Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a ballast unit
for gas discharge lamps, in par~icular for fluorescent
tubes, and mercury and sodium vapour lamps.
The object of the present invention is to provide
a ballast unit for use with various types of gas discharge
lamps, which unit is technological]Ly better and/or less
expensive than known units and which fulfils at least some
of the following:-
(a) the starter part of the unit is not to have any
mechanical or moving parts and is thus to minimize wear andmaximise dependability; .
(b) the unit is not to have any additional trans-
formers, iron cores other than the iron core requlred for - .
the choke9 in order not to increase welght, size and iron
losses needlessly;
(c) the unit is to use an increased preheating
- current higher than the rated current of the lamp in order
to ensure flicker-free starting of the preheated lamp;
(d) cut-off of the preheating is not to depend on .. ~ ;
static conditions, such~as a favourable ambient temperature :.
which may trigger a cold start~but instead a certain minimum.
safe quantity of preheating energy is necessary to ensure ...
the cathoda glow temperature in any conditions; and
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(e) the unit is to eliminate any possibility of
overheating or burn-out in the event of a fault arising, i.e.
` iD the event of a faulty electrode, impaired gas fill etcO
According to the invention there ls provided: a
ballast unit f.or a gas dlscharge lamp having preheatable
, ~ .
' electrodes, said unit comprising: a choke for limiting the
30~ current to the lamp after:firing of the lamp and connactible
in series with the lamp electrodes; a semi-conductor switching
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means,having a control electrode, for pre-heating of the -
electrodes of the lamp, said switching means being connectible
in an inductande-free circuit path in parallel with said lamp
electrodes; and a control circuit operating in response to
an AC voltage supplied to said lamp electrodes and connected
to said control electrode for triggering said semi-conductor
switching means during successive half cycles or alternate ~ :
half-cycles of said AC voltage to heat the lamp electrodes .
by the current passing through the circùit path in paràllel
with the lamp electrodes, the control circuit being arranged
to reduce the switch-on time of the switching means and hence :~
the preheating periods during successive half cycles or
alternate half cycles of the AC voltage until the lamp fires,
said control circuit further prov~ding progressively increas-
ing voltage peaks across the lamp during each preheating
period prior to reaching the lamp ignition voltage, thereby
avoiding cold starting of the lamp; said control circuit
means including: a storage device capable of storing a charge
. dependent on the voltage supplied to the lamp, and a discharge
resistor connected to said charge storage device, the com-
bination of the storaga~device and said discharge resistor
bein8 arranged to govern the point in time at which the
control voltage reaches a sufficient level to trlgger th :~
switching means during each successive half cycle or alternate
:ha~lf cycles of the AC voltage; and an impedance nètwork
which in use, is coupled to the choke through the preheatable ~:.
lamp electrodes and which, when the switching means is non-
conductive, provides protection against ignition of the
discharga lamp whilst, in use, the preheatable electrodes
~ of the lamp are~cold, said network being further arranged to .
: eleminate the effect of high frequency harmonics of the ~C
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voltage on said control voltage.
Embodiments of the invention will now be described
by way of example with reference to the drawings, in which:
Figs. 1 and 2 show circuit diagrams of two embodi~
ments.
The embodiments of Figures 1 and 2 have a ballast
coil 104 with an inductance which is 2-3 times smaller than
that in known circuits. A capacitor 103 is arranged in the
discharge path in series with this inductance 104. As the
current or voltage wave form of the fired tube is approximately
square, consisting in a first approximation of a fundamental
frequency component and a third harmonic component9 the values of -
~the inductance 104 and the capacitor 103 are chosen so that the
:.
! impedance of this network is inductive for the fundamental fre-
quency component of the mains supply and capacitive for the third
hsrmonic of the mains supply. With consideration of the amplitude
with which those signals occur an almost complete compensation of
reactive current components thus occurs. As the series inductance
is two to three times lower than that in known units, the firing
voltage of the lamps is higher, permitting two lamps to be
connec~ed in series, each of which formerly required a ballast
unit of its own. Thus, the reduced inductance and the pos-
~slbility of operating two tubes in series with a single
baIlast unit offers substantial savings.
The ballast unit shown in Figure 1 comprises the
series capacitor 103, the series inductance 104, which may
have the form of an air gap choke, for instance~ and two
~series connected tubes 105a and 105b. The outer electrodes
of the series connected tubes are connected in series with
the inner electrodes via saturable chokes 106a and 106b.
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~; Inst~ad, of course, there may be only one saturable choke 106.
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The arrangement also includes an interference suppressor
capacitor 121. To protect the ballast unit from overload,
the circuit of the saturable choke Ls provided with voltage
dependent resistors 107a and 107b, while the main circuit
includes a thermal switch 108 bridged by a resistor.
The saturable chokes 106a and 106b are so dimensioned
that because of their steeply rising reactance they at first
only present a very low resistance to the heating current.
