l i n u x - u s e r s - g r o u p - o f - d a v i s
L U G O D
 
Next Meeting:
October 20: Web Application Hacking: How to Make and Break Security on the Web
Next Installfest:
TBD
Latest News:
Oct. 10: LUGOD Installfests coming again soon
Page last updated:
2002 Mar 26 11:35

The following is an archive of a post made to our 'vox mailing list' by one of its subscribers.

Report this post as spam:

(Enter your email address)
Re: [vox] [OT] Smoothing out the powerline signal?
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [vox] [OT] Smoothing out the powerline signal?



On Tue, 26 Mar 2002, Mark K. Kim wrote:
> On Tue, 26 Mar 2002, ME wrote:
> > How close is your fridge to your computer and other devices?
> 
> Umm... it's a mini fridge... doubly serving as my printer stand :)
> 
> I can't believe an EM from the fridge can disturb a huge device like an
> electron gun inside the CRT, especially when the monitor is shielded.
> Plus, the power line is fluctuating all the time (60Hz) but that's not
> very apparent at all; why would it become so apparent only when the fridge
> turns on/off?  Can you convince me it's at least plausible?

Sure. It does not take much in the way of a strong magnetic field to
impact a CRT. A simple example can be found when taking a magnet and
bringing it close to the screen. (Please avoid doing this, as it can
damage your monitor over time.) Newer CRT are capable of "undo"-ing *some*
of the problems created with a magnet, but older ones can be damaged
severely. In this simple demonstration, you can see the image distorted by
the presence of a magnet. The stronger the magnet, the greater the
distortion. (Bring a large rare-earth magnet and you can completely
destroy your monitor's image - leaving it warped and distorted.) When this
kind of damage occurs, you can sometimes, leave the monitor off for a
while, and turn it back on, then perhaps degauss it a few times, and cycle
this process over several days, but you never really return to 100%
without actually opening up the beast (and that is beyond my abilities.)

Now, the amount of EMI generated by a powered device is (for the most
part) rather steady. (Yes, I know it is not entirely true, but just for
simplicity let us assume it is.) Since the field does not change too much,
devices will not experience a notable "jump" for the most part. (Of
course, with older laser printers, or locating two monitors close to each
other, you can start to see distortions and pulses but usually at a
predictable rate.)

Now, in the case of a device shifting on, there is (assuming no other
devices, and ignoring the earth's own field, and that of other devices
beyond the CRT we are looking at and a high-power device such as an old
laser printer or a fridge) a state where one field working by itself is
rather steady. When the second source shifts from a void (nothing) to being
on, the new field may interact (depends mostly on proximity and then on
power) with the original field from the CRT.

It is possible to generate electron flow with magnets (magneto, or think
turbine with hydro-electric power), but it is also possible to create a
strong magnet with electron flow. They are actually linked. (You probably
know all of this, but others may not.) When an electron moves a magnetic
field is a result. Magnetic fields, can move electrons. (Ever made your
own electromagnet by wrapping wire in a cyll like a spring around a
screwdriver, and apply a DC current through the wire?)

Now, CRT are much more at risk for EMI than almost any other common
household device. The reason is obvious: an electron gun fires a stream
that itself generates a magnetic field and is impacted by magnetic
fields. Plates on either side of the stream can pulse, and shift their
intensity to alter the path of the stream to "scan" a phosphorus
back-surface of your screen. The strength of the plates is not
exceptionally strong (relative) but is stronger than many other household
devices.

Once you can convince yourself of the impact of EMI on a CRT with your own
magnets and see how distance plays a very strong hand in it wit the source
and destination, then you may be able to accept other EM interference as a
possible problem. I do not recall all of the equations or constants
(physics was many years ago), but I seem to recall it being something like
  q(1)q(2)
K---------- = F
    r^2

With K being some constant, and q being charges, while r being radius. The
charge of course could change with power, but the more powerful part is
the radius/distance between the two charges. Halving the distance
increases impact by 4 times while doubling the power only increases it by
2x. (Notice how this equation closely matches that for gravitation :-D )

Increasing distance by 4 times, drops effective forces to 1/16th that of
"original."

Though, a fridge and a CRT are not simple charges (fluctuations due to the
changes in pictures on the screen, and the fridge changing cycles) you can
think of them as broadcasting EMI. The shielding does help protect other
devices from the shielded devices, and helps protect the shielded device
from other devices EMI, but it is not perfect.

Set two monitors next two each other with one left on and the other left
off (say for a day.) Then make sure the one that is on, has an image
displayed. Turn on the one that is off and watch the effect on the image
on the one that is on. Repeat the experiment moving the monitor a apart
from each other each time.

We have had laser printers with powerful enough EMI to cause shakes in CRT
parked over 5 feet away. Left like this for 3 years, the CRT became perm
damaged - trying to compensate for the predictably shifting EMI and would
actually shake when taken away from the printer. Newer laser printers use
less power, and some have better shielding leading to less EMI.

