On 7/22/2011 5:43 AM, Erik Christiansen wrote: > On 17.07.11 06:35, Dave wrote: > >> Does anyone have any experience in testing for condensation issues? >> > When I finished off the development of the first automotive digital > clock used by Ford here in Australia [1], certification testing included > temperature, humidity, vibration, and dust testing. > > The humidity testing was done in a one cubic metre environmental > chamber, with circulating fan, heating, cooling, water supply, and a > humidity controller. The latter just requires both dry-bulb and wet-bulb > thermometers to measure humidity. ISTR that we set the dry-bulb > temperature for chamber temperature, then looked up the wet-bulb > temperature required for the desired humidity, on a graph pasted to the > side of the chamber. The whole thing was about the size of a > refrigerator. The clock only had to work up to 95% RH, IIRC. > There was a bung-hole on each side, with a screwcap, for admitting leads > from test equipment. > > For the destructive temperature test, I built an insulated wooden box, > and used a bank of electric jug heating elements, controlled by a cheap > thermostat. The clocks' plastic front lenses distorted before the > electronics failed. (50°C [122 °F] was the basic test, run for some > months, but the clocks were then still fine after weeks at 80°C [176 °F]) > > Dust testing just used cement dust. It is truly amazing how that stuff > finds its way in when agitated by a fan. > > The automotive environment raises another relevant consideration; > electrical ruggedisation. The clock had to withstand a "load dump" from > the 12v alternator. If the load is suddenly removed, output jumps above > 80v for hundreds of milliseconds. (Wikipedia has seen worse: "The peak > voltage of this surge may be as high as 120 V and the surge may take up > to 400 ms to decay.") > > Instead of individually protecting each electronic unit, I proposed and > built a load-dump preventer, which clamped the whole 12v supply to 16v, > absorbing the load-dump. That was however several dollars per car more > expensive, so couldn't fly. > > External inputs can also be educational. During prototype testing in > pre-production vehicles, it was found that "the clocks died the second > time the car was started." Being an LED clock, the display was turned on > only when the ignition switch "accessories" position was energised. Not > surprisingly there were several accessories relay coils connected to > that, but the magnitude of the back-EMF on switching them off was a > surprise for an inexperienced engineer: about -800v, IIRC. Fortunately > it was easy to protect against that. > > Erik > > [1] Back in '78 > >
Interesting.. so did you have to make any design changes to meet the 95% RH requirements?? I agree, cement dust is really bad stuff... I do some work around concrete processing machinery sometimes and the dust just gets everywhere. After only one day with my laptop around a processing machine.. I will be cleaning and re-cleaning the keyboard and screen many times afterwards. It also builds up on machinery and causes mechanical issues. I wasn't aware of the load dump situation. Fortunately my past mobile work has used a 12 to 24 volt inverter to drive the control circuits, otherwise that probably would have been an issue. Dave ------------------------------------------------------------------------------ 10 Tips for Better Web Security Learn 10 ways to better secure your business today. Topics covered include: Web security, SSL, hacker attacks & Denial of Service (DoS), private keys, security Microsoft Exchange, secure Instant Messaging, and much more. http://www.accelacomm.com/jaw/sfnl/114/51426210/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
