[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: More MH lighting questions....

Bill Wichers wrote:

"The ballasts I use are made by Advance and, like all commercial
transformers/ballasts/chokes I have seen are potted in a heavy nyleze
varnish to prevent such vibrations. Over time the varnish sometimes
crach and permit the coil to vibrate. In the larger transformers I have
worked with (several kVA) the local magnetic field becomes so strong
the units to aduibly hum even though there is no discernable mechanical
vibration (meaning physical movement in this case, not just the
causing the sound).

Not that there aren't some cheapo ballasts out there that aren't potted

Most of the electronic ballasts operate in the 10-40 kHz range, the
of which is audible. Remember that many people, myself included, can
the ~15 kHz horizontal scan line frequency from TV sets. The nice thing
that the switch-mode electronic ballasts normally use a heavily potted
taped small transformer that makes essentially no sound no matter what
do to it.

The unfortunate thing with the electronic ballasts is that many of them
not properly filtered and emit RF hash that interferes with radio and
television operation. I haven't had any problems with this myself,
I do have the equipment to find and solve these problems if they come
(working on an extension to my EE in RF/Microwave design).

     -Bill  "
Not all coils are potted--one reason is that the stuff dries out and
cracks, retains heat.  I believe more expensive coils generally are not
potted.  But a ballast can be designed well with potted or non-potted
coils provide the power factor is high enough--at least .9

If a unit is humming, it's vibrating--you can't transduce the
electromagnetic energy to kinetic energy in the air just by emanating
an electromagnetic field.  I don't think quantum electrodynamics
precludes this in principle; but no one has invented a device that
moves air without itself moving.  In very good audio equipment, the
good transformers (coils wrapped on coils) are specially designed to
prevent stray field effects, vibration, etc.  In a ten or twenty dollar
Advance ballast; well that's a different deal altogether.  But that's
why Advance ballasts often hum and Fulhams don't.

Yes, poorly designed electronic ballasts feed rf in to the air and onto
the household AC lines.  Regular ballasts do the very same things but
at different frequencies.  Also, proper grounding can usually solve
most problems if the ballast, plain or electronic, is designed well to
begin with.  A ground wire wrapped around the AC feeds and connected to
the reflector can seriously reduce or eliminate the fields.  To keep
the he AC lines clean, filtering is necessary.  The required limits on
how much harmonic distortion a ballast can put on AC lines is rather
high.  So better ballasts are cleaner than cheaper ones that just meet
UL standards.

You're right that freqs below 20KHZ are audible and some electronic
ballasts run at those lower frequencies. (Although 15 kHz is the more
likely upper limit if you are over 50--even lower if you're older than
that). If the coil vibrates at 15kHz or higher, you definitely won't
hear a hum, you'll hear an extremely high pitched squeal, and if you
open the front door, you'll probably find half the dogs in the
neighborhood yelping to be let in. ;-)

I'm not sure but I believe the much lower inductance of the coils in
electronic ballasts (a lower inductance in required to impede higher
freqs) also means that the potential sound energy level form vibrating
is lower.  Anyhow, squealers are rare in true electronic ballasts--once
you made a ballast that expensive, you might as well anchor and shield
the coil properly.  

Some ballasts have timer circuits to cut the current that goes through
the filaments (as opposed to the arcing current).  This saves energy in
rapid start lamps, and these ballasts are sometimes called electronic
ballasts.  But that's a misleading use of the term. 

Also, I hope you are not hearing a 15kHz squeal from your TV whenever
it's on.  It should not be eminating a 15kHz audible frequency.  That's
too much energy leaking.  Many sets will do this until they have been
on for minute or two and warm up.  But if a set does it constantly, it
needs to be fixed or replaced.

Scott H.

Do You Yahoo!?
Make international calls for as low as $.04/minute with Yahoo! Messenger