It’s fine with me. Conversations take many paths along the way.
Are you documenting your build? If so could you paste link? I am very interested in following your project and would love to join in on any discussion.
I have two feet of this in my possession right now. I was planning on using the rear compartment to pass air across the fins of a heatsink mounted inside. I played with different locations for an all weather fan, but I was unable to think through a solution that could be worked with a limited access to the required tools.
@zpinch - Meh, maybe the XHP series can handle 100% drive current in that application. You could easily be correct about that, my way of thinking about lightbar drive currents is probably simply out of date following the release of the XHP series. RE: copper… I think that if you were to tear down one of the Baja Designs XL80’s you’d be very unlikely to find a slab of copper in there (beyond maybe MCPCBs). RE: those graphite thermal pads from Cutter - Cutter is only advertising a fraction of the number you mentioned. What cutter advertises is 5 W/mk on the Z-axis, the only axis that matters here. That number falls in line with conventional TIM claims. I haven’t seen any benchmarks/tests demonstrating this material as compared to other TIMs - I wouldn’t get my hopes up. As far as Panasonic’s big number, that applies to in-plane rather than Z-axis (which Panasonic calls the C-axis I think). My advice to you is to assume that this is all a bunch of hocus-pocus and to stick with conventional TIMs until you see test numbers to back up any claims. Those numbers are available in profusion for many other TIMs.
I wouldn’t be surprised to learn that the XL80’s feature a driver which thermal-throttles a lot of the time - especially for the low-speed stuff or when at a stop. Depending on your application that’s often not going to be a problem with the XL80. Many operators wouldn’t even notice. On the other hand, building a similar light without thermal protection could become a problem.
@PPDB22 - Have you seen any professional automotive lightbar units with that heatsink configuration / a fan in them? Enclosing a fan and a finned heatsink inside a box which vents to the outdoor environment sounds like asking for trouble. In addition to that as you’ve already pointed out - the extruded aluminum setup you’ve got also multiplies the number of thermal joints. In ref to your earlier question about my post #48 - I was probing for both reasons. There’s really no reason not to share calculations like that on the forum - lots of good knowledge gets thrown around and we often end up correcting one another…
Ya, I understand. I have not seen any light bars with active cooling. There are many things to think through and it may not hold up to outdoor conditions. However, if I were to use the enclosure above, I would drill 16mm holes through the extrusion that divides the compartments. The LEDs would then be mounted with 16mm sinkpads directly to the copper heatsink. The heat path would then be limited to two junctions, led to mcpcb and mcpcb to heatsink. This way the majority of the heat would be ratiated directly to the copper fins and blown out using a fan to create a mini wind tunnel. It does seem unlikely that I could implement a design that would keep out the dirt and outher elements. Again, idk, these are just ideas that I’ve been playing with trying to find the best thermal regulation possible in order to build a compact light bar with a good deal of power.
I will be documenting the build yes, but keep in mind it will be taking months to complete as I want to do a bunch of testing of optics and thermals, see how far the LEDs can be pushed.
I see your idea about using a heatsink in the two compartments, but if I am understanding it correctly, how is the heat going to be removed from the back compartment where the fins are? Will this be outside vented? There are some pretty SICK high performing PC heatsink out there (I have Noctua NH-D14 on my PC and I can run it fanless with a heavy OC’ed CPU), and I have played with the idea of using them, but I just can’t seem to find a way to make it work.
It really doesn’t matter if they throttle the light when not moving, I don’t care as I would not be ever using my light when not moving. I know their lights are designed very well and can handle being run 100% indefinitely, with no air flow, but not much info or reviews are out there yet on the XL-80.
As far as those thermal pads, it looks like on Digikey, the thicker the graphite thermal pad, the lower the thermal conductivity, so how can it be the in-plane rating if this is the case? The thinnest pad is 0.0007” with a 1850W/m-K, while the thickest pad @ 0.0039” has a 700W/m-K rating. Am I misunderstanding this, because how can a thicker pad have a lower in-plane thermal conductivity if that is how you think they are rating them. Please show me where you are reading this.
I have played with a number of ideas to move air across these fins. To do it, part or most of the back compartment would not be weather proofed. Meaning outside air would be sucked in or blown out. One way is to put an intake underneath and vent out the air through a number of openings around the top of the sides and top of the back of the assembly. I started doing some airflow simulations and saw I would need to manipulate the orientation of the fins some, but it is definately possible to get air flowing.
Also, placement of the driver and thermal regulation/fan control circuits will need some creative adjustments. Maybe using the sides, part of the back, and some on the led side to get it all to fit. I have a list of premade circuits that fit well in the enclosure above. You are getting me thinking of this again. I don’t know if that is good or bad? 
Heat pipes (phase change pipes) are an idea I have entertained using in the design to spread the heat around. I haven’t looked too hard to find single heat pipes you can purchase, but it could potentially be a cheap addition to reduce junction temps further. Just having a simple loop of liquid cooling plate right off the LED pads is another idea, but that may as well be taken to the extreme of making the whole light bar liquid cooled with an external rad. (thinking a small engine oil radiator). But I keep going back to KISS moto.
I just thought about my design… instead of keeping a bottom to the aluminum case, where it would couple to the heatsink, why not get rid of the bottom (CNC machine it away) and have the LEDs sit right on the heatink, with the copper bar still having to be used on the XHP-50s, as the elliptical optics are shorter than the spot optics and need to be flush with the front of the spot optics.
