[time-nuts] Line Frequency

Bob Camp lists at rtty.us
Mon Feb 10 13:34:44 EST 2014


Hi

Let’s say you use a magnetic pickup loop. The most likely source of local 60 Hz field is the very same transformer you noticed in that well designed wall wart. Rotate the wall wart, rotate the field, flip the phase ….

————

Ok, how about a capacitive pickup. Close to one side of the 240 I get one phase, close to the other I’m 180 degrees out. Move this and that … the phase wanders. Of course if you wire the cap to the line you *know* what’s what. That sort of defeats the safety aspect ….

Bob

On Feb 10, 2014, at 12:57 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:

>> You should really have isolation from the mains. The transformer scheme
>> is fine. I don't like the high value resistor approach.  
> 
> I know what you mean. I too used to think isolation and safety meant using a transformer, like a low voltage AC wall-wart.
> 
> Then out of curiosity I opened some of them to see what was inside. Sloppy design. Poor soldering. Child labor. Solder drips. No waterproofing. Fingerprints. Vinyl tape. Compared to that, a DIY, carefully constructed, direct connection of two AC lines to PIC input pins via a pair of 1 Meg flameproof resistors has merits.
> 
> Perhaps what we need is transparent wall-warts, like hospital-grade power cords.
> 
> Alternatively, has anyone considered grabbing 50/60 Hz from the air? Just about any dangling unshielded wire will act as a pickup. Maybe not as robust a signal, but this is not so much a problem for solutions already using a hardware or software PLL. And it certainly wins the safety prize. Call it the iClock Air...
> 
> /tvb
> 
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