Byron A Jeff wrote:
>
> >
> > PMFJI, I have not seen Appl. Note 521, but a transformer is considered to be
 a
> > separately derived voltage source by the National Electrical Code(N.E.C.)
> > and the voltage
> > across the secondary windings is of course ungrounded unless you connect one
>  of
> > the windings to ground.
>
> App Note 521 specifically talks about wiring the 115V directly to the PIC
> without the use of an isolation transformer. After reading here I've decided
> that it's a bad idea and I plan to use a transformer.
>
> > I hope this helps.
>
> It really does. I'll go ahead and step the voltage down to 9VAC and detect the
> zero crossings from there.
>
> BAJ


I'm glad that you folks all have a healthy respect for mains potential,
and transformers have lots of wonderful properties, especially for those
working on the circuit itself.

However, zillions of PRODUCTS operate safely without transformer
isolation, benefiting from improvements in size, weight, efficiency, and
cost.  These products are properly INSULATED, of course.  Remember, even
transformers depend on INSULATION for their isolation.

I don't hesitate to design without transformer isolation.  Many of my
products use series capacitor power supplies, and any mention of this
always brings out dozens of responses of the "You're crazy if you don't
use a transformer!" variety.  I take comfort in the simple fact that the
vast majority of electrical products in the world don't use a
transformer.  However, it is too true that knowledge and care are
essential.  Electricity can kill.  I use isolation transformers, battery
operated scopes, differential probes, etc.  Take care.
--

Paul Mathews, consulting engineer
AEngineering Co.
optoeng@whidbey.com
non-contact sensing and optoelectronics specialists