Thermal considerations in LED lamp design - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 03, 2016, 09:20:21 pm
content: Hello!My mind has spawned the idea to build a LED lamp for my nightstand and the whole thing has gotten out of control now.The current plan is to contruct the whole thing out of 450 LEDs and each of them should be controlled individually. Not RGB, just a simple and plain warm white.For that i have designed a PCB, bottom layer only solid copper with a bunch of vias:There should be 25 of these, they are 48mm in diameter and each one holds a WS2803 LED driver with 18 channels and 18 LEDs.They should be stacked together with spacers that hold the assembly together.And the spacers function as big voltage bars and a bit of a heat sink, one for positive supply and one for the ground connected through all PCBs.The whole thing should be put in a acrylic tube, maybe with convection holes at the end:Here starts the problem. I have tested a single PCB and how warm it gets and i fear with 25 of them in an enclosed space it will get really warm. So i dit some math:25 PCBs18 LEDs per PCB15mA current per LED5V Supply25 PCBs x 18 LEDs = 450 LEDs450 LEDs x 15mA = 6,75A6,75A x 5V = 33,75WBesides some really basic math regarding these things i have no clue or aptitude but 35W is a whole lot of heat.Most times, is suspect, only 50% of the LEDs will be lit but that still is a lot of power.I had a small test PCB running on my desk for the whole day and it dit get a bit warm but not hot.Of course i could just put a temperatur sensor in there, and i think i should, but still...Anny thoughts?Greetings,Peter

author: tszaboo
author_rank: Super Contributor
timestamp: January 03, 2016, 10:18:41 pm
content: You can DC-DC down the supply voltage to say 4V and save 6W. If it is possible, have some venting holes.You have something like 1W per PCB there. I think it should be able to handle it. Warm and below 85 or 125 degrees is two different things. Once it is finished, attach a thermocouple and measure it.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 03, 2016, 11:04:06 pm
content: 34 W is not a lot of power.Will you be using steel threaded rod? Steel is not a good conductor of electricity; I wonder if the voltage drop at the end of the stack will be OK.For mechanical stiffness, I would use 3 or 4 threaded rods; otherwise, the assembly will not be stiff in bending (in one direction).With more rods in parallel, you will also reduce the resistance, and therefore the voltage drop.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 03, 2016, 11:05:56 pm
content: You can get some easy power back by supplying less than 5V to the stack. WS2803 is rated down to 3.3V according to the datasheet I found. You have to figure out your IR drop from 400mA across your power bus, but I'm guessing you could supply 4V.Acrylic has a thermal conductivity of about 0.2 (source). Looing at the render, I guestimate that's a 400mm long, 70mm ID, 80mm OD tube. I'm going to be a little pessimistic and look at just the heat dissipation radially for once slice, which I estimate to be about 13mm. k*Area/Thickness works out to about 0.065 W/K. That slice is 1.355W at 5V, so about 21C rise presuming ideal convection outside the tube. (It will be warmer in practice.) You might be ok with that; even 30C is well within the specs of the parts (presuming ambient around 20C.)

author: mikerj
author_rank: Super Contributor
timestamp: January 03, 2016, 11:27:56 pm
content: Quote from: pmbrunelle on January 03, 2016, 11:04:06 pmIt's a shitload of power for an LED bedside/reading light. A few watts should be more than adequate unless you plan to have a football stadium in your bedroom.

author: georges80
author_rank: Frequent Contributor
timestamp: January 04, 2016, 12:54:09 am
content: For a reading light a 1W LED will be MORE than enough light and you would want to dim that if it is directed at what you are trying to read.Is that what you are building or something to light up the entire room and then some. A modern XM-L2 is around 1000 lumens at nominal 3A (ONE LED) and that is a LOT of light for a 'nightstand' light...cheers,george.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: January 04, 2016, 08:45:53 am
content: My entire room is well lit by 15W (18W at the plug) of neutral white CREE LEDs - same for the bathroom and my sister's room. For a nightstand, 2W are more than enough - in fact I retrofitted an abat-jour for my grandpa and originally started with 3W and had to reduce it to 1W because he complained that it was too bright; it's still brighter than the original 20W tungsten lamp.34W are *PROBABLY* overkill (unless your LEDs are really inefficient) - and if you end up with the design you made, consider using frosted acrylic because looking directly at the LEDs is at the very least annoying if not dangerous.

