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
--
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