T&T: Fluorescent lights

[Candy Chapman and Gary Bell]

Todd Lanning asked:

 
We have recessed 12v florescent lights in the salon (4) & aft  cabin (2), 
they are up under the wood window trim.  Last year the ones in  the salon all 
stopped working at the same time <snip>  I  have tried changing out the tube light 
bulbs but to no avail.  Is there  something in one of these lights that would 
cause the other(s) to not  work.....i.e. why would both lights stop working at 
the same  time?? 
 

<>I believe that your lighting all failed simultaneously when the tiny 
little inverter circuit portion of the 'ballast' circuit board lurking 
in the housing of each light was faced with dropping DC house battery 
voltage, meaning it drew higher and higher current until a semiconductor 
or transformer failed.  This 12V DC ballast must convert the input DC 
voltage to a much higher AC voltage to start the arc in the mercury 
vapor inside the lamp, and then automatically switch to a somewhat lower 
AC voltage while limiting the AC current flowing through this now very 
low 'resistance' arc.  Imagine leaving the lights on in the saloon one 
evening when the house battery voltage dropped overnight -- when you 
return the lights are off, and the next time you try to turn them on, 
nothing.  That's what happened to me on the window valance lighting on 
the 'mezzanine' on our PDQ.  I found that the ballasts fail regularly on 
this type of lighting, and Campers World (the chain of RV supply stores) 
no longer will sell these ballasts because they were getting most of 
them back from irate amateur installers who either got the DC polarity 
wrong  or had a repeat of the low voltage condition, or failed to 
properly match the ballast to the lamp bulb.  I just Googled "12 volt 
fluorescent ballast" and got a slew of hits, with a wide range of 
pricing.   I have also refrained from replacing my ballasts, and am 
investigating a much more costly LED solution.  Problems there -- beyond 
the cost -- are the color of the light, select the circuitry to power 
the little beasties, and finding time to fabricate the strips of enough 
of the right LED's to do the job properly.  Benefits of course will be 
tiny current draw and virtually limitless bulb life.  I'll report the 
results when I get round to the project.

Mister Science