Indoor Furnishings User Manual

21
2000-11-07
5) The data given for the additional heating by means of the Lead-
High current is directly related to the lamp current.Ballasts
which do not make use of this direct relation,should be tested
in all practical lamp operating conditions.
6) Information is only given for dimming to 10% of the rated value
of the lamp current.It is observed that,at lower dimming levels,
the temperature profile of the electrode is different from the
one at higher lamp currents.Therefore the rules that determine
the required extra heating at dimming levels > 10% cannot be
extrapolated to lower “deep dimming” levels.The rules that
determine low dimming are still under study.
However Philips, as well as other manufacturers of electronic
gear, already bring ballasts to the market which permit deep
dimming.Since no general rules can be given as yet,extensive
life testing should be done with those lamp-ballasts combina-
tions.For Philips dimming ballasts,the combination with Philips
lamps was extensively tested and released to the market.
4.3 DC operation of ‘TL’5 lamps
With DC operation of fluorescent lamps the current is flowing in
one direction.The positive mercury ions are transported to the
negative cathode.This might lead to the situation that all mercury
will disappear from the discharge area near the anode,leaving that
part of the lamp red burning.
For this reason DC operation of ‘TL’5 lamps during longer periods
must be avoided, especially when the non-stamp side, opposite of
the cold chamber, is anode (red burning will occur rather quickly
as not much mercury is present near the anode).
4.4 End-of-life behaviour and associated risks
The following four possible situations can be identified,and should
be taken into account by ballast designers:
1) The lamp does not start but both electrodes are intact.
If,for whatever reason, the lamp does not start, the ballast may
continue to supply the preheating current to the electrodes.
This may cause overheating of the lamp cap.Ballast and
luminaires should take care of maximum preheating currents
(see section 4.2.2),and IEC requirements on maximum current
in any lead,and maximum lamp cap temperature (see section
2.4.2) to avoid this overheating.
2) The lamp operates, but one of the electrodes is de-activated or
broken.
The normal end-of-life situation for fluorescent lamps is emitter
depletion of one of the electrodes.In most cases the discharge
will extinguish and the lamp will not start again, i.e.the above
situation arises.However, if the ballast is capable of sustaining
the discharge, a new condition arises.Because of the absence of
emitter material the voltage drop at cathode will rise sharply,
resulting in an extra power dissipation.It is concentrated in a
very small region in front of this electrode and is highly asym-
metric. It only occurs in that half phase when the depleted
electrode has to act as cathode and emits electrons.The same
applies when the cathode breaks and a lead wire acts as
cathode.
Especially under HF conditions the discharge is easier main-
tained,because no high re-ignition voltages occur as for
50/60 Hz operation.The extra power in the cathode fall region
may lead again to strong overheating of the cap and its surroun
dings,i.e. the glass and the lampholder.So the ballast should
limit the power in the cathode fall to a safe level,or switch-off.
3) The lamp operates, but with both cathodes deactivated or
broken.
In the above case of rectification,the ballast does not have to
switch off,but might limit the power in the cathode region, for
instance by sensing the asymmetric in the voltage.The lamp
then continues to operate.After some time also the emitter of
the other cathode will become depleted.Now on both sides an
increased cathode fall is present.The situation is symmetric
again,with a higher lamp voltage.If the ballast senses asymme-
tric only and does not switch-off,it may return to ‘normal’
operation,with extra local power dissipation in both cathode
falls.Again this will lead to overheating of the cathode regions.
So the ballast should switch off at too high values of the lamp
voltage or limit the power.
4) The lamp operates, but with a strongly increased lamp voltage.
The lamp voltage may also increase by slow leak-in of
impurities.
If the ballast is capable of sustaining the discharge, with for
instance a constant current,the lamp wattage will increase sym-
metrically together with the voltage.This higher lamp power is
not localized as above, but may still lead to overheating of the
lamp and parts of the system. So the ballast should switch off at
too high values of the lamp voltage,or limit the power it can
deliver to the lamp.