/diy/ - Do It Yourself

>>973598
Yup, no problem.

Heat
Heat is often a problem

>>973604

I don't see a problem with the heat. These little LED rings don't put that much heat out even at full wattage.

>>973623
Not the same guy, but the resin generates heat when curing.
I doubt enough to cause damage to the LED's, but definitely something to keep in mind.

>>973623
the resin will act as an insulator, need to place something under the leds to help dissipate the heat

>>973598
As long as they arent uv ones youll be fine
They use clear cast resin on some led strips anyway
Jusr rhe UV led srrips make the resin yellow and crack

>>973632
Seriously? I doubt these LEDs generate that much heat. But you may know better. They will be embedded in pretty deep. Not much chance of using sinks

>>973633
Not uv. Generic RGB LED rings.

So I'm guessing I can embed no problem?

>>973623
There are 16 leds in a ring, and the maximum each LED can draw is 50mW per colour, so with all colours turned on 150mW. (2*0.018 + 3*0.018 +3.2*0.018) Leds are about 40% efficient (best case) so about 90mW of each LED is actually heat being generated. Lets be generous and pick 50mW of heat to work with. That means that the entire ring (assuming heat conduction over pcb is significantly lower than to air due to the resin) is producing about 0.8W of heat with all colours turned on. Note that the chip inside the LEDs is not even considered.
If we now consider that the LEDs max junction temperature is 125 degrees, so for long and reliable life they should not get above 80 degrees in practice. Assuming an ambient of 30 degrees. This means that the conduction from junction to air should be lower than 50/0.8=62.5 celcius per watt.
I looked in the datasheet of the similair LED with the same package, and the junction to ambient without being soldered is 170C/W, and from junction to solderpoint is 50C/W.
The ring gives a large surface area for heat to release, but as you can see it is quite possible that they will get too hot during operation.

>>973598
does resin shrink?
I know there is clear potting compound. you might be better off with that.

>>973635
Are you doing many or just one?

If you're doing many, just do a test one first, run it for a while to make sure the heat build up isn't too extreme and make sure the resin you're using cures slow enough that the heat from the chemical reaction doesn't damage the LEDs, if it works then it works, if not then you have an issue on your hand.

If you're just doing one, then just test it and see what happens, I doubt any of the materials are that expensive, a single LED ring and a small amount of resin. If it works it works, if not go back to step 1

>>973658
Thank you for breaking it down like that. I will do what>>973697 suggested and try it out. They will not be on all the time and each led will flicker for a micro second for a few minutes. So I don't think that much heat will be generated.

Another reason I asked was to know if the liquid resin could seep in and damage components? Is that possible? I don't know shit about smds

>>973733
>Another reason I asked was to know if the liquid resin could seep in and damage components? Is that possible? I don't know shit about smds
No you should not have to worry about it. The two main concerns are resin temperature when curing, and heat dissipation during operation.

Tip if you are going to test first: you'd need to have a reliable way to probe the led temperature (for example a small hole in the resin that reaches up to an LED solder joint, through which you can stick a thermocouple or point an IR thermometer) so you can estimate the dye temperature (that shouldn't exceed 80 degrees as stated earlier).

>>973733
>Another reason I asked was to know if the liquid resin could seep in and damage components? Is that possible? I don't know shit about smds
No you should not have to worry about it. The two main concerns are resin temperature when curing, and heat dissipation during operation.

If the LED is indeed only pulsed for a short amount of time each minute, you should be OK operating temperature wise. Although i'd still check if i were you just to be sure.

Tip if you are going to test first: you'd need to have a reliable way to probe the led temperature (for example a small hole in the resin that reaches up to an LED solder joint, through which you can stick a thermocouple or point an IR thermometer) so you can estimate the dye temperature (that shouldn't exceed 80 degrees as stated earlier).

You need to know this temperature because the temperature on the outside can be deceiving due to the insulating properties of the resin, and a LED may seem ok for a while whilst running too hot, even though it may break after a week because of the thermal stress.

>>973658

You're ignoring the fact that the resin itself is much better at conducting heat than air. It's effectively a heatsink at this scale. Not to mention the traces and the PCB itself will distribute heat much more evenly than if the LEDs were just magically powered without being connected to anything.

Even a couple watts would only get the whole thing a little warm. There's no way you're going to have an issue with LEDs this small.

>>973733
Just jumping in here. A couple of years ago I built two bar tables. Basically I lined the inside edges with smt rgb leds, then poured about an inch or so of epoxy to fill the table. Here's what I learned:

No problem with epoxy soaking in to the leds. They have worked great for two+ years now with no indication of failure. I often run them with all three colors turned on with a constant current supply, so a duty cycle of 100%, for hours at a time. No indication of any heat issues.

Avoid overheating the leds by using a little less hardener in the epoxy. That way the epoxy will take longer to set, but will run cooler as it's curing. Be sure to mix the epoxy thoroughly though, so you dont end up with any soft spots.

