Rear of the heatsink with 2 x 40mm fans. I think they need to be bigger.
To answer your question on an earlier post @dbrn32, I did look at Heatsinks USA before I bought the ones I have. They were pretty expensive, so I looked elsewhere. I found the these at parts express. I think they would work ok with one COB. I’m going to try mounting one as soon as I can and test the temps without active cooling. I like what @Daddy is doing with the aluminum angle on your thread “let’s talk about DIY lights”. I can get that at the scrap metal place fairly cheap. I think I’ll get some next week and modify my test fixture to use that. The Heatsinks I’m using have holes already drilled and tapped in the side for mounting. The aluminum might even help with cooling. @Alton66 @1BigFella
The aluminum frame will help in cooling. And I think one cob per heat sink would help alot if it’s cost effective to go that route. I would skip the computer fans if you can and use one or 2 oscillating wall mount fans if you have room and run over the top of all your heatsinks?
I believe he’s using the frame to hold his sinks, but I don’t remember if he posted what he was using for heatsinks. I prefer a frame, and to have as little wiring as possible. But some prefer the adjustability of single hanging cobs. It’s too big of a mess for me though, especially if your active cooling the sinks.
How deep are the fins, and what’s the spacing from fin to fin? If you go to one cob for sink I think they’d be fine with the fans you have. Otherwise I would recommend going with a slightly larger fan, to at least cover it from one side to the other. If you can get a half assed measurement of the surface area of the heatsink, those numbers I posted earlier are pretty solid. Will at least give you an idea of where you’re at.
The heatsink USA stuff is pricey. It depends on what you have wrapped up into the all the other stuff. A 36” piece of 5.886 and a single 140mm fan would be adequate for 4 of your cobs. I’m guessing that would run about $70 or so. If you go to a single heatsink and fan per cob, how does that stack up price wise? Another option would be cpu coolers at 1 per cob, but I don’t think that would be any cheaper.
The better you make it, the price is most likely going up. In your case, I realize the occasional cob failure isn’t very concerning due to their cost. But there’s probably a happy medium somewhere between cost and piece of mind. Hopefully we can track it down.
I have 4 x 9" fans (one in each corner) and one big oscillating fan pushing the homemade co2 generators . Moving a lot of air won’t be a problem. Because I’m growing sour diesel and it’s so easy to get mold, I went with a lot of air flow. I love that winter is coming because I can pull free cold air from outside instead of expensive Air conditioning. It really helps when you’re on a shoestring budget.
So really your biggest cost is going to be heatsinks I’m using 140mm pins and they are about $21 a piece. But I can move them were ever I want. @dbrn32 is correct I mount the sink to the frame.
I was referring to fans on the heatsinks sorry. Not sure about climate where you’re at, but winters here make me more nervous. Summer sucks too, but easier to manage for me.
I think heatsink USA was $30 a foot for what I was looking at. I have 20 of the Heatsinks I bought so I could do one 1,000 watt light. I was hoping to do an 800 watt for flower and 600 for veg with 2 COBs per heatsink. Might not work out that way. I would like it if I didn’t have to cool them actively.
Any chance you can return? I think you can get passives for $10-$12 each buying in that quantity. Not sure they would be drilled and tapped for those cobs. But it’s way cheaper than a custom run.
Can’t return them. I bought them 4 months ago so I’m stuck with them. I thought they would be ok because they are made for power amplifiers and supposedly for 2 x 100 watt COBs. They’re not. Maybe 2 x 30 watts. Should have done more research. I could put them in a closed fixture and pump cold outside air to them. I have the stuff to do that. As for your question on fin to fin spacing -
I suspect the issue is that they’re autos lol. Tag me there, I’ll see if I can help. Consider yourself warned if it’s a nutrient type of issue, I usually go looking for help myself.
That looks about like a 300 square inch heatsink, so the very minimal size COB you could run without fans right on it would be 30 watts. That would probably work okay with your other fans stirring the air up in your test. With very good airflow down the fin channels, I think you could run about 120 watts of COBs. This heatsink is too small to run even one 50 watt COB without a fan on it.
And I would suggest that if you lose one COB due to overheating, you are going to lose them all very quickly. You really don’t want to run any electronic component right on the edge of failure. It shortens their live exponentially. It could be days instead of years.
About your fan placement, I worry that the ends of your heatsink might not get much airflow through the fin channels because the air can just flow up and out of the fin channel. I think it would work a lot better to add a top to the heatsink to force the air to stay in the fin channels. Your COB will blow up if the fan stops working, but it would blow up anyway without the fin duct if the fan stopped working. You could experiment: Use a non-contact thermometer (Harbor Freight has one cheap) to measure the heatsink temp like it is now. Then construct a heatsink duct out of cardboard and duct tape so all the fan air goes down the channels, and measure it again.
It’s useful to look at ebay for surplus heatsink extrusions. I got my 500 square inch extrusions for something like $10 each and new retail they are way more expensive.
I definitely have to blow air on both COBs or the one that’s not getting air will fail. I used to be in engineering and should have done my homework on heatsinks. I kind of jumped in with both feet without checking what I really needed to run these 50 watt COBs. I should know better. As I told @dbrn32 I will try one cob on it and see how it runs. I’m sure it’s still going to need cooling.
Another update on these driverless 50 watt COBs. I’ve had 2 fail since my last post. They don’t fail completely. Just almost no lumens out of the lights. Looks like all the LEDs are lit but very dim. Looks like all they have is a rectifier, driver, and current limiting resistor. Not a lot of components to fail. @1BigFella. @Daddy. @dbrn32. I don’t think these things are made to be on 18 hours at a time.
I’m using 140mm pins and they are about $21 a piece. But I can move them were ever I want. 
