[time-nuts] Line Frequency

[M. Simon]

The big picture is that I want a true zero crossing detector (or as close as possible) to minimize variations from line voltage variations. Thus the op amp. 

 

Engineering is the art of making what you want from what you can get at a profit.



On Monday, February 10, 2014 1:46 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
 

>> Maybe a couple of back to back diodes (zeners?) across the capacitor to make
>> sure the voltage stays in the input range of the op amp/comparator. 
>
>The standard approach in the digital world is Schottky diodes.  They come 
>with a pair in a tiny 3 pin package.
>
>Most digital logic has built-in protection diodes.  Look at the fine print in 
>the data sheets.  There is usually a max current and/or max voltage/time/area 
>spec.  It may be off in the general info sheet for the logic family.  It's 
>common to take advantage of those diodes by using a current limiting resistor.
>
>
>> I like the LT1122 for speed. It is not too pricey. But there is always the
>> ubiquitous LM111 family. 
>
>I'm missing the big picture.  Why are you adding another chip?  What are you 
>going to connect this signal to?
>
>As tvb has shown, you can connect the AC line directly to a PIC input pin 
>with a big enough current limiting resistor.  I'm using an AC wall wart and a 
>pair of resistors to get within range of the modem control signals.  I've had 
>no noise events in the past 50 days.
>
>I did have noise problems with boxes running off an isolated power brick.  I 
>never tracked it down.  I assume it was a missing green wire ground so the 
>chassis ground jumped around when interesting stuff came in over the power 
>line.
>
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>You can feed the signal into the audio input and capture the raw data and 
>look for glitches.
>
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>-- 
>These are my opinions.  I hate spam.
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