On the topic of mixing, use slow steady strokes to mix the epoxy. The best way to avoid bubbles in the finished product is to introduce as few bubbles as possible in the first place. You can pop bubbles that rise to the surface using a hair drier or hot air tool. The hot air causes them to pop. Never scrape the epoxy from the sides of your mixing vessel, you're just asking to introduce unmixed epoxy in to your pour if you do.

You will have bubbles in the epoxy unless you put the curing epoxy in a vacuum chamber of some sort. It may look like you are totally bubble free, but when you turn the led on, the light will illuminate bubbles that you couldn't previously see. This may be ok though. Since the leds are pointing inward in my tables, they really wouldn't have too much of an effect if the light wasn't being diffused by the micro bubbles.

If you are using wood or another porous surface as a base, first apply and cure a thin layer of epoxy over the porous surface to trap bubbles in the base, then start pouring and curing additional layers of epoxy until you reach the desired thickness. Don't pour more than about 1/8" at a time, again to keep the heat of the reaction down.

It sounds like you wont have any problems. Just go slow and steady.

Additionally, jesus fucking christ, stop giving advice if you have no idea what you're on about. Resin doesn't get that hot unless you fucked something up bad and it goes into thermal runaway before catching on fire. At worst, it should only get unpleasantly warm.

These things have to survive a trip through a 200°C+ reflow oven. Heat from resin curing isn't going to do shit.

>>973743
Who are you talking to?

Additionally, stop fucking looking both ways when you cross the street. People cross the street all the time and don't get hit by cars. Unless you fuck something up big time you won't get hit by a car.

>>973746

Shit analogy is shit. Fucking up the resin and worrying about the LEDs would be like getting hit by a car and worrying about the Subway coupon in your pocket. The LEDs would still almost certainly be fine, but you'd now have a ring completely covered in ruined resin. Take a wild guess on how easy it would be to get that off.

>>973740
>You're ignoring the fact that the resin itself is much better at conducting heat than air. It's effectively a heatsink at this scale.
This is not how thermal calculation work buddy. You fail to see that copper is actually way better in dissipating that heat into the air than silicone would do. All the heat eventually needs to be dissipated to the air otherwise it would only get hotter and hotter. This ability to transfer heat to other materials for a given area is given by the Thermal transfer coefficient of the particular material. This coefficient scales linearly with the thermal conductivity coefficient Example: The thermal conductivity of silicone cast resin is about 2000x lower than copper. That means that for a certain area of copper to dissipate heat, if you encase a given area in silicone you'd need AT LEAST 2000x the given area in silicone exposed to the air. That is also the reason why heatsinks are made of aluminium and copper, and never from other materials as cost saving method, because that just would not work and could even worsen the problem.
That is the entire property of isolation, As an analogy to demonstrate where your logic fails: according to you you could put on more jackets in the summer to increase your ability to lose warmth, because the jacket material conducts heat better than air (it does, but that does not matter here).

>Not to mention the traces and the PCB itself will distribute heat much more evenly than if the LEDs were just magically powered without being connected to anything.
And? The thermal resistance from junction to solder and junction to case always exist, and will always be in series with other thermal resistances. Nothing you can do about that.

>Even a couple.... e with LEDs this small.
Have you ever tried dissipating a few watts in any package at all? The thing will burn your fingers. It is also bad design to not even check.

>>973747

Wow, you're really just not that bright, eh?

First off, if you actually knew anything about the electronics biz, you would know that a typical reflow profile brings the board up to 200+ C for less than 10 seconds. Epoxy with too much hardener can idle at 90C for a couple of hours which is easily enough to destroy an led or at least greatly reduce its life.

It's also clear that you're not exactly a lit major. The point of the analogy is "why take unnecessary risks when it's soooo easy to just avoid them?"

Seriously, you're bringing down the iq of the entire board.

>>973749

I was oversimplifying because your post read like someone who Googled it and then interpreted what he read poorly.

No duh the heat has to be dumped all the same. But the thermal conductivity of your typical casting resin is no worse than a small enclosure with poor/no air circulation, which a low-powered LED ring will certainly cope with.

The part I took issue with was quoting the junction-to-air resistance, in a situation in which the LED isn't being cooled by air. It's being conducted away by the resin, which will do a much better job than air at initially removing heat from the LED even if it can't just be blown away, taking that energy with it. It's a useless specification in this case. The junction-to-pad resistance is much more relevant. At 50°C/W and 150mW, you're looking at a rise of all of 7.5°C. The resin would have to be maintaining a substantial differential (in excess of like 30°C) between its surface and the solder joint in order to seriously impact the lifetime of the LEDs, which I seriously doubt is going to happen with under 2W of input power on a ring nearly 2" in diameter.

FFS, the little test board I stuck a 3W RGB LED on doesn't even get particularly hot at full power, and, even if it's exposed to air, that thing's a fraction of the size of an LED ring and the heat is much more concentrated since it's all on a single LED.

>>973755
>"why take unnecessary risks when it's soooo easy to just avoid them?"

Then why the fuck were you trying to make that point when MY point is that there wasn't a risk?

