[time-nuts] Line Frequency

[M. Simon]

Hal,

The resistor H bridge with a capacitor in between sounds like a good idea. Yes - I'm starting with a wall wart. About 12 VAC.  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. I like the LT1122 for speed. It is not too pricey. But there is always the ubiquitous LM111 family. I do get the ECL biasing trick you pointed out. 

Chris,

Experimenting
 is good. Except that to do a valid test I'd have to run the experiments
 in parallel. I'm not just using this for triac triggering. I want to 
take measurements in real time. i.e. a PC running an FFT will not get me
 what I want. 


I may ultimately have to create a test set that generates 60.000 Hz and injects a noise signal ( +/- .1V pulses at 100 KHz say) that can be turned off. The 60 Hz need not be extremely accurate. Just stable. 

My counter does time triggering so I can do averaging if necessary without the counter swallowing pulses. 

Simon


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



On Sunday, February 9, 2014 8:38 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
 

>msimon6808 at yahoo.com said:
>> Does any one have a circuit (tested - operational) for monitoring line
>> frequency? I'd like something that checks zero crossing so that it is
>> relatively insensitive to line voltage variations.  
>
>I'm assuming you are starting with an AC wall wart and 2 resistors to divide 
>the voltage down to something within range.
>
>The easy to understand way is to use 2 more resistors to bias your input pin 
>at the switching threshold and a cap to connect the middle of both pairs.  
>The circuit would look like a H with 4 resistors on the vertical bars of the 
>H and a cap on the horizontal bar.  Top left of H connected to AC in, top 
>right to +V, bottom left and bottom right to ground, and center right to 
>input pin.
>
>You can do it with 3 resistors.  Replace the lower of the 2 resistor setup 
>with a pair, one to ground and the other to +V.  Adjust the size of those 
>resistors so the parallel resistance is the same as the one you are replacing 
>and the middle voltage is the switching point.  Mumble.  There is a word for 
>this that I can't remember.  It's used for things like terminating ECL input 
>signals.
>
>If you are going in to a RS-232 port, you can probably get a useful +V from 
>one of the modem control signals.  I forget the polarity.  You may have to 
>hack your software to set it to the right polarity.
>
>
>-- 
>These are my opinions.  I hate spam.
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