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Aubrey Kagan

# Potentiometers: Mechanical & Electronic, Part 1

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Victor Lorenzo
8/18/2014 5:13:37 PM
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Re: Early lessons in electronics
Hi @Blaine, I totally share your feelings about Aubrey's posts, "they are straightforward and contain clear examples".

My dad, now retired, dedicated most of his live to electronics. His tools, instruments and component boxes delighted me like no other toy did. I remember my first incursion in the pots hacking arena. Pots used to be open by the terminals side so dust and hummidity could find an easy way in. That produced a high level of noise when the wiper slided over the dust, audible in the case of the radios and HiFi amplifiers. My task was unmout the cover, clean the whole thing and close it again.

It was instructive and fun for me... and kept me busy, which was good too ;-)

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antedeluvian
8/18/2014 9:03:13 AM
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Re: Figure 3
Dennis

These are good basic pot concepts. One way to think about why 3(c) is better than 3(b) is that the divider off the wiper attenuates not only the supply voltage across the pot but also the voltage noise variations caused by the wiper motion. Thus the resolution is increased by 1/ the divider ratio.

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D Feucht
8/17/2014 10:03:51 PM
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Figure 3
Aubrey,

These are good basic pot concepts. One way to think about why 3(c) is better than 3(b) is that the divider off the wiper attenuates not only the supply voltage across the pot but also the voltage noise variations caused by the wiper motion. Thus the resolution is increased by 1/ the divider ratio.

"I have only seen figure 3c in a 1987 Maxim seminar ..." Take a look at older Tektronix instrument circuit diagrams. This is the usual method of applying pots in Tek equipment, and in particular, in oscilloscopes.

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antedeluvian
8/15/2014 2:36:25 PM
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Re: Volume control
GSKrasle

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GSKrasle
8/15/2014 1:54:28 PM
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Re: Volume control
Well, I HOPE the math is right, but here are my solutions to making logarithmic and exponential tapers from a linear pot. It's rougher than my usual work, but all I really wanted to do was to work-out the math. The solutions appear to be close to ideal, but were found with "Goal Seek," not analytically.

But I can't seem to get a picture to paste!

This is the equation I used (check my work?):
Vo/Vi=((1/RP1)+(1/R1)-(((RP2+RP1)*((RP2*R3)/(RP2+R3)))/(RP1*RP2*(RP1+((RP2*R3)/(RP2+R3))))))/((1/R1)+(1/R2)+(((RP2+RP1)*((RP1*RP2)/(RP1+RP2)))/(RP1*RP2*(R3+((RP1*RP2)/(RP1+RP2))))))

In your five-resistor circuit above, if Rp = 1k, the best-fit to a "log" or "audio" taper is when R1 = ∞,  R2 = 944.7, R3 = 0. "antilog" requires R1 = 126.6, R2 = ∞, R3 = 9.8.

Just a fun little math exercise!

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antedeluvian
8/13/2014 4:53:26 PM
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Re: Volume control
GSKrasle

anybody else care to try their hand at the five-resistor circuit? It's harder than it looks!

Given my history with Excel, I have regularly thought I should develop a model of this circuit, and every time I have decided it is too complicated.

I got real close using the 'goal-seek' utility,

I am very impressed on your use of the gaol-seek, and perhaps that is the way to go- rather than develop a generic solution use the solution you want and solve backwards.

But let me add some practical caveats re using them in the real-world:

And especially thanks for the caveats. There is no substitute for experience!

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GSKrasle
8/13/2014 3:48:41 PM
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Re: Volume control
How curious! Last night, I was playing my favourite video-game ('Excel') with a model of your circuit (c), which is a bit more complicated than you'd think. The additional resistors change the function Vo/Vi vs. X (X being the wiper index) from simple linear to a ratio of polynomials. I was trying to approximate a perfect LN function by manipulating R1,2,3 (where R4+R5=Rp=const). I got real close using the 'goal-seek' utility, though I suspect using circular references and iteration could be used too.

I HOPE my algebra is correct: anybody else care to try their hand at the five-resistor circuit? It's harder than it looks!

The motivation is to approximate the behaviour of an expensive 'logarithmic' or 'audio-taper' pot with a cheap linear one. (Cheap 'log' pots are poor in their approximation, and, particularly, in their matching.) Remember that, because 'most everything biologicalous is logarithmic or exponential, volume-control, brightness, etc. is always best-handled logarithmically.

Wikipedia has a pretty good reference on pots, and there's this: http://www.bourns.com/data/global/pdfs/OnlinePotentiometerHandbook.pdf

But let me add some practical caveats re using them in the real-world:

Don't ignore the wattage ratings, and remember that if you are using only half the resistive element, the rating is half as large! Watch what happens at the extremes!

The extremes of the adjustment range are usually messed-up. The wiper might jump to a short to the end-contact, it might reach a limiting value that's not 'zero,' or it might go open or flakey. Take that into account! (This is one reason to connect the wiper to one end.) Many pots that have been used for experiments have damaged elements at the extremes (see above). Often, you can FEEL the messed-up-ness.

Pots are expensive. Specialler ones are generally expensiver, exceptions being certain ones used in consumer gear, including some 'tapped' ones, which, incidentally, offer a way to do thermal compensation.

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Maciel
8/12/2014 2:13:38 PM
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Re: Early lessons in electronics
Very good article AnteDeluvian. Some electronic issues never go out of style lol! Pot is certainly a component of many applications. I confess that I've burned several in one setting, in my time in high school. This configuration is very common since operational amplifiers connected to a high input impedance to current flowing through the POT is very small ....

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samicksha
8/12/2014 2:04:33 PM
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Re: Volume control
I remember potentiometers were used to control picture brightness, contrast, and colour response but just wanted to check how well they work in motion controls systems.

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antedeluvian
8/12/2014 8:42:04 AM
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Re: Early lessons in electronics
Blaine

I for one will give you a pat on the back and not a jab to your chin.

Thanks for you kind words.

Your article reminded me of a long time ago when I was not yet a teenager.  We had electronics around the house, and I did a project once where I somewhere obtained a couple of speaker drivers, and decided I would add them as a remote output from our "audio system" which was all contained in a nice piece of furniture which had fold out doors, lift up lids for the turntable, and built in speakers.  These features give you a hint as to how long ago that was.

I wonder how many of us on Planet Analog got into electronics because of (or at least as an early indication) music and the attempts to record it. Unlike you I had no expertise to call on and living in a technical backwater in deepest darkest Africa did not help. I started out recording with a microphone and when I hear certain oldies (like "Have I the right?" by the Honeycombs) I can still hear my kid brother screeching. After that I tried recoding by connecting across the speaker of the radio (there was no audio socket) and into the mic input (early reel to reel- only had a mic input) with varying degrees of success.

The tape recorder was replaced and the new one came with an audio cable, and that worked OK (again I thing across the speaker, no stereo yet) until the cable broke. Hand made replacements nver worked well becasue it seeems there was some kind of impedance matching withing the cable. Of course the noise from the potentiometers always found its way through as well.

Finally I caught up with technology when both the radio/record player and the tape recorder had matching input/outputs.

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