Gas station without pumps

2016 February 4

Kitchen lighting

Filed under: Data acquisition — gasstationwithoutpumps @ 19:43
Tags: , , , ,

My wife has been unhappy with the lighting in the kitchen for many years now, so I finally got around to designing a new lighting solution for the kitchen and hiring  a contractor to install it.  We previously had rather ugly fluorescent tube fixtures over the stove and over the sink. The over-sink lighting looked like it was done in the 60s and had 2 20-watt tubes; the stove lighting was a replacement in the 1980s and had 2 40-watt tubes.  The lighting on the countertops was never very good, and nothing illuminated the interior of the cabinets.

My wife thought that can lights in the ceiling might be a good solution, but I didn’t care for putting that many holes in the already poor insulation, nor for the very high price of can lights. Instead, I proposed a series of little LED pucks along the series—the sort of puck lights that are usually used for spot illumination under kitchen cabinets.

LED pucks are available in a wide range of prices, and I tried out three different ones to see which gave the best light and looked least offensive.  Then I didn’t buy more of those, because the lighting store wanted $40 per puck, not including the non-standard connectors needed for wiring it up.  Those pucks claimed to have 240 lumens output, and I wanted about 3500 lumens for the kitchen, which would have meant 15 pucks ($600).

Instead I went with a cheaper set that was easier to wire up but only had 165 lumens per puck, needing 22 pucks. I bought 8 sets of 3 LED pucks from TorchStar through Amazon for a total of about $240.  I printed a bunch of circles the same diameter of the pucks and laid them out on the ceiling, adjusting the spacing and the distance from the walls to avoid shadows. After we’d lived with the paper circles for a week, I called in my favorite contractor to install the pucks. (Of course, the labor charges for patching the old holes in the ceiling from the previous lights, repainting, installing the new lights, and replacing the wooden soffit panel where one light was removed far exceeded the cost of parts.)

 

There are 11 pucks on each side of the narrow kitchen, lined up across from each other.

There are 11 pucks on each side of the narrow kitchen, lined up across from each other.

I did not use the power supplies that came with the pucks, which are only adequate to power 3 or 4 pucks each, and which seem to be very cheaply made (and likely unreliable).  The provided power supplies take 2.5W for lighting one puck, 4.5W for 2 pucks, and 0.3W when not providing any power.

I replaced the power supplies with 2 MeanWell SGA40U12-P1J 12V power supplies each on a separate wall switch and each powering 11 LED pucks.  MeanWell claims an efficiency of 86.5% for the power supplies on their data sheet and considers these supplies “high reliability” supplies. Each power supply takes 21.7W  with a 48% power factor, according to my KillAWatt meter. A 48% power factor is pretty low, but seems common for small switched power supplies—larger supplies often have power factor over 0.9, but that requires more expensive circuitry. The 21.7W/11 pucks means that each puck accounts for 1.97W from the AC supply.

Each puck supposedly puts out 165 lm with 3000°K color, for a total illumination of 3630 lm for the kitchen, which I find a bit too bright, but which my wife finds about right—she’d like the light to be a bit yellower, though. I couldn’t find 2700°K LED pucks that were well made at a reasonable price, so we had to go with these 3000°K lights, which are OK, but not as soothing as 2700°K lights.

If the rated light output is correct, then we’re getting 83.6 lm/W, which is a pretty good efficiency—according to Epistar, these would be their premium chips (120 lm/W), not their standard chips (100 lm/W), if the system efficiency is over 80 lm/W.  (Of course, I’m having to trust TorchStar about how bright the chips are—I don’t have a calibrated illuminance meter.)

The pucks each have  18 Epistar 3528 SMD chips—in 6 chains of 3 LEDs each with a 150Ω current-limiting resistor on each chain. I was interested in characterizing the I vs. V curve for the pucks, and (with a little computation) for the individual SMD chips.

I measured the voltage and current at a number of operating points by hand with a cheap DT-9205A multimeter, and set up a circuit for measuring the current and voltage using a Teensy board and PteroDAQ.

The first circuit I tried was not successful—the LED puck would not turn all the way off! I looked at the waveforms for the voltage and for the current with PteroDAQ, and determined that there was 60Hz pulsing when the LED was supposed to be off. I think that I had a ground loop problem among the three power supplies that was amplified by the bipolar transistor.

The first circuit I tried was not successful—the LED puck would not turn all the way off! I looked at the waveforms for the voltage and the current with PteroDAQ, and determined that there was 60Hz pulsing when the LED was supposed to be off. I think that I had a ground loop problem among the three power supplies that was amplified by the bipolar transistor.

My second attempt used the transistor in a different amplifier configuration:

Here the emitter voltage follows the base voltage, and I had no problems with ground loops—there is no voltage gain from the NPN transistor, just current gain.

Here the emitter voltage follows the base voltage, and I had no problems with ground loops—there is no voltage gain from the NPN transistor, just current gain.

I did have to play around with the voltage swing on the function generator, as the base current was larger than I expected and the 50Ω output impedance of the function generator did seem to matter. I changed the 20Ω resistor to different values to get different current ranges—from 0.5Ω (which resulted in data too noisy to be useful) to 10kΩ.

Here are the I vs. V curves for the puck as a whole:

On a log scale, the different ranges of measurement for the different current sense resistors can be clearly seen. I cut off the low-voltage end of each set of measurements, where the noise got to be too large.

