Calculating how much metal I need to dissipate heat for an LED?

Flashlight Modding and DIY Parts
1

Hi all,


Anyone know how to do this? Basically I need to know the actual formula to. For example I need to know how many BTU per watt ( or what ever) a Cree XML2 will emit at 3000ma and how many cubic inches of aluminum it would take to dissipate that heat at a rate fast enough the light would be sustainable at an ambient temperature of XX?



Thanks in advance.

2

I don’t have the formulas to offer but there are many more variable to consider that mass. There are thermal resistances at junctions, surface area, geometry, emmissivity, etc.

I did a few minutes of googling and thinking and found these links. I’ll get back to this thought at a later time if somebody doesn’t chime in with a more sophisticated response. Just busy now and really shouldn’t be on here. lol

http://www.efunda.com/formulae/heat_transfer/convection_forced/calc_lamflow_isothermalplate.cfm#calc

http://www.thermoworks.com/emissivity_table.html

http://www.freebyte.com/cad/thermal.htm

Just a start…

3

Back in my university days in physics I could have done this… but I have not used that for 30 years. Many variables, first of off the amount of heat generated by the LED, the heat transference from the LED to the material (star) it is mounted on, assuming it is attached to the base metal with fasteners and some kind of heat transfer grease then the thermal conductance of that material must be known and finally the amount of air flow around the heat sink.

This could be done, but I do not think I could do it now. It is sad to think that my choice of jobs has dumbed me down to the point that I could not do basic metallurgical physics. But, on the upside I bet someone here could.

You might hit google, your main concern is calculating the heat conductance of each junction and determining what your desired limits are and last of all determining the size of head sink that will allow for heat dissipation required for a given time of operation or heat production. Your alloys will also make a difference, but it will be minor in the grand scheme of things.

A couple of pages on heat transfer theory… Wow, where was google 30 years ago??? :slight_smile:

Themal conductance

Matt

4

This is not the answer you are looking for, but the calculations are similar to the kirchhoff’s voltage laws that you are probably already familiar with.

As far as being able to figure it out, you could do what I have done since my university days. Use something that is called “Dimensional Analysis” Believe it or not, the formulas for calculating your result are “encoded” in the constants that you use in your calculations! With “Dimensional Analysis” there is no need to know formulas, only the factors that will influence the outcome. Here is a link to how to use “Dimensional Analysis” to solve some simple problems.
Dimensional Analysis

In your case if you are looking for how many BTU/Watt then take all the constants and equalities that are related and force the units to cancel out on the right hand side of the equation so that only BTU/Watt is remaining. That number will be your answer. All without the need to know the equation.

5

Thanks for the link, gotta study that more.