[time-nuts] Simple AC mains zero-cross detector

Charles Steinmetz csteinmetz at yandex.com
Sun Dec 21 17:15:48 EST 2014

Didier wrote:

>A commend regarding your ZCD. You propose to use a dual 120V primary
>transformer to generate the isolated 120V AC needed by your circuit.
>Unless specifically designed for that purpose, the isolation between the
>two 120V primaries of a common transformer is probably not as good as the
>isolation between primary and secondary, which could be a safety hazard.
>Since small transformers with a 120V primary and a true 120V secondary are
>hard to find, a better way would be to use two "regular step-down"
>transformers back to back, like two door bell transformers: 120-24-120. You
>would then get double isolation.

The transformer I used is a dual C core "pseudo-toroid" -- it has one 
primary and one secondary winding on one bobbin and the other primary 
and secondary winding on the other bobbin (on the opposite side of 
the core).  The primary-to-primary isolation of any transformer wound 
this way should actually be better than its primary-to-secondary isolation.

Of course, not all small power transformers are built this 
way.  However, the primary-to-primary isolation of any small 
commercial power transformer should be sufficient not to cause any 
safety problems in the ZCD application.  Hipot ratings are regulated 
by standards and are generally greater than 1500v from any winding to 
any other winding, and even if there were an effective interwinding 
capacitance of 200pF between primary windings, the 60Hz current in 
the ZCD ground would be no more than single-digit uA at a 
maximum.  (By comparison, ground-fault interruptors trip at 4-6mA -- 
1000 times greater than this.)  The actual effective 
primary-to-primary capacitance is likely to be very much less than 
200pF, and the capacitive leakage current from the 120v to 120v 
isolation windings should be comparable to the capacitive leakage 
from the Vcc supply.

All that said, there is certainly nothing wrong with using two 
transformers "back to back," as you suggest, to improve 
isolation.  It is also how anyone in a 200 or 240v country would 
generate isolated 120v:  240:12 ==> 24:240.

Best regards,


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