On a log scale, the different ranges of measurement for the different current sense resistors can be clearly seen. I cut off the low-voltage end of each set of measurements, where the noise got to be too large.

On a linear scale, the difference between the hand-held multimeter and PteroDAQ measurements is visible, and the roughly linear increase in current with voltage over a threshold is also clear.

On a linear scale, the difference between the hand-held multimeter and PteroDAQ measurements is visible, and the roughly linear increase in current with voltage over a threshold is also clear.

I can rescale these plots to remove the effects of the serial and parallel connections and of the 150Ω current-limiting resistors, to get an approximate average I-vs-V plot for a single Epistar SMD chip:

The operating point for these chips on the puck is 24mA, which means that the chips must be the "premium" Epistar line, which goes up to 30mA. To get 30mA, we'd need 2.93V, making these chips 88mW chips.

The operating point for these chips on the puck is 24mA, which means that the chips must be the “premium” Epistar line, which goes up to 30mA. To get 30mA, we’d need 2.93V, making these chips 88mW chips.

 

Going back to the puck as a whole, at 12V the current would be about 139mA, for a power of  1.66W, which means that the power supply is operating at an efficiency of about 84%, slightly lower than the rated 86.5%. But the current-limiting resistors are causing a voltage drop of about 3.475V, so the puck is only 71% efficient.  The combined efficiency of the electronics is about 60%.  If the 165 lm output of the puck is correct, then the individual chips would be putting out about 140 lm/W, which is pretty impressive—I suspect that the 165lm is a slight exaggeration, as it is difficult for the customer to measure and complain about.

 

5 Comments »

  1. This is a fascinating project – thank you for posting!

    I, too, don’t normally like the color that LED lights output. I read somewhere that they’ve got a single, pure element that’s actually radiating the light and thus with LED lights you’re getting a single wavelength (or a small number of single wavelengths), whereas the incandescent bulbs tend to not be quite so narrow.
    Anyways – I was wondering about the idea of using an LED to supply the light, and then putting something over it to change it’s color (e.g, a piece of paper, or one of those ‘gels’ they use in theater productions to change the color of spotlights). The idea is that one could change the color and/or change the wavelength distribution awa from being a spike and into being more of a bell curve.
    (Is the thing about the single wavelength(s) true? Does the bell curve idea make sense?)

    (Also – my understanding is that LEDs put out very, very little heat so it should be safe to put gels/paper over them for long periods of time)

    I thought I’d mention it because you’ve got a theater background so might be able to find and try out a gel reasonably easily

    –Mike

    Comment by mikethetall — 2016 February 7 @ 13:02 | Reply

    • Single-color LEDs do indeed have a fairly narrow bandwidth (generally given in the specs—for example, the wavelength of LTL-4273 is 600nm±35nm).

      I don’t have a theater background (my son does), but gels are easy enough to find online if you want them. Adding a gel to an LED doesn’t help, as the gel just absorbs some wavelengths—it doesn’t emit light in wavelengths that are missing. All that putting a green gel over a red LED would do is block the light.

      White LEDs consist of a blue LED (around 460nm) surrounded by a coating that contains green and red fluorescent molecules. These are selected to have a very broad response, and the amount of each can be adjusted to give different color balance to the emitted white. You can see spectra for a few different color temperatures on the Luminus MP-3030 datasheet, the 1W white LED I’ve used for desk and table lamps.

      LED lights for theater use don’t use gels (subtractive color), but use additive color mixing—lighting up red, green, and blue LEDs (sometimes violet, yellow, or other colors as well) to get the color desired. The most common lights are RGB or RGBW: red-green-blue or red-green-blue-white. The addition of white LEDs gives better color rendition than just RGB, because the fluorescent molecules of the white LED have a wider bandwidth than the red and green LEDs.

      Low-power LEDs put out little heat, but LEDs can still get hot. The 100W–150W theater light we are planning is going to need heatsinks and fans to keep it cool enough not to burn out the LEDs. Even the little 1W LEDs I used for the desk lamp needed heatsinks. If LEDs were 100% efficient, they would produce about 683 lm/W (well, that’s only true for green LEDs—100% efficiency is fewer lm/W for other colors), but existing chips and systems are around 60–150 lm/W, so only 10–25% of the power in comes out as light—the rest is dissipated as heat.

      At very low power, LEDs can be very efficient (there is a claim of over 100% efficiency, converting some heat into light at extremely low power levels), but they get less efficient as they get warmer, and practical LED lights have to get rid of a lot of heat to remain reasonably efficient.

      Comment by gasstationwithoutpumps — 2016 February 7 @ 14:51 | Reply

      • I’m glad I asked – this is a really informative answer! :)
        Thanks!

        Comment by mikethetall — 2016 February 8 @ 14:30 | Reply

  2. […] than fluorescents, and LED fixtures are often more expensive than fluorescent ones (though the LED pucks I put in my kitchen cost very little for the fixtures—all the expense was the labor of patching and painting the […]

    Pingback by Make magazine | Gas station without pumps — 2016 March 19 @ 12:01 | Reply

  3. […] lighting in the cove around the skylight in bathroom replaced with LED strip lights. Unlike the kitchen LED lights, I did this project entirely […]

    Pingback by LED lighting for bathroom | Gas station without pumps — 2016 July 4 @ 20:52 | Reply


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