author: SeanB
author_rank: Super Contributor
timestamp: January 04, 2016, 09:09:36 am
content: For video.photography I have some high power (125W, 150W) discharge lamps, and they are very bright. For really bright I can pull out a 400W mercury lamp and use it, though it does look like a mini sun when lit.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 04, 2016, 08:42:16 pm
content: Hello and thank you everybody for the reply!As for the part about the number of LEDs and the power consumed, i know that it will be really, really bright.That is a intentional design choice that has to do with how it might be used and what it should be able to do in regards to displaying light effect and what difusor material i will use.The lamp will indeed be enclosed in either a semi transparent acrylic or a clear acrylic tube with some sort of light difusor fixed to it.For the most part if would not expect the lamp to run with more then at max 1/4 of its potential power but i do not want to go blindly into construction without considering the worst case scenario.I suspect the whole thing to produce about 2000 Lumens, when fully turned on, potentially 4000 Lumens with proper LEDs.I am gooing with cheap ebay ones without a datasheet for my first try, proper LEDs would cost my an arm and a leg for starters.Quote from: NANDBlog on January 03, 2016, 10:18:41 pmThat is a option i considered, sadly the only suitable powersupplys i get in the 5V range with at least 10A are only capeable to be trimmed down to about 4.5V and i do not want to put a rather beefy DCDC converter into the whole thing. Allthough TI makes a few nice modules they are not that cheap and thend to get rather warm themselves on a really small footprint.Quote from: pmbrunelle on January 03, 2016, 11:04:06 pmThe material of choice in this case is brass in form of 10mm motherboard spacers with a M3 thread:http://www.cybergooch.com/tutorials/images/buildsystem/IMG_6805.jpgQuote from: Galaxyrise on January 03, 2016, 11:05:56 pmThank you, that sounds good!All the parts would be very happy to work at higher temperatures but the acrylic tube is a bit sensitive to too much heat and i can not replace it with glass since i can not get a suitable tube in that diameter.Greetings,Peter

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 04, 2016, 09:27:50 pm
content: Quote from: PeterFW on January 04, 2016, 08:42:16 pmJust a power resistor will move some heat out of the tube. I'm thinking a big ceramic resistor likehttp://www.digikey.com/product-detail/en/CB5JB2R00/CB5JB2R00-ND/1740800orhttp://www.digikey.com/product-detail/en/SQP10AJB-2R7/2.7W-10-ND/18721probably wouldn't even get too warm to touch.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 05, 2016, 04:27:24 pm
content: Quote from: PeterFW on January 04, 2016, 08:42:16 pmThen just mod the supply to a tad lower. The feedback resistor divider network is extremely easy to find in this case as it has the trimmer pot, which is a part of the feedback divider. Typically a 15-minute job of replacing one resistor.Also try to source some brass rod for better electrical and thermal conductivity. Even if you couldn't remove the heat from the tube through the rod, it will help avoiding hotspotting at the middle PCBs. At least if your PCB design allows thermal transfer to the terminal(s).Finally, don't feel too bad to add a small fan at the end of the tube. If it is "normally" used at lower power levels, the fan doesn't need to run at all. Program it to only turn on during extended time of high power.

author: rdl
author_rank: Super Contributor
timestamp: January 05, 2016, 05:15:35 pm
content: Quote from: PeterFW on January 04, 2016, 08:42:16 pmOff the shelf 60 watt equivalent LED "bulbs" run around 9 watts and 800 lumens or thereabouts, so you're probably in the ballpark. I guess you must be planning for the lamp to do things not possible with regular LED bulbs.

author: tszaboo
author_rank: Super Contributor
timestamp: January 06, 2016, 09:10:53 am
content: Quote from: PeterFW on January 04, 2016, 08:42:16 pmVishay makes something called microbuck. It is a DrMOS kinda device with built is controller. I think it covers the voltage and current range you are looking for. It is very cheap, though I havent used them yet. The DC-DC could be a extra pcb similarly stacked. I suggest this because it also improves your efficiency.In my experience, power supply datasheet is saying +/- 10% but the actual adjustment range is usually higher than that.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 07, 2016, 09:47:32 pm
content: So... i set to work today, even though it is such a simple circuit i have a bit of a problem with a few of the outputs not working. But it still is really, really bright even at 15mA, i may be even be able to go down to 10mA per LED, run at 4V i would be down to "just" 18W full power, good news!Soldering is a bit of a pain in the arse without soldermask and with abusing the PLCC2 package for vertical mounting:My soldering is way, way better usually... it looks horrible.Quote from: rdl on January 05, 2016, 05:15:35 pmYes i do, it is more of a "why not, i can do that" then an actual need.Maybe it is more of an art piece instead of a lamp.I tried describing what the lamp should do but i fail at it, i will have to show you a video of the diming and light direction selection when the thing is actually done.Quote from: NANDBlog on January 06, 2016, 09:10:53 amOn that note, i just tested the whole thing and the lowest i can go is about 4V without suffering anny loss in term of brightness.Quote from: Siwastaja on January 05, 2016, 04:27:24 pmMhm, that is indeed a good option, i addet a footprint for a temp sensor on the boards!

