, 2013, 06:04:16 am
content: Quote from: vk6zgo on March 25, 2013, 04:21:10 amI didn't say that - I just said it was originally the name for a meter to measure the electric potential or voltage. The measuring device happened to be a resistor with a calibrated variable tap, and that is how the name transfered to the more common potentiometer. All I said was that the use of the word rheostat as an alternative name for a potentiometer has being disappearing recently. Back in the 60's and 70's, potentiometer and rheostat were pretty interchangeable. The electronic industry more commonly used "potentiometer", the electrical industry often used the word rheostat for the same device.A potentiometer is still a rheostat today - the definitions have never changed. Just modern usage.QuoteNo disagreement there. The 0.00001% accurate Fluke 720a Voltage Divider uses over 30 potentiometers as variable resistors for calibration.QuoteNot sure what you mean. Of course a designer follows their judgement - that is the job of a designer.If you look at a device from the 60's or 70's, you often see a mass of trimmer pots and most of them will be used as variable resistors rather then voltage dividers. You just have to be aware that in the variable resistor mode of use, the resistance includes the contact resistance which can be erratic, and the temperature coefficient that is often worse then a cheap fixed resistor. It is really only the advent of the microcontroller and digitizing that has eliminated most of the trim pots in designs and replace them with fixed resistors and electronic calibration instead.

author: robrenz
author_rank: Super Contributor
timestamp: March 25, 2013, 12:36:29 pm
content: Hereis a very usefull reference from Bourns. Definitely worth a read through. Beware many of the pages are out of sequence in this pdf.Edit: pdf attached of a less detailed but still very usefull version