If you use the model of a piece of wire as being a resistor that heats 
     up when current passes through it (as it is and as it does), then 
     there should be no problem reading any graph that indicates rise above 
     ambient.  This is where degree rise above ambient comes from.
     
     Ralph, correct me if I'm wrong, but the assumption I've always heard 
     is that the air surrounding the wire is an infinite heat sink and its 
     temperature NEVER increases.
     
     The other assumption is that temp rise above ambient can be roughly  
     linear.  That means, if the chart says with a particular current you 
     get a 10 degree C rise above 20 degree C ambient, then you will get 
     basically (forgive me here) a 10 degree C rise at 40 degree C ambient 
     with the same current.  Meaning your ABSOLUTE trace temperature is now 
     50 degree C.
     
     When selecting hookup wire with insulation rated at a particular 
     temperature, these rise above ambient charts become critical.
     
     Using the above info on traces (wire) covered with laminates 
     (insulation) becomes a little easier.
     
     OK with you Ralph?
     
     doug mckean
     [log in to unmask]
     
     
______________________________ Reply Separator _________________________________
Subject: Re[4]: heavy copper...
Author:  [log in to unmask] at internet-mail 
Date:    4/18/96 3:52 PM
     
     
     
Ben Davis wrote:
     
> ...So now I'm confused.  Does the chart indicate the rise of the trace
> temperature or does it indicate the rise above ambient of the environment? 
> Perhaps someone from the IPC committee that worked on the 275 (Gary are you 
> there?) could comment and/or elaborate on the charts...
     
Note 3 of figure 3-4 says:
"For general use, the permissible temperature rise is defined as the 
difference between the maximum safe operating temperature of the laminate 
and the maximum ambient temperature in the location where the panel will 
be used."
     
I agree that this and the other notes could have been written in a less 
ambiguous way, however it seems evident to me that what you are seeing 
as you look at the chart is that as the current rises the temperature 
of the trace rises.  The trace itself is the only thing defined in even
the most superficial of ways here.  There is nothing said of the rest of 
the environment (i.e.: thermal mass of surrounding parts, thermal 
conductivity of the enclosure, air flow or heat transfer efficiency, etc...), 
so, to
me, I don't see how any chart could ever hope to map the rise of the 
ambient temperature due to current changes in a given conductor.
     
I would also welcome any comment from Gary or other authoritative source. 
Or maybe just anyone with an opinion. :-)
     
     
     
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