brightnessled
What is a suitable candela value for a LED to have that doesn't blind you if you stare at it, and will still allow you to view silkscreen markings on the PCB but not to dim that you can see the internal workings of the LED ?
I suppose this question is more of an experience question.
This question originally had "lumens" in the text but what I was really asking was for candela. The question has been edited with candela.
LEDs are current driven devices, not voltage driven. Hence, as long as the drive current requirements are met, the voltage involved is merely a dependent variable, so to speak.
For both types of LEDs mentioned, a constant current buck LED driver would probably provide the lowest cost Bill of Materials for a very high efficiency design.
For instance, the Texas Instruments LM3407 constant current, floating buck converter will allow a supply of 4.5 to 30 Volts, and a LED string forward voltage of anywhere between 10% and 90% of the supply. Taking the voltage numbers provided in the question:
0.9 x 11.8 = 10.62 V0.1 x 28 = 2.8 VSo, as long as each string is between 2.8 and 10.62 Volts Vf, this regulator can support the application. Typical implementation, taken from the datasheet:
By using switching current regulators, the issue of getting rid of surplus voltage as heat is pretty much eliminated. As can be seen from the example of the LM3407 above, switching regulators aren't too fussed even if 90% of the supply voltage needs to be dropped within the regulator: The device does cycle-by-cycle current regulation of the load (the LEDs), and simply switches off the rest of the time, hence not needing to dissipate the excess voltage as heat.
This simplifies the series / parallel LED decision significantly: Simply put as many LEDs in series as you like, up to the maximum determined by the supply voltage lower limit (10.62 Volts as calculated above).
Thus, 3 LEDs of 3.4 Vf, or 5 LEDs of 2.2 Vf can be supported per string, in this particular application.
Most lumens per watt is a moving target. It's even more of a moving target if you care what they cost. As such, browse your supplier's website/catalog and enter
into a spreadsheet, if, as is too often the case, they don't have a handy sorting tool that will list the results in lumens-per-watt directly. Make a formula for lumens/watts, and a formula including the cost if you care about cost, and see what you get. In 6 months, it will all be different.
As for the types you list, I'm pretty sure none of them are even in the running for that competition.
Best Answer
Without knowing the number, I recommend looking at some examples.
I find the raspberry pi LEDs to be Just Right, and the Arduino nano LEDs to be much too bright.
Looking at the resistors (and LED models) should give you an idea about the current you want.
After that, as Erik says, you'll need to experiment. Different colours are perceived differently, the color of your silkscreen matters to the eye, etc.