Power Dissipation of 2512 SMD Resistor, Theta (C/W)?

[Jason White]

Hello PICList,

I have a design where Tambient=75C. I need to dissipate 560mW through a
resistor.

I have a 4 layer PCB board in an enclosed cast aluminum box with no air
flow. Duty cycle is 15% - 3 minutes on 17 minutes off.

I am considering using 1 or 2 resistors in series. Specifically 1W rated
components in the 2512 package.

I am having trouble estimated the theta (C/W) value for a 2512 resistor. I
am looking for guidance on selecting the correct value. I am tempted to use
110C/W.

Source [1] has huge errors in the presentation of graphs and test data*,
but by reading the tests one can extrapolate theta=282C/W.

Source [2] states worst case theta=110C/W and best case is 52C/W

Source [3] states theta=44.5C/W with no further clarification

[1] https://www.seielect.com/Catalog/SEI-RMCF_RMCP.pdf (Page 7 and 8)
* The "temperature rise at 100% power" is actually temperature rise at 53%
rated power which is extremely misleading.

[2] https://www.vishay.com/docs/53048/pprachp.pdf (Page 3)
[3] http://www.resistor.com/assets/pdf/2512std.pdf (Page 1)

[Denny Esterline]

As you've discovered, thermal is non-trivial.
I would, more or less, discard the sources you've quoted.
If you read though them there will be many assumptions and conditions
to their data that does not apply to your application. Free air, amount of
copper
it's connected to, etc. None of which fit your application for the
information to be
directly relevant.
With minutes of on time in a no airflow situation, the thermal mass
surrounding
the board is a more dominant factor. Which then requires an understanding
of
other heat sources in the unit.
There are mathematical tools that can find answers to this, but I've found
that
they require more specific details than I can usually provide, leading to
more
uncertainty than I'd like.
In practice, you'll probably get better information faster by putting a some
resistors in a sample of your enclosure and hooking it to a power supply.



On Fri, Jul 20, 2018 at 9:25 AM, Jason White <

Post by Jason White
Hello PICList,
I have a design where Tambient=75C. I need to dissipate 560mW through a
resistor.
I have a 4 layer PCB board in an enclosed cast aluminum box with no air
flow. Duty cycle is 15% - 3 minutes on 17 minutes off.
I am considering using 1 or 2 resistors in series. Specifically 1W rated
components in the 2512 package.
I am having trouble estimated the theta (C/W) value for a 2512 resistor. I
am looking for guidance on selecting the correct value. I am tempted to use
110C/W.
Source [1] has huge errors in the presentation of graphs and test data*,
but by reading the tests one can extrapolate theta=282C/W.
Source [2] states worst case theta=110C/W and best case is 52C/W
Source [3] states theta=44.5C/W with no further clarification
[1] https://www.seielect.com/Catalog/SEI-RMCF_RMCP.pdf (Page 7 and 8)
* The "temperature rise at 100% power" is actually temperature rise at 53%
rated power which is extremely misleading.
[2] https://www.vishay.com/docs/53048/pprachp.pdf (Page 3)
[3] http://www.resistor.com/assets/pdf/2512std.pdf (Page 1)
--
Jason White
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