Choosing a heatsink
The lamps will each consume 10 Watts (about 1 Amp at 10 Volts). These LEDs use about 30% of the electrical energy to produce light, with the remaining 70% converted into heat. So the four lamps will create 40 x 0.7 Watts, or 28 Watts, of heat. Generally, heatsinks are rated as degrees Centigrade per Watt. I found in previous tests that around 70 degrees is a good maximum temperature to aim for. It’s not so hot that it damages fingers if it gets touched briefly and it’s well below the absolute maximum for components, which is usually 120-150 degrees. So, with a room temperature of, say 30 degrees, that leave a maximum increase of 40 degrees. If the heatsink is being heated with 28 Watts then it needs to be no more than 40/28 = 1.43 degrees / Watt. It must also be reasonable compact (e.g. not very long and thin) and have a large enough flat area to mount four 24x44mm LEDs.
Looking in the RS catalogue, this seemed a nice one. It’s 10×12 cm and 3.7cm high, costs £7.32, and has a nice flat base on which to mount the LEDs. However, there is one little problem. With the LEDs on the base, pointing upwards (it’s an uplighter!) the fins will be horizontal. Although it’s rated at 1.1 degrees/Watt (which is better than the 1.43 we calculated, and would only raise the temperature to (28*1.1+30) or 61 degrees, that rating is for the fins being mounted vertically. Mounting them horizontally will reduce their effectiveness and increase the temperature. How much? That’s very hard to calculate. Experience suggests that it may well stay below the 70 degrees I decided as a maximum, but I would need to try it to find out.
So I started looking around for alternatives. I found this on ebay for £6.81. It claims to be an “ARCTIC Apline M1 Passive – 40W Silent CPU Cooler for AMD (AM1 Platform)” It measures 9.4×9.4cm and 7.9cm deep. I liked it because it’s about 10cm square, rather than 10×12, meaning a round lamp can be 2cm smaller. It also has more material, so will heat up more slowly. I have in mind that, even if it can’t handle all 28Watts continuously, it would still allow a “dawn simulator” function to work for an hour or so – like a regular alarm, it only need stay on for a while. (Of course, in this case it would need a temperature sensor to reduce the power / turn it off if the temperature does rise too much.)
As described in a previous post, there are two kinds of heatsinks, the black ones with widely-spaced fins, designed for convection cooling, and the silver ones with closely-spaced fins, designed for fan-assisted cooling. So, despite the eBay one being advertised as a “Silent CPU cooler”, it’s designed to have a fan! Nonetheless, I decided to use that one. We will find out at the end of the project whether it was a good choice…
Building a frame
There needs to be some kind of frame the hold the heatsink and circuit board in place, with respect to the shade (the plastic pipe that’s used as the outer case). Previously I had used 1cm L-shaped aluminium strip to build the frame for the Bearing Witness project. I had a length of that remaining, so decided to build a frame from that.
The heatsink needs to be held horizontally, about 10 cm from the top of the shade. Since the shade is 35 cm high, that means there needs to be a 25cm long support. Except, we need to allow air to circulate, so the shade needs to be held away from whatever it’s resting on. I cut four pieces of aluminium strip, each 40cm long (10cm to screw onto the heatsink +25 for the remaining length of the shade +5 to give it some “ground clearance”. I screwed two pieces onto each side of the heatsink, and added three small pieces in an “H” shape towards the bottom to brace it.
So, this is the basic unit. The shade, being basically a long tube, will fit around this frame with a few mm to spare. (See drawing – looking from above.) The main PCB can be attached to one of the uprights, the power and clock data sockets can go in the horizontal bar at the bottom. The series resistors for the LEDs can be attached to the vertical struts [Later note: there wasn’t enough ventilation to put them there, so they were moved to above the LEDs]. The four 10W LEDs (red, yellow an 2 x white) will be attached to the top, pointing upwards.