Chuck wrote:
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.
I frequently recommend small 120:12v or 120:6.3v power transformers as 600 ohm line to voice coil transformers for audio applications. I have tested scores of them, and have yet to find one that is not flat from at least 20Hz to 15kHz -- often significantly better -- if operated at no more than 1/2 its rated 60Hz power. The distortion is typically < 1% at that power level. [I have not measured transformers from wall warts, but I expect that many if not most of them conform to the same general specs.]
Typically, they are running very near the edge where the core is entering saturation,
That depends on the current you are drawing. The ZCD circuit doesn't draw anywhere close to the transformer's rated current, so core saturation is no worry at all.
Because of the nature of transformers, a transformer isolated ZCD will propagate every of the various frequencies it passes, with a different delay.
The transformer's group delay is not an issue at the 1uS level. However, the input filter I specified has non-constant group delay, which varies about 40uS from 10Hz to 600Hz (the range of frequencies where I observed significant components of grid transients). So, there is a tradeoff. If accurate timing of transients is more important than some spurious noise responses, the ZCD should be built without the filter capacitors (C1 and C5 on the schematic), as noted in the description. In that case, the group delay is within 1uS for all frequencies above ~10Hz.
Note that this also applies to any other detector, including those using optoisolators -- any input filtering will create non-constant group delay.
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
