DIY driverless COBs test update

I’ve been running a test fixture with 110v AC 50watt full spectrum driverless COBs for about 10 weeks. I am working on a 800 watt light and I am learning what I need to build a reliable and cool light.

I bought predrilled heat sinks, so I’m stuck with them.

When both lights are running with no cooling they get to 200°F. With cooling fans blowing 25cfm on them, temps are around 120°. I have had 2 lights fail due to high temps. A much bigger heat sink would be better. The Light is good according to the light meter app on my phone whatever that means. The light is almost full color spectrum and at 18" it’s 25,000 lux. I can’t afford a good PAR light meter right now. I just got a cheap sunlight meter today that’s supposed to measure PAR. If it doesn’t work I’m only out $14.


At a quick glance, I have some questions and a comment. First of all, what is the big box on top of the lumber? Secondly, why is the lumber there? Also, are you measuring temps of the heatsink or is there a dedicated place on the cob to measure case temp?

I’m thinking the heatsink should be plenty sturdy enough to just attach the hangers to it and do away with the lumber. That would allow a little better airflow and possibly help with temps some

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@OldSkunk I agree with @dbrn32. You are at a point where your heat takes away from your efficiency. Lose the lumber (it holds heat and blocks airflow). Free hang the heat sinks or build an aluminum frame instead? But I love the direction you are going in. Keep it up !!


I left that out, please don’t take it as criticism. I think getting it done with low cost options is awesome!

I do think those sinks are on the small side for the wattage. Depending on the cost, I’m not sure if you could do better or not. I suppose we could look at that if you’d like. I am wondering if placing the fans on top of the heatsinks and blowing air down and through the channels would cool them more evenly. But that’s just a guess.


The box is a computer power supply to run the fans. The heat sinks are probably ok for 20watt cobs x 2, but not 50watt x 2 without cooling. The lumber is there to mount the fans and heat sinks. I have measured the temp right at the COB without cooling and it’s about 95°c. I need to find the time to put all of it on a big piece of aluminum. I bought three pieces of much bigger heat sink for $30. When I get the time, I need to mount 2 COBs to them.



I have 2 40mm fans on the heat sink directly above each COB blowing on them. I think they’re too small to do much. I can’t find a 75mm which would probably get the job done. I bought a 3" wide x 1/4" thick piece of aluminum about 6’ long. I’ve been thinking about how I could use it (or pieces of it) to mount this equipment for further testing. I just made the fixture in the picture because I wanted to see how the cheap COBs would hold up. They seem to be fine if you keep them cool enough. Don’t worry about criticism. I’m a good listener. Especially if it can help keep me from making boo boos.


I’m a huge fan of COB’s

In my opinion COB’s are currently the best LED tech.

I’m running Cree CXB 3590’s and 3570’s with a mix of 3500 and 4000K.

I’m very interested with what your doing and will be following along!

Excellent work!!!:slightly_smiling_face::+1:


Those bigger sinks would do better. And you’ll have more luck looking for an 80mm fan. 40,80,120mm are all standard sizes.

Probably a lot of ways you can make it work. Have you looked at heatsink usa? They have a bunch of different extrusions, and you can buy by the inch. It’s not exactly cheap, but you can certainly just size it to do the job you need. It’s way easier than trying to buy something undersized and hoping it will work anyway.

For active cooling (with fan) you want about 4.5 square inches of surface area per watt on the heatsink. For passive cooling (without fan) you need to be closer to 20 square inches of heatsink per watt. There will be some variables like base thickness and how much room there is between fins for air to move. Even surface prep and how good of transfer material you use can play into effectiveness. Maybe take a look at your current sinks and see if you can estimate how much surface area it has.

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@OldSkunk I think your doing awesome and we want you to succeed. But we want you to understand that as heat goes up efficiency goes down. So hotter means less light. You are on the right path though and I think small 4-6 inch fans mounted right to your light frame blowing across the heat sinks might be your solution.

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Assuming you mean cxb 3590 and 3070? I’m pretty familiar with Cree line, not aware of a 3570 unless it’s new or something?

I think I remember when you got your lights, they are prebuilt fixtures using the crew cobs correct? Just out of curiosity, did they give you the binning info of the cobs?

We can’t see the back of your heatsink, but I assume it has fins. You want the fans blowing air through the fins, not down from above. Or you could use a top fan if you duct it so the air moves down the length of the fins. Trouble with ducting is that if the fan stops for any reason, POOF go your COBs. If you do duct, it would be a good idea to get a thermostat so if the heatsink gets too hot the COBs turn off.

Over 85 C is too hot! In fact, I think that you want the COB LEDs to stay under 85 C, so the heatsink has to stay lower than that. It does you no good to have a bigger heatsink if it just slowly gets up over 85 C. Bigger may just mean it takes longer to equilibrate. The real thermal bottleneck is from the heatsink fins into the air moving over them.

I have about 500 square inch heatsinks for each 70 watt COB and driver, so I definitely need a fan. Right now each pair of heatsinks has a small table fan blowing down it and they run really cool, like maybe 40 C. Eventually, I plan to duct them all into a big plenum with all my air exhausting from that. I will include thermostats.

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Why do you say a top mounted fan needs duct? I disagree. It works the best of any way I’ve tried. Not that I’ve done it with every fan or every heatsink. But it’s always worked better than blowing air from one end to the other.

The problem with the way he currently has fans mounted is that very little of the air moved actually goes down the channels. And the air that does, is heated from the near end prior to making it to the far end. As opposed to a top mounted fan about the diameter of the heatsink width. It wall draw air in and then channel most if not all of the air from the center down all channels and out.

