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multiplexing LEDs

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RGBrainbow

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Hi,
when multiplexing LEDs they are normally driven with a higher current to achieve same brightness as in normal operation.
1 LED: 1/1 nominal current for 1/1 time
2 LEDs: 2*nominal current for 1/2 time
3 LEDs: 3*nominal current for 1/3 time
.
.
.
Now the question:
How far can one go this way?
For example: how many 7-segment-digits can be multiplexed @ full brightness without toasting the LEDs?
What is the limiting factor? Overheating the LED-chip? Frying the bonding-wires?

best regards
joachim
 
The thermal time constant of the bond wire will enter into the computation. Very short times (nanosec) can support higher currents than long times (millisec). The internal resistance of the LED might pose arcover problems at high currents.
 
Since this is a common practice, many of the LED's used for such displays, have this information worked out for you in the data sheets. I doubt you will find it for garden variety 5mm LED's, but for something like a 7 segment display, or superbrite surface mount LED's the information is available from the maker. This rating is often called Ifp or allowable forward pulse current.

Other than that, you get to experiment. Problem is, it can be very hard to tell when you are driving a LED to the point where it will survive a few months, versus a few years.
 
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