Lot’s of things to consider.
I have also though about simply putting a polycarbonate ‘bubble’ on top of the heatsink as a lense, doing away with a case (expensive part). But the bubble might be expensive too, and it maybe costs alot to get custom made?
Ya… I’ve thought of that as well. I thought about using them in the same design I talked about earlier. Actually, I was looking at some pc heatsinks with vapor chambers. But then I would have to use all adhesives as screws/bolts would puncture the chambers.
i have on hand some aluminum radiator runners taken off of a snowmobile. They kind of look like the old running boards on a truck or van. One side is flat and the other has extruded fins and the center is hollow. It is divided up into chambers that allow liquid to be pumped through the center. I could see this being used somehow, but like you said KISS:)
When I first discovered these graphite sheets from panasonic and such, I was very enthusiastic about the thermal conductivity, until I found out that it’s anisotropic.
I think that you folks are trying to reinvent the wheel. The most effective way to handle this is exactly like those XL80’s that were brought up earlier. The housing is the heatsink. Heatpipes are expensive for no gain here and they significantly increase complexity vs the monolithic case/heatsink scenario.
As far as the graphite stuff - I’ll let interested parties check the datasheets on their own.
IMO the important questions are:
- How to produce an inexpensive housing similar in function to the ones seen on professional lights? With a lathe or mill of the appropriate size this isn’t a big deal, but what about everyone else? It might have to turn into a 3-piece sandwich rather than 2-piece for those people: Heatsink - gasket - spacer - gasket - front plate. I do not think that adapting products like the one seen in Post #61 for operation with 19W LEDs is practical.
- Building a controller / driver which will keep the lights from killing themselves thermally. It’s all well and good to say that you won’t turn the lights on except while in motion. It’s another to hop out of the vehicle for a while due whatever and then the lights cook.
EDIT: when I write “spacer” above I mean one in the style of a carb spacer.
But this isn’t round! hehe
I agree, that is exactly how I figured I would use the heatsink, part of the housing. About gaskets though, I have looked and found custom made gaskets but I wonder if it would be better between heatsink and housing/spacer to use a high quality RTV ‘type’ sealant such as “Honda Bond”, it is a high temp long lasting engine gasket sealant. At the same time I would also like to be able to disassemble it for whatever reason… only makes sense. But then, you also want to take advantage of the housing as part of the heatsink too, use it to soak up heat so then an thermal adhesive like Arctic Alumina should be used.
Thermal adhesives are poor compared to greases/pastes. I see no reason to use adhesive in either the 2-layer or 3-layer sandwich (XL80 type or carb-spacer type setup). A careful builder can have both a flush mating surface (metal-to-metal) and a gasket/o-ring compression area (a lip or ledge). As long as we’ve got a metal-to-metal mating surface like that (preferably on the inside of the gasket!) we can use a grease or paste. I understand that MX2 is very stable and easy to apply, that might be a good choice. Many builders underestimate how much a good application can affect performance. If you don’t know you are getting a good/great application of TIM, you probably aren’t. Just to be clear: an ideal lightbar build should have more than enough wiggle-room in the design to allow for a “bad” TIM application. We can still strive for the best, but if I was going to calculate things I’d definitely calculate based on a poor TIM application.
One of the good things about a pre-formed gasket or o-ring is that it has a uniform thickness. That way a person can setup their ledge/lip to allow for the correct amount of compression while ensuring the metal-to-metal mating surface I mentioned above. With liquid gaskets I don’t think that it would work the same way.
I don’t understand this part.
That was a poor attempt at a joke… in response to you saying we are trying to reinvent the wheel.
Yes! A 2 tier mating surface is what I need, duh. I didn’t think of that. So the next question is where do you get these gaskets? Are you talking about a very large round gasket, which you then just force into the rectangular shape?
Another aspect I am trying to work out is moisture and pressure. Just a simple pressure/moisture relief valve or something of the like? Ideally I wouldn’t want anything so as to not allow any moisture from getting in because in a situation where the light is just turned off (hot) and it is raining heavily outside, then the pressure relief valve opens up to equalize pressure, can it not suck in moisture? My thinking is if the unit is built solid enough (including thick lense), along with a moisture blocking electrical cable, then it should just take the ups and downs in pressure, which can’t be too terribly much in the first place. That is my theory anyways, let me know what you guys think.
Cheers.
I think that “no valves” will be best. I stand ready to be corrected. If you are truly concerned but unsure of how necessary a release valve would be you could potentially add simple burst disk valves.
There are many gasket options. Cut your own from sheet rubber products. Order them CNC cut from a service like Big Blue Saw or Ponoko. Use o-rings stretched around an external trench. The last option leaves your gasket exposed to the environment and is probably not best IMO. With a lathe or mill an o-ring trench could be cut in a better place… but if you had one of those we wouldn’t be having the same discussion at all.
I get the joke now.
I have found many silicone o-rings the size I need (~10 inches diameter, 3/16” CS). I have looked up trench or gland guide dimensions, you need approx. 20 - 30 % compression before bottoming out of the aluminum.
The same will be done with the lense seal (o-ring also), but the o-ring will be under neither the lense in a trench, while the housing bezel/trim will compress it.
I am looking for a waterproof cable pass-through, something with a nice cable strain relief on it as well.