Talk:Potentiometer/Archive 2

Rheostat/Rheotomes
The reference to an early Edison patent for a carbon potentiometer in 1872 needs work. Historical web sites such as www.tomedison.org/invent.html refer to Edison's 1872 invention of the "carbon rheostat" which consisted of powdered graphite in glass tubes that changed resistance by increasing and decreasing pressure on the graphite with a screw/knob. Edison abandoned this because it was too sensitive to vibration. I could not find his patent for this. Edison got a US patent 131,334 titled "Rheotomes or Circuit-Directors" but this is for a selector switch, not a rheostat, although a simple modification could change the switch into a multi-tap potentiometer as was used in radio broadcasting studios and audio recording studios where they were called "pots". In 1877, Edison revived the vibration-sensitive rheostat for use as a carbon (graphite) telephone transmitter. On the web site edison.rutgers.edu/sernote3.htm near the bottom there is a reference to Edison's notebook entry for a "carbon rheostat" in 1873. Rheostats using a screw/knob to vary current by varying pressure on graphite were used in power supplies for vacuum tube radios during the 1920's and 1930's and they were labeled rheostats. Greensburger 02:10, 5 January 2007 (UTC)

Requested improvements
Remove the invalid redirection "Redirected from Potentiometric analysis" --208.114.161.196 15:50, 18 February 2007 (UTC)

Loudness
"(...) a setting of 5 sounds half as loud as a setting of 10", it says in the Audio Control section about logarithmic pots. Shouldn't that be: "5 sounds half as loud as 6?". I don't know a lot of maths. Dafferson 23:10, 3 June 2007 (UTC)

Differences
Tell whats the difference between linear, tapered, and logarithmic types. i am a electric engineer, and i sometimes rely on wikipedia to do my electronics!! i consider this piece of article VERY important!! XU-engineer 21:35, 10 June 2007 (UTC)

Requests
I would like a correction on digital pots. The statement "they allow small adjustments to be made to the circuit by software, instead of a mechanical adjustment." is not correct. It implies that you cannot control a digital pot any other way, when in fact, you can. We use them, but use an electronic interface that has no software or no firmware. It is very simple and some digital pots will allow and up/down switch. The serial ones can be interfaced electronically too and without software or firmware. Not all use I2C bus, many also use a simple two wire serial bus. So, I would suggest this be corrected. Danpeddle 04:38, 4 October 2007 (UTC)

It would be nice to explain Johnson noise and ways manufacturers have designed the resistors to reduce this or at least reference to the fixed resistor, assuming it mentions it. Danpeddle 04:38, 4 October 2007 (UTC)

Can someone add some photos of linear high power potentiometers to show that they are not just circular? (the linear body might be a tube, but they are still linear - see Ohmite) Danpeddle 04:38, 4 October 2007 (UTC)

Rheostat vs. Potentiometers
It seems their is too little understanding of what is a rheostat. You have stated a rheostat "is usually much larger, designed to handle much higher voltage and current" when comparing to a potentiometer. Please, understand what you are writing so the wiki world doesn't look so foolish! This is a statement that an uneducated novice would make.

A rheostat is a two terminal variable resistor used to control current. It has one fixed end and one variable end (the wiper). They can be high power high voltage, low power low voltage or anything in between. They can also be linear in shape or any shape a potentiometer comes in. They can be linear taper, log taper, any other taper and even digital.

A Potentiometer is a three terminal variable resistor used to control voltage (variable voltage divider). It has two fixed ends with a variable middle (the wiper). They can be high power high voltage, low power low voltage or anything in between. They can also be linear in shape or any shape a potentiometer comes in. They can be linear taper, log taper, any other taper and even digital.

Basically, they are the same component, except the rheostat has either a wiper shorted to one fixed end or the second fixed end of a potentiometer is not connected to anything. Try it, take any potentiometer and short the wiper to one fixed end and you have a rheostat or do not connect the second fixed end in your circuit! Please note, that it is better practice to short the wiper to one fixed end of a potentiometer or purchase a rheostat.

BOTH a potentiometer and a rheostat can be seen as varying the resistance (depending on where the measurement is made). After all, they are VARIABLE resistors.

