Cooling of high density Neopixel blades

We discussed heat pipes in another thread, and it turns out they depend on capillary force to redistribute the liquid coolant to the warm spots after condensation in the cool spots. Therefore, they will only work across a length of about 30 cm – less if you don’t put a good enough wick inside. Building your own heat pipe is perfectly possible, but impractical, so if I were to try it I would use several ready-made shorter heat pipes placed with overlap in some suitable geometry to span the length of the blade.

Water has weight, sure, but it’s still a lot lighter than aluminium, and you don’t have to fill the entire volume inside. A thin layer of liquid near the surface is enough if you have enough flow. My current build in progress is a tri-star type blade with three strips on an equilateral triangle profile, so the volume is somewhat smaller still. I’m using a silicone hose inside the triangular tube to pump the liquid into the blade, and let the fresh coolant flow from the top, where the heat is more of a problem than near the hilt. The hose has a thick wall as well, which displaces some liquid. The blade is heavier than a standard blade, sure, but it’s nowhere near the “baseball bat feeling” I got from the failed version with the solid rod.

I got sidetracked by experiments with fluorescent coolant (rhodamine 6G) flowing in direct contact with some 50 green 3W LED chips mounted on combined aluminium reflectors/heatsinks, but I might abandon that and go for overdriven green 12V 5054 strips instead. They can reach the same power density, over 100W in a 90 cm blade, with much less work.

My progress is often slow, because I don’t always find enough spare time to have fun the workshop, but I will post updates here.

Hi, new here. Wanted to reply because I will be attempting to build a three-strip, 240 leds/m COB saber blade as an upgrade to my two-strip 144 leds/m SMD blade. I built my first functioning lightsaber last year and it works great (although I killed one of the batteries because I f*'d up the battery voltage checker) but I want to push the brightness to the limit this time round.

The ideas in this thread are definitely worth checking out but I thing a feed line with air from a miniature pump going to the tip of the blade and flowing back through the aluminium triangular profile (on which the three strips will be placed) will be my way to go, with vent holes in the top of the hilt.

Air cooling is a lot (and I mean a LOT) less efficient than liquid cooling. Consider how much heat sink surface and how much air flow you need to cool a CPU or a GPU of about the same power rating, and those are designed to run hotter at the core than what is advisable for an LED strip.

I haven’t done the math, because my skills in thermal design are not up to it, but a rough guess is that air cooling would be noisy and problematic, bordering on insufficient. It’s better than no active cooling at all, of course, but it might not be enough to handle the power dissipation. The inside of the blade is smooth, and does not offer a lot of surface area for air cooling. Liquid cooling is really a whole different ballgame, and might be worth the trouble.

Put some temperature sensors in there and implement a thermal shutdown? Three high density LED strips are expensive and worth protecting.

My double 144 led strip design consumed about 40 Watts on 80%-100% brightness (flickering, so let’s say 90%), same for red with green consuming only 14 Watts (also less bright). I don’t have any burned-out leds yet, but then again I don’t have it turned on for longer than a few minutes at a time.

I don’t know the TDP of the COB strips I ordered (I don’t trust aliexpress sellers to state accurate specs), but in general I believe they are more efficient per led. With a triangular alu profile I would have three times the mass of a flat strip but only a 50% increase in power consumption if I use the SMD5050 as a reference.

Regarding air cooling: a CPU has a lot of heat generation on a small surface area which then has to be spread through a block of aluminium with fins, which isn’t great efficiency in terms of heat transfering, I think. But if I have a triangular profile with leds mounted on it the heat from the led only has to travel through something like 1mm of aluminium before reaching the air-exposed surface. I mean, in your simple example at the start of this thread it works, right? If a little airflow could keep the temperature manageable at sub-100% brightness or just cool the blade quickly when turned off I’d be happy. I’d rather do that than mess with water-filled heatsinks, liquid cooling loops (which needs a pump, radiator AND fan) or evaporative cooling (alcohol and electronics: only need one spark to blow up your saber).

Anyway, thanks for your input :slight_smile:

I should say that I have not yet built a full length blade with triangular profile. The increased mass of the heat sink doesn’t really help a lot here, because the blade is enclosed in a heat insulating PC tube, and possibly some diffuser foam to make things even worse. For anything beyond a short test run, the dissipated heat will build up rapidly and needs to be forced out. The 3x larger mass for a 1.5x increased power doubles the time it takes to build heat, but the maximum temperature without cooling will be higher. There is more power being dissipated inside a tube of the same size and with similar thermal properties.

