I suspect that the lower frequency ones aren't being driven with as much
LED current. If anyone has the schematic they should be able to
determine if that is true or not. Faster speed doesn't typically mean
greater duty cycle (more heat) but greater drive current into the LED
will create more light output, and that drives the output transistor
harder, which would make it turn on faster. The phototransistor
wouldn't necessarily turn off slower due to the higher drive unless it
was being heavily saturated.
Heat is a significant factor in LED degradation but in my experience
high current is even worse. We used to test LED's with excessively high
currents (like 10 times max ratings) in a cold chamber with the chip on
a heat sink ... the LED never got very hot but the high current would
degrade the LED anyway. The current puts a LOT of stress on the lattice.
By the way, the degradation mechanism is called dark line defects. If
you run really high current through an LED with lattice defects, and do
so to an unpackaged chip under a microscope, you can literally watch the
dark line defects grow.
Assuming that the output phototransistor has three leads available
(base, emitter, collector), you can get a good idea of which LEDs are
brighter (and therefore give you more margin for degradation) by reverse
biasing the collector-base junction and using it like a photodiode while
driving current through the LED. The collection efficiency of
collector-base junction will be primarily determine by the device
geometry and is therefore pretty consistent from device to device. The
currents will be small (microamps) but are measurable. Plus the
brighter LEDs often mean they have fewer lattice defects in the first place.
73,
Dave AB7E
On 6/8/2021 6:24 PM, Kevan Nason wrote:
Dave, AB7E, wrote:
"It doesn't make sense to me to worry only about the
two higher speed positions, since if the LEDS are degrading the slower
positions are going to see the same problem soon after the higher speed
positions. The only difference might be if the LEDS in the higher speed
positions are being driven significantly harder."
You know far more than me, Dave, but I thought the same thing and had
to ask about that too. Answer was the other five are all low speed
operation and that somehow made them not susceptible to failure like
the other two. From that I thought lower speed likely means lower heat
generation and therefore less problems. I bought extra optocouplers
just in case they go out too.
Kevan N4XL
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