They are also represented slightly different schematically. And, speaking of schematic symbols, the rheostat actually has two; one that shows it's wiper connected to one end and one that shows an arrow THROUGH a resistor (no connection of the arrow to the fixed end)

Danpeddle 04:09, 4 October 2007 (UTC)
 * OK, sounds good. You sound well qualified to update the article to make the distinction then. --Hooperbloob 22:04, 28 October 2007 (UTC)
 * A rheostat is definitely a two-terminal device - the essence of the "potentiometer" is the variable voltage divider, three-terminal configuation. --Wtshymanski (talk) 16:11, 17 December 2007 (UTC)

I still think the two terminal / three terminal distinction is wrong. I find it ironic that the picture of a rheostat in the article clearly is a three terminal device, which pretty much proves my point. --ssd (talk) 00:38, 2 October 2008 (UTC)
 * It's how you use the device. If you use one fixed end and take current out the slider, it's a rheostat.  If you use both fixed ends and use the slider as an adjustable voltage divider, it's a potentiometer. A rheostat is an adjustable resistance, a potentiometer is an adjustable voltage divider. Sometimes those who need a rheostat use a potentiometer and tie the slider and one fixed terminal together. IEEE Standard 100 is unusually useful in these defintions, and if it's good enough for the IEEE, it should be good enough for Wikipedia. I should fix the caption if it's misleading.  --Wtshymanski (talk) 15:45, 2 October 2008 (UTC)

Digital pot section
I've removed the following text from the digital pot section, because there really isn't any tie in to this article. A digital pot could be used to create a hybrid digital and analog control system, but nothing in the removed text really describes how a digital pot would be used in such a system or the advantages/disadvantages of using a digital pot in such a system. Sagsaw 18:20, 28 October 2007 (UTC)

''For the most part, the universe of feedback system design separates neatly into two fundamental design paradigms: analog-based designs and digital-based designs. Both design strategies are characterized by advantages and liabilities that generally make one or the other a clearly superior choice for any given application. Pure analog designs, for example, tend to display design simplicity and low development costs that play well in the context of smaller system applications. Digital-based designs, by contrast, have essentially unlimited flexibility in computation of the feedback function and therefore dominate in more complex feedback applications in which higher development costs (generally including software implementation) can be justified. Sometimes, a feature of particular utility in digital designs is the availability of digital memory with its ability to store feedback parameters for arbitrary lengths of time without drift. This makes possible a variety of system solutions including multiple system operating modes in which feedback values can be acquired in some modes and then applied in others''

There is also an "analog" potentiometer which can ge referred to as "digital". This is a thumbwheel look a like has been on the market for countless years. For example the 3682S or 3683S series made by Bourns. Other series which could be refrred to as "digital" is a multiturn potentiometer with a digital dial mounted like the CT-26-6A or the H22-6A.

Galdang (talk) 16:26, 27 July 2008 (UTC)

Pots as Measuring Devices
How are they used to measure anything? The only thing I can think is that they are used in conjunction with some other device -- I would bet that this other device is what is actually used to measure... thought? Jheiv (talk) 01:09, 26 June 2008 (UTC)


 * This is explained in the article 3rd paragraph: "A potentiometer instrument for measuring the potential (or voltage) in a circuit taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The sliding tap of the potentiometer is adjusted and the galvanometer briefly connected to both the sliding tap and the unknown potential; the deflection of the galvanometer is observed and the sliding tap adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology."
 * The potentiometer (pre digital) I used in an electronics lab was a drum about 40 cm long and about 16 cm diameter. There was a spiral notch around the drum and a 10 meter resistance wire in the notch which wrapped around the drum 20 times.  Printed on the drum above the wire/notch were millimeter marks with centimeters numbered.  I moved the slide wire until the galvanometer was about zero and then slowly rotated the drum by hand until the galvanometer was zero.  If the reading on the drum was 500 cm, the voltage being tested at the slider was exactly half of the total voltage on the 10 meter wire.  Before doing a measurement like this, you first had to calibrate the potentiometer using a mercury standard cell with the slider set at the cell voltage. Greensburger (talk) 01:55, 26 June 2008 (UTC)


 * Although it seems that it is, in fact, the Galvanometer, that is doing the indication, the integration of the potentiometer with the Galvanometer seems like a fair use of the term measuring device -- thanks for the explanation. Jheiv (talk) 04:10, 26 June 2008 (UTC)

merge
I suggest merging the stub digital potentiometer into the "digital control" secton of the "potentiometer" article, until it grows large enough to need its own article. (p.s.: When it does get big enough to split, remember to redirect Digitally controlled potentiometer to the appropriate place). --68.0.124.33 (talk) 03:49, 7 July 2008 (UTC)


