Building LED lights from facts, no theories

[knna]

Regarding Cree Vs Luxeon: Isn't a higher VF better? Since you will be running a higher wattage at same current.

With LED, light output is determined by current, not wattage. Any curve of light output is given vs the current level, independent of the Vf associated to that current.

So for a givent current, a given light output. As higher the Vf, higher the consumption, for the same light output: lower efficiency (more watts burned for same light output). This analysis is valid when comparing different Vf bins of same model.

When comparing different LEDs, obviously they havent the same light output at a given current level, so its necessary to compare energy efficiencies of both. But in general, for leds having similar light output at a given current, the lower the Vf, the more efficient. As manufacturers always report light output at a given current (350mA for 1W leds and 700 mA for 3W), when you compare them, the lower the Vf, the better. As higher Vf increases the wattage burned but not the light emission.

We need to forget using wattage to estimate light output of LEDs, because the wide differences between brands and models, and still between different bins of the same LED (both of flux output and Vf).

That way has worked pretty well with HID lights, because efficiencies of them are very, very similar. Saying "i have 50w of HPS per sq ft" gives a very good indication of the light used, because HPSs have near constant energy efficiencies (400-1000W).

But it dont happen with LEDs. 10w of a given LED may gives 1.5 PAR watts while others (or same with way superior bins) may give perfectly 3 PAR watts (double light for same watts burned). So watts burned dont say nothing about light emited when working with LEDs. At least, currently.

I try to be pretty conservative on the current. Don't like to run at more then 60% of max.

I think its the right way: easier to cool. more efficients, more light points. But its more expensive (more drivers or components and heatsinks, more leds for same light).

The array I am building will have several panel current meters, so I can monitor overall current plus each string.

I agree with that way. Its the only way to take conclusions of how the array works better.

I think current adjustable systems are almost a must. Way more versatility, specially if each color has its own control. Once you do a constant current driver, doing it supporting current limit adjusting is as simple as adding a variable resistor to the current sense circuit.

What are your thoughts on where to operate on current curve? What do you think is the sweet spot where you are trading off efficiency for better penetration? I am setting my rig up so I can play around different current settings and see what produces best yield/watt.

For the increased efficiency, the lower the better.

From cost/performance perspective, it depends of each LED model. On most cases, below 600mA. I think 400-500mA is a good compromise on most cases. Some top LEDs support higher currents keeping decent efficiencies, but currently they are little. I believe in just 1-2 years LEDs released are going to support 700mA very well. But not yet.

It wouldn't surprise me if we have to run these guys as close to max as possible and use aggressive cooling.

If you find that you need more light, you can only do that or add more LEDs. Anyway, id dont think we are going to use our experimental arrays for many years, so running them hard is doable.

I think with LEDs we are going to have to start tailoring the grow to fit the light. We need to start thinking a little more 3D about productive growth area then we do with HPS. Maximizing available light in the Z axis is the real limiting issue with LEDs. You probably only have a couple of inches of decent penentration with LEDS. If you can somehow increase that to 4 or 5 inches you should be able to double yield in the same square footage.

I am currently playing with growing horizontal /flat plants, trying to keep the canopy depth within 6 inches.

Fully agree. I noticed this many time ago. Thats why im insisting about the need of use side and/or intracanopy lighting. That of only grow SCROG. As i dont like SCROG at all (just personal preference), i tend to seek for ways of allowing the grow of tall plants with LEDs. And BTW, increase the light's productivity.

My crude method for heat management:
I look for hotest spot on array with IR thermometer and make sure it is not more then 10 degree F above ambient temp after it has been running for a few hours. Max ambient temp is 80 degrees.

I know there are some pretty good ways of estimating junc temp with basic equipment. I just haven't got around to doing it.

Measure temp at solder point or just next to the led base when heatsink temp is stabilized.

You must know the thermal resistance of the LED package you are using. Its a figure on K/W (or ºC/W, its the same) that says how many ºC junction temp raises for each watt burned by the led. Multiply it (add 1 before if there is a thermal adhesive layer) by watts drawn by the LED (If*Vf, current multiplied by voltage (A*V=W)).

Add the result to the base temp and you get a close approximation of the junction temp.

More accurate measurements implies using very short light pulses (microseconds) and compare Vf of them to Vf on continous operating condition. But this requires to know accurately how much drops Vf for each ºC more, and manufacturer's data often is not precise enough.

I would like to set my test array up so I can continously monitor temp of each array. Any ideas?

Using a thermocouple attached to each heatsink, always on the same position (preferably, the center). But if all arrays uses same LEDs, same power and same heatsink, differences shouldnt be large enough to justify measuring each one, IMHO. But doing it together with current control is going to give very valuable info. :thumbsup: