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  • EEVblog #50 – Solid State LED Lighting, and How Thermal Design Sucks

    Posted on December 19th, 2009 EEVblog 42 comments

    Dave lights up his new backyard deck with Cree XPG LEDs, and then winds up measuring the wind from a storm instead. And who can spell efficacy?

    LINKS:
    Cree XPG LED Datasheet
    Arctic Silver Thermal Adhesive
    Luminous Efficacy Graph
    Graph #1
    Graph #2
    LED Strip, Mounted LED
    700mA 35W Mains LED driver

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    • http://kaqelec.dyndns.org Kathy Quinlan

      Hi Dave, Love the Blog :)

      I have never tried to work out thermal designs in engineering, I like you look at the datasheet that usually says you need a heatsink of X/W, and then just use a heatsink slightly better ;) or if I have the ability I use the case (assuming it is metal)

      Although I believe there is a thermal cad package out there, which can take into account all the various points of say a PCB with a multi layer stackup and then work out say a Dpak package which is dumping X watts into a PCB….. I personally just prefer to “Build it” and then measure heat, if the magic smoke comes out it needs more heatsinking, if it is not too hot to touch then it has enough ;)

      Regards,

      Kat.

    • http://kaqelec.dyndns.org Kathy Quinlan

      BTW I love LED lights, but people resist them as they are too “White” ie they have a high blue content, people prefer a warmer or redder light.

      Me personally would prefer to pay the power supply company less money ;)

      Regards,

      Kat.

      • karma

        Actually, nowadays power LEDs are avilable in various white flavours, often called, cool white, warm white, and so on.

      • Sean

        The latest offerings from one of the Phillips data sheets give you Kelvin ratings. Choosing LED’s will soon be like choosing T8 fluorescents. Even the CFL people are starting to wise up. Proper lighting color and rendition is rather important in work spaces and the technology is starting to catch up.

    • http://www.danielbjohnson.net Daniel / septer012

      Fun idea for you. Microcontroller up the Cree’s individually. Dimmer? patterns? audio sensor? proximity sensor?

      I was under the impression the Phillips Luxeons were better then the CREEs on the market.

      I actually own two Luxeons but haven’t tried them out as ive been missing the thermal adhesive material.

      I have an idea for a smart flashlight thats pretty rediculous: Luxeons mounted on a heatsink with a fan and thermistor:

      http://www.danielbjohnson.net/digital-flashlight-800-lumens/

    • JIm

      Oh dear I’m going to have to disagree.
      For 8 LEDs on a single heatsink the first 3 terms (LED,Base & Adhesive) should be represented by 8 parallel paths, only the heatsink is a single path. As your measurements show the heatsink temperature does not vary much with distance the approximation is valid.

      Secondly a cool heatsink does not always imply a safe design.
      Take the extreme case of a transistor with a 15W rating (at 25 C case) and a maximum junction temperature of 175 C mounted on a 0.5 C/W heat-sink.
      If the transistor now dissipates 15W, the heat-sink temperature will rise by only 7.5 C to 32.5 C but the junction temperature will be out of spec at 182.5 C.

      Jim

    • karma

      Hi Dave,
      good talk on LEDs.

      It would be nice to show how to perform easy datalogging and charting.

      What software can be used and a kind of demo would be really thankful.

    • http://blog.wickedengineer.de tobi

      …and a warning :

      avoid looking directly into the lightsource !
      i am fiddling around with a ostar led, running 24V at 750mA, producing massive heat and pumping out lumens !
      i blinded myself for several minutes when i was accidently switching it on when pointed at my face !

      …and dave you’re right: thermal design sucks and is often done wrong !
      the best thing: build the setup that fits into your desired space, measure the temperature under extreme conditions. getting hot ?! – add more heatsink area if possible or include a fan !
      measure again :)

      i’m an audio guy and thats how you do stuff in my business… listen and repeat :)

      keep up the good work !

      looking forward to have a beer with ya in the future :)

    • http://www.idea2ic.com/ Don Sauer

      The real question is lifetime. If junction temperatures are not getting too hot, lifetimes should be high. But spending the extra money to replace all your ceiling flood lamps with something like fluorescents does not make as much sense when the fluorescents are burning out twice as fast. The lifetime for LEDs appears to be rated from 10khrs to 50khrs.

    • Anthony

      Hi Dave,

      Really enjoy the video blog, I am a novice but still enjoy it.

      I had a question about the chart you showed with the efficiencies of the different color type LEDs. I was under the impression that the white LEDs like your using are actually a UV LED with the semiconductor having a phosphorus coating, and the white light is a product of the phosphorus coating reacting to the UV. So wouldn’t it work to just look at the UV LEDs efficiency to figure out what your LEDs efficiency is?

      Me and my brother discuss electronics and he is my source on how this high wattage white LEDs work. I guess the way to Identify them is the yellow color of the emitters.

      But I am a novice like i said.

    • walter delbono

      awesome…

    • bmwm3edward

      Love these blogs – Dave.

      Now, please tell me how efficiency (or is it efficacy?) varies with input voltage, with regard to temperature as a function of ambient temperature and humidity (humidity has got to play a role in heat dissapation, no?!)

      Just kidding.

      Thanks, see you next week…

      ~E

      • http://www.eevblog.com EEVblog

        There is no such thing as “input voltage” with LED’s, they are driven from a constant current. But yes, the voltage drop will vary slightly with temperature, around -2.1mV/degC which can be ignored in this application.

    • David

      I don’t think the calculation is difficult. Just the basic heat equation with 5 heat sources (or whatever the number of LEDs) and a massive conducting heat sink.

    • Lionel

      Thermal calc’s = guesstimate, then suck it & see. ;^)

    • Adam

      I have a question about the current used here: At full power, these LEDs use nearly twice the current of a 60 Watt bulb. Using 16 of them, even when they are only running on 700mA, adds up to 11.2Amps. That’s probably all the load you want to put on a 15Amp breaker. did you have to add a breaker line just for these lights?

      • http://kaqelec.dyndns.org Kathy Quinlan

        Hi Adam,

        Not quite, the led’s run at say 700mA, this is with a forward voltage drop of 3.2V (this voltage was taken from the LED data sheet), at this point the led use (W=V*I) 2.24W.

        If you had 16 of these led’s, depending on your power source, you could put them all in serries, with a terminal voltage of at least 51.2V with internal current limit, or about 60V with an external resistor (12 ohms 10W)

      • http://www.eevblog.com EEVblog

        That’s not how it works.
        The LED’s only take 2.2W each (remember, they only have a voltage drop of 3.2V), so that’s 35.2W total power for all 16 LEDs, or half of a single 60W incandescent bulb. Add in the efficiency loss in the controllers and it’s probably say 45W at most. At 240V that’s only around 190mA from the mains.
        So they are incredibly efficient, more than twice as good as CFL’s.

        • Blacksmithdog

          I understood Dave’s maths on the current draw and how he calculated approx 190mA, but I’m having difficulty understanding this (I’m a 40-year old young-player) and I’m hoping Dave can explain. How can there only be 190mA drawn from the mains if the LEDs require 700mA to drop 3.2V and produce the 2.2W?

          • AngeloQ

            Blacksmithdog, sorry this is late, but in case you are still wondering, the way this works is that the power supply is switchmode, not linear. So you have to think in terms of power, not current. There is a step-down regulator that delivers a low voltage but higher current than drawn from the mains supply. I hope that helps.

    • http://cvandewater.com Cor van de Waterf

      Hi Dave,
      Thanks for putting the spotlight on the amazing progress in LED lighting – I agree that they should get much more attention and use, in particular to reduce energy wasted while the longevity (especially under mechanical shock and vibration) is an added bonus.
      I am still wondering why you said that thermal modeling sucks, after you explained that you found data about the thermal behavior of an alu strip 3mm thick and 25mm wide saying it is about 3 deg C per Watt.
      If I plug in the waste heat of a single LED, assuming that the LEDs are far enough apart to not influence each others temp much, then I arrive at 4.2 deg temp delta. Taking the total power of all LEDs at 11.2W the temp rise would be 34 deg, so apparently the truth is in the middle – only the adjacent two LEDs influence the temp of each LED (total power 3x 1.4W = 4.2W) so total temp rise is about 12.6 deg C.
      By the way: in your model you included the Junction to Base thermal resistance in the total 13 deg difference you measured, but I doubt that you measured the junction temp. Most likely you measured the temp at the base of the LED. However, you can easily calculate the temp of the junction after measuring the temp of the base and the total power dissipated in the LED, knowing that the Junction to Base thermal resistance is 6 deg C per Watt and the heat dissipation is 1.4W it means that the Junction only runs 8.4 deg hotter than the Base.
      So, during your hot Sidney day with 37 deg C ambient the Base apparently runs about 50 deg C with the particular Alu strip you chose and the Junction will be well below 60 deg C. That means it is pretty cold and should last a very long time. Enjoy the lights and I look forward to more interesting Vlog posts. Cor.

    • spencer

      I think this is brilliant. I just bought some of the 6500k pure white ones. After looking at it, I can’t wrap my head around how you can get these to be dimmable. A microprocessor can use pwm for a normal low watt led, but this high watt led would burn out a micro processor.

      • http://www.eevblog.com EEVblog

        You can dim them either by PWM, or simply driving them from an adjustable current course.
        There are mains drivers available that have a dim-able output, like this:
        http://www.cutter.com.au/proddetail.php?prod=cut684

      • signal7

        @spencer: The answer to that question is to interface the microcontroller to the LED’s via a transistor or mosfet. Of course, the amount of power being switched is going to have an impact on the design of the switching circuit, but I don’t otherwise see why it wouldn’t be possible. In fact, if it’s decently designed it would be much more efficient in terms of power regulation than any current limiting resistor could/would be.

