arcing in a pot

[pucho812]

what would cause an electrical arc in a standard non sealed pot?

 

[JohnRoberts]

what would cause an electrical arc in a standard non sealed pot?

??
Arc from what to what...?

Wipers generally have multiple fingers to insure that even if one or two are bouncing up off the surface of the resistive element the rest will still make contact.

To arc there needs to be an air gap... and a voltage potential. I don't see that scenario existing inside a typical potentiometer.

JR

PS: A scratchy sounding pot could be caused by DC across the pot.

 

[pucho812]

well it arcs when the pot is adjusted. You can here it go snap, inside the pot and this is a brand new pot. It's visible too if I am looking at the pot from the bottom. DC across the pot makes since and since I have no schematic it's totally possible.

a typical amp pot.

http://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Potentiometer.jpg/225px-Potentiometer.jpg

 

[JohnRoberts]

There may be some weird static voltage thing going on with some internal plastic parts, but it seems unusual..

Contact the pot manufacturer if there is a problem with their lube they will probably know about it and have advice.

JR

 

[kpearsall]

I got a pot to smoke once in a circuit I was breadboarding. Something to do with making the input go to the wiper instead of the outside... Crackled and then just started smoking. Corrected the wiring and it behaved as normal.

 

[isophase]

you don't mention, what the pot is meant for in the first place?
i'v seen a pot arcing once wired as an attenuator between a little power amp and a headphone set.
i think i replaced it with wirewound (3W?) and it solved the problem.

 

[pucho812]

the pot is labeled threshold and it looks to be an afterthought mod as fit's labeled with p-touch tape and not engraved.

 

[JohnRoberts]

Creating a spark requires a lot of voltage. To jump a few ten-thousandths  of an inch in a spark plug requires a few ten thousand volts. Most audio paths do not generate large enough voltages for sparking/arcing.  Primary wiring involves relatively large spacing to protect against arcing from voltage spikes in mains.

Static electricity "could" generate large enough voltages, but that seems pretty rare inside a mass produced component. I have seen static effects inside pots but mostly it expressed as noise when the pot shaft was turned, not actual sparks.

JR

 

[PRR]

> Creating a spark requires a lot of voltage

You can get a small but visible spark running 300V through resistors *and pot wiper* and there's crud on the tracks.

As this is a mystery mod, who knows what is going on? But threshold (is this a limiter? Don't be stingy with info!) might be a divider off B+. The "designer" may not have realized (or cared) that it would spark.

 

[pucho812]

hmmmmmm. I guess that leaves me wondering will it eventually short out the pot?

 

[MagnetoSound]

It will eventually open the pot, as the arcing burns a hole in the track.

 

[JohnRoberts]

Speaking of arcing it is remarkable what becomes a conductor with enough voltage... I was on my lawnmower the other day, cutting through a patch of thick clover and all of a sudden the motor died. My first thought was wet grass in the safety switch wiring, but troubleshooting 101 is to look for something obvious first. Glancing at the spark plug I noticed a piece of clover sitting on the end of the spark plug. After brushing that off, the mower started and ran smoothly. In my decades of cutting grass I never saw that, but most spark plug wires have an insulated end cap, the cheap magneto I bought last year had an exposed cap.

It always something...

JR

 

[gemini86]

Spark plug wires break down really fast. Usually faster than the plugs themselves, these days (since most manufacturers use 100K+ rated plugs). If you were to touch that wire with your hand, I bet you'd be in for a bit of a shock (literally).

 

[emrr]

If it's threshold with dc in a tube comp, and it's wire wound, it may spark as moved.