SV: Re: [sdiy] midi optocouplers

[harrybissell]

Most "slow" optos are for going across a barrier (two isolated power
supplies)...
for instance if you wanted to drive a AC (120-240V) lamp from your
LFO... it would be
prudent to have the LFO drive the opto, and the opto drive the lamp
circuit (SCR-Triac etc).   The 'slow' response is way fast enough for
the 8-10ms until the next half cycle
comes along.

You could just tie the synth ground to one side of the AC line (on
120V)... its incredibly
risky and stupid. I did it once to drive some theatre marqee lights.

Slow optos are used everywhere in industrial controls...

H^) harry

Karl Ekdahl wrote:

> dammit, i have all these optos laying around but no idea of what to
> use the for if not for midi... whatever do you do out of slow optos??
>
> Karl
>
> harrybissell <harrybissell at prodigy.net> skrev:
>
>       imho its too slow.
>
>      I don't know if this is correct of not, but a 32KHz signal
>      would have a
>      half cycle time of 16us... in effect the frequency is really
>      twice
>      32KHz.
>
>      With the rise/fall times you have there the opto would be in
>      an analog
>      range all the time, never high or low. That would mean like
>      zero noise
>      margin...
>
>      You could try it if you have one, but I would not count on
>      it.
>
>      The lower the value of resistor on the drive (LED) side, the
>      better off
>      you are
>      except for the power you need to drive it. On the receive
>      (transistor)
>      side...
>      a smaller resistor will give faster response, but you need a
>      higher
>      drive current
>      or good CTR (current transfer ratio - ie gain, beta, etc).
>      Larger
>      resistor values
>      will be slower.
>
>      Why not a part with digital output drive ???
>
>      H^) harry (who just got burned on the PC816/817. The proto
>      had gain
>      range
>      "B"... 1st production had gain range "A" and the optos would
>      not
>      saturate)
>
>      Karl Ekdahl wrote:
>
>      > A quick question: would a PC814 opto with rise/fall time
>      max of 18uS
>      > be good for midi? I know the usual suggestions are 6N13X
>      with way
>      > lower fall/rise but according to my calculations;
>      1/32150=31uS is
>      > absolute minimum rise/fall, so about half that should be
>      good, right?
>      > Also, what resistor to VCC should i use (And why)?
>      >
>      > thanks
>      >
>      > Karl
>      >
>