Another part of this can include RF interference. AM radios seem to be at
greater risk for RF noise cause by weird power and fields. (Driving a car
under power lines while listening to an AM station may cause blanking of
the signal, or allow you to "hear" the interference on the station and
notice it increase as you drive towards the power lines. FM radio is not
impacted as much by our power lines.

My memory is not as good as it was, but I seem to recall spark gaps with
current applied and running as notoriously noisy. Part of this has to do
with a change in difference in electronic potential between two surfaces
(images two ends of wires) being sufficient to allow the electricity to
use atoms in gaseous form as conductors. As the current passes
"through" the air via atoms in a gas state, they heat up the gas causing
it to rise. (Notice how many spark gaps tend to arc upwards? Also, think
Jacob's Ladder.) In cases where spark gaps pulse or click, the charge is
not as small as when a good conductor is available. Why? A charge must
"build-up" on one side to such a degree to make use of air as a
conductor. The larger the gap, the higher the
charge/electronic-potential. The higher the charge/eV, the higher the EMI
and the higher the RF interference.

RF can impact electronic devices, as many circuits resemble antenna
(length, and metrics as well as charge). It is possible for RF to impact
our CPU, and our CPU broadcast RF / EMI.

Want to look at some fun stuff?
Do some online research about HERF guns and EMP bombs. Most of what you
read will probably be trash. (People claiming to have portable HERF guns
or what-not.) People have made devices that produce string magnetic
fields, and some that can damage computers, media and circuits, but what
most govt classify as HERF and EMP devices are not so portable AFAIK.)

There was a presentation at the con about 2 years ago (maybe 3) on EMP
bombs and their construction/design. It is impressive what they have been
able to do. The two big parts of them seem to be:
1) working a tradeoff for power vs. time (have a very high intensity, but
at a very short duration.)
2) shaping the emitter to limit the spectrum to target mostly certain
kinds of devices.

In the cases of these devices (assuming they exist and are not urban
legends) the idea is to make a device that produces EM/RF interference on
purpose. When raise to a sufficient power, a strong EM/RF "signal" can
damage physical circuits - especially those with charge/current. Weaker
ones may cause temporary failures, but allow for a "reboot" of the device
to return function to normal.

The idea proposed by the other fellow about improper grounding producing
EM/RF is a good idea to examine - especially if you have your fridge next
to your CRT. 

As an experiment, you can help determine if it is a power or EMI/RF
issue. Physically move your fridge  to the other side of your room away
from your CRT and system. If you notice the impact on its power-on is
seriously decreased, it is more likely it is an EMI/RF issue. If the
problem is just as intense, then I would look at power.

Also, since circuits for signalling control (say to your eletron gun) can
be impacted by EM and possible other RF interference, the gun itself need
not be the victim. The EM will naturally find the weakest
link.

Sorry I assumed your fridge was in your kitchen and far away from your
computer stuff. Hopefully you can get a refund on the UPS if it is not the
solution to the problem. See if your UPS does convert AC to DC and then
synthesizes its own clean AC signal from the DC current. (Should be in the
manual, and I would expect most to do this.)

People here with actual credentials in engineering with RF generation and
EM, or physicists can probably come up with some better examples. My
physics is rusty and has not been exercised in years.

-ME

-----BEGIN GEEK CODE BLOCK-----
Version: 3.12
GCS/CM$/IT$/LS$/S/O$ !d--(++) !s !a+++(-----) C++$(++++) U++++$(+$) P+$>+++ 
L+++$(++) E W+++$(+) N+ o K w+$>++>+++ O-@ M+$ V-$>- !PS !PE Y+ !PGP
t@-(++) 5+@ X@ R- tv- b++ DI+++ D+ G--@ e+>++>++++ h(++)>+ r*>? z?
------END GEEK CODE BLOCK------
decode: http://www.ebb.org/ungeek/ about: http://www.geekcode.com/geek.html

_______________________________________________
vox mailing list
vox@lists.lugod.org
http://lists.lugod.org/mailman/listinfo/vox



LinkedIn
LUGOD Group on LinkedIn
Sign up for LUGOD event announcements
Your email address:
facebook
LUGOD Group on Facebook
'Like' LUGOD on Facebook:

Hosting provided by:
Sunset Systems
Sunset Systems offers preconfigured Linux systems, remote system administration and custom software development.

LUGOD: Linux Users' Group of Davis
PO Box 2082, Davis, CA 95617
Contact Us

LUGOD is a 501(c)7 non-profit organization
based in Davis, California
and serving the Sacramento area.
"Linux" is a trademark of Linus Torvalds.

Sponsored in part by:
Appahost Applications
For a significant contribution towards our projector, and a generous donation to allow us to continue meeting at the Davis Library.