Hi Charles, I have a question about the accuracy of your scheme, given transient effects.
Transformers, such as are in wall warts, etc..., are wound in a way that is pretty good for 50Hz/60Hz operation, but have had nothing intentionally done to normalize operation at any other frequency. Nor have they had anything done to improve the fidelity of the signal they pass. Typically, they are running very near the edge where the core is entering saturation, not because it is a good thing, but rather because it minimizes the amount of copper and iron, and the physical size necessary, for a given amount of power output.... The trade off being efficiency... a little more heat is generated, and that is the customer's problem to deal with, not the manufacturer's... but I digress. In the 99 and 44/100 th's percent of the usage of a transformer coupled ZCD, the positive and negative zero crossings are going to come chugging along predictably about every 8.3 milliseconds. And, the degree which their arrival is unpredictably 8.3333... milliseconds, is what I believe to be the the realm of the grid-nut. One facet of that unpredictability is what I am interested in, for the purposes of this post: Suppose, that one of the grid-nut persuasion is interested in the timing of the occasional crash transient where somewhere during the course of a cycle, an unintentional zero crossing occurs due to a transient that drags the grid voltage through ground. With an opto isolator protected ZCD, the transient will be propagated to the logic side by way of the usual speed of light, and will remain true to the fixed delay introduced by the optoisolator ZCD... The optoisolated ZCD has no ability to affect where the crossings occur, or for the most part, how often the crossings occur; it will faithfully register and send the glitch along to the logic side for measurement. A transformer isolated ZCD, is different in this regard, however. Because of the nature of transformers, a transformer isolated ZCD will propagate every of the various frequencies it passes, with a different delay. What this means, is that as long as the zero crossings keep chugging along at a nominally 60Hz rate, you will have your touted sub-microsecond timing accuracy; but, introduce one crash transient that causes a significantly early zero crossing, and you will be introducing frequency components other than 60Hz, and will cause the crash transient's time-of-occurrence to be misrepresented, and will also cause the subsequent zero crossing's time-of-occurrence to be misrepresented... all due to the transformer's inability to induce all frequencies with the same speed. This same uncertainty will occur even if the so called crash transient does not pull the sine wave all the way to zero, but only wounds it a little. Thoughts? -Chuck Harris _______________________________________________ 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.