 * Do not merge. The article is large enough, and the two devices are different enough to warrant two different pages.  I really don't think its a good idea.Jheiv (talk) 04:27, 7 July 2008 (UTC)
 * Do not merge. Same. This article should stay as it is. Ginbot86 (talk) 21:26, 17 October 2008 (UTC)

RV onpotentiometers what does it mean?
Ive found this on fender guitar amps.zdoes anyone know what it means? megaarxidas Unsigned contribution by 00:10, 11 May 2009


 * Please post questions at WP:Reference desk/Science. This page is for discussing improvements to the article.  Sp in ni  ng  Spark  13:59, 12 May 2009 (UTC)

Pot mod
Is up for deletion if anyone knows anything about it or can help determine whether it's a notable subject. ChildofMidnight (talk) 06:24, 20 June 2009 (UTC)

Redirect
Other languages have a separate page for Rheostat, but in English it's just a redirect here. Am I allowed to link the Rheostat redirect to the equivalent article in other languages? Flying fish (talk) 19:10, 2 August 2010 (UTC)
 * Seems a reasonable thing to do to me.  Sp in ni ng  Spark  06:36, 3 August 2010 (UTC)

Is the potentiometer only a voltage divider?
Wtshymanski, I puzzle out how to explain to you the difference between potentiometer and voltage divider. Well, let's try again... Potentiometer is the name of the "pure" unconnected element (a three-terminal resistor with a sliding contact) while voltage divider is the name of one possible application of this element where the input voltage is applied between the ends and the output voltage is taken between one end and the wiper. So, you may connect a potentiometer as a voltage divider but, for example, with the same success you may connect it as a current divider. For this purpose, ground the pot ends and connect a current source to the slider (note the slider is an input here while the ends are outputs!). By moving the slider you can change the proportion between the current flowing through the two legs. Does this potentiometer form a voltage divider as your source claims? No, it is not a voltage divider at all! It is something absolutely different; it is a current divider! Another similar application is to apply voltage to the wiper; think of this arrangement as of two complementary rheostats with a common point... That is why I have added this "...can form a voltage divider..." instead "...forms a voltage divider..." A potentiometer can be a voltage divider but it can be something else as well... Please, find a more reliable source for the definition. Circuit dreamer (talk, contribs, email) 22:23, 4 August 2010 (UTC)

Unfortunately, you have not taken the trouble to read my explanations... Instead, you have blindly copied&pasted the definition from the IEEE source. Well, let's continue this discussion with the hope that you will finally realize that the definition is confusing and misleading (although I respect this institution, I prefer to rely on my reasoning).

If we accept that "a potentiometer forms an adjustable voltage divider" (although it is more than obvious that it can act as a current divider, voltage summer, etc.), then we may say also that:
 * "a transistor forms a common-emitter amplifier" although it can act as a common-base, common-emitter and differential amplifier,
 * "an op-amp forms an inverting amplifier" although it can act also as a non-inverting and differential amplifier,
 * "a resistor forms a voltage-to-current converter" although it can act with the same success as a current-to-voltage and resistance-to-voltage converter,
 * "a capacitor forms an integrator" although it can act as a differentiator as well, etc.

As you can see, each of these elements has many possible applications and no one will identify the element with only one of its applications. Why do we then identify the element potentiometer with its application voltage divider? Circuit dreamer (talk, contribs, email) 00:17, 7 August 2010 (UTC)


 * Perhaps because the literal meaning of the name potentiometer is a measured potential. Current, unlike voltage, cannot be described as potential.  Sp in ni  ng  Spark  10:31, 7 August 2010 (UTC)

Parallel voltage summer is another interesting application (in addition to the examples above) where two grounded voltages are applied to the ends and the output voltage is taken between the wiper and the ground. Indeed, this arrangement may be thought as two united voltage dividers but it is not a simple voltage divider. Circuit dreamer (talk, contribs, email) 12:46, 7 August 2010 (UTC)


 * This is just more POV and OR like we have seen before. It is good that you are no longer assigning the benefits of negative resistance to virtually every electronic circuit but it appears you have just embarked on some other taxonomy. I thought we had disabused you of all this. Zen-in (talk) 15:52, 7 August 2010 (UTC)

What is (not) a potentiometer?
(a new viewpoint at the old element)

I intended to discuss the legendary name in the talk page of Potentiometer (measuring instrument) but I have nothing against to start this interesting discussion here (we may continue or move it later to the other page). It is interesting not so for the very name but for the great idea behind it.

