Notes: |
Well, this was a first for
me...first time I've ever bought a complete lighting fixture just
because I wanted the lamp! This was spotted in our local
Tesco supermarket. A complete table lamp, complete with the
PL-C lamp pictured here and a ballast unit...all for a few pence short
of £5. The package consisted of a small wooden based table
lamp, the PL-C lamp itself, and a magnetic ballast built into a
somewhat large plug in unit, similar to they type commonly containing
low voltage power supplies.
This lamp is one of Philips' extensive PL range of non-retrofit compact
fluorescent lamps. When compact fluorescent lamps started to
become a common sight, Philips were the first to spot the idea that the
lamps need not be directly connected to the ballast as with the type of
lamps you see designed to replace conventional incandescent
ones. This meant that a whole new world could be opened up to
the designers of the opportunity to use these extremely compact lamps
in applications where previously only miniature fluorescent tubes or
incandescent lamps could have been used. The PL series are
available in a variety of styles, the PL-C as shown here, a two-tube
version in high and low wattages (The PL-L and PL-S respectively), a
recently launched high output version in wattages of up to 120W (PL-H),
a version making use of U shaped tubes rather than the linear ones
here, the PL-T, and even a special version of the PL-L designed for use
in low temperature environments. All of these are available
in both 2 and 4 pin versions - the significance of this is explained
below.
This lamp is fitted with a G24d cap - which will look exceedingly odd
to you if you've not seen one before! The two pin cap on this
lamp contains a conventional glow-bottle starter and RF suppression
capacitor in the square section. This has the added advantage
that a new starter is fitted with each lamp - an important feature as
the cycling as the end of lamp life when connected to a magnetic
ballast is very hard on a starter, and a faulty starter could end up
sticking and destroying a perfectly good new lamp. It also
has the potential to cause ballasts to overheat and fail. The
fitment of a new one with each lamp negates this problem entirely
however. Having the starter and capacitor wired internally
into the lamp does preclude this from operation on electronic control
gear though. It was for this reason that four pin versions of
the various lamps were released a few years later, with direct
connections to each of the lamp electrodes, and no internal starter.
The unusual straight four tube design of this lamp (and others in this
range) is one of my favourites when it comes to lamp design, not least
because it was one of the first that I started seeing. It
does help in functional terms too, due to one of the characteristics of
fluorescent lamps (and in fact all metal vapour lamps). The
efficacy of these lamps is determined by the vapour pressure in the
lamp, this in turn is determined by the temperature of the coldest
point on the lamp wall - as the metal dose will condense
there. In the case of a low pressure mercury vapour lamp such
as this, a low vapour pressure is desirable - hence as low a cold spot
temperature as possible is desirable. In the case of this
style of lamp, that cold spot is created by leaving a small dead space
at the end of each of the parallel tubes. Due to the fact
that this area is not part of the path followed by the discharge -
hence it is further from the centre of the discharge column, the wall
temperature is far lower than anywhere else. This is a simple
idea to implement, and works very well for small lamps such as
this. Unfortunately on higher wattage lamps, this dead space
needs to be quite large due to the higher temperatures present, meaning
that the lamps tend to be slightly longer than those using other
methods to control vapour pressure such as external amalgams.
The ability to control the cold spot temperature like this however does
lend a certain flexibility to lamp designers though, in that they can
alter the size of the dead space at the end of the tubes to aim for the
correct vapour pressure to be achieved at different ambient
temperatures. The PL-L range for instance has a smaller dead
space at the ends of the tubes, allowing it to operate best at lower
ambient temperatures. I have noted in Aberdeen City, that the
PL-L lamps are particularly popular choices for use in the fixtures
used to provide night time illumination inside bus shelters. |