Without an oversized heatsink, fan failure will almost always lead to led failure. Depends on how fast you catch the failure is all. It’s half the reason I use passive sinks.

A bigger heatsink will indeed disperse more heat faster. A big enough heatsink will not require the use of a fan at all. I’m sure there has always been air movement from my exhaust, but I’ve never had an led fail and never use cooling fans on them. Goes right back to the pay now/pay later instance. You can definitely spend a little more on heatsinks and never have to worry about fans, ducting, or any of that stuff. There are people doing it all over the place.


Yeah, if your fan is as big as your heatsink, that’s great. If you have a smaller fan, then the air needs to run down the channels not just bounce off the middle of the heatsink. And I’m sure a fan pointing down into a pin heatsink would work fine because the air has plenty of ways to exit going by all the pins.

You make a good point about making the air move over the cool parts of the heatsink first and putting anything hot at the far end. Otherwise your air gets preheated and doesn’t do any heat transfer over the cooler parts of the heatsink. This why I put my big driver transistor at the far end of the heatsink from the fan.

A big heatsink will work okay without a fan, but it depends on convection to move the hot air away from the fins. So without a fan your heatsink has to be about 4 times as big. For high-powered COBs that can be huge. With no fan, my 500 square inch heatsink that mounts a COB and the driver transistor for both COBs gets a little too hot to touch. Turn on the fan and it is barely warm.

Lowes has a 22 Amp simple thermostat for less than $14. Controls 120 or 240 VAC.

I only know for sure what I’ve done, and that’s based off what I’ve seen others do. If you have 5 3/4” heatsink A 140mm covers it, and when top mounted blowing onto the heatsink, it will channel the air down the fins, not bounce. Obviously a smaller sink would require a smaller fan. And you need to make sure there’s enough gap on fins for air to travel. If the fins are too tight it won’t cool as well with or without a fan.

The point I was making about the pin heatsink, is that they run plenty cool with no fan at all. Sure there is natural convection from my exhaust, but that’s going to be there anyway. I have zero additional fans, power supplies, or wiring that pertain to lighting. Forget the cost of running the stuff, and worrying about failure. It’s just soooo much easier and neat to not have to deal with it. All you have to do is make sure it’s big enough for thermal load you’re planning.

500 square inches seems small for a 70 watt cob and the heat watts from your supply. Doesn’t surprise me it gets hot without fan. I have a chart on surface area of the heatsink usa profile surface areas. I can track it down if you’d like?

No thanks, Vero documentation has a whole 31 page thermal design guide for their COBs and I read that cover to cover. Basically it comes down to around 10 square inches per watt unless you use a fan and then your heatsink is about four times better. (That’s for finned heatsinks, not for pin.) As long as you are using a decent TIM grease between the COB and the heatsink, all the calculating of thermal transfer between the COB and the heatsink is something you don’t need to bother with. The overwhelming thermal bottleneck in the design will be between the fins and the air.

If you have a breeze around your lights from the exhaust or air circulation fans, that will help a lot, but if you are depending on those fans you have the same problem: If those fans stop your chamber gets hot and the heatsink works much worse. Without a fan right on the heatsink, the ambient air temperature is critical. Heat will not transfer if the air around the fins is the same temperature as the heatsink. The only good news is your COBs will probably die before your plants do.

Actually, for my design I think I will use an air flap and microswitch in the exhaust duct to shut down the COBs if the air is not flowing. Much simpler and I only need one of them.

No lol, the chart I have gives you the surface area per inch of their extrusions. So rather than having to get it and physically measure, you know how much of what size to hit target.

By Vero, you mean bridgelux correct. I’ll have to find that document, because everything I’ve seen with that low of area fails to maintain case temps.

I’ve ran my fan on recycle timer before without seeing anything more than a slight increase, like 2c. So it could happen, if I was one of those guys trying to cram Watts cheaply and over driving my cobs, then my exhaust just happened to fail the day I decided to go on vacation for a week. But since I generally run more cobs softer, and can’t remember a time I’ve left a single grow unattended for more than 24 hours, I’d say the chances of a that leading to one my cobs failing nonexistent. It hasn’t happened in 3 years anyway. Not even when running a sealed room with co2 and cycling my exhaust.

Maybe a lot of that has to do with luck, or just with using quality tried and true components. Not really sure to be honest. But it seems a lot less likely to happen with less components to rely on or encapsulating the heatsinks with anything. If you go that route, I would certainly install some sort of interlock to protect them. But for anyone growing in a small to medium sized tent, additional ductwork, wiring, and interlocking isn’t a good or even feasible option. For that reason, I will continue to recommend the passive pin sinks.

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You can search for Application Note AN30
Thermal Management for Bridgelux Vero Series LED Arrays, That is a cut and paste of the exact title. When you find it the other Ap Notes there are useful, too. We both agree that having an exhaust fan is a very good idea. Lights are going to generate heat and if you don’t have a means to remove that heat your tent is going to get too hot.

You’re saying that the hot air floating around the top of a tent is going to go out the exhaust. But all of that heat was generated by the lights. I’m saying you can use that very same exhaust fan to pull air right over and through the heat sinks and thereby use 1/4 the size heatsinks or have the heatsink run a lot cooler. Kind of like the HID fixtures that have a duct to connect to the exhaust fan.

Both ways work. Mine will give me a cooler heatsink and a cooler tent. When I get it hooked up I’ll take some temperature readings and post them.

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Ha!!! Typo!!!:joy::grinning:

Yep 3070.

Sorry about that.:slightly_smiling_face::+1:


I figured, but wanted to make sure I didn’t miss something new. Hard to tell these days at the rate things come out.


I’ll get you all a picture of the whole thing with fans. I’m out of room to work where I live so I am working on the lights at a friend’s shop.