The cap is there to make a smooth transition from "short circuit to Vcc" (at the moment when the cap is fully charged and you write the pin LOW) to "no load" (when the cap is fully discharged). XY graph would be better but I did not take it. You must imagine it from the trace or make your own measurements.
The current does not drop enough to prevent heating beyond the danger point. (IIRC, it IS enough to make parallel MOSFETs much more "self-balancing" than bipolar transistors.)
Theoretically, anyway. I've seen a large number of projects that drive LEDs directly from pins, and don't seem to break (or at least not quickly.)
Interesting questions to consider:
- does the internal ON resistance increase with decreased Vcc? How much?
- does that mean that for a given LED Vf, you can pick a supply voltage where no external resistors are required? (eg white LED with Vf=3V means I = 1/40 = 25mA, with a 4V power supply...)
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Well perhaps you COULD but the intensity would be very dependent on the supply voltage (and temperature)
I've seen different ideas of LED characteristics - and to be fair there are so many different combinations of compound semiconductors used.
This one would indicate the slope of the IV char is very non-linear
This indicates the slope is more or less linear
Which means it can be treated as a fixed voltage in series with a resistor.
The "knee" roughly corresponds with the band gap voltage - which is also temperature dependent
From a "proper" data sheet
this LED can be represented (at 25C) as a Thevenin Eq Cct with a voltage source of 2V and series resistor of 10 ohm (approx).
So if Vs was 2.2V you would get 20mA; but at 2.4V you would get 40mA.
But that’s a smaller contributor to current limiting than the 40 ohms of the gpio pin. I’m not trying to get the voltage to hit a point on the Vf curve of the LED, I’m trying to adjust things so that the gpio internal resistor is all you need…
Isn't that shown in the output curves in the datasheet for 5 V and 3 V?
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