Electro Bob

Why?

About half a year ago I installed a 3 meter warm white led strip above my desk. Although I didn’t spend a lot of money on it I am really happy with the light quality and intensity. Lately I was thinking about adding more LED strips and was checking the options. One important aspect that comes to mind is about the lifetime of these products, are they better than bulbs, overall? Obviously there’s no specification you can trust anywhere. So, how do I know if cheap strips are good enough or it is worth spending the money on more expensive ones? Measure some data for the cheap ones and see if they are good enough. The experiment was designed to be simple, not extremely accurate.

Update:

The measuring method has been criticized, and some of the claims are fair: there’s no control and the sensor may degrade. I have stopped the experiment after 1400 hours to upgrade it. But first, i replaced the measured strip with an identical, unused one and the sensor reads exactly the same as in the beginning. So, there is NO sensor degradation. Later, when the measurements will reach the 70% threshold i will replace the strip with another identical unused one and compare the sensor readout. This should eliminate the sensor variable and provide control. See new experiment, LED logger v2.

The Logger

I setup a very simple device, for first measurement attempt: a metal project box houses 15 cm of unused strip that shines light on a TLS2550 sensor, everything being isolated from ambient light. The strip is placed on the aluminum front panel of the box, providing a great amount of cooling for the strip, much more than real life applications where it could be mounted on wood or walls. The box is sealed so there’s no ambient light going in of dust.

This sensor is really great: it has I2C interface, high dynamic range and approximates the human eye response, giving the result directly in lux (after some math). I’ve used a microcontroller and an EEPROM memory with a couple of years of space, storing data every 6 hours. There’s a serial port that provides a simple interface with an instant read, memory erase and memory dump. Current is measured over a 2 ohm resistor and stored as ADC counts. The used board is Arduino compatible, but it was not used with the Arduino environment, just your regular C.

Results:

The light has been on continuously for about 700 hours, almost a month 1400 hours, two months.. The graph shown below will be updated periodically.  So far, the LED strip has dropped about 12% 18% in brightness compared to the beginning. Current has stayed the same, meaning that they are actually reducing their efficiency.

Although it is too soon to tell exactly I decided to try to predict the life of the LEDs. Since they don’t burn but rather fade, I think that the point where they dropped to 70% of initial intensity is a good mark of their end of life in applications where they are used for lighting and not decoration.

Using an exponential projection I found that it will take just 2200 hours until the LEDs drop to 70% of their initial intensity. This is rather disappointing, even for cheap LEDs. In about 2-3 more months I should know for sure how good this initial prediction is, but I doubt it will be too far from the truth. UPDATE: after about 1400 hours of use, the prediction still stands, the strip will fall to 70% around 2200 hours.

How bright is a LED strip?

A while ago I made an experiment which tried to find out how much light a 3m led strip could give. Obviously lacking any specialized equipment I tried to compare it to some references. I used a room and cut the strip in 50 cm pieces and grouped them on the ceiling. I used my camera to determine the correct exposure of the whole room. Next I proceeded to replace the LED strip with bulbs that could give me the same exposure. I found that I need a 25W and a 40W incandescent to arrive to this. According to the bulb packages they each give 200 and 400 lm, totaling 600 lumens.

Using my modified power meter I found that the strip consumes 14.3W from the mains. This means that the LEDs are about 42 lumen/watt, which, as an idea, is comparable to the worst CFLs and is still rather poor for LEDs. But, do remember that this takes into account the PSU, which is the right way to evaluate for practical reasons.

LED strip vs incandescent: Preliminary conclusion

Based on the price I paid for the LED strip (~4.5 EUR/m), local electricity price of about 0. 09 EUR/KWh and bulb price, but excluding the LED PSU and workmanship for installing everything it takes 2800 hours for the LEDs to become cheaper than the incandescent.

Since my strip is not considered useful after 2200 hours as it becomes too dim, it means that it is not a cost effective solution for illumination, compared to incandescent bulbs. LEDs are supposed to save energy and money. It appease that low cost strips are not a good way for that.

What about more expensive LED strips? I used the same thinking as above and found more expensive strips with Nichia LEDs require about 7000 hours before they become cheaper than incandescent, again excluding the PSU. This is a rather rough estimation considering the brightness and efficiency specs are right. Again, the manufacturer of the strips doesn’t give any data about the intensity decay, so the cost effectiveness of the solution is still unknown.

Download

The code for the logger is available for download here. It’s free for non commercial use and rather unpolished.