[time-nuts] Line voltage frequency interface

ed breya 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 
applications.

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.

Ed





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