[Diy_efi] Photocouplers

[Mike]

<sigh> The paradigm I was trying to put forth was one of
not downgrading because you think you can get away with it.

You are also making the assumption - like a lot of companies
that should know better, that nondescript 1N diodes do in
fact work reliably all the time from all manufacturers.

Having been in power systems engineering for over 20 years,
I can say with experience, get the best part to do the job
that is not likely to be made down to a cost.

In particular I have specific experience of 1N diodes
failing far far before they should have - there are cheap
ones around from really bad QC manufacturers, not anywhere
near so much the case as the UF series.

I mentioned Nasa, because there is an organisation that
made seemingly simple assumptions that on two occasions
lost lives and space craft - vis a vis the shuttle.
When on each occasion they should have known better had
they followed one simple axiom of management and that
is "If you dont know for sure then find out", in reference
to:-

1.	Challenger
	Seals on solid boosters oeprating below 32deg F,
	guesses made about rate of temp rise to allow
	seals to become fully flexible no effort to confirm.
2.	Discovery
	Damage to tiles on left wing, guesses made
	about degree of damage - no effort to take photos.

I dont like making guesses when something can so easily
fail - especially from past experience regarding
*non-descript* components such as 1N diodes.

That is why for a few additional cents there is a higher
level of confidence in using the UF series.

But sure, hey use the downgraded common cheap parts and
worry later why the optocoupler fails - ok for you, not
for me and you can resist learning from my experience...

rgds

mike




At 07:15 PM 26/2/2003 -0600, you wrote:
>26 Feb 2003 Mike <erazmus at iinet.net.au> writes:
>> It is important.
>> 
>> You want to make sure it switches faster then the portion
>> of the waveform across the opto which could exceed its
>> reverse recovery time.
>
>Well yes the extra diode does need to conduct in reasonable 
>time.  A 1N switching diode recovers in about 4ns.  If we got 
>a few volts change in that time, we would be talking dV/dt of 
>1,000,000,000 V/sec.  Gee, that kind of stuff on injector wiring 
>would cause the FCC to come after us.  Fortunately those 
>things are built to be a lot slower.
>
>But never mind the reverse recovery time.  The damage will 
>be caused by reverse voltage across the LED, not reverse 
>recovery.  If the parallel diode fails to conduct with 10 V or 
>so forward bias, the reverse biased LED will be in trouble.  
>Fortunately just about any diode will start conducting in the 
>forward direction so fast, they don't even list numbers.  Of 
>course if the circuit application is not operating with 
>nanosecond speeds, the whole discussion is moot.
>
>But everyone is entitled to express their opinion.
>
>> So, to be sure I use the UF series, for two reasons,
>> 
>> 1.	They are faster than 1N series
>> 2.	Because they are required to be built to a faster
>> 	spec there are less cheap nondescript versions
>> 	around and are therefore more reliable.
>> 
>> By all means make a suggestion to use lower spec parts on
>> your own system if you dont care about the outcome but
>> dont give people advice to take shortcuts with systems
>> you are taking responsibility for - unless of course
>> you risk assumptions and happen to work for nasa <sigh>
>> 
>> rgds
>> 
>> mike
>
>Gee mike, I forgot to check my qualifications to comment 
>on the list.  Guess I need to either get your approval first, or 
>work for NASA.  Well I don't work for NASA, but my company 
>did invent the telephone.  I missed that event, but I did 
>manage to give a hand fielding the first practical cell phones 
>a century later.  Actually I've been doing designs for them 
>for almost 1/3 of their history, never had a single field 
>problem yet.  
>
>But wait a minute, I'm entitled to my opinions too.
>
>Bruce Roe
>
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>Diy_efi mailing list
>Diy_efi at diy-efi.org
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>
>

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