On Sun, 21 Dec 2014 17:37:07 -0500 Charles Steinmetz <[email protected]> wrote:
> Ed wrote: > > >It seems to me that a low voltage secondary should be OK by using a > >fast comparator IC rather than a transistor to decide - the gain of > >the IC allows for much smaller detection levels, so the equivalent > >zero-crossing velocity could be the same. An IC tripping in a 10 mV > >band should provide the same effective ZC velocity at 12 V input as > >a transistor working around 100 mV with 120 V input. Or am I missing > >something? > > When the switching band gets that small, device noise, input offset > voltage drift, and other errors have a proportionally greater > effect. I actually built a similar circuit with a 12v transformer > and an LT1720 comparator, and it had worse jitter than the > two-transistor circuit with a 120v feed. In this case, there is no > substitute for starting with a higher-slew-rate signal. (Yes, the > LT1720 did marginally better than the two-transistor circuit when > both were fed from 120v -- but the fussiness of working with a fast > comparator and the small gain over the two-transistor circuit made > the latter the better choice, particularly in a design being put "out > there" for others to build.) > > Best regards, > > Charles > Looking at the data sheet of the LT1720, 1mv would have about 8ns delay. Call it 10ns. A Vp of 29 volts should be sufficient to put the delay around 90ns, making 100ns error or target percent of the 1uS target. _______________________________________________ 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.
