Smoking Pot

[guitarify]

So I was playing around with the Devi Ever Aenima circuit on a breadboard and decided to put a 500K pot in the spot where you can sub different resistors(10L, 100K, 220K) to get different pedals. I jumpered 2 of the lugs to make a variable resistor. Worked fine till I turned the pot all the way down. Then I got lots of noise, some sparks and smoke coming out of the pot.
Now is this because I reduced the resistance to zero? Should put a small value resistor in series with it? Or is the pot bad?

[Nasse]

some DC over your pot
ohms law
few volts and few ohms => if more than 0.25 watts (or whatever the spec sheet says) trough pot = smoke comes out

you are on right track with that extra series resistor

[jefe]

I used to, back in my college days. 

[dougman0988]

I know this has nothing to do with your post, but did anyone else chuckle when they saw the thread title? lol

[Mark Hammer]

Well I did.  Though it was more of a "Boy, that guy sure knows how to attract attention to his query!" grin.

[rousejeremy]

I was expecting a different topic

[flintstoned]

I'll do some heavy research on this topic and get back to ya

[guitarify]

Well I did.  Though it was more of a "Boy, that guy sure knows how to attract attention to his query!" grin.

For some reason this is the quickest response I've gotten on this forum, thanks for the help.

[MikeH]

I've smoked a pot or 2 in my day.  Happened to me when I accidentally but 9v DC into the middle lug of a volume pot.

[Mark Hammer]

At their heart, pots are little more than 2W fixed resistors with a movable contact.

Q: Are larger diameter pots higher wattage than the smaller types?

[CynicalMan]

At their heart, pots are little more than 2W fixed resistors with a movable contact.

Q: Are larger diameter pots higher wattage than the smaller types?

Alpha 16mm:
taper A, C, D = 0.06W
taper B = 0.125W

Alpha 24mm:
taper A = 0.25W
taper B = 0.5W

www.alphapotentiometers.net/html/16mm_pot_1.html
www.alphapotentiometers.net/html/24mm_pot_3.html

[Gordo]

Then I got lots of noise, some sparks and smoke coming out of the pot.
Or is the pot bad?

There's no such thing as bad pot. As long as you didn't inhale...

[Mark Hammer]

At their heart, pots are little more than 2W fixed resistors with a movable contact.

Q: Are larger diameter pots higher wattage than the smaller types?

Alpha 16mm:
taper A, C, D = 0.06W
taper B = 0.125W

Alpha 24mm:
taper A = 0.25W
taper B = 0.5W

www.alphapotentiometers.net/html/16mm_pot_1.html
www.alphapotentiometers.net/html/24mm_pot_3.html

Well geez, no wonder they burn up!  I always thought they were rated higher than that.  An 1/8 of a watt for most of the smaller pots I use? 

[CynicalMan]

That's one of the reasons why 9mm pots are a PITA. They're rated at 0.025W to 0.05W, which means that they burn out like crazy.

[R.G.]

It's worse than that.

The pot power rating is for the entire element. It is easy to burn out one section of a pot by subjecting it to more than it's share of the total element rating. The actual burn out of a small section of the pot is smaller than the rating for an entire element. It's not quite as linear as 1/10 of the element burns out at 1/10 of rating - nothing about heat transfer is that linear!

It's simpler to set a current limit for the whole pot from the whole-pot rating, and never subject any section of the element to more than that current. This keeps the per-unit i-squared-R under the burnout dissipation.

[Mark Hammer]

It also suggest that anyone looking to introduce "sag" into a circuit might want to use a 3-position toggle and 1/4w resistors, rather than a pot in some cases.  F'rinstance, a 4049-based overdrive.

[kurtlives]

It's worse than that.

The pot power rating is for the entire element. It is easy to burn out one section of a pot by subjecting it to more than it's share of the total element rating. The actual burn out of a small section of the pot is smaller than the rating for an entire element. It's not quite as linear as 1/10 of the element burns out at 1/10 of rating - nothing about heat transfer is that linear!

It's simpler to set a current limit for the whole pot from the whole-pot rating, and never subject any section of the element to more than that current. This keeps the per-unit i-squared-R under the burnout dissipation.

Very interesting, yet totally logical.
Thanks for sharing that.

[Paul Marossy]

I smoke a lot of pot(s). I did one hundred twenty of them very recently. They smoke when I solder the wires to them, but are still fully functional when I am done. 

[Ronsonic]

Or is the pot bad?

Now it probably is.

RG nails it (as usual). And this is exactly the sort of situation where we find out about that.

As for the thread title, I'll just say I turned 20 in 1975.

[Quackzed]

I hit it.