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 07, 2016, 10:23:23 pm
content: Quote from: PeterFW on January 07, 2016, 09:47:32 pmWhere are you measuring that 4V? It's not obvious from those pictures how you'd be supplying a variable voltage to your board. If it's not right at the inputs on the LED board, you may be dropping voltage someplace you're not accounting for. If you're also cutting supply to the logic board, that might have other side effects.

author: ajb
author_rank: Super Contributor
timestamp: January 07, 2016, 11:05:01 pm
content: Quote from: Galaxyrise on January 07, 2016, 10:23:23 pmAccording to the datasheet, the WS2803 is intended for VDS of 0.4-1.0V, and going from 5V to 3.3V supply drops the maximum output current from 30mA to 20mA. So given mystery white LEDs at 3ish volts, "about 4V" sounds like a reasonable lower bound.

author: Fungus
author_rank: Super Contributor
timestamp: January 08, 2016, 12:48:52 am
content: Quote from: PeterFW on January 07, 2016, 09:47:32 pm

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 08, 2016, 03:17:12 pm
content: Hello and thank you everybody for the reply!Today i put together as much as i could with the parts i had at hand, i am one PCB short of what i wanted but one edge of the PCB got mangeled during development:Now i got to write some code and put a few controls on that.Quote from: Galaxyrise on January 07, 2016, 10:23:23 pmOnly cut the power to the LEDs and measured ~4V directly at the LED footprint.Quote from: Fungus on January 08, 2016, 12:48:52 amOnly picture i had at hand right now, pretty old though. No motivation to take a current one :-P

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 12, 2016, 07:01:38 pm
content: Everything looks good so far, i ordered all the parts including the PCBs.And 50 bucks for 30 PCBs is a decent price all things considered.I redesigned the PCB a bit and hope it will not bite me in the arse, but the prints looked pretty great fitting.Just have a hard time finding proper studs/bolts/nuts for a price i am willing to pay without waiting 8 weeks for the mail from china.Annyone interested to see how it will look when put together and why i make it so hard for myself?

author: sleemanj
author_rank: Super Contributor
timestamp: January 12, 2016, 09:27:16 pm
content: I like it, reminds me of a kerosene lamp shape. Add battery power and I reckon it would make a good campsite light.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 12, 2016, 09:58:24 pm
content: Quote from: sleemanj on January 12, 2016, 09:27:16 pmFunny thing to say, that is the inspiration and one reason for the big bunch of LEDs.A really soft and warm flicker as if there was a fire as light source in there.Besides some directional and dimming effects

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 21, 2016, 04:35:42 pm
content: And the assembly nightmare begins... 30 of these. Yay!

author: tszaboo
author_rank: Super Contributor
timestamp: January 21, 2016, 08:06:18 pm
content: That is why we have subcontractors...

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 22, 2016, 03:56:28 am
content: Do one per day... it will be ready in a month!You should test the boards individually first. It would be a pain if a board in the middle of the stack didn't work, and you had to disassemble everything to access it.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 23, 2016, 11:18:19 pm
content: Quote from: pmbrunelle on January 22, 2016, 03:56:28 amI test every thing, multiple times, yes.If the board in the middle would fail that would be quite the pain in the arse.Today was a fun day!Lessons learned so far:Do not trust manufacturer website and allways build prototype with new stock.It gets way too warm, but that is not a problem annymore.The threadet rods+spacers work like a charm as bus bars with voltage drop that is not worth mentioning.It is nicely bright.

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 28, 2016, 01:10:13 am
content: Quote from: pmbrunelle on January 22, 2016, 03:56:28 amBEHOLD! It is done!It is too late for me to work with line voltage, so the full power test will have to wait till tomorrow.The whole thing draws 9A on paper, my bench power supply only can deliver five and the 5V at 10A supply i bought for this still needs to be wired to the mains.No problem with a voltage drop so far, so id say this is a viable way to use threadet rod as a power bus.But more detailed measurements will have to wait til tomorrow.Greetings,Peter

author: PeterFW
author_rank: Frequent Contributor
timestamp: January 31, 2016, 06:01:10 pm
content: Been writing code for the damn thing for a few days now, you get one last picture.Maybe ill come back when its finished and in a case.