tomp wrote:
Not true. When in the forward condition which is what happens in the normal operation of the LED, there is no high impedance related to turn on speed . . .
I probably wasn't explaining myself well. There is clearly some sort of destructive effect going on since P Elliott reported the LEDs were blowing up on turn on without the parallel 100K resistor. The question is what is the cause of this.
I was merely pointing out that the impedance of any diode is a function of the current passing through it. If you look at I-V curve of the LED that you attached a few posts earlier, the impedance of the diode is just the inverse of the I-V slope and that varies with current. When the LED diode is turned on and light is emitted (current around 20mA or higher), it's a relatively low impedance. When the voltage across the diode is lower, say <1.5v, then the current through the diode will be <<1mA and the impedance of the diode will much higher.
But as I think about it further, the more important factor is the change in current through the LED with time. When the LED turns on, there is a sudden increase in current through the diode. That in turn could give a voltage spike due to the inductance of the dropping resistors and the associated wiring. It's the standard effect where V = L dI/dt.
Experimentally, a parallel resistor solves the problem. And I suspect that Roscoe's parallel capacitor would also be highly effective. One can do both if one believes in belt and suspenders.