Although I am not a beginner I realize that this topic is not my strongsuit.
I have decent knowledge about matrix schematics multiplexing rows and columns with shiftregisters and transistors,
I do have some questions about using standard white leds for this purpose, the led has the following specification:
Inominal (nominal current) 50mA and 20 Lumens
Imax (maximum current) only 1/10 DC (10%) 150mA and 50 Lumens
So what if I want to drive this specific led in a matrix and want to have the maximum brightness of 20 Lumens per led ?
At Imax I need to operate it for 40% DC in order to get to the 20 Lumens.
Does this mean I can only operate it at the 10% DC which will give me max 5 Lumens ?
So the questions:
- does this mean this Led can not be operated with higher brightness than 5 Lumens in a matrix?
- are there special leds for matrix operation that can withstand lets say 1Amps at 10% DC ?
- are there other practical solutions available (for instance not multiplexing but driving each and every single led)
You can't generally get the max possible brightness on a multiplexed display. You also have to take thermal considerations into account.
Where brightness is really importand I'd normally not use more than a 4:1 multiplex.
Bear in mind that perceived brightness ( and usuelaly LED output vs. current) is not linear, so, for example, a quarter of the current is typically only going to be approximately half as bright.
White LEDs in particular are very efficient, so unless you want particularly insane brightness it's rarely an issue. Do some tests to see what current you really need.
Thanks Mike,
do you recommend using a higher refresh/scan frequency with lower amperage or a lower frequency with higher amperage ?
Frequency is less important than % duty - if you can increase the percentage time the LEDs are on for, that'll do more for making them look brighter than increasing the current by the same amount.
Frequency doesn't generally matter brightness-wise, it needs to be high enough to avoid flicker - I usually aim for 250Hz for a full scan if possible, but you may be able to do with half that in some cases.
For an X/Y matrix, interlacing the row scan order can help reduce flicker.