Barring fucking up, of course, but there's no accounting for that.

>Epoxy with too much hardener can idle at 90C

He's not pouring a gallon of the shit into a huge mold. He's making something (I'm assuming) roughly the size of a hockey puck. There's no way that little resin is going to remain that hot for that long. How do I know? Because I actually made just that mistake with just that amount of resin before. Cooled in 20 minutes.

>>973766
>word limit

And again, this was me FUCKING UP. This is a non-issue barring any stupid mistakes like that.

>>973766
>there isn't any risk.....except for the risk

>he's making something (I'm assuming)

Seriously, just stop talking.

>>973772

I will, but only because there's nothing left to talk about.

>>973766
>No duh the heat has to be dumped all the same. But the .....o worse than a small enclosure with poor/no air circulation, which a low-powered LED ring will certainly cope with.
Shifting goal posts now eh? You seriously described something entirely differently.

>The part I took issue with was quoting the junction-to-air resistance, in a situation in which the LED isn't being cooled by air
If you actually posses some reading comprehension, you'd note that i did not use that number anywhere in the calculations. And it is still relevant in a comparison, because that is a thermal resistance in parallel with the copper dissipation in the case without resin. And the situation IS one that the leds are cooled by air. That is the entire comparison.

>It's being conducted away by the resin, which will do a much better job than air at initially removing heat from the LED even if it can't just be blown away, taking that energy with it.
Only on short duration. It insulates the LED and as described before, will only heighten the temperature of the LED during normal operation.

>It's a useless specification in this case. The junction-to-pad resistance is much more relevant.
Not true at all. As i said, you need AT LEAST 2000x the surface area of the pcb top+bottom to have the same dissipation. Not in the least irrelevant.

>The resin would have to be maintaining a substantial differe.....ubt is going to happen with under 2W of input power on a ring nearly 2" in diameter.
I demonstrates with actual calculations that this was the case. You are assuming AGAIN that the thermal conductivity of PCB to air is the same as silicon to air. Which you cant do. That is the entire point of insulation and why heatsinks are for example painted black.

The more you comment, the more i am under the impression that you are just googling shit and trying to damage control. I am certainly unsure if you have done ANY thermal design in your entire life.

>>973766
>FFS, the little test board I stuck a 3W RGB LED on doesn't even get particularly hot at full power, and, even if it's exposed to air, that thing's a fraction of the size of an LED ring and the heat is much more concentrated since it's all on a single LED.
Also not how thermal design works
Also 3W+ leds always have aluminium core boards which the ring does not have.
Also this only confirms what i said, thermal conductivity is important.
Also if they are not on a heatsink and not running hot, you are either lying or doing something seriously wrong. 3W leds get HOT. That is why they need some form of heatsinking.

>>973658

Seeing that I use these things in all sorts of applications in many different situations I have to say that you don't have to worry at all about heat. Also, the max power draw is is 90mW total. So yeah I don't really see a problem with using the resin.

>>973799
>Seeing that I use these things in all sorts of applications in many different situations I have to say that you don't have to worry at all about heat.
No proper design is that you test that shit. Also, trusting people on their word on the internet is something you should never do. Not every situation is equal and LED strips are certainly not equal to rings. I have done a lot of LED work myself (even car headlights), and you even have to watch out with heatsinks attached to them, because often the ambient is a lot higher than you'd expect.

>Also, the max power draw is is 90mW total.
See above, that is pretty much what i did my calculations with.

>>973803 well seeing what you did here >>973740 I'm going to go out on a limb here and say you don't understand heat transfer. The layer of copper is incredibly thin and dissipates heat quite quickly unto the next medium it comes in contact with. That next medium's k value and area are the deciding factor for the heat transfer. We're not talking about conducting heat here, we're talking about transferring it.

Also in your calculation is not true is that you're assuming 90mW of electricity is going to generate 90mW of heat, which it does not. If you're correct that they run at 150mW but are only so efficient that means that other power is being used elsewhere, probably in heat. This means at max theoretical these can only be putting out about 60mW of heat each. Again, this is not a one to one ratio of power to heat.

>>973815
>>>973803 (You) well seeing what you did here >>973740 I'm going to go out on a limb here an
That is a different person. I am the first, the second is somebody else who i was arguing with. It was an argument between us two. And i also said to him that he doesnt really understand heat transfer.

>Also in your calculation is not true is that you're assuming 90mW of electricity is going to generate 90mW of heat, which it does not.
Read the calculations again. I took 100mW power dissipation, and 40% efficiency. To be generous i picked 50mW power dissipation.
And that is being really generous. Most green and blue leds respectivly 15 and 35% efficient.
I never took a 1-1 ratio of power to heat, but a reasonable number, even on the generous side.
Also, this is all without considering the chip in the LEDs that converts all of its power into heat.

tldr; i dont think we are disagreeing at all with each other, i think you've got me and someone else confused.

>>973870
>To be generous i picked 50mW power dissipation.
typo, must be power dissipation to heat, of course.

>>973870

Yeah I think I am getting the conversation confused.