I wanted some LED up-lighters for 3 of the rooms in my house that have vaulted ceilings – I like the diffuse and non-glare lighting you get from these type of lights, however I couldn’t find domestic products that would suit -aside from bespoke commercial lighting companies where you could be paying $$$ for each unit!, so I decided to DIY them.
After a search of the market I chose these Bridgelux LED modules which looked to be suitable and were so relatively cheap that I decided it was worth buying some and experimenting with them to see if they would be fit for purpose since at the time (post Brexit announcement – I’m UK based) they were a little over $3 each in one-off quantities.
Basic specifications are –
CRI 90; 1000 lux @ 350mA; 116 lm/W
These LED’s are 4000K colour space, or ‘daylight’ in colour which I personally prefer since it mimics natural daylight rather than candle or incandescent lighting which I know we humans are used to but which lets face it are hangovers from the days when we didn’t have much choice as to light rendition after sundown.
Another feature of these LED’s which differentiate them from most that High St stores sell is that they have a high Colour Rendition Index (CRI) of around 90, where a CRI of 100 represents a perfect black-body radiator. What’s more, because they work by exciting a phosphor coating covering the LED emitters, they can be dimmed with no loss of colour rendition.
In contrast typical LED lights sold to the general public are RGB based, where you can change their colour by biasing each individual LED, however because each of the individual LED’s have different power <-> lumen output curves, if you say balance the colour to suit your tastes at one particular light intensity, and then dim or brighten the lights by altering the power across all 3 LED’s the colour spectrum will shift and in some circumstances this quite dramatically changes the overall colour of the light.
So having chosen these LED’s, I needed to find some suitable way to power them –
LED’s are constant current devices, so typically you would have a mains to low DC voltage converter, and then a DC-to-DC constant current device to limit and control the brightness by adjusting the drive current through the LED.
Again after a quick search I found these controllers and power supplies which fitted the bill
Meanwell electronically power factor corrected Power brick ELG-100-24B
From the manufacturers spec sheet the LED’s need –
21-32V forward voltage at a maximum of 700mA to drive them to maximum output, so one of the constant current controllers could safely power 2 LED’s, and the 96W power supply is obviously overkill, but if I have say 4 x 20W-max LED’s per room and 3 rooms then allowing for diversity that PSU should be sufficient.
To explain the diversity decision, the 3 rooms I have to illuminate are a living room, a bedroom and a 2nd bathroom and so I figure that the chances of any two or in extreme all 3 being required ‘on’ at the maximum illumination and at the same time quite low, especially since the max rating of 4 LED’s @700mA represents over 7.5K lumens per room, which in ‘old money’ is equivalent to similar lumen density per room as 800W of incandescent (tungsten) light bulbs – i.e. very bright!
One feature I hadn’t considered is that the DC->constant current controllers can not only be controlled by analog voltage, say from a potentiometer but they also support Pulse Width Modulation – PWM and hence could be controlled by something like a Raspberry-Pi or similar SoC
So the bottom image shows how one can power 2 of these LED’s from a single constant current regulator.
Next I’ll explore how we could use a Raspberry Pi to control the regulator current and hence brightness