[CQ-Contest] Mk2R+ optocouplers

[David Gilbert]

Yeah, that would be 5 milliamps for all of them, which certainly is not 
an excessive amount of drive current.  Most application notes for 
optocouplers tended to spec about 10 milliamps for a typical 
application, and as best I can remember 50 milliamps was often the max 
rating.  If those optocouplers were degrading at 5 milliamps they really 
had poor quality LEDs in them.

I still don't buy the idea that the lower speed slots should make much 
difference, though.  I can't think of any reason why speed should make a 
difference in LED lifetime at 5 milliamps unless those slots simply were 
activated significantly more often ... higher duty cycle, not higher speed.

73,
Dave   AB7E




On 6/8/2021 10:19 PM, Ken K6MR wrote:
>
> From the info I have it looks like they are all driven through a 1k 
> resistor, supply voltage is either the USB 5V bus or the system 5V 
> bus.  Drive current looks to be well below the max shown on the data 
> sheet.
>
> My unit is pretty old and has all Fairchild (white devices).
>
> Ken K6MR
>
> *From: *David Gilbert <mailto:ab7echo at gmail.com>
> *Sent: *Tuesday, June 8, 2021 21:20
> *To: *cq-contest at contesting.com <mailto:cq-contest at contesting.com>
> *Subject: *Re: [CQ-Contest] Mk2R+ optocouplers
>
>
> 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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