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 From:  David W. Hess <dwhess at banishedsouls dot org>
 To:  m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Hardened Platform -40C to +85C
 Date:  Sun, 30 Jul 2006 15:33:21 -0500
On Sun, 30 Jul 2006 08:30:40 -1000, you wrote:

>You'll also want to think about the very real possibility of a "cold 
>start".  That is, if the power fails, and the entire board gets to 
>sub-zero temps (esp -40C (which is the same as -40F,)) then the crystal 
>oscillators(s) on the board can freeze.  Without the crystals, you get 
>no clocks.  Without the clocks, you get nothing from the CPU and other 
>components that require at least one stable clock.

When I have done designs for low temperatures in the past other then thermal
expansion the only problem has been electrolytic capacitors freezing.

Do you happen to know what the actual failure mode was with the crystals?  I
have seen oscillators fail to start but it was because of low gain and not
mechanical issues with the crystals.

>At the high(er) temps, you'll have difficulty with the decreasing "theta 
>T" or  difference between the case (and junction) temperature of the 
>components on the board and the temperature inside the enclosure, as 
>well as the decreasing theta T between the temperature inside the 
>enclosure and that of the ambient outside of it.

Do you mean the allowable temperature rise from junction to ambient temperature?
If the power dissipation and thermal resistance does not change then the
difference should not change.  You still need to derate everything to keep sane
junction temperatures or course which gets tricky at a Ta of 85C.  My rule of
thumb was generally a maximum Tj of 85C but that would lead to zero power at a
Ta of 85C.  You end up with odd things like TO-220 regulators using 20 watt heat
sinks to operate at 1/10th of their maximum load.

>Be careful, you'll find many companies who claim "-40 - 85C" who can't 
>really do it.  Test it yourself before you accept more than a small 
>quantity of any offered solution.

I suspect it comes down to relying on a company making products for the
industrial market instead of the consumer or business market.