On Sat, 27 Dec 2014 14:21:21 +0000, David wrote:

On Sat, 27 Dec 2014 13:39:46 +0000, David wrote:

I am trying to fault find a fluorescent light on the 12V circuit in our
motor home.

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.

There is certainly a lot of circuitry inside.

Firstly, do camper/caravan/boat 12V fluorescents usually run at 240V
with a built in inverter (or sometimes)?

Secondly if so is there usually clear marking of the 12V and 240V
parts?

I don't particularly want to probe a 240V AC circuit with the 50V DC
part of my little multi-meter.

http://i817.photobucket.com/albums/z...ttleGreyCatUK/
Hymer/20141227_140052.jpg

A bit blurry but hopefully shows enough.

http://i817.photobucket.com/albums/z...ttleGreyCatUK/
Hymer/20141227_143106.jpg

shows the whole thing.

One thing doing my head in is that the wiring is blue and brown, (which
should mean 12V negative earth, I think) but to the light the blue seems
+ve and the brown -ve. If you look at the pictures you can see a blue wire
going to the switch and a red wire coming back from the switch to the
circuit board. I've put a multi-meter on it twice and both times confirmed
that blue is +ve and brown -ve. The positive is switched.

The fitting has been in the van and working since we bought the van in
2008 and only recently became progressively more difficult to get started,
and often ran at half brightness at least for a while.

It failed completely a couple of months ago and it proved not to be the
tube as a new tube didn't make any difference.

I assume the circuit board has failed but there are no obvious brown and
smoky bits.

Oh, and just found

http://en.wikipedia.org/wiki/Fluores...s_of_operation

"Fluorescent lamps can run directly from a direct current (DC) supply of
sufficient voltage to strike an arc. The ballast must be resistive, and
would consume about as much power as the lamp. When operated from DC, the
starting switch is often arranged to reverse the polarity of the supply to
the lamp each time it is started; otherwise, the mercury accumulates at
one end of the tube. Fluorescent lamps are (almost) never operated
directly from DC for those reasons. Instead, an inverter converts the DC
into AC and provides the current-limiting function as described below for
electronic ballasts."

So presumably it is likely (but not certain) that the circuit board in the
photo includes an inverter.

Cheers

Dave R

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