[mythtv-users] HDHR in harsh enviroments (was:OT: OTA HD )

Brian Wood beww at beww.org
Fri Dec 26 21:21:25 UTC 2008

On Friday 26 December 2008 12:30:35 David Brodbeck wrote:
> On Fri, December 26, 2008 9:40 am, Brian Wood wrote:
> > I would expect that sort of high temp environment not to cause immediate
> > failure, but rather to decrease the MTBF and result in failure before you
> > might otherwise expect it. It might be difficult to trace this directly
> > to the operating environment.
> That's the most likely scenario, but in extreme cases you might get
> thermal runaway, which can destroy transistors very quickly.  It depends
> on whether there are any semiconductors in the HDHR that dissipate a lot
> of power.

In case anyone's interested:

"Thermal Runaway" happens because the current through the device causes 
heating, and increased temperature causes the internal resistance to drop, 
thus increasing the current flow, which increases the heating effect which 
increases the temperature, which increases the current, which...

You get the idea. It is not a good thing.

I wouldn't think an attic would get hot enough to cause this, but if the 
airflow was poor, it might. Of course for most of the year attics here in 
Wyoming probably cause more cryogenic effects than overheating.

I wonder how a superconducting HDHR would perform.

I recall early reports of HD-PVRs getting very hot, people describing them 
as "too hot to hold". I never had this problem with my unit.

For most electronics, not designed to operate at very high temperatures (like 
vacuum tubes are), a good general rule is that if it is too hot to hold your 
finger on it is too hot.

I'd guess that folks running Myth systems do not utilize too many vacuum 
tubes, and some of the younger members of this list might be vague on exactly 
what a "tube" is :-)

beww at beww.org

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