Effect of and on ambient temperature?

  • Thread starter The little lost angel
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The little lost angel

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On reading spec docs, most parts appear to be specified for X ratings
at ambient temperature of 25C. Then they have a thermal derate to zero
at some temperature like 275C.

The problem now is, ambient temperature right next to the part will
get hotter too right? So does the spec mean, as long as I start with
25C ambient, it's ok to run the part up to 275C.

Or do I have to figure out the temperature rise of the ambient air and
at what temperature it will stabilize given the heat dissipated by the
part. Then see how much temperature rise am I allowed?

In addition, if I'm constantly pushing 25C air through using forced
air cooling. Does this means I can assume 25C constant temp or does it
still mean nothing since the air next to the part will always be
hotter than 25C?

Thanks!

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The little lost angel wrote:
On reading spec docs, most parts appear to be specified for X ratings
at ambient temperature of 25C. Then they have a thermal derate to zero
at some temperature like 275C.

The problem now is, ambient temperature right next to the part will
get hotter too right? So does the spec mean, as long as I start with
25C ambient, it's ok to run the part up to 275C.

Or do I have to figure out the temperature rise of the ambient air and
at what temperature it will stabilize given the heat dissipated by the
part. Then see how much temperature rise am I allowed?

In addition, if I'm constantly pushing 25C air through using forced
air cooling. Does this means I can assume 25C constant temp or does it
still mean nothing since the air next to the part will always be
hotter than 25C?
Click to expand...
Some specs indicate a case temperature of 25 C, and some specify
ambient temperature. The ones that specify ambient temperature
usually have a note on device position and any air flow requirement
(still air, X feet per minute, etc.) for that spec to make sense. If
ambient is speced, and no other details are given, I would assume
still air and normal mounting orientation. If still air is assumed,
the air temperature is that some distance form the device, and does
not take into account the air heating near the device. Natural
convection is assumed.

--
John Popelish
 
On Wed, 17 Dec 2003 02:58:50 GMT, John Popelish <jpopelish@rica.net>
wrote:

Some specs indicate a case temperature of 25 C, and some specify
ambient temperature. The ones that specify ambient temperature
usually have a note on device position and any air flow requirement
(still air, X feet per minute, etc.) for that spec to make sense. If
ambient is speced, and no other details are given, I would assume
still air and normal mounting orientation. If still air is assumed,
the air temperature is that some distance form the device, and does
not take into account the air heating near the device. Natural
convection is assumed.
Click to expand...
Thanks!!

If I may ask, at
http://www.ohmite.com/catalog/pdf/appnotes_res_select.pdf

This documents has a table (pg 7) for calculating the wattage rating
needed. Of interest is the airflow factor.

Are these estimate valid for the general case (since their chart on pg
6 says typical) or only specifically for Ohmite stuff?

--
L.Angel: I'm looking for web design work.
If you need basic to med complexity webpages at affordable rates, email me :)
Standard HTML, SHTML, MySQL + PHP or ASP, Javascript.
If you really want, FrontPage & DreamWeaver too.
But keep in mind you pay extra bandwidth for their bloated code
 
The little lost angel wrote:

Thanks!!

If I may ask, at
http://www.ohmite.com/catalog/pdf/appnotes_res_select.pdf

This documents has a table (pg 7) for calculating the wattage rating
needed. Of interest is the airflow factor.

Are these estimate valid for the general case (since their chart on pg
6 says typical) or only specifically for Ohmite stuff?
Click to expand...
The concept is generally applicable, but the exact limits depend on
the materials the resistors are made of and their peak temperature
limits. Beside, there is no specific hard limit, only a reduction in
reliable life as temperature goes up. If you are making a circuit
that is part of a bomb's proximity fuse, you have less concern in this
area than if it is part of the life support on a multi generational
space ship.

--
John Popelish
 
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