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20th Jan 2013, 7:48 pm | #1 |
Triode
Join Date: Jul 2006
Location: Thetford, Norfolk, UK.
Posts: 22
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Tin whisker growth on potentiometer
This Cossor 1052 oscillograph had various faults; no Y shift, focus and brightness problems. This was found to be due to a short circuited Y shift pot. On removing it, it was still shorted, so I removed the back cover. The fault then went away. There are tin whiskers inside the cover which were shorting both the ends of the pot together, but not the wiper.
Anyone had this fault?
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He's gone to make his meter. |
21st Jan 2013, 7:15 am | #2 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,554
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Re: Tin whisker growth on potentiometer
There have been many posts about fly by wire car accelerator pedal tin whiskers including lawsuits about cars going full throttle due to the same problem.
Take a web search and have a read. The pot on the pedal has the same problem. |
21st Jan 2013, 9:46 am | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
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Re: Tin whisker growth on potentiometer
It's a very well known problem and has got a lot worse since the switch-over to unleaded solder. Though the Cossor scope far pre-dates that.
Some applications (aeroplanes but not cars) are still allowed to use lead in their solder. Tin dendrites will grow along surfaces quite easily, and even upward across fresh air. All they need it the presence of tin and a bit of electric field to impel them. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done Last edited by Radio Wrangler; 21st Jan 2013 at 9:47 am. Reason: forgot one sentence! |
21st Jan 2013, 12:22 pm | #4 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,865
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Re: Tin whisker growth on potentiometer
No, I've never had it on a pot. Rather scary about car accelerators, as I think the vast majority if not all are "fly by wire" nowadays.
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21st Jan 2013, 1:01 pm | #5 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: Tin whisker growth on potentiometer
I spent my career as a systems engineer working on military aircraft fly-by-wire flight control systems. I remember a visit back in the mid-80's by a couple of engineers from one of the motor-car manufacturers, I think it was Jaguar, looking for any advice we could give on using similar technology for steering and braking sytems on their products.
They were quite taken aback when it was pointed out that a military aircraft flight control system typically uses four lanes of sensing and computing, with cross-monitoring between lanes to identify and isolate failures. They didn't see how a system in a motor car could ever be more than simplex, on cost grounds. The thought frightened me then, and it still does! Andy |
11th Feb 2013, 10:08 pm | #6 | |
Octode
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,086
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Re: Tin whisker growth on potentiometer
Quote:
I'd not come across this property of tin before, it's fascinating! What age do you estimate your pot to be? -ie what growth rate do these whiskers have? Cheers, Scott
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www.scottbouch.com |
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12th Feb 2013, 6:12 pm | #7 |
Hexode
Join Date: Jul 2004
Location: Paris, France
Posts: 350
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Re: Tin whisker growth on potentiometer
Hello to All, 3NF,
Read the full story on tin whiskers and by none less than NASA itself! http://nepp.nasa.gov/WHISKER/background/index.htm http://nepp.nasa.gov/whisker/photos/index.html#semi Makes you shudder... Best Regards jhalphen Paris/France |
12th Feb 2013, 6:24 pm | #8 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,088
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Re: Tin whisker growth on potentiometer
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12th Feb 2013, 8:27 pm | #9 |
Triode
Join Date: Jul 2006
Location: Thetford, Norfolk, UK.
Posts: 22
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Re: Tin whisker growth on potentiometer
Hi Scott
The tin whiskers on the Cossor 'scope had 50 odd years to grow. The set has only been running for a couple of hours in the last 5 years, so can't be attributed to an electric field. The first time I heard of this was when bronze anodes on 100KV electron beam microlithography machines grew whiskers and shorted out the EHT. These bronze anodes were quickly replaced with stainless steel. End of problem. chris
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He's gone to make his meter. |
14th Feb 2013, 12:36 am | #10 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,274
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Re: Tin whisker growth on potentiometer
Most car accelerators have 2 pot tracks that have to track each other at all times. If the signals disagree then on goes the engine light and limp mode soon after!
I think military and medical devices must change to ROHS ?
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Kevin |
14th Feb 2013, 10:02 pm | #11 |
Dekatron
Join Date: Dec 2004
Location: East Yorkshire, UK.
Posts: 3,989
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Re: Tin whisker growth on potentiometer
Just seen an interesting article on the Net, Please Google, 125mph nightmare.
John. |
21st Feb 2013, 2:58 pm | #12 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: Tin whisker growth on potentiometer
I don't know if this NPL webinar is of interest to anyone. You need to book, but it is free.
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