Variable Colour Temperature Lamp – VCTL Mk2

Several years ago I made a lamp for Graham, my artistic brother. This was actually  lamp with 2 LEDs; warm – 5300C and Cool – 6300C. Both were 2000lm. They were both adjustable in brightness by potentiometers so the effect was variable colour temperature. A simple PWM circuit controlled the power to the two lamps.

What use is this? My brother sells prints of his pictures which involves taking a photo of them. If you take the photo in warm light it looks too warm when displayed in warm light. The same effect with cool light so you take a photo in cool light to show in warm light and vice versa.

Problems: When moving house Graham dropped the lamp down the stairs – complete wreck. Well, it was only an experiment anyway :(. Trouble was Graham liked it. He hasn’t complained that it hasn’t been replaced yet – he wouldn’t.  Also the original lamp flickered with some settings so a bit of redesign was needed.

My replacement is only a little bit more sophisticated and has taken a long time to produce as I spend a lot of time in the garden and walking the dog.

I was going to go completely mad and use a micro controller.  Partly to annoy Graham who believes in KISS (Keep It Simple Stupid) as do I really.  I succumbed.  The end result used a 7555 timer, as it is more reliable than the 555, driving a dual op-amp from pin 6 which on a 12V supply gives a 4V pk-pk sawtooth wave.  For the op-amps I used an LMC6482 dual amp as I have a few of them in my bits box and they’re pretty good.

Choosing the brightness of the lamp involves a couple of choices:  The power of the lamp being the main concern.  I am using 4 off 12V 10W COB lights; 2 warm and 2 cool which I obtained from Deal Extreme but you can go up to 200W if you use adequate heatsink and power supply.  If you are going to upgrade the power you will probably have to use a different MOSFET as well.  Deal Extreme can supply both the LED’s and the power supply.

The LED’s need a heatsink and this will determine the physical design of the whole lamp.  Heatsinks can be a bit of a dark art.  Trial and error come into it a lot and it is always best to err on the conservative side.  The heatsink was the most expensive part of this project and in many respects the most complex:  You can use a piece of aluminium sheet or for the lower powers you can buy one online.  I advise that you get one of those cheap infrared thermometers so that you can see how hot it all gets.  50°C is a good maximum temperature to aim for though you can go up to 70°C with slightly reduced reliability. You will need heatsink compound between the LED and heatsink otherwise the LED will get very hot and the heatsink will do nothing.  A rough guide to the size of the heatsink in cubic centimeters can be made by multiplying the power by 10; so, a 100 Watt LED array would need a 1000 cc heat sink.  This will keep the heatsink below 70°C if the air temperature is 20°C.  This is a bit hazy as a specification if you are using aluminium sheet.  As a guide my 40W LED’s needed a sheet of 2 mm thick aluminium about 25 cm on a side.  If you are going for much more power you will need thicker aluminium as well as a larger area.

There are ways to reduce the size of the heatsink.  One is forced air cooling: If you blow air over the heatsink with a fan then a really fierce and noisy fan can reduce the size by a factor of 10, and a decent, moderate fan can reduce it to a quarter.  If you increase the ambient or reduce the maximum temperature, then you will need a bigger heat sink.  The key is the difference in temperature between the ambient and the heatsink.  The above calculations are based on a difference of 50°C.  If you really want to go into it then you buy a heat sink rated with a Өj of so many °C/W from technical supplier and pay.  Calculations aren’t that difficult it’s getting the heatsink specs in °C/W.  When it comes down to it it’s still more of an art than a science.

The lamp will be able to run off mains or a 12V battery. When both are switched on then the battery will be charged from the mains. The battery can be either a backup (car) battery or much preferably a deep discharge (leisure) battery. At full brightness my 40W lamp takes about 3.5Amps so a 60Ah battery should last about 15 hours; if it is a leisure battery. If you try this with a car battery it will become useless fairly quickly as they are not designed for deep discharge. You can however get away with discharging them to about 40% repeatedly. To make your 60Ah car battery last without dying on you, you need to treat it like a 36Ah battery.  Connecting the battery, power supply and lamp is quite simple though I recommend that you put  rectifier on the 12V output of the mains power supply so that the battery doesn’t operate the power supply crowbar.

The controller and the lamp will be connected together by a 2 m long 4 core cable with plug on the lamp end

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