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, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
