[time-nuts] Line voltage frequency interface
eb at telight.com
Mon Nov 28 14:03:22 EST 2011
How about a new thread with a more appropriate name to replace what
was: "Clocking a PIC16F628A from a Rubidium Standard?"
I don't think anyone will be interfacing anything associated with
their Rb standards directly to the AC line, except for unusual cases.
Regarding components: There are many resistors and capacitors rated
for line voltage withstand capability. Line and safety rated
resistors are typically specified as "flame-proof" or "fusible" -
they will safely blow open without much material turning into a
fireball. What you do not want to do is use small-bodied resistors to
drop line voltage, regardless of the resistance value - the weakness
is in the smallness - they can arc and light up. So, never use tiny
or 1/4 W resistors of any type for this application - even a bunch in
series (although this is sometimes done with SMT resistors in newer items).
A very common type used is a 1.0 to 4.7 megohm 1/2 W carbon
composition. Believe it or not, you probably have one of these in
most pieces of AV equipment with a two-wire power cord - it goes from
the neutral (wide spade on the plug - in the US) to the chassis,
which is at earth ground - your ground - to discharge static charge
buildup. If the line is inadvertently reversed, the resistance limits
the leakage current to a non-hazardous level. UL approved. Most of
these resistors I have seen appear to be very cheap, poorly formed
parts with 10 or 20 percent tolerance, although it's possible they
use special body materials for the application. I prefer good quality
1 W carbon composition resistors for high-resistance line-dropping
In the old days, the two-wire cords weren't polarized with the wider
neutral spade, so the equipment manuals would say to try reversing
the plug orientation if excessive of hum was encountered during
initial hookups - this would put the resistor on the neutral side,
reducing ground loop currents between the equipment. You had a 50-50
chance with each item.
So, never make a low impedance connection to neutral - treat it just
like the line side because it can indeed be reversed.
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