GregS said:
On an AC line driven Xmas light st, the LED is on for 50% of the time
If the peak current is 30 ma. then the average is 15 ma. !! ??
greg
OK Greg, I see what you're saying, but I think that really, you are playing
with words and math here, rather than exploring a concept that's relevant to
speccing the diodes. Average diode current is not a good way to be choosing
the device, because it relies on the limiting factor being power related,
rather than a possible physical limitation of the construction. Any diode
that's going to be correctly rated for this job, must be capable of handling
the peak instantaneous current. How long it's passing that current for,
followed by how long it's then got as a 'rest' period, is neither here nor
there. So, if we were discussing a power rating, rather than a current
rating, then your reasoning would have some validity.
If, in your christmas tree light example, the LEDs were specced for a max
forward current of say 20mA, then by your reasoning, with an average current
through the string of just 15mA, we should be OK. I would venture to suggest
that this would not in fact be the case, as the peak current of 30mA through
the string would over-run the diodes by some 50% at the peak of the cycle.
If the reason for the 20mA max figure was one of power dissipation in the
LED die, you might get away with a15mA average current figure, but if the
limitation is on say the maximum current handling capability of the
connections to the chip, then the likely outcome would be a blown LED. I
think we're probably talking the difference between a guaranteed reliable
setup, and a 'getting away with it' setup.
Arfa