Little LED Project

[rustyruss]

Good Evening everyone,

I'm making a little project, pretty simple, its a LED light for underwater.
The LED cobs are going to be reasonably powerful, Im looking at the Cree LED CXA2540-0000-000N0HV250H, XLamp CXA2540 White CoB LED, 5000K
I want to add a Thermal Couple to the LED on the TC solder point, and monitor via a Picaxe. Couple of questions,

1) What type of TC should I use with a Picaxe?
2) What would be the best way to link the Picaxe TC circuit to the power supplier to throttle the power should the LED temp get to high?

I want to power a total of 4 lights, which I calculate as 75w each, so 300w total.
Im thinking of using either 4 of these - Mean Well LED Driver, 36V Output, 75W Output
or 2 of these to power 2 at a time - Mean Well LED Driver, 36V Output, 150W Output, 5A Output
Do these drivers seem suitable?

Another option I was thinking was, could I run all 4 off a PSU like this - AC/DC Enclosed Power Supply (PSU), ITE, 1 Outputs, 320.4 W, 36 VDC, 8.9 A


Many thanks in advance
RH

 

[inglewoodpete]

You don't mention the maximum temperature that you want the LEDs to operate at. If using a PICAXE, it would be easier to use a DS18B20. You say you are operting the LEDs underwater - a modest heatsink should keep the temperature stable anyway.

Personally, I would simply use a current regulator with no added 'smarts', but those Meanwell LED drivers already have that built in. In my experience, Meanwell power supplies are pretty reliable. I've used them in some pretty tough conditions.

 

[rustyruss]

Hi Inglewoodpete,

Thanks for the reply.

I was thinking of putting the TC on the given solder point of the LED, do you think this is overkill, and to simply put a DS18B20 with in a drill hold of the Heatsink Housing. The main purpose of the TC circuit is if the water drops below the light, convective colling simply will not be good enough to prevent these from over heating. So looking to add this protection.

I would like to keep the LED sub 35C to ensure good life span. As you mention, given the thermal mass of the water, the cooling will be very efficient, and the Ta of the water will rarely get above 20C.
If I want to push the light output to the upper capacity, I note, and Im very much an elec noob, that the LED curves suggest the voltage needs to run at 40 / 41 when at 25c Tj. Do I have that correct, and if so do I need to look at a LED driver with 41v output and not 36v??

Many thanks

RH

 

[inglewoodpete]

I think a 35C limit is a bit low. I would allow LEDs to work up to around 50C, otherwise there would be occasional summer nights (here in Australia) when they would not be lit until 10 or 11PM, when the ambient temperature drops below 35C!

The voltage across an LED is more about the junction temperature than the applied voltage. While incandescent lamps were very voltage dependent, LEDs are controlled by current. So you need to be able to regulate the current being applied to an LED. You can't safely apply 36 volts to a "36 volt" LED without current control. As the LED heats up, the voltage across it drops reduce causing the current to rise, resulting in thermal runaway and ultimately self-destruction. If you connect two or more LEDs in parallel without current limiting on each LED, one of the LEDs will heat more than the others resulting in thermal runaway before failing. Then another LED will heat and start 'hogging' the current before it, too, self destructs. And so on.

I limit the current of high-powered LEDs to 80% of their rated current using a current regulator circuit. There is very little discernable reduction in the light output. Using a simple series resistor (like you might use on 3mm or 5mm LEDs) is NOT suitable for high-powered LEDs.

You will see that the Cree XLamp CXA2540 Series is a LED Array. Under that yellowish covering are threads of LED elements. Each LED element will drop about 3.5 or so volts, so the CXA2540 probably has around 10 LED elements in each thread. With the specified maximum 2.1 Amp drive current, a CXA2540 could have 10 or more threads of LED elements.

 

[rustyruss]

Thank you for the reply. That all make sense to me, much appreciated.

Would I be better, based on 4 light units, to use a PSU like this AC/DC Enclosed Power Supply (PSU), ITE, 1 Outputs, 320.4 W, 36 VDC, 8.9 A, and make a small current limited for each output (1 per LED), or would it be safer to use the Madewell LED Drivers?
Just thinking the PSU would be cheaper at £90.00 vs 4x £43.87

Many thanks
RH

 

[inglewoodpete]

I have not personally used Meanwell LED drivers. I have used many Meanwell power supplies on commercial artistic projects, though.

I had a speed-read on the Meanwell MDC-80 specifications and it seems to meet your requirements. By the time you have built, housed and tested a current limiting LED driver for each LED, the MDC-80 could possibly be a better solution. It depends on the purpose of your project and your budget.

 

[rustyruss]

How would I combine the thermal protection with the LED driver. Would I need to make a circuit after the driver to cut the power if it goes above the temp limit?

 

[inglewoodpete]

Having a heatsink that is undersized for air cooling is problematic. To me the simplest solution would be to use a heatsink that is large enough for air cooling. Even with thermostatic control and with no water for cooling, the system would turn on, heat up to a threshhold, turn off until a lower threshhold is met and turn on again and continue to cycle off and on until water becomes present again.

I have successfully used the attached circuit which I developed for high power RGB lighting control in public sculptural artworks. This used 3 x PWM channels from a 28X2, one for each of the LED colours. While intended for use with PWM, it could be used to simply switch the a large LED on and off. Q4 is a P-channel MOSFET, which requires a gate voltage more negative than its source pin to turn on. Current limiting is performed by Q44: when the current through R47 and/or R48 develops a voltage that reaches Q44's Vbe turn-on threshhold, it causes Q44 to start to turn on, applying a more positive voltage to Q4's gate. This causes the MOSFET to start to turn off. Q4 and Q44 reach an equilibriam at the calculated (designed) current. At this point we have the maximum current controlled. Switching on and off is controlled by Q41, Q42 and Q43. With the input (marked as PWM control input is the diagram) pulled low, Q41 is turned off so its collector voltage rises resulting in Q42 turning on. Turning Q42 on pulls its collector low, turning on the PNP transistor Q43 which in turn applys a strong +ve voltage to the gate of Q4 resulting in it turning off and cutting the drive durrent to the LED. Applying +5v to the PWM input allows the opposite to happen, allowing MOSFET Q4 to turn on with the maximum current being limited by Q44 as described above.

If you want to go down this path, you would need your PICAXE to monitor the temperature of the LED and only have its output set to high while the temperature is lower than your chosen limit. The power source would just need to be power supply of a little more than the LED's requirement: 38 to 40 volts in your case. The MOSFET will generate a reasonable amount of heat, so will need a suitable heatsink.