Help me choose a heatsink for LED lighting

[burnit]

Hello there.

I'm pretty new to electronics and I have no good excuse for this. As it is I have bough me a solder iron and gone wild on stuff I have at home to either repair them or make some minor changes. I have now decided to build my own LED lighting since they are really cheap to run and the price of electricity can get a bit out of hand where I live. I have now purchased all the components and only need to find a heat sink! Before this project I have worked some with Power LEDs where I used a single LED per heat sink. This was no problem but these items are pretty expensive when you want to have more then one LED and just do something for fun so I want to use a single heat sink for all the LEDs.

I'm now looking at building a LED lighting for my kitchen table, I have made some rough calculations and decided to go with a few Luxeon rebels with a good mix of Warm White and White. The LEDs are at about 3.5watt each when run at 1000mA and I will be running 7 of these guys to light my kitchen table. The problem is now that I don't know which heat sink to go with, fact is that I don't even know where to begin when choosing one. I have taken some time to read up on this and looked at the YT-video (which is how I found this forum) and have not gotten any closer to find a suitable heat sink.

My problem is how I go about making the calculations when I have more then one LED on a heat sink, having a single LED is no problem for me to calculate. Anyone out there who could help me out?

Lets assume I will need to get rid of 21 Watts (3W/LED) of heat.
T_J: 100C I prefer to run them at this level to spare the LEDs and give them longer life. T_{J max} is 150C
T_Amb: 25C pretty standard
R_JC:  10K/W I have been told to use this figure by a supplier to be on the safe side. He didn't help any more then this.
R_CH: I will be using ArcticSilver thermal adhesive* (thermal conductivity 7.5 W/mK, thermal conduction factor 350.000W/m2)
R_HA: ?

How would I choose my heat sink from these figures? I really don't know how to handle more then one LED. Do I add them all together and calculate using a single LED of 21watts or do I make the calculations to the heat sink and then calculate for each LED on the heat sink? I I'm really lost here and need some help.

* The ArcticSilver thermal adhesive is stable up to 100C.

[alm]

Think of the ambient air as the circuit ground, the LEDs as positive potentials to ground, and everything between the LED's die and the ambient air as resistors. So the thermal resistance of the heat sink (usually indicated by the manufacturer) is in series with all of the LED's, and the R_JC and R_CH are in parallel. If the max. ambient temperature is 30°C, the max temperature rise is 70°C. You're dissipating 21W, so the max. thermal resistance is 3.3 K/W. The thermal resistance from the junction to the ambient air is (R_JC + R_CH) / 7 + R_HA. Calculating R_HA should be straightforward.

R_CH sounds quite optimistic (~0.1 mK/W?), doesn't it depend on the pressure and flatness of the surfaces? The CPU sockets it's usually used for exert a few millions Pascal or so of pressure on the heatsink. This determines the the thickness of the layer of thermal grease, the thinner the better. I would expect something in the order 0.1-1 K/W.

I'm quite sure Dave made a video about this exact topic when he was building LED lighting. Should be easy to find through the episode list.

[burnit]

Thanks for your response, I have already had a look at the video about heat sink but still don't understand how I would go about when applying it when I have 7 heat sources. I do know how to do it with one though. From your post I calculate that I should use a heat sink of about 0.9 K/W. Does this sound realistic?

I have found a few heat sinks that I would like to use but I still don't know if they are good enough for my LEDs. Since I have spent some money on getting good LEDs as opposed to China LEDs to save power I would not like it if they broke due to bad heat dissipation. It is really no problem getting a good heat sink but I do not want to pay to much for it.

[Psi]

for a real world comparison, here's what i have

It's
9x 3.2W White 176lm each
10x 1.5W Red 67lm each
4x 5W Green 147lm each
9x 2.5W Blue 30lm each

The headsinks get to the point where it's quite painful to hang on to them for more than 10 seconds. (they have some resistors on them too)
So probably around 50deg. They have been running as my room lighting for 3 years without any failures.

[IanB]

for a real world comparison, here's what i have

Holy cable spaghetti, Batman! 

[Psi]

It does have a cover

Here's what it actually looks like in the room

[IanB]

Thanks for your response, I have already had a look at the video about heat sink but still don't understand how I would go about when applying it when I have 7 heat sources. I do know how to do it with one though.

If you put more than one LED on the same heat sink, you simply add up the heat loads from all the LEDs. So for instance, if you had two LEDs attached to the heat sink dissipating 3 W each, then that is equivalent to a single LED dissipating 6 W. If all the LEDs are identical you can assume they are all operating at the same temperature and combine them in your calculations as if they were one huge LED.

[alm]

Maybe this diagram will help. The battery symbols indicate power sources, so 3W of power flows through each of the legs, and 6W through R3. If power flows through a thermal resistance, there's a temperature drop (T=P*R_Th), similar to the IR drop of current flowing through a resistor. You can apply Kirchoff's voltage (temperature) and current (power) laws, and analyze it like any normal circuit. The temperature at the bottom is your ambient temperature, which is a known quantity, as is the (target) junction temperature.

Since the power and thermal resistance of both legs are the same, the junctions should be at the same temperature, and you can connect the top of R1 and R4 in parallel and calculate the parameters for 6W of power through (R1 + R2) // (R4 + R5). This is for only two LEDs, extending it to seven should be easy.

[ivan747]

It does have a cover

Here's what it actually looks like in the room

Awesome. Do you have a video showing it, or a tutorial about it?

[ivan747]

I do know how to do it with one though. From your post I calculate that I should use a heat sink of about 0.9 K/W. Does this sound realistic?

According to my research a few days ago on power dissipation and heatsink size, yes 0.9K/W seems realistic given the relatively high thermal resistance of the LED's.

[burnit]

Maybe this diagram will help. The battery symbols indicate power sources, so 3W of power flows through each of the legs, and 6W through R3. If power flows through a thermal resistance, there's a temperature drop (T=P*R_Th), similar to the IR drop of current flowing through a resistor. You can apply Kirchoff's voltage (temperature) and current (power) laws, and analyze it like any normal circuit. The temperature at the bottom is your ambient temperature, which is a known quantity, as is the (target) junction temperature.

Since the power and thermal resistance of both legs are the same, the junctions should be at the same temperature, and you can connect the top of R1 and R4 in parallel and calculate the parameters for 6W of power through (R1 + R2) // (R4 + R5). This is for only two LEDs, extending it to seven should be easy.

I will look into this, thank you very much for your help.

@ PSI: A beauty!

[Psi]

It does have a cover

Here's what it actually looks like in the room

Awesome. Do you have a video showing it, or a tutorial about it?

yep, i made a video ages ago

[ivan747]

Nice, I always wanted to do something like that. So far I have only received SIRC commands with a PICAXE, and made a little serial receiver for the computer, search for LIRC.