How much heatsink for a 5W?

I'm getting ready to put together a 5W light, but I'm unsure of how much heatsink it's going to need. It will be in open moving air pretty much all the time, but will run for multiple hours straight, hands free.

So... let's say that I have a chunk of aluminum the size of a C cell, with fins lathed around it, and a 5W emitter on one end. It sounds like overkill to me, but is it? How much shorter could that be made, if at all?

Thanks.

[ QUOTE ]
marklein said:
I'm getting ready to put together a 5W light, but I'm unsure of how much heatsink it's going to need. It will be in open moving air pretty much all the time, but will run for multiple hours straight, hands free.

So... let's say that I have a chunk of aluminum the size of a C cell, with fins lathed around it, and a 5W emitter on one end. It sounds like overkill to me, but is it? How much shorter could that be made, if at all?

Thanks.

[/ QUOTE ]
Once you get beyond a certain thickness, it is mostly surface area that matters. This link should give you something to work with: Luxeon thermal design guide

I'm afraid that I have nowhere near the math skills needed to make that PDF useful.
PDF-> /ubbthreads/images/graemlins/twakfl.gif <-Me
Anybody care to crunch the numbers for finned surface area for a 5W? /ubbthreads/images/graemlins/bowdown.gif Is it vaguely 10 inches???

In reality I'm hoping to get some rough ideas based on what other people have used in the past.

The mass is only important for time to steady-state... cooler steady state is directly proportional to the surface area (and airflow)... probably the best if not the only way to find out the answer is a prototype.

My megalight has just about 25 sq. inches of surface area =-O but it only gets to a steady-state of about 115 deg or so running 3 emitters at 3W... and it's 'internal'... for convection to work well the fins have to be vertical... is this light going to be the basic shape of a flashlight? emitter out the end of the cylinder? if the light will be used horizontally then i could see using this method... but i'd be aiming for close to 12 sq inches or more based on my 9W and 25 inches being 'just about perfect'.

The outside of a cylinder the size of a C battery is just bout 8 sq. inches... without grooves..

if you cut 1/16th grooves (admittedly a little thin).. 1/8th deep... you end up with .7091 sq. inches per set (high n low.. 1/8inch)... so.. 12 sq inches would be 17 1/8ths or 2 1/8th inch... where a C batt is 2.42 inches...

You could increase the amt of convection by having a non-radial symmetry using slots left n right rather than having radial grooves.. think of ribs n a backbone.

I think you are just about 'in the zone' with your size guess... it'll get quite warm but not too hot to handle would be my prediction..

The heatsink of Megasonic gets what would not be fun to hold steady-state but it's never hot enough to hurt.

Good luck let's see some pictures.

-awr

speaking of pictures: here are visuals of 2 different 12 sq. in. heatsinks

Thanks a lot. The rib thing is good thinking. I'm going to try that at 2" long and see how it does. It will be that, plus the sealed 1" can that the emitter/optics will live in on one end.

It's going to be a bike light, so air flow should be very good at all times, but will be perpendicular to the fins. We'll just have to see how it goes. It's only money! /ubbthreads/images/graemlins/grin.gif

with moving air, the direction of fins not all that important... probably could be 1/2 the size.. when you said 'not hand held' implies stationary, where moving through air would be better cooling than hand-held....

If i was building the light... just out of 'elegance' i would probably make the fins at an angle so they would work from forced air as well as stationary with convection when not moving... just a thought.

-awr

the purpose of a heatsink is to take the heat from the emitter slug, and move it to the outside of the flashlight body where it can be taken away into the air or your hand.

to do this, the most important thing is going to be surface area.
more surface area will result in more points for the heat to transfer from the heatsink to the flashlight body.
for this reason, finned heatsinks are generally counterproductive, as the fins only serve to remove surface area that could otherwise be contacting the light body itself.
on external areas, or maybe areas where weight savings is important, than finning or ribbing of some sort may prove useful, but again it just depends on the design.

a little confused about the apparent contradiction here... in LED technology getting heat away from the slug of the emitter is the important thing... I may have made an incorrect assumption and didn't make it clear... a finned heatsink ONLY helps if it's exposed to the outside which is what i was assuming...

Most of the mods do follow the principle of really 'heat conductor' (to the acutal heat-sink... the outside of a light)... but of course when 'in open moving air' was mentioned i assumed marklein was talking about the heatsink being out in the airflow... if that's the case than of course fins are the way to go.

Depending on the design, the fins can stick out the top like the bike light in the class 2 contest light.

Another thought: the cylinder could be vertical like in the pics above, and have the emitter out the 'side=front', but have to play with how to put on an emitter and reflector counter-intuitive on the 'side' of the cylinder.... i'd look a little like the phalanx gun.

-awr