Indoor Furnishings User Manual

19
2000-11-07
4.2.2 Conditions for proper operation of the
electrodes
Electrodes in ‘TL’5 lamps consist of a coiled construction of
tungsten wire which is filled with emissive material.The lifetime of a
fluorescent lamp is determined by the lifetime of the electrode. In
order to ensure sufficient electrode lifetime, its temperature should
be kept within certain limits.Above a certain temperature the elec-
trodes will be too hot,leading to enhanced evaporation of the
emissive material and severe end-blackening.Below a certain tem-
perature the electrode is too cold and sputtering of the emitter
occurs.This may lead to extremely short life of the lamp.
If ‘TL’5 lamps are dimmed by reducing the lamp current,the power
dissipation in the electrodes will decrease, resulting in a lower tem-
perature of the electrode.Thus,in general, additional heating should
be supplied to the electrode to maintain its proper temperature.
The temperature of an electrode is primarily influenced by three
currents.A simplified diagram of electrode currents is depicted in
figure 4.4.2.
The currents I
LH
and I
LL
can be measured with a current probe
around the lead-in wire.
By definition the higher of the two currents is called I
LH
,the lower
of currents I
LL
.
If the two lead-in wires are taken together through one current
probe,one measures the lamp current I
D
.
Electrodes in ‘TL’5 lamps are designed in such a way that the lamp
current can be varied around its rated value within certain limits.
Within these limits of the lamp current,additional heating is not
strictly required.If, however,the ballast does supply an additional
heating current,the currents in the lead-in wires (I
LH
and I
LL
)
should be kept within the limits given in the tables on the following
pages (Normal operation).
If the lamp current is to be dimmed over a broader range, addition-
al heating should be supplied to the electrode for it to maintain its
optimum temperature.Also in this case the currents in the lead-in
wires should be kept within limits (Dimming operation).
Best lifetime and minimum end-blackening of the lamp is obtained
when the “target setting” for I
LH
and I
D
is observed during dim-
ming (see figures 4.2.3 - 4.2.6).With this “target setting”
(relationship between lamp current and Lead-High current), the
electrode will have its optimum temperature.
I
LH
I
D
I
LL
Figure 4.2.2: Lamp electrode
I
LH
= Lead-High (“total”) current
I
D
= lamp (discharge) current
I
LL
= Lead-Low (“heating”) current
Figure 4.2.3: “target setting” curve ‘TL’5 HE range
I
D
I
LL
I
LH
Normal operation 130 - 205 mA < 165 mA 130 - 240 mA
Dimming operation 15 - 130 mA < 165 mA 140 - 240 mA
I
D
I
LL
I
LH
Normal operation 270 - 435 mA < 365 mA 270 - 450 mA
Dimming operation 30 - 270 mA < 365 mA 310 - 450 mA
For ‘TL’5 HE range:
Note: Also the requirement I
LL
max.=165 mA should be met.
For ‘TL’5 HO 24W and 39W:
Note: Also the requirement I
LL
max.=350 mA should be met.
Figure 4.2.4: “target setting” curve ‘TL’5 HO 24W and 39W
0 100 200 300 400 500
0
100
200
300
400
500
I
LH
- Lead-High current (mA)
TARGET setting
I
D
- Lamp current (mA)
to be
published