IMO the literal meaning of potentiometer is a device for measuring a potential difference (potentiometer = potential + meter) and this means exactly voltmeter. More than a century ago, the situation was well-known - a new device had appeared and it needed a name. They named this 3-terminal resistor potentiometer because they were using it in an arrangement for measuring voltages; the application has given the name of the element. But I would like to show that even then, in 18th century, this name was wrong, misleading and at least inexact. Let's see why (click  [show]  button on the extended content bar below). Circuit dreamer (talk, contribs, email) 16:39, 9 August 2010 (UTC)

The potentiometer as an element of a meter


What the potentiometer is not. Potentiometer is not a meter; it is only an element, a part of a meter. So, the title Potentiometer (measuring instrument) is not correct; more precisely speaking, it has to be Potentiometer (element of measuring instrument). But what is this element? What does it do? What is its function in such a measuring instrument? Let's look at this old-fashioned arrangement from our nowadays viewpoint to see what the role of the potentiometer actually was.

Why the potentiometer is not a meter. The bare potentiometer is not a meter at all and it cannot measure anything; it can only produce (more precisely, regulate) reference voltage. But voltmeters receive voltage; they are only-input devices. For example, the classical needle voltmeter is a device with electrical input and a mechanical output; so, it acts as a voltage-to-displacement converter. But our device (named initially potentiometer) is just the opposite - it has a mechanical input and an electrical output and it acts as the opposite displacement-to-voltage converter.

What the potentiometer really is. So, the potentiometer is only a regulating element like a resistor ladder that, in conjunction with the constant reference voltage source, forms a regulated and precisely graduated reference voltage source. It is equivalent, for example, to a multiplying digital-to-analog converter (DAC). This resemblance is full in the case of a step potentiometer.

What the role of the potentiometer is. The role of this regulated reference voltage source is exactly the same as the role of a DAC included in the system of an analog-to-digital converter (ADC) - to produce varying reference voltage in the range from zero to the reference voltage. An analog-to-digital converting system consists of DAC, supplied by a reference voltage, a comparator and a control unit. Our potentiometric measuring system consists of the same elements: a potentiometer (DAC), a galvanometer (comparator) and a man (control unit).

How the potentiometric system operates. The reference voltage at the potentiometer's output is placed contrary and in series with the input voltage so that the two voltages are subtracted according to KVL and their difference is applied to the galvanometer acting as a zero indicator. The man monitors continuously the difference looking at the galvanometer and adjusts the output voltage equal to the input one.



But this is exactly the negative feedback principle! It seems our potentiometer arrangement is a system with series negative feedback. So, it acts exactly as a voltage follower and a non-inverting amplifier with only difference that here a human being plays the role of the op-amp. How interesting it is! Imagine it turned out as early as at the 18th century they invented, were using and deriving benefits from series negative feedback! But what are these benefits?

Obtaining infinite input resistance. As a result of the equalizing the two voltages, the resistance seen from the side of the input voltage source is virtually increased from a few hundred ohms (the actual galvanometer's resistance) up to the infinite resistance of an ideal voltmeter. Thus, thanks to Miller's virtual infinite impedance idea, the low-resistive ammeter (galvanometer) is converted to an ideal voltmeter (in respect to the internal resistance). So, the whole measuring system (the voltage source, potentiometer and galvanometer) behaves as...nothing; it is invisible for the input voltage source that "sees" nothing. This is a remarkable property of systems with series negative feedback - to increase virtually the impedance seen from the side of the input voltage source up to infinite. This phenomenon is known as bootstrapping and is based on Miller theorem.

But this is a reversed potentiometer! If you are curious enough (and not tired enough), you may finally note that the whole arrangement is some kind of a "reversed potentiometer" and this is exactly the idea of using a potentiometer as a meter. The standard usage of a potentiometer as a varying voltage source implies moving the slider as an input quantity to obtain a voltage as an output quantity (i.e., the displacement controls the voltage and we have a displacement-to-voltage converter). But here the man adjusts the potentiometer's slider so that to make its output voltage equal to the input voltage (figuratively speaking, the input voltage makes the slider stand in the position where the output voltage is equal to the input one). It turns out the input voltage controls the displacement and we have obtained some kind of a voltage-to-displacement converter. This was exactly the electromechanical meter that they wanted in the 18th century where the slider acted as the needle of the conventional meter. I remember that in 60's such automated meters (data recorders) implemented as servo systems (where the galvanometer was replaced by an amplifier and the man - by a motor gear) were very popular in industry. Remember also the vintage analog X-Y plotters from these years using DC servo motors and precise potentiometers as feedback sensors where the input voltages moved the pens. Such a reversing is a unique property of negative feedback systems.

