Hi Russell,
Thank you very much, that was exactly what I was missing.
Best Regards
            Luis

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of RussellMc
Sent: 10 June 2010 12:33
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: Heatsinks

I need to get a resistor to replace a damaged one on a board where is
used to measure current, it is a 50 mOhm 5W resistor. This is a one off
job.

_

Graphs are for flange temperature and Temperature rise Tjc - junction to
case.
You care about Tja - junction to air.

Data sheet implies max junction temperature is 155 C.
At 25C ambient that's a delta T of  120 C.
TO220 pkg has a nominal Rjcthermal of about 60 C/w.
So at 2W you'd use up that 120 C.
They say 2.5 W so that suggests a slightly better Tjcth of 120/2.5 = 48
C/W.

You can improve that quite a lot with vertical mounting and  a very
modest
heatsink. Chances are you can fit SOME heatsink here - even if not a
traditional one. Thought followed by common sense design should make
quite a
lot of difference.

At 40 C you can have (155-40)/48 =~ 2.4 Watts.


Russell



On 10 June 2010 19:28, Moreira, Luis A <Luis.Moreira@ccfe.ac.uk> wrote:

>
> Hi All,
> I need to get a resistor to replace a damaged one on a board where is
> used to measure current, it is a 50 mOhm 5W resistor. This is a one
off
> job.
> I found this one http://www.farnell.com/datasheets/6367.pdf . Now this
> is a resistor that normally needs a heatsink but I do not have the
space
> to fit one hence I would like to use it without it. Looking at the
> graphs given for derating and calculating for 5W dissipation on it,
and
> 40 degrees C ambient,
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