[sdiy] optocouplers/Vactrols

[Scott Gravenhorst]

Tim Parkhurst <tparkhurst at siliconbandwidth.com> wrote:
>> -----Original Message-----
>> From: Paul Higgins [mailto:higg0008 at tc.umn.edu]
>> Sent: Tuesday, October 19, 2004 10:49 AM
>> To: Metrophage
>> Cc: synth-DIY
>> Subject: Re: [sdiy] optocouplers/Vactrols
>> 
><snip>
>> Well, that explains why I couldn't find any optocouplers on the
>> Hamamatsu website, only LDRs.  And I have heard that Clairex has
>> gaps.  I've also seen people use 35mm film canisters with holes punched
>> in the bottom for the leads; kind of bulky, but it works.  With the
>> right LDR and construction, you can get pretty good performance even
>> from DIY "Vactrols".
>> 
>> -PRH
>> 
>> On Tuesday, October 19, 2004, at 08:51 AM, Metrophage wrote:
>> 
>> > I don't prefer any yet, I am just getting into them, like it sounds you
>> > are. Each manufacturer makes units with various respective resistances
>> > and response times.
>> >
>> > Clairex and Hamamatsu appear to be dropping their production of many,
>> > if not all, analog photocouplers. There are still quite a few out
>> > there. Compare specs for the values you need!
>> >
>> > CJ
>> 
></snip>
>
>So, what are the big variables in DIY optocouplers? As far as I can tell,
>there are three main areas:
>
>1) LED current vs. output. Manufacturers variation from one part to the
>next.
>
>2) Photocell illumination vs. resistance. Again, variation from one part to
>the next.
>
>3) "Manufacturing tolerances." I.e., variations in position, distance,
>opacity, when the whole thing is being put together.
>
>Okay, so #3 is at least under our control. Getting things positioned and put
>together consistently just takes some care and practice. But how bad are the
>variations in items 1 and 2? Could a simple trimming circuit (say a pot in
>series with the LED) be used to get a fairly close min/max match between
>several DIYOCs? (Note the new acronym. Remember where you saw it first ;) )

I would add:

#4 linearity
At least for H11Fx parts, (this is a LDR/FET optocoupler) there is a
linearity issue when the input causes a larger than 100mv signal across the
FET terminals.  This linearity problem caused me to abandon a filter design
using it.  But I may revisit this with the low signal caveat, since that
seems to cure it in large part.  In fact, the OTA methods have a similar
issue, I note that an OTA SVF often uses a 100:1 (or similar) voltage divider
before the signal hits the OTA input terminals.


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