In article ,
charles writes:
In article , Andrew Gabriel
wrote:
In article , David
writes:
I am trying to fault find a fluorescent light on the 12V circuit in our
motor home.

What size fluorescent tube are you thinking of?

I would look for LED lighting. In particular, LED retrofits for 12V
electronic transformers will just work without needing anything else.

Opinion seems mixed, but there are several postings which claim that
these are actually 240v AC lights with a built in inverter so that
they can run off 12V.

That seems unlikely to me. It would make the unit unnecessarily expensive
and reduce efficiency.

Well, they do exist - as emergency light fittings. Usually the battery
supply is 3v or 4.5v, not 12v.

Different thing altogether.

The electronic ballast runs from the battery voltage.

Maintained ones (i.e. can also run light off mains) either have a
completely separate ballast for mains operation (most common case in
my experience), or the low voltage charger circuit has enough spare
capacity to run the lamp and charge the battery at the same time.

A common design for the two ballast types uses a 50Hz ballast for
mains running, and a high frequency ballast for battery running.
An advantage with this scheme is both are permenently connected to
the tube and no relay is required. When operating off mains, the
50Hz can't get back through high frequency capacitor ballast into
the high frequency ballast, and when operating on the high frequency
ballast, that can't get back through the 50Hz ballast as the impedance
is far too high at 20kHz+.

Emergency lights are in any case horribly inefficient (at least, the
minature fluorescent tube type with integral battery). They charge
the batteries at high current (has to fully recharge in 14 hours),
but they don't have any smarts inside to stop doing that when the
battery is charged, so it just goes on to waste several watts after
that. They use high temperature batteries so the resulting excess
heat doesn't destroy the batteries.

--
Andrew Gabriel
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