Friday, November 07, 2014

Trying out a cheap Chinese 100W LED array

Recently I found that Chinese 100W LED arrays cost less than $5 on eBay, so I had to try one out. The LED array arrived in less than a month in a padded envelope with no other protection. This is not right, because white LEDs are static sensitive, but I guess you can't expect much for $5, and the LED works. Here's the LED at very low power:
Note how the LEDs have unequal brightness. One whole row is brighter, presumably because the dark LED in that row has greater forward leakage and a low voltage drop across it. This all looks bad, but it's normal at low power. Using higher current and a low PWM duty cycle would probably produce more uniform results, if that was needed.

The biggest heat sink I had was a Slot-A Athlon heat sink. After mounting it the first time, without paste just to check the fit, I could see light between the LED and the heat sink. There were high spots at the plastic-filled holes and slots in the LED's metal plate. I don't think the plastic was high; it seems more like the metal was distorted by punching operations. The whole thing was also warped on a larger scale. After sanding it with fine sandpaper on a piece of glass, I got a much better fit and mounted it.
Note how the LEDs have very similar brightness now at higher power. The LED is a bit dirty from Brasso, which was probably unnecessary. Fine sandpaper was good enough.

Then there was the question of how to drive the LED. I have a power supply from an old Fujitsu SMD hard drive, with -12V and +24V outputs, giving 36V, which is more than enough for the LED. I set up some primitive linear current regulation, using a 0.22 ohm resistor and Vbe of a small transistor. The transistor controlled the gate of a power MOSFET with a big heat sink. This works surprisingly well, though note that Vbe changes with temperature. It regulated the current to 2.7A and I measured 35V accross the LED.

The heat sink required some serious airflow to stay cool enough that I could keep my finger on it indefinitely near the LED. Fortunately that same SMD hard drive had two 8 cm fans, to which I added some cardboard ducting to concentrate the air. The power supply even monitors the fans, not via RPM but via thermistors which are cooled by airflow and heated by resistors.
The light coming from the LED is surprisingly hot. My fingers quickly feel burning hot within a few centimetres of the light emitting face, and heat can be felt much further away. It's not the soothing heat of an incandescent, but something more like the feeling of steam escaping from a pot. I guess that's because the infrared light from incandescent bulbs penetrates deeper than visible light.

The LED is also incredibly annoying to look at, and worse than the sun or even badly aimed HID headlights. It's totally unacceptable to have this LED even in the farthest part of my peripheral vision. At the same time, the room doesn't really feel very bright.

In the past I was thinking of retrofitting a powerful LED into a halogen torchiere which had a bad bulb socket. I didn't do it because I wasn't sure I could deal with the heat in an acceptable way. Two high speed fans are fine for experimentation, but a light I use every day should be fanless or at best have a slow speed fan. Instead I ended up fixing the torchiere, making new socket contacts from brass pluming screws.

Comparing light from the 300W halogen torchiere and the 100W LED operating at around 90W, I definitely prefer the torchiere. It makes the room seem brighter even though the LED might actually be a bit brighter. It's hard to compare brightness due to different colours and lamp positions, but I don't think this LED is capable of 9000 to 10000 lumens at 3A. The LED's colour is also somewhat weird. Colours which would normally seem close to white seem yellowish or purplish.

Overall, I'm not too impressed. The light I'm getting doesn't make me want to create a more permanent lamp or flashlight using this LED. It was fun to play with though, and certainly worth the $5.

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