[Diy_efi] Photocouplers

[bcroe at juno.com]

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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