How hot is too hot?

[MikePL]

So I've been doing a few projects with LEDs, but there is still one thing unclear for me. How good should the heatsinking be? I always run the LEDs within their specs, which is from 350 to 750mA. I assume that the stars are basically not a cooling element but an aid to mount the LEDs with ease to surfaces. Am I right? I usually use heatsinks from shipsets or from old CPUs like pentium II. They are flat and have an area of around 5x5cm. One such heatsink, when cut in two, serves well for two LEDs and keeps them cool. Am I being anal about cooling or not? When a LED is hot to the touch I start to panic and when it is about to burn I turn it off.

I wonder if they really need that much cooling? I recently connected a 3W LED to a 2x3cm (2cm high) heatsink and the LED got quite warm at 400mA. Am I worrying too much?

 

[modamag]

Mike the maximum junction temperature is 145*C for Cree XR-E.

Take a look at pg6 of XR-E datasheet which gives you the relative Flux.
Thermal Magnment Apps Notes

I use pretty much the same rule for LED vs CPU overclocking.
1) Try to keep it as cool as possible
2) When the heatsink temperture > 65*C throttle back.

Now there are exception cases depends on how you use the LED (eg. duration, purpose, etc)

A quickie subjective rule of thumb is if it's too hot for your hands to handle the heatsink then it's too hot for the LED.

A CPU heatsink is ussually plenty. IF you're using old PII HS, those are rated for ~35W with the aid of force air fan cooling. Let's say taking away conservatively derate that to 40% you still got 14W of heat that it can dissipate. 2 3W LED driven @ 1A with converter loss will be ~ 9W. You're still good.

If you are to use recycle CPU HS, I highly recommend P4 since those puppies were notorious for >100W power consumption as they tried to keep up with AMD in the MHz race.

For your 2x3cm HS, that might only be enough to cool 1 LED @ 350mA. I would recommend more surface area or other means of getting the heat out of the system (conduction, fan, hand, etc).

Hopes this helps.

 

[Melchior]

Indeed old P3 and AMD Athlon heatsinks are cheap, plentiful and large enough that even without a fan they have enough surface area to work as a passive heatsink.

As these are CPU heatsinks and designed to work with fans, they don't radiate quite as well without one. Also Anodized black, is a good thing. (not painted)

 

[TorchBoy]

This is one of those subjects that I always intend to pay a bit more attention to, but never quite get around to. I got a multimeter a few months ago with a thermocouple, so I should be set... But what does a °C/W figure for a heatsink mean? My guess is that the whole heatsink will raise its temperature by a certain number of degrees for each watt of heat energy put into it. Is it that simple? :thinking:

 

[modamag]

TorchBoy, your assumption is basically correct.
However, usually they used thermal conductivity k (W/m*K) instead of thermal resistivity (C/W).

dQ = k * Area * (Tobject - Tenv)

So the better the k or increase area the better it can remove heat.

 

[TorchBoy]

Thanks modamag. Most heatsinks here in NZ that have a rating are marked in °C/W. Does that mean we're behind the times? Is there a straight-forward conversion between the two?

 

[modamag]

You guys are just backward J/K

It's the inverse relationship.

From wiki

thermal conductance = kA/L, measured in W·K−1
thermal resistance = L/kA, measured in K·W−1 (equivalent to: °C/W)

 

[TorchBoy]

You guys are just backward J/K

It's the inverse relationship.

It comes from being upsidedown Down Under, I'm sure. :naughty:

So if a heatsink has a thermal resistance of 5 °C/W that's a thermal conductance of 0.2 W/K, but how do I get to its thermal conductivity in W/mK? Its area or thickness? Do I even need to do that? What's a reasonable figure for a heatsink for a Cree or four?

:duh2:

It's late. I'll have another look at this tomorrow.

 

[MikePL]

Damn... The more replies appear the less I understand.

 

[modamag]

PII - 1-2 LED You're good
2x3cm HS - 1 LED @ 350mA or less

A CPU heatsink is ussually plenty. IF you're using old PII HS, those are rated for ~35W with the aid of force air fan cooling. Let's say taking away conservatively derate that to 40% you still got 14W of heat that it can dissipate. 2 3W LED driven @ 1A with converter loss will be ~ 9W. You're still good.

If you are to use recycle CPU HS, I highly recommend P4 since those puppies were notorious for >100W power consumption as they tried to keep up with AMD in the MHz race.

For your 2x3cm HS, that might only be enough to cool 1 LED @ 350mA. I would recommend more surface area or other means of getting the heat out of the system (conduction, fan, hand, etc).

 

[Mash]

Damn... The more replies appear the less I understand.

Fantastic quote!
Can I borrow (steal) this one??????

:twothumbs:twothumbs:twothumbs

 

[MikePL]

Feel free to quote me

 

[Edjusted]

Take a look at pg6 of XR-E datasheet which gives you the relative Flux.
Thermal Magnment Apps Notes

All of modamags info is pretty much right on, read through those links for more info, and his rule of thumbs are very good also.

For those of you too lazy to read the .pdf or those who don't understand. Here it is in a nutshell.

You never want to exceed the junction temp of an LED! Ever! It will color shift and fail in a much shorter time than it is rated (depending on how hot, it may fail in minutes, or even seconds!)

So take the Cree XR-E which has a max junction temp of 145 deg C

Tj = Ta + (Rthj-a*Pd)
Tj = Junction Temp
Ta = Ambient Temp
Rthj-a = (Spoken as R theta j a (or j to a))Thermal resistance from junction to ambient
Pd = Power Dissipated

So lets figure out the max ambient temp for the 5 C/W heatsink mentioned in one of the posts.

Tj = 145
Ta = ?
Rthj-a = (Thermal Resistance from junction to solder point on LED) + (Thermal resistance from heatsink to ambient)
Rthj-a = 8 (From CREE datasheet) + 5 (from above) (sticklers may also want to add that there is thermal resistance between the LED and Heatsink, even with thermal grease this can be up to 1 C/W, but I'm going to neglect it for this example)
Pd = LED Vf * LED Current = 3.7 * 1 = 3.7W

So...

145 = Ta + (13*3.7)

Ta = 96.1C

Which means you could use this heatsink and LED combo up to a max ambient temp of about 96 degrees before the junction temperature was exceeded. So your LED would stay lit even after your skin starts to blister, hooray!

This is just a quick example, but gives you an idea what Thermal Resistance means, and how it's related to junction temp. I highly suggest all of you read the CREE thermal management guide for more info. And to confirm all this is accurate, who knows... I may have made all this up. :twothumbs