No worries. I’d be thinking same thing if I didn’t know any better.
@OldSkunk sure they can run 24/7 there is no rule that says they cant…except heat 
. Once you can figure out how to keep them cool you will have it figured out. It really just depends on how much time you want to spend messing with them (your time is worth money too)
No it’s my fault I didn’t take the extra time to read from the beginning of the thread lol @dbrn32
@Daddy is right too, get a much larger heat sink 4times the size of the LED and possibly air circulation can help with temps
It’s ALWAYS heat, when electronic parts fail so fast. They are probably running them right on the edge of failure so you have to supply great heat sinking.
You might first try to measure the temperature of the COBs when they run for a while. I have mine mounted on 1/4 inch copper blocks so I can tell the temperature just by putting my finger on the copper block. If you have very good thermal contact with a heatsink, you could just let them run for a couple of hours and then measure the temp of the bottom plate of the heatsink. If you are using one of those non-contact optical thermometers (EG. Harbor Freight) then you have to turn off the COB so the light does not interfere. You can look at the datasheet but max temperature is probably around 85 C.
I they are running hotter, then I would guess one parallel LED string fails and then all the other strings have that same current divided among them and another fails, etc. It is a thermal runaway effect. You could try running a bigger fan on your heatsink to keep it cooler.
Or you could get a big power resistor to put in series so they get a little less current. Do you know the current they draw? I assume you are running them with a 12 volt computer supply. If not, then ignore all of this:
If they really are 50 watt, then the current would be 50/12 = 4.17 amps. Their total resistance would be 2.88 ohms, and we can guess the COBs drop about 80% of that power and the resistor about 20%. So it would be dropping about 2.4 volts and would be a 0.6 ohm resistor. Assuming the COB voltage doesn’t change much, you could add a 0.1 ohm 2 watt power resistor in series to drop the current by about 15%. See if they run longer with lower current.
Fry’s electronics sells 0.1 ohm power resistors. Just make sure it’s 2 watts or more or it will burn up.
They are driverless cobs, all hooked to ac wall circuit. Thermal runaway shouldn’t be an issue. I asked if they were wired in parallel because he said they all came on but were dim. If they’re wired in series I’m sure there would be an issue with too low of line voltage to the onboard electronics.
Wouldn’t you think @1BigFella?
If they are 120 VAC then there’s nothing to do except wire them all up to the wall power directly. But that means they each have their own resistor, etc. Nothing you can do to wire in series or to change the current. Since they do have their own resistors, that’s not really the “in parallel” wiring everybody warns about.
I guess all that’s left is to keep them cooler. That’s pretty much all they let you do, and if you do a poor job of it they will self-destruct. Maybe they are dim because there is still some current flowing and it’s enough to heat the phosphors. I noticed my COBs stay dimly lit after I turn them off because they are still warm. After several seconds they cool off enough to go out entirely.
Lots of electronic parts will only last a few seconds without adequate heat sinking. If they last a few hours, that tells you the heat sink is getting too hot. If the heatsink gets too hot to touch, there’s the problem. Heat transfer out of the COB is a product of the temperature difference between the LED junctions and the heatsink.
Well said! I had a feeling you missed what he was working with.
@OldSkunk just wondering if you did have the ac wiring in parallel? The fact that they are on but dim has me curious. If they are indeed “burned up” it will usually end up with part of the circuit open, which should halt electron flow. But anything is possible.
Have you tried a single cob per heatsink with fan directly over top?
I got my COB fixtures plumbed so the exhaust fan pulls air through the heatsinks. Each Vero29 is running at about 82 watts and has 24 cubic feet per minute of air moving through it’s heatsink. Cool as a cucumber. I can touch the copper heat spreaders and they are barely warm. Just a small amount of airflow through the heatsink makes it 3 or 4 times more effective. Before doing the plumbing, I was using table fans to blow down each heatsink and they were definitely warmer.
Sorry for the late reply. Been working on mounting just one 50watt cob on the Heatsinks. Lots of work drilling and tapping. Also using 2 fans blowing on the COB instead of 1 like before. Also going to try a small Sqirrel cage blower per heatsink. To answer
Your question about the draw. It’s .5A at 110v. The test fixture I had made was 4 x 50watt cobs. The total draw is 240 watts which includes 4 x 80mm fans and the computer power supply to drive them.
![IMG_20171203_172208889_HDR|281x499]
(upload://28rIkVeouodYIbzzAggDlJagy6b.jpg)
This time I’m building a 300watt test fixture with reflective mylar to see if it helps improve the weaker light from these COBs.
