Pots.....which do you like best?

[slacker]

....what would cause a pot to smoke?

Same thing that would cause a resistor to burn up. 16mm pots are rated for 1/4Watt if you put more than that through them they can smoke. This is quite hard to do in a pedal but if you accidentally get 9 volts between the wiper and one of the outside lugs and the resistance between the wiper and lug is low enough you can do it. Which maybe answers Duck's question.

[duck_arse]

it's easy enough to do. get a pot, put one end to ground, then accidentally wire +9V to the wiper instead of the other end. now rotate the pot shaft (try the other way, as well), and you should soon smell the magic escape.

[armdnrdy]

it's easy enough to do. get a pot, put one end to ground, then accidentally wire +9V to the wiper instead of the other end. now rotate the pot shaft (try the other way, as well), and you should soon smell the magic escape.

Hey Duck,

Do you think that you could do a video tutorial on this?....as it might be helpful to those that are new at letting the smoke out! 

[Mark Hammer]

What allows a pot to work as one is the pressure holding the wiper in contact with the resistive strip.  Every time you move the wiper, you are rubbing the strip.  After a certain amount of rubbing, particulate matter, rubbed off the strip, begins to accumulate inside the pot, requiring cleaning, and eventually resulting in a pitted surface on the strip.

YMMV, but my own experience is that the larger the diameter of the pot, usually the higher the wiper quality and the less abrasive it is.  If it is a control that rarely gets moved, in a pedal/device you don't use all that often, or don't expect to need in tip-top shape for the next decade, or is easy to clean, then that's not a problem.  If it is in a location that is hard to get at for cleaning, needs to be absolutely crackle-free, despite tens of thousands of rotations, and needs to last a long time, then go with a 24mm unit, unless space demands otherwise, or unless you simply prefer the convenience of smaller-scale pots.

I'm looking at a 9mm unit right now, and it doesn't seem there is any way to get inside and clean the resistive strip, without destroying the unit.

[amptramp]

There was a patent granted back in the 1960's that showed a pot with a cylinder that had two resistive strips on either side of a conductive strip with photosensitive material between them.  There was a prism connected to the shaft which rotated with it and positioned the output of a light on the back of the pot.  The middle conductive strip was the output (slider) and the outer two strips had connections at opposite ends.  Wherever the light hit the photoconductive material, there was a low resistance joining the slider to the position on the resistive strips at one location.  This pot could never wear out, lose contact or get any more noisy than the ideal noise of a resistance at the same temperature.

I am surprised no one went into production - this would have been a noiseless pot with no wear mechanism and audiophools would have lined up in droves to buy it.  The use of photoconductive material would have made it suitable for high values (>50K) but values above that find plenty of use.  The taper is set by the resistive strips and could be linear, log, audio or anything else you can get in an ordinary pot.  Back then, the only light source would have been incandescent.  It is time to revive this with LED's.

[snap]

http://www.diystompboxes.com/smfforum/index.php?topic=691.msg4539#msg4539
http://www.diystompboxes.com/smfforum/index.php?topic=87566.msg736691#msg736691
http://www.diystompboxes.com/smfforum/index.php?topic=42534.msg307464#msg307464

[duck_arse]

it's easy enough to do. get a pot, put one end to ground, then accidentally wire +9V to the wiper instead of the other end. now rotate the pot shaft (try the other way, as well), and you should soon smell the magic escape.

Hey Duck,

Do you think that you could do a video tutorial on this?....as it might be helpful to those that are new at letting the smoke out! 

well, my huge stock of pots could obviously bear the loss, but, someone's already done it, by accident, and videoed it, by coincidence, and posted it, by jove. [I have no idea of how to copy a utube link from another post .....]

https://www.youtube.com/embed/JrhHdIHgDkk

I'd be a fool      to repeat it.

[snap]

[duck_arse]

geeze, ^ that's what I was trying to post! snap!

[armdnrdy]

You have to love how that circuit was screaming at the end! It reminds me of a heart rate monitor that flat lined! 

[snap]

geeze, ^ that's what I was trying to post! snap!

yep - that was obvious. so I just copied and pasted your link:
"https://www.youtube.com/embed/JrhHdIHgDkk"
in a new window or tab, and in that youtube vid in the left upper corner there appears the title of the vid:
"Atari Punk potentiometer blow up", which is a clickable link that opens up a new window or tab when being hit.
This new tab shows a new URL: "https://www.youtube.com/watch?v=JrhHdIHgDkk",
which you copy and paste in your post or reply between the two brackets that show up when you click
the "YouTube" button
like this: "[ youtube ]between the brackets[ /youtube ]"