20M2 Drives RGB LED sans Resistors

[erco]

I've mentioned this before, but here's some evidence how a Picaxe deals with a dreaded "pin burnout" situation. Conventional wisdom has us use Ohms law to calculate the right resistance to limit current thru an LED, but in practice the internal resistance of the output pin limits current to ~22mA or less for all 3 colors. Red LED is slightly higher because of its lower forward voltage. A resistor saved is a resistor earned.

10mm common-cathode RGB driven with no resistors by PICAXE 20M2 just barely pushes the current limitations for individual pins and the port (group of pins). Not suggested for continuous duty in a mission-critical application, but fine for intermittent use.

 

[PhilHornby]

I recently built a project, where I decided to drive two LEDs (from a 14M2), using PWM (because I fancied having them 'pulsate', rather than just blink). I don't know what muddled thinking I was following, but I made the PCB without dropping resistors...

It had been in use for quite a while, when it occurred to me, that using PWM would only alter the average current, not the peak.

Now I know how the Picaxe has survived

 

[erco]

It had been in use for quite a while, when it occurred to me, that using PWM would only alter the average current, not the peak.

I would think average current would be the critical factor. If heat/overheating is the issue, then seems like the silicon is cooling down and recovering on the off cycles.

SO ADD ANOTHER BATTERY, you gotta push the envelope !!!

 

[erco]

But don't try my no resistor "solution with a Piranha superbright RGB LED, which draws ~50 mA per pin! Note that this is a common anode LED, versus the common cathode standard 10mm RGB LED shown above.