Newbie Question -guidance requested

[itolond]

I am new to LED scene, last time it was luxeons and 1-3 watt discussions, which leaves me somewhat disarmed for the way in which LEDS are rated these days- and some what confused about this.

I have been looking at MFRS specs for (mostly) 5050 chips and either I am not getting or the information sometimes is absent? - As I understand the current ~300-3000Ma is a good indication of the capacity for brightness of an LED whilst Lumens is the actual output.

Hows does one determine the brightness of a LED from MFRS spec sheets - i have seen lm per watt - what do I look for here (bit lost) max output, max watt ????

also I am looking to have a few 5050's on a board. - mixing types can cause issues as the power supply would be the same, just the 5050 led would change ...risk?

 

[DIWdiver]

Not sure what datasheets you are looking at, but I just picked 3 at random and all three had luminous flux (lumens) ratings on or about page 3. This is the total light output at rated current. Typically they also have a curve of output (percent of rated output) vs current in case you want to use something other than the rated current (it isn't quite linear).

The max current rating is somewhat an indicator of what the max lumens will be, as long as you are comparing single-die white LEDs. Max power would also be a good indicator. But these can both vary quite a bit, especially if comparing different generations or quality of devices. Your best bet is the rated lumens adjusted for actual current.

Lumens per watt (AKA luminous efficacy) is somewhat akin to efficiency, so long as you are talking about the same light color. But lumens are weighted according to the sensitivity of the human eye. Since the eye is much less sensitive to red than to green, it takes more power in red to seem the same brightness as green. In other words it takes more power to generate a lumen of red than of green.

So different color temperatures in the same family of LED will have somewhat different efficacies. To get a warmer color, you have to add more red, thus reducing the efficacy. This is true whether you are using RGB dies to get white, or phosphors. If using phosphors, there are other effects too.

Mixing LEDs can be safe if you follow some rules. LEDs in series all get the same current, so as long as they all can handle the current you want to drive them at, you're good. LEDs in parallel all get the same voltage. This can get pretty tricky. It's generally safe if they are all from the same manufacturing lot and have pretty similar heatsinking. Otherwise, generally not a good idea.

 

[itolond]

Appreciated - i now know what to look for. there does seem to be quite a range in luminous flux - Cree X series is very high, where as some of the other 5050 chip and as you refer to non-white are dramatic...

Not sure what datasheets you are looking at, but I just picked 3 at random and all three had luminous flux (lumens) ratings on or about page 3. This is the total light output at rated current. Typically they also have a curve of output (percent of rated output) vs current in case you want to use something other than the rated current (it isn't quite linear).

The max current rating is somewhat an indicator of what the max lumens will be, as long as you are comparing single-die white LEDs. Max power would also be a good indicator. But these can both vary quite a bit, especially if comparing different generations or quality of devices. Your best bet is the rated lumens adjusted for actual current.

Lumens per watt (AKA luminous efficacy) is somewhat akin to efficiency, so long as you are talking about the same light color. But lumens are weighted according to the sensitivity of the human eye. Since the eye is much less sensitive to red than to green, it takes more power in red to seem the same brightness as green. In other words it takes more power to generate a lumen of red than of green.

So different color temperatures in the same family of LED will have somewhat different efficacies. To get a warmer color, you have to add more red, thus reducing the efficacy. This is true whether you are using RGB dies to get white, or phosphors. If using phosphors, there are other effects too.

Mixing LEDs can be safe if you follow some rules. LEDs in series all get the same current, so as long as they all can handle the current you want to drive them at, you're good. LEDs in parallel all get the same voltage. This can get pretty tricky. It's generally safe if they are all from the same manufacturing lot and have pretty similar heatsinking. Otherwise, generally not a good idea.