    • http://www.cutter.com.au Mark Riley Cutter Electronics

      the star boards are =< 2 deg C/W
      Cheers
      Mark

    • Brian

      Use caution when comparing CFL fixtures to LED solutions; the power factor (PF) of cheap CFL ballasts are terrible, often as low as 0.5. This means that the CFL that boasts its power consumption is only 25W, you are paying for 50W because the PF is so awful. You pay for dirty power.

      And Dave, don’t forget to add in the losses from the LED mains powered driver in your total power consumption, lest anyone forget that that effects the total solution efficiency.

      Great blog!

      Brian

      • Jason

        Residential customers pay by kWh. The 25W CFL with a low PF is still using 25W, not 50W. I’ve seen a lot of so-called “power saver” devices claiming to “increase your power factor”, which does nothing for your utility bill. Dan Rutter has a good article about PF correction here: http://www.dansdata.com/gz088.htm

    • http://citybeeblogs.org/theshineofyellow/ Quinton Sukovich

      catchy little title, LOL

    • http://www.compactfluorescentbulb.info Gracie Hayes

      Led lights are great because they are long lasting and consumes less electricity.*;~

    • http://www.steviaplantextract.com Ethan Jones

      Incandescent light bulbs will soon be phased out because they waste a lot of energy.’.~

    • Richard Borchardt

      Hi Dave,

      I recently discovered your EEVBlog and WOW! I’m super-impressed. Everyday at work I listen/watch a few episodes to catch up with what you’ve done so far. Well done on the informative, down-to-earth and practical information you present.

      Now, regarding the LED lighting you discussed in this blog – I have just two questions.

      1) How did you power the LED strips you constructed? Did you build a dedicated, low voltage DC power supply for them, are you PW-Modulating them or are you driving them directly from 240V AC via an unregulated PS or capacitor/zener-based voltage reducer? Can you provide details on the power supply.

      2) The heatsink. You used 3mm flat aluminium bar for mounting the LED’s onto. Would it be possible to mount them onto a steel flat bar or square tubing (25x25x3mm or 50x20x3mm or whatever) instead? What is the thermal dissipation of steel versus aluminium?

      I’m just thinking that using steel would be more practical in such an application – easier to come by (not really, but I have heaps of steel tubing lying around, whereas I’d have to go and buy the aluminium specially), and it seems a waste to sacrifice the aluminium for a couple of lights).

      Best Regards
      Richard

    • Peter Perez

      Dave,
      good day, I can’t watch this episode 50. Please can you give me info were can I watch this episode. Thanks a lot.

      • http://www.eevblog.com EEVblog

        It is just an embedded youtube video like all the others, there should be no problem watching it.

    • http://americanarchitectureservice.com/blog1 Gary Wheeler

      Brilliant (pardon the pun).

      Only qualification, though, is what happens when there is a flyspeck, insect, or bit of dust or feather on the element? Does it burn off, light up, or imbed itself into the plastic?

      Wonder if a bit of black tissue paper on the element would burn a hole, like a low power laser?

      In the worst case a larger glass or plastic lens could be put over the LED to keep objects a short distance away. There was problem with quartz bulb “torchiere” in the range of some hundreds of watts in the US 10 years or so ago, they ended up having to have a metal guard or something installed, in case someone left a paper on the top. Not always likely, but stranger things have happened.

    • Raj

      When I put together a 50W white LED setup for a DIY Projector, I just stuck a GPU heat sink and fan to the back of it. Runs touch cool (not the led, that thing goes through a couple Fresnel and can set fire to your hand), but the back of the LED. I see people stick massive heat sinks on the back of these LEDs and it’s crazy. A good GPU or CPU fan is the same width and height of the LED, dead silent and looks really funky. Some even through some more LEDs into the mix :P.

      I tried doing thermal calculations with it but it was one of those no name LEDs from china so no data.

      I suppose it would be interesting to see how easily these multiple source designs could model in Matlab or something.

    • chuck

      What earlier episode do you mention the temperature loggers (thermal cron eye-bun (sp?))

    • samgab

      to chuck… it’s the iButton.

      To Dave: How do you solder the XP-G’s? They seem to be mounted onto a sort of base. What is the base, and where can you get them?
      I’m making a ceiling bounce lighting system simlar to this, but I think I may have trouble soldering the XP-G emitters… Any help appreciated! Ta.

      • http://www.eevblog.com EEVblog

        You just solder a wire to the pad. The base is alloy for the heat. In Oz you get them from http://www.cutter.com.au

    • pierre

      Cree broke 230 lm/W in the lab and just put out a new part that’s above 140 lm/W, using Silicon Carbide… and it cots half the price of the previous ones ! Gotta love these guys.

      Also there are lots of lower-wattage but high efficiency LEDs that you can buy on pre-assembled aluminium PCB for diffuse lighting, more LEDs spread the heat and means less junction temperature rise…

    • K5tor

      One question,

      Wher is the video.???

      73 de k5tor
      torsten