A CPU heatsink has the dual purpose of spreading the heat out and cooling, yes, but there is a lot of air flowing across a lot of surface area to make it cool off about the same amount of total power as a light saber blade. Also, with a long air channel, the air will gradually heat up as it passes through, making it necessary to maintain a higher flow to keep the air reasonably cool even towards the end. This is why I was aiming for a water cooled blade. I haven’t found the time to build it yet, though. I have the material, or at least enough to build a prototype to test the cooling, but reality attacked and forced me to put my light saber experiments on hold.

I’m a “problem focused” person, meaning I tend to point out things that could go wrong. I would really like to hear further about any experiments and prototypes from you. Perhaps it’s fine with just a fan and a smooth triangular tube – I don’t have the necessary skills to work it out in theory.

Keep us posted!

Based on some quick calculations and assumptions such as ~80/90% efficiency for the used leds (going for 60W TDP here), temp diff. of about 35C, tuber inner surface area of 0.024m2, flow velocity of 0.53m/s using this fan (it’s so cute) and this website for doing the “flat plate” calculation, I would be able to dissipate about 2/2.4W of heat from the tube (assuming air doesn’t heat up which it certainly does). But with this efficiency I would have to dissipate 6/12W of heat… Which can be done but you need a fan that moves 20x as much air as the one I shared, hehe…

But maybe it’s doable if you limit the amount of diffuser material (I’m talking COB led strips) and take into account the passive heat dissipation of the blade. Wishful thinking, I guess.

I’ll keep you posted. :+1:

Edit: reading back my post I guess I’m a bit of an optimist

Thinking aloud here.

You might be able to increase the cooling area inside the tube with some porous metal structure, like a curled-up mess of aluminium thread or maybe drill shavings, but then the air would be harder to push through, and I’m not sure the thermal contact between the tube and the stuff inside would be all that great.

A small pump can probably push (or pull) a lot more air than a fan through a long, narrow channel, if the air flow is the limiting factor.

Evaporation cooling without circulation is another option. Just filling that tube with water will keep it at a threshold of 100 degrees C for a while. It takes quite a lot of energy to create steam, so the battery will probably run out before enough water has evaporated to cause anything to burn.
I’m not clear on how to let the steam out but keep the water in, but I’m sure there is a solution.

If the LED strip takes damage at 100°C, alcohol boils at lower temperatures, but a blade emitting ethanol or methanol vapor is not entirely safe, and the smell would be… lore inappropriate.

A water pump is small and fairly silent, and the water in circulation will heat up quite slowly, particularly if you add a small water tank with cooling fins to dissipate at least some of the heat from the hilt. The hilt itself can be a good heat sink as well. No need for a properly dimensioned radiator, because you don’t need to design for sustained operation.

Heat pipes!

Yes, a sequence of heat pipes would get the heat out of the blade, but the heat still needs to be cooled off somehow. Or maybe a body of water inside the hilt is enough, with the final heat pipe dipping into it?
Some hilts might even have enough metal in them to just absorb the heat without becoming uncomfortably hot. Again, I haven’t done the math on this.

At least hilts have the metal on the outside, so it has a chance of bleeding away the heat. WIth a little design flair, it may be possible to pass of heat sinks as greeblies/tsubas/hand guards/blade breakers.

I have collected scraps from the old generation of LED light bulbs, the ones that required a heat sink. Some are really good greeblies for that sci-fi futuristic look.
There are modern heat sinks as well, for high power LED spotlights, that have a round, finned profile with suitable inner diameter, from 1" and up.

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Another idea I’ve had over the weekend is to use a CO2 canister (such as those use in airsoft) to occasionally blast the inside of the blade/led strip girder with gas. I don’t know how much gas is in those canisters but it should be able to quickly cool a warm metal tube/profile. You could build a system to quickly change canisters that is based on airsoft guns.

I had one other crazy idea based on something I read on here which I’ll post later. It’s rediculous but you probably wouldn’t have to cool it - even at full brightness.

Edit: posted it here.

A blade that gives off a puff of CO2 once in a while. That would be kind of steampunk. Love it. A small CO2 cartridge, the ones originally made for seltzer bottles, could be enough for short runs. And short runs are probably all we would get anyway from a battery-powered 50W blade…
Electric valves for high pressure gas tend to be kind of clunky, though. Another option could be a manual release, a pushbutton you need to press once in a while to keep the blade cool, say, in response to a temperature alarm sound.

I like it. The “psshhhtt-trigger”. lmao