What a potentiometer can be
I hope I have managed to convince you that a potentiometer was not a meter in the distant 18th century; it was only an element of a meter. The three-terminal resistor (the bare element) can act as a potentiometer (displacement-to-voltage converter) or voltage divider (voltage-to-voltage converter) but they are not the same. In addition to these applications, this element can be used also in many other applications. Let's list some of the possible applications of the element named inaccurately potentiometer.


 * Voltage-to-voltage converter (voltage divider)
 * Displacement-to-voltage converter (the potentiometer discussed above)
 * Voltage x displacement multiplier
 * Parallel voltage summer with weighted inputs
 * Split supply (resistive virtual ground)
 * Bipolar varying voltage source
 * Current divider
 * A system of two complementary varible resistors (rheostats) with a common point...

(add more applications)

Circuit dreamer (talk, contribs, email) 16:23, 9 August 2010 (UTC)


 * In what way do you expect to convince without citing a single reference? The calibrated scale of the potentiometer is the meter.  The galvanometer is only used to find the null.  Arguing that the scale has to be calibrated with something else and it is therefore not a meter is a completely false argument; try applying the same argument to a ruler or a clock.  Negative feedback? utterly perverted way of looking at it - your circuit is merely a classic bridge circuit.  Sp in ni  ng  Spark  11:53, 22 August 2010 (UTC)


 * I rely on human common sense, basic electrical laws and circuit concepts to convince readers. The calibrated scale of the potentiometer (and of any analog meter) acts only as a simple mechanical analog-to-digital converter and, in the case of a non-linear device (e.g., an ohmmeter), as a functional converter. The scale (like the humble ruler) only converts the mechanical analog quantity (slider or needle displacement) into a digital one. All the rest (the potentiometer, the galvanometer and the man) acts as an electromechanical analog voltage-to-displacement converter. Thus the whole potentiometric measuring system acts as a voltage-to-digital converter' consisting of two consecutively connected subsystems: a voltage-to-displacement converter and a displacement-to-digital converter.


 * The first of them (the potentiometer, the galvanometer and the man) acts as a manual servo system (i.e., a negative feedback system with a displacement as an output); it is a true automated servo system in such applications as analog strip or XY chart recorders. In this negative feedback system, the man produces (output) voltage by means of the power supply and the potentiometer. Then, he/she compares the output voltage with the input voltage with the help of the galvanometer and changes the output voltage so that to make it equal to the input voltage. As a result, the slider's position and voltage correspond to the input voltage. In an electromechanical servo system, an amplifier amplifies the difference between the input and output voltages and drives a motor gear that moves the potentiometer slider. I remember I built such a servo in 60's by using two potentiometers, a polarized relay and a toy motor gear. This experiment helped me to become familiar with the concepts of balanced bridge circuit and negative feedback. Actually, it was my first meeting with negative feedback systems.


 * It is extremely useful to see and to show the general idea behind a few apparently different devices (in this case, a potentiometric measuring system, a potentiometric servo system, an emitter follower, an op-amp follower, etc.) This will allow conveying the common general idea (negative feedback here) from simpler to complex devices. In our case, if the young people have discerned the negative feedback in this primitive 18 century's arrangement, they will understand later how a potentiometric servo system operates, then how an emitter follower operates, after how an op-amp follower operates... because this is exactly the same arrangement: the op-amp's output stage acts merely as an electronic potentiometer...


 * Indeed, if we drive this arrangement with another potentiometer, the two potentiometers will constitute a bridge circuit. But it is not merely a classic bridge circuit; it is a balanced bridge circuit consisting in addition a man and a galvanometer. The man with the help of the galvanometer balances the bridge. Circuit dreamer (talk, contribs, email) 22:14, 22 August 2010 (UTC)

I'm sorry, but this is not a general discussion forum for what potentiometers "should" be or for any other aspect of electronics. It's strictly for discussing improvements to the article, and there appears to be broad consensus that redefining the subject matter does not fall under that criterion. I collapsed the above discussion as an alternative to simply deleting it, but if additional off-topic material is posted here I may do just that. Chris Cunningham (user:thumperward: not at work) - talk 09:28, 23 August 2010 (UTC)