someone said:

> > the safest way is to use an opto-coupler.
> >

  I've heard this topic sooo many time it definitely bored me. :(
  The isolation tehnique ( any one would be ) is not necessary if
the final product it's a stand alone ( or it's communicating via
optoisolated i2c, RS485, Rs232 etc ). It's also not necessary even if
the product have a LCD and a keypad and *all* isolation requirements
to the operator's hands and to other commands are fulfilled ( ie plastic
chasis, isolated buttons, optocouplers or transformers for other special
inter-modules connections etc. ) It's also not necessary isolation to
triacs or tyristors gates driving, but sometime boosting it's required.
Think just a little what you'll do when 10 or
20 triacs must be driven. For mains zero cross detection, I'm using
succesfully an external schmitd trigger gate (cmos, CD4093 ) input
directly to main ( 220V -15%...+20% ) via a 220K resistor and one
protective zenner ( 5V1, 4093 supply at 5V ) to ground from the same
input. Optionally a 10nF...100nF can be used for filtering, but usually is
not necessary.
For the system I've described, conecting the system ground to the
neutral or to phase does not change anything in normal functioning
conditions. And this one was tested.

best regards,
Vasile

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