Tuesday, October 15, 2013

Techy Tuesday - LED Lighting Doesn't Get Hot?

I've been replacing lights in the house with LED lights on sliding time scale.  When 800-ish lumen LED bulbs, which are equivalent to 60W incandescent lights, go on sale, I'll pick up one or two.  There are four in a fixture overhead right now, two in a Y outlet in a lamp in the living room (which together give more light than a 100W incandescent), and a couple in isolated fixtures.  I've removed all of the curly tube fluorescent lights I had and gone solid state.  

One of the claims you'll hear about LEDs is that they don't radiate heat like a regular bulb does.  Hot filament bulbs radiate in the same way black body radiators work, as the physicists say.  They produce more infrared (heat) than visible light.  Most of us have grabbed a hot light bulb before, so we don't need to be reminded!
(source)  But LEDs aren't black body radiators and aren't radiating by incandescence, so one of the common things you hear is that you don't have to be concerned about heat coming from the front of the bulb when you mount it.  Writing in the trade magazine EDN, Ed Rodriguez tells of an investigation into heat coming from the front of an LED. 
What you will further seem to know for sure is that the PN junction is the hottest point in an LED (or any other power semiconductor for that matter) with the LED mounting substrate and or heat sink being somewhat cooler. How much cooling is needed can be rather accurately calculated if you know a) the heat sink temperature (easy to determine) and b) the junction-to-case thermal resistance (easy to determine for the data sheet).

You likely agree with what you have just read.  Having been involved over several decades with silicon power semiconductor and power LEDs in one way or another I also would have agreed... until I did not.
He goes on to describe his surprise at burning himself on an LED he was working with.  The junction was certainly around 40C, and that's just barely above body temperature - it shouldn't burn skin.  So he measured with instrumentation and it read 125C!  A sliver of wax formulated to melt a 70C melted instantly confirming the measurement.
To further determine if this perceived high temperature was a “phantom”,  I proceeded to place over all devices at one time or another a small (1” by 1” by  .032) piece of aluminum.   Without exception, each became a hot plate, elevating the aluminum temperature (as measured by a thermocouple on the top illumination-immune side away from the light). Without exception, the temperature got to between 125-150ºC.
The article is techno-geeky to the extreme, but I know some of my readers will read and enjoy.  The bottom line is that despite the press, LEDs do generate infrared and radiate it out of the front of the light.  When LED makers were struggling to reach 1W input power, it was less noticeable.  With today's much higher powers, a few percent turning into infrared is noticeable.  Heat can affect phosphor life and therefore the life of any bulbs you buy.  And if you want to put an LED bulb in a sealed fixture, you should be aware of the heat.  A sealed fixture designed for a large incandescent bulb should be fine, because the LED will still radiate less heat than it was designed for.  But if you're starting from scratch, don't assume all the heat goes out of the metal base of the bulb or convenient heat sink.


  1. Ill be damned.

    I totally bought into the "LEDs dont produce heat" thing. Had to test this for myself. Walked into the kitchen and grabbed one of the GU11 3w LED bulbs I have and nearly yelped.

    Cooler than an incandescent, but still could burn if ignored.

    Oh, and if your looking for cheap LEDs, The Egg has all you need (I use Roswell and like the "Natural White", but YMMV): http://www.newegg.com/LED-Lighting/SubCategory/ID-2896?Tid=17125

  2. There is a reason the LED Bulbs I just put into the bathroom look like ordinary bulbs but have a BIG heat sink on the socket side!

  3. Hmm. Giving it very little thought, it would seem that the maximum heat would be proportional to the wattage used. I guess it makes sense they'd get hot to some degree (heh!) since power is going in, and not all of it turns to light.

    1. Just to be clear, we're talking about infrared coming out of the front of the bulb like the old-fashioned heat lamps. But, yeah, the heat was less noticeable with lower wattage bulbs. The new thing is that the mechanism that produces light in an LED is different from a filament and no one expected this to be happening.

  4. I purchased two LED replacement "bulbs" at two different times. Each one lasted less then a week. The "bulbs" cost perhaps 20 times as much as an old style bulb but they last 1/1000th as long.

  5. I've never run across anyone saying that. Mine are months to over a year old and I fully expect to get years out of them.

    I suppose a bad problem with the AC power could do that, but like I say, I've never even heard of that happening to anyone.

  6. it be safe to use a "60 Watt" LED bulb in a fixture calling for a maximum 40watt incandescent?

    1. Before this article, I would have felt comfortable doing that. 60W LED bulbs use about 8 to 10 watts, and the ratings on fixtures are usually for heat in the parts. If you're putting the bulb in a sealed fixture, say with a plastic globe over the bulb, I'd be less comfortable. In a lamp with a bare bulb and lampshade, I'd do it.

      Hope that's useful to you. Since you're replying to a nearly three year old post, I gather you're having a hard time finding an answer. Unfortunately, without getting some way of measuring the temperature out of the bulb, it's hard to know for sure.

      I put four of the curly compact fluorescent bulbs in a fixture overhead that's rated for four 60W bulbs. Those use about 15 Watts, so 1/4 of the fixture rating. The fixture was fine, but every one of those burned up within less than two years. They didn't catch fire openly, but they all smoked and stank like burning electronics. I put another one in a ceiling fixture, so it hang upside down. Those burned out in a few months.