high voltage cause thin hairs of metal to grow a short inside potmeter - Page 1
Forum section: electronics / projects

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 26, 2022, 08:26:09 am
content: if you have never seen tin whiskers before ?please have a look hereHP 711A Power Supply 0-500V 100mA all tubes test repair teardownhttps://youtu.be/O8O6-OlvCAA

author: mikerj
author_rank: Super Contributor
timestamp: September 26, 2022, 10:59:00 am
content: FWIW High voltage isn't needed for tin whiskers to grow, it's thought to be caused by internal stresses within the tin. Nice power supply BTW!

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 26, 2022, 07:19:54 pm
content: i think this is accelerated by long time usage at high voltage,since other pots running at less voltage, did not have it,they must be all same age. or maybe different manufacture process, metal combination, and such,maybe also storage at humidity ? is a thing ?but then i expext to see it all over ?at least a very interesting topic

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2022, 08:26:42 pm
content: You'd be amazed how long those Tin whiskers can grow without any high bias applied...https://duckduckgo.com/?q=tin+whiskers&atb=v328-1&iax=images&ia=images

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 26, 2022, 09:33:30 pm
content: That's one of the reasons it was alloyed with lead, the wishkers happens mainly when the tin concentration is high.Rare to see in <25 years old equipment, more likely to happen in 70s and older electronics?Probably that pot was plated with pure tin or some high tin % alloy.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 27, 2022, 12:10:09 am
content: Had tin whiskers cause some problems in some old (1970s) radio equipment. They had used tin plating on the inside and outside of the RF shielding "cans". Some would grow long enough they would occasionally short a weak RF signal to ground, or stop a crystal oscillator from oscillating.If there's any current available in the circuit though, even just a few mA, that's usually enough to fuse the tin whiskers open when they do make contact. Mostly causes problems with small signal, high impedance circuits.

author: fuzzoli
author_rank: Regular Contributor
timestamp: September 27, 2022, 02:32:59 am
content: ...had a similar issue with the tuning capacitor in an old Fischer FM tuner.My dad built the tuner from a kit and it worked great. When we were old enough to "touch it", we of course wanted to tune the radio to our music. Well, the darn thing wouldn't tune down past 100MHz or so. It would squeal like a banshee! Since 105.1 was his station of choice, we never really looked into why the thing behaved as it did.Fast forward 20 or so years...My parents replaced the old Fischer with a modern stereo and gave me the tuner, for which I then set out to fix that banshee once and for all.After carefully looking at the tuning cap, it looked like bits of "dust" were on the plates in just the right spot -- hmm, maybe the "dust" was shorting out the cap? Well, a bit of tuner cleaner and scrubbing and things were back to normal!The buildup of a couple of decades of electronic and radio theory made it obvious that when listening to the banshee, the noise wasn't being made by some a spirit bringing doom and gloom to our house, but rather was simply the radio doing what it was told to do -- tune the band -- in this case randomly and quickly as the "dust" shorted the plates....and another few years before Google was able to index pages with people having similar issues confirmed those "dust" particles were in fact whiskers.And a mystery 25 years in the making had solved.Gotta love it.