@dbrn32 the AC is wired parallel to each COB, but only one has failed. The other COBs are ok, but one has all the diodes on but very dim.
@Daddy the last failure definitely wasn’t heat. I monitored that one for days and never got above 125°F. Most of the others were though.
@AmnesiaHaze I’m going after as minimum cooling as I can get by with. I realize the heatsink is too small, but it’s what I’m stuck with, so it looks like I’m going to be using some watts to cool unfortunately.
Your lens is sopping up some light so you can project a narrower beam. When you have a bunch of plants under a bunch of COBs I don’t think the hot spots generated by the lenses are useful, and it’s less total light on the plants. As long as your lights are not too far away from the plants, I think it’s better to let the light spread out uniformly. Likewise, I doubt COBs project any light behind them, so a reflector is useless. Just as an example, you can look at the Bridgelux Vero29 datasheet online. It has a projected light pattern and nothing gets projected in the area you would put a reflector.
If your lights are too weak, it might be more useful to build a smaller grow chamber with flat white painted, white panda plastic, or aluminized mylar walls. Get more light hitting the walls reflected onto the plants.
Where and how are you measuring temps? Do the cobs have a dedicated spot to measure case temp? It doesn’t appear so, is why I ask. Which leads me to believe you measuring the heatsink temps. Knowing the sink temps is generally a good thing, but it’s not how the cobs the cobs are rated. Depending on how good your transferring heat, will tell you more or less.
Not being exactly sure how and what you’re monitoring, I suspect the possibility you maybe didn’t have as good of a thermal joint on that failed cob. If you’re measuring heatsink temps, and the others were higher but didn’t fail, that’s a likely cause. It’s entirely possible that cob wasn’t as good as the others, or there was a manufacturing defect too. I’m just throwing out info that may be able to help you through the project.
What kind of differences are you seeing with the two fans? I’m sitting here wondering if one fan pushing air while the other is pulling would help. Or possibly raising them up a smidge off the heatsinks a bit with some standoff’s or something.
I’ll try 3 without lens and 3 with one to see what happens. My flower area is 4’ x 6’ and it’s lined with panda film that has reflective mylar over it.
I’m monitoring the temp on the Heatsinks right behind the COBs. I have used the push/pull on the fans and they run cooler when I’m pushing air. On the COBs I use a thick enough layer of thermal paste that when I mount them, they push out a pretty good amount of paste that I have to clean off.
Copy that. Was just trying to think about potential things to check.
I’ve been running a single COB mounted on the heatsink for several hours now with the dual fan setup on the back of the heatsink and a small squirrel cage type blower.
With just the heatsink fans, the temp gets to 130°F. With all fans running the temp is 110°. With no fans, the temp gets to 155°.
@1BigFella
@Daddy
@AmnesiaHaze
That’s a great build you got there good job. I’m just suggesting though I think those magnifying lenses don’t do any good for your foot print, as you want the COB to naturally let its full spectrum out. That lense looks like it’s constricting your foot print, and it also prevents heat from releasing, the lense absorbs heat and it reflects some back, as it is a magnified glass 
@OldSkunk
The cooler you keep the cobs the better efficiency not considering premature death and @AmnesiaHaze is right about removing the lens, it will help with heat. we need a simpler and more efficient way to cool though. you are using more electricity cooling then making light lol.
155 F should not kill a COB, I think. You should check the real part’s datasheet if you can. My COBs can run at 185 F before they release the magic smoke and die. Of course, that is the LED junction temperature. If you have a good thermal connection to the heatsink, the drop in temperature should be less than 10 degrees. So a heatsink temperature of 175 F should be survivable. I would still run at least the two fans. Those little fans are really cheap to run.
Maybe your COBs just have very poor quality control, and they failed because of defects in workmanship? Or maybe they are really cheap because they are some good factory’s rejects? Just buy a bunch of spares and replace those that fail early. Once they run for a week for 18 hours a day, I bet they run for years.
If you want cheap whisper fans look at Electronic Goldmine. (I can’t post a link but you can search for that.) They have tons of surplus stuff including fans galore for just 2 or 3 bucks.
Here’s my latest test fixture with 6 x 50watt lights. I’m going to use it to monitor temps and see what the light is compared to my 300watt LED lights I bought on Amazon. So far the temps are really good with just the squirrel cage blowers on. The temp at the COB is 110°F The fans mounted on the back of the heatsink don’t seem to make much difference. All fans running draws 43watts, so I’m giving up 1 lights draw to cool everything.
I want to be sure these driverless COBs are reliable before I waste a bunch of time building a “real” light out of them. @dbrn32 @1BigFella @Daddy @AmnesiaHaze