# Description

I want to use a 50 Watt LED like this one (42mm*40mm)

The space to integrate this component is limited. I have only 55mm*55mm*30mm in total. So the LED + Heatsink size must not be higher than that.

I found one that is just with these dimensions: 55mm*55mm*13mm

However the description says Fit High Power Led 5 watt 10 watt.

The second option is to use a passive 40mm*40mm*20mm heatsink like this

The LED will be put in a tight spot: plastic under and metal (don't know which one) on the sides.

2cm away from the back (from the heatsink) would be some wires.

I can leave the top cover of my device open in order for the heat to go away, no problem for that.

# Questions

Is one of these two heatsink appropriate in my case? Which one would be better? I have no idea how much heat produces an LED like this

What is the risk of using such heatsink with a 50 Watt LED? Could the LED blow up? Burn?

Is it possible the sheath of the wires on the back melt?

• What is the specified thermal resistance of the heatsink in the (no doubt) excellent product data sheets? A 50 watt LED will probably produce somewhere between 49 watts and 51 watts of heat but, in order to verify this you should provide a link to its data sheet AND NOT the bang good page that you are buying the heatsink from. – Andy aka Jan 27 at 12:33
• @Andyaka I just updated my question to add a 2nd option with another passive heatsink. However I can't find any data sheet for both products and no mention of thermal resistance in their description – Jérôme MEVEL Jan 27 at 12:37
• Without that data it is guesswork. – Andy aka Jan 27 at 12:46
• Is it possible the sheath of the wires on the back melt? Sure, the wire isolation could melt. But even if it didn't your LED will not last long. LEDs suffer from heat, it limits their operating life. If you do not keep the LED at a low enough temperature (like 70 - 80 degrees C) it will develop "black spots" that will go open circuit, then more current will flow through the other LED chips so they will fail sooner. I suggest to use temperature control and never let the LED become hotter than a certain temperature. You will get less light but that's the price for bad cooling. – Bimpelrekkie Jan 27 at 13:05

If ambient temperature is 30°C and you want to keep the LED under 80°C, ignoring thermal resistance from junction to heat sink, you'll need a bit less than 1°C/W heat sink. Passive heat sinks with this thermal resistance exist but they're going to be huge and heavy (about half a kilo, and 200x70x50mm).

Using Mouser search engine, passive heat sinks with dimensions similar to your 40x40x20mm example score about 8-12°C/W so that won't work.

So you have to use a fan, like a CPU cooler.

The example heat sink with fan you link from banggood has no specs but it is way too small anyway.

This one could work but it is too big.

I have no idea how much heat produces an LED like this

About 50W, and the LED should be kept reasonably not too hot (like 85°C maximum).

What is the risk of using such heatsink with a 50 Watt LED? Could the LED blow up? Burn?

With the heat sinks given as examples it'll fry in less than a minute.

Is it possible the sheath of the wires on the back melt?

The LED will probably fry first.

You have to do the thermal design first because that will set quite a lot of constraints on your whole design, like the shape of the enclosure, air flow, etc.

If you don't do the thermal design first then you end up painting yourself in a corner.

Your options are either to pick a cheap cooling solution and then design around that, or use something a lot more expensive that will fit in your current design, probably something like a 1U server cpu cooler with heat pipes and a flat turbine fan that will be very loud.

If you need flat, but can tolerate length, then a GPU cooler from an obsolete GPU could work too.

You could put the fan somewhere else and duct the air through the fins of a passive cooler similar to the one you posted, but you'll have to check its thermal resistance versus airspeed.

It's always a good idea to add a temperature control, or at least a failsafe which turns the LED off if it is too hot, in case dust or cat hair clogs the vents.

Remember the air has to flow, so the enclosure needs enough holes to let the cool air in and the hot air out... and it needs to prevent the hot exhaust air from being sucked back into the fan and recycled... I'm not sure how you intend to build this but if the LED has its back against a wall, then you're going to have to blow the air from one side of the heat sink for example.

• Ok I see. Given that I want to go cheap, those Chinese products on BangGood don't have any data sheet available so I can't do the thermal design. I could actually cut a small hole in the plastic at the bottom and put a 50*50mm computer fan on the top of the heatsink. The LED + heatsink are limited in a 50*50mm space but I could add up another extra fan over that to suck air up I think. I will try that and at worst the LED will fry... Seems acceptable to me because it's very cheap. I just requires time. Thanks for your answer – Jérôme MEVEL Jan 28 at 7:59
• You can try and measure the temperature of the heatsink... – bobflux Jan 28 at 9:18

I'm concerned that you have packed the LED in so tightly that you won't get any airflow around it. Leaving just the top open won't be enough to establish good convection...you need a way for cool air to get in at the bottom. You might be better off using the metal sides as the radiator, and thermally connect the LED to the sides with thick metal.

Is one of these two heatsink appropriate in my case? Which one would be better? I have no idea how much heat produces an LED like this

If you had the datasheet you could estimate its efficiency, but probably somewhere around 10-15% of the power becomes light and 85-90% becomes heat for a cheap COB.

The space to integrate this component is limited. I have only 55mm*55mm*30mm in total. So the LED + Heatsink size must not be higher than that.

You've picked the wrong COB for your application. Using a high efficiency COB will more than halve the input power requirements as compared to low end models, while further reducing the fraction of your input power that becomes heat. With a more appropriately chosen COB, you could probably get your thermal load closer to 15W at the same optical power radiated as a cheap generic unit. This will be much more manageable in a small space.