Re: Line Noise Interference Question

[nobody@xxxxxxxxxxxx (Dave Houston)]

"Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote:

>Does the chopping of current near the zero crossing of switching power
>supplies affect X-10 or is it the filtering of such choppiness in the power
>supplies themselves that causes the problem?  I have a lot of switching
>power supplies that cause problems, but a also a few that don't.  I'd like
>to know what differentiates the two.

You're over complicating things and looking at the wrong factors.

Switching power supplies are similar, in concept, to the power supplies that
were used in car radios prior to the transistor. They used a vibrating reed
(designed to vibrate at 60Hz) to switch the 6V (prior to ~1955 most cars
used 6V) DC voltage on/off, generating a square wave that was fed to a
step-up transformer to create 120V/60Hz. A standard power supply circuit
then converted this into the high DC voltages needed by the vacuum tubes
used in the radio. (Many baby boomers exist because of vibrators and back
seats.)

Switching power supplies have replaced the vibrator with electronic
switching operating at much higher frequencies (20kHz-1MHz, no
standardization). The higher frequencies mean any transformer and other
components can be much smaller. They can be highly efficient (but many are
not) in terms of energy use and they can use a wide range of input voltages
so one PS can be used worldwide. Linear power supplies use heavy
transformers and are inefficient energy users.

Switching power supplies can cause problems for X-10 in two ways.

1. They can output noise in the 120kHz neigborhood (You've seen such noise
from a CFL.) which an X-10 receiver sees as continuous logical 1s at each
half cycle. This tends to jam all X-10 signals. (NOTE: An X-10 receiver
counts transitions in a 650µS window starting at ZC+250µS. 48 or more
transitions is a logical 1. See http://www.mbx-usa.com/xtc798.txt - first
paragraph.)

2. They may filter their output to prevent noise from reaching the mains.
The filters may also filter 120kHz thus draining the X-10 signals.

There's no easy way to tell whether a switching PS is X-10 friendly other
than trial and error. They seldom provide much information beyond
input/output voltage/current.

The noise you saw from a CFL using the ESM1 is probably ~120kHz from its
switching power supply. Since it's a square wave, it contains a lot of
harmonics so it may be a harmonic of the fundamental switching frequency.

AFAIK, all of the diagnostic tools available have similar frequency
responses. If Dan Lanciani is right, its possible to have a noise source
that will not show up on them that will still jam X-10. He said he had a
~200kHz noise source that interfered with X-10 but did not register on the
ESM1. I'm not sure whether he was using the ESM1 that I loaned him
(fullscale=10Vpp) or an ELK model (fullscale=5Vpp). It might be that it
would register on the latter but not the former. It's hard to say because he
really didn't provide much detail about the actual frequency and amplitude.

John Galvin indicated that the Monterey responded to out-of-band noise that
did not affect the ESM1 (again, my ESM1 on loan) but he didn't provide any
further details on frequency/amplitude.

X-10 specs are for 120kHz ±2kHz so most test equipment is designed around
that. With the relatively recent proliferation of switching power supplies,
currently available test gear may prove incapable of detecting all the
frequencies that will jam X-10.

For further reading:
http://www.electronics-tutorials.com/basics/switched-mode-power-supplies.htm
http://en.wikipedia.org/wiki/Switch-mode_power_supply
http://www.consumerreports.org/main/content/display_report.jsp?WebLogicSession=QskjsPdgjUnIkrGTKBQs1gH2ur22N7aX2ctPO7Qj4mw9i3w3U8z2|8882122804884371552/169937909/6/7005/7005/7002/7002/7005/-1|5298542958250215617/169937910/6/7005/7005/7002/7002/7005/-1&FOLDER%3C%3Efolder_id=538847&ASSORTMENT%3C%3East_id=333133&bmUID=1120478128741

NOTE: If you follow the link in the first URL to the ATX power supply
schematic, the 220nF capacitor used across the mains presents 6 ohms
impedance to 120kHz.

If a PS causes problems only when it is on, it's most likely a noise
problem. If it still causes problems when off, it's most likely a capacitor
across the mains. However, there may be cases where the capacitor is behind
a switch so that complicates the analysis.

A bandstop filter (e.g. all of the X-10 filters) deals with both noise and
the capacitor. It blocks noise from reaching the mains and blocks X-10
signal from reaching the capacitor.

If it's a noise problem, the noise will be constant. It will not come and go
in sync with the ZC in patterns that repeatedly turn on selective lights in
the middle of the night. The X-10 noise fairy does that.