Main ballast functions
In chapter 2.1 of this Guide: General aspects, section 2.1: Main ballast
functions, the main functions of ballasts have been described.The term
‘ballasts’ is generally reserved for current limiting devices, including
resistors, choke coils and (autoleak) transformers. Other pieces of
auxiliary equipment are compensating capacitors, filter coils and
starters or ignitors. Some systems use an additional series capacitor
for stabilisation.
With the components summed up, all control functions which are
necessary to operate standard fluorescent lamps can be carried out.
Special arrangements, including sequence start, constant wattage or
dimming circuits will not be described in this Guide, as such circuits
are more and more being replaced by the modern high-frequency
(HF)systems.
Stabilisation
In section 3.2: Stabilisation, the need for current stabilisation in
fluorescent lamps has been described, resulting in the following two
formulae:
I
lamp
= (V
mains
- V
lamp
)
/
Z
ballast
and:
P
lamp
= V
lamp
. I
lamp
.
α
lamp
where
I
lamp
= the current through the lamp
V
mains
= the mains voltage
V
lamp
= the voltage across the lamp
Z
ballast
= the impedance of the ballast
P
lamp
= the power of the lamp
α
lamp
= a constant called lamp factor
From these formulae it can be concluded that the power of the lamp
(and therefore the light output) is influenced by:
- the lamp voltage V
lamp
, which in turn is highly dependent on
the operating temperature (see section 5.3.12:Ambient and operating
temperatures) and on the lamp current, according to the negative
lamp characteristic (see section 3.2: Stabilisation).
- the lamp current I
lamp
, which is dependent on the mains voltage (see
section 5.3.13: Effects of mains voltage fluctuations), the lamp voltage
and the linearity of the ballast impedance.
In order to avoid undesirable variations in light output as a consequence
of mains voltage fluctuations, the lamp voltage must be not more
than approx. half the value of the mains voltage (100 to 130 V) and the
impedance should be as linear as possible.
Ignition and re-ignition
In chapter 3: Lamps, section 3.3: Ignition, the need for ignition of a
fluorescent lamp has been described.
E L E C T R O M AG N E T I C L A M P
5
1 1
5 1
1 3
BALLASTS
1 2
107
C O N T R O L G E A R
