LED matrix, get uniform brightness

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matrixofdynamism

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I have found that the human eye responds nonlinearly to color brightnesses and the response is different for each color. OK, neve mind that.

There are multiple single color LEDs on the board and also some RGB LEDs. In order to get uniform brightness on the single color ones, is it sufficient to connect the same (value & tolerance) resistor to all the LEDs or should I develop a constant current source using transistors? Doing so will require me to have a lot of matched transistors which I do not think can be found. There are 120 LEDs. I ask this since LEDs are current controlled devices.
 

matrixofdynamism said:
In order to get uniform brightness on the single color ones, is it sufficient to connect the same (value & tolerance) resistor to all the LEDs or should I develop a constant current source using transistors?
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Putting several LEDs in series, it turns the current control easiest, ensuring that flow the same current across everyone, therefore just 1 resistor per net suffice. When you associate each one of these arrays in parallel, the average drop voltage tends to overcome the individual variations.

In addition, the choice of a supply voltage much greater than the total diode voltage drop makes the bright not so dependent of the individual direct voltage drop.

Not necessary to say that it is quite important to get all the diodes from the same manufacturing lot.
 

Well calculated resistors should give sufficient uniformity for standard requirements. LED display modules are mostly using constant current driver ICs like TLC5926 with different current programming for each RGB color.
 

I have also read that since the different LEDs may have slightly different forward voltage drop and I-V characteristics, all LEDs of the same type may not have the same brightness level when connected to the same voltage source with the same resistance in series to limit current; or connected to the same current source and thus have the same current through them. Also, nonuniform temperature deviation can also lead to difference in the brightness levels.

When you say constant current driver ICs, I take this to mean that there are like dozens of matched tranistors inside the IC that are connected together such that a current mirror with a lot of transistors can be made?
 

When you say a "LED matrix" it is assumed numerous components. In fact, there is an expected variation even among the LEDs becoming from the same manufacturing lot, but perhaps almost negligible for the human view, particularly if we consider that will be assembled near to other LEDs having the same specifications. In general we don't pay much attention to a single LED, unless specificaly intended to inspect it brightness, but in the overal context of the aspect of the panel, small variations do not matter.
 

When you say constant current driver ICs, I take this to mean that there are like dozens of matched transistors inside the IC that are connected together such that a current mirror with a lot of transistors can be made?
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Not specifically current mirrors, but multiple programmable constant current output stages on a chip.
 

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