The LED burnout urban legend...

Actually I have seen this many times on public forums. There is information that LED resistors must be used. Otherwise, the LEDs will simply burn out, as well the microcontroller will take damage.

I have found these informations to be largely untrue. Various explanation can be found among the most exotic for instance a theory that electromigration would happen inside the IC, or the bonding wire.

I have found many microcontrollers and 74HC TTL chips to be quite tolerant for LED matrix and 7segment driving without any kind of resistors. It is true that if you drive a cathode sink using one I/O bit only you get some brightness variation.

One measure indeed is to reduce the operation voltage. For instance I made a minute timer, using a 16F54, and a 2-digit LED display. No resistors obviously. It runs about 3 days from a CR2032 battery!

How can high currents capable of damaging LED display and/or microcontroller occur at this power level? That is a question which very likely only has the answer that LED burnout explanation is actually an urban myth!

It is however true you should use resistors for single LEDs that are permanently ON.

I have seen calculators from the early 1980s which had LED displays, and even nearly 30 years ago, no resistors were used!

So what is all that "engineering knowledge" on public forums, and numerous websites?

Commercial electronic circuits were done differently even 30 years ago.

I don't know if this is done on purpose, or if they really don't know better, urban legends actually being nourished and maintained inside the education system!

Not that 7-segment displays are that important these days. Often small LCDs are used- cheaper. But LED matrix is still used. Why actually use more components than neccessary? Why waste so much electrical power? Even at full 5 volts, LED burnout is a total non-issue, I have never observed it for LED matrix.

But actually, there are also some projects which don't use LED resistors.
For instance this LED matrix circuit:

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