Chris Loiacono wrote:
>
> Yes, as anyone who bothers to read data sheets would be aware. If it's funny
> to ask the original poster a Q in order to gather information needed to
> provide a useful, helpful reply....then it's also a bit funny thatI have
> units in the field for over 15 years with original optos and no noticable
> performance degradation. I guess it all depends on part selection and
> application knowledge & experience.

> My question was originally intended to flush out application reasons that
> optos were a no-no in this project....Which apparently has yet to be shared
> with the list. Is the reason cost, lack of familiarity with opto's, product
> longevity, or whatever???


Chris, I don't doubt that you have well designed
products using optos and yes the poster seemed
a bit quick to categorise as optos==unreliable.

But since you specifically asked about opto
reliability i'd like to say that optos are one of
the lesser reliable semiconductors I have come
across in the TV/VCR repair industry.

It's not the emitter that is the problem but the
photodiode, and the problem seems to apply to most
(older) single photodiodes and the photodiodes
within optocouplers. We were taught that the
silicon has to be doped to an "unstable" level to
get good photo response.

The properties of the photodiodes change over time
and are affected by voltage and current. This is
a big problem with older VCRs where the opto pairs
sensing spool rotation start to fail in an
intermittant fashion as the Vr (or Ir) of the
photodiode becomes different for the same light
level.

In the National brand VCRs it is weird as it is
always the left spool sensor that fails, even though
the right one is closer to the capstan motor and PSU
and runs hotter. BUT the left opto runs at a slightly
higher reverse current through the photodiode.

Testing them is as simple as measuring the Vr on the
photodiode with a series resistor to reg 5v, when
not illuminated, and comparing the voltage to a good
one.

The optos in TV set PSUs fail in similar fashion
slowly over time and surprise it is usually the
sets that run a slightly higher Ir through the
photodiode.

Using an opto for on/off digital use will avoid
noticing the ageing problem until it gets a lot
worse. Modern optos are a lot better too, and
designers that use a minimal Ir will get a lot
more life compared to cheap products pushing the
opto Ir to avoid using higher-gain support
components. So obviously you took the care to
design your product to last, but unfortunately
many designers don't. :o(
-Roman

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