It~is possible to operate this circuit even without the
control circuit indicated in the right hand part of Figure 1,
the tubes lOSa, 105b being ignited by the voltage occurring
across the saturable chokes 106a and 106b as well as across
the positive temperature coefficient resistors 107a, 107b.
However, to improve the firing properties of the
arrangement shown in Figure 1, ~n accordance with the
invention, a thyristor 112 is connected in parallel with the
chokes and is arranged to conduct an additional heating
current on alternate half cycles of the ma~ns supply, accord-
ing to the charged state of the capacitor 116 which i8 ,:
'20 charged during- the remaining half cycles of the main supply.
The charging of the capacitor 116 is effected via a diode
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~ lO9 and a resistor 111. The resistors 110 and 113 ensure
;~ corresponding voltage potentials at the control electrode
o~;t~he thyristor 112. The limiting diodes 114 and 115, ;
which are controlled across the resistors 118 and 120 and
acr~oss a diode 119 under the influence of a capacitor 117,
act as overload protection for the thyristor 112.
For a 50 cps mains~supply, exemplary values of
the~;c~i~rcuit components used in Figure 1 may be as follows: ~ 1
~ Capacitor~103 ................... 4 VF j!
Choke 104 ................ 2=300 Ohms
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75~51
Resistor 110 ............. 1 megohm
Resistor 111 ............. 150 kilohms
Resistor 113 ............. 330 kilohms
Capacitor 116 ............ 47nF
Capacitor 117 ............ 350uF
Resistor 118 ............. 10 kilohms
Resistor 120 ............. 1 kilohm
The embodiment shown in Figure 2 comprises an
auxiliary winding 136 which is connec~ed across the inner
electrodes of two tubes 105a and 105b connected in series
and which is wound on the same core as the inductance 104.
Owing to a heavy heating current which flows through the
outer electrodes of the two tubes the winding 136 develops
a relatively high heating voltage for the inner electrodes
of the tubes. The electronic circuit developing the heating
current comprises a triac 122, a resistor 131 and a diac 129.
A timing circuit 125, 126 i8 connected via a rectifier circuit
127, 128 to a resistance network 123, 124~ a capacitor 132
and a further capacitor 130. This network functions as
. 20 follows: On positive half cycles of the ma~ns voltage, the
capacitors 132, 130 and 126 are charged through diode 128.
As oon as the voltage across the capaci~or 132 reaches a
:- ~ . - .
certain value the diac 129~triggers the triac 122. Firing
of~the~triac shorts the circuit and the capacitors 132 and
130~will~discharge~rapidly ~hrough diode 127 whereas the
capacitor~126 wil1 discharge only very 910wly due to its
para;llel connection with the resistor 125.
After~the mains voltage~ crosses zero-voltage and
becomes~negative,~ the;t~riac will beoome non-conduotive.
30~ Durin~g the negative half~oyoles, the capacitors 132 and 130
will be charged to respective negative voltages, through the
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diode 127. When capacitor 132 reaches a suitable value of
negative voltage, the triac will be fired again. As the
conductive triac approximately represents a short-circuit
path, the capacitors 132, 130 and 126 will then be connected
in parallel. Consequently, when the trlac 122 conducts the
capacitors 130 and 132 will not discharge completely and
there will be a charge exchange between the capacitors until
the sum of the voltages of capacitors 130 and 132 reaches
the same magnitude as the voltage across capacitor 126.
When the mains voltage again swings positive, the
triac will become non-conductive. As the capacitor 126 still
has a positive voltage, that is the voltage at the anode of
diode 128 is positive, and the capacitors 130 and 132 still
have a negative voltage, that is the voltage at the cathode
of diode 128 is negative, this latter diode will only conduct
after the difference between the mains voltage and thè voltage
across capacitor 126 reaches a value equal to or greater than
the sum of the voltages of capacitors 130 and l32. Therefore
diode 128 will start to conduct after the zero crossing of
the mains voltags and, as the voltages of the capacitors 132
and 130, have first to be reversed before reaching a positive
f~ring value the firing of the triac 122 will be shifted
ln the positiYe half cycles of the mains voltage towards the
apex of the mains voltage. Therefore the more the capacitors ;
132 and 130 are charged to a negative voltage during nega~ive
half~cycles of the malns, the greater the delay before the
voltage of capacitor 132 reaches a sufficiently positive
vala~e~ in~the subsequent positive half cycles for triggering
the diac 129~and hence the~triac 122. This shifting of the
30~firing point of the trlac 122, is accompanied by, as already
described in relation to the previous embodiments3 a
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shortening of the heating periods durlng which a heating
current flows through said triac, and also by an increase
in the voltage induced in the choke 104, each time the triac
conducts, for the purpose of igniting the tubes 105a, 105b.
If the heating time in the positive half cycles gets so short
that the capacitors 132 and 130 still keep positive voltages
when -the mains voltage swings negative, the voltage of the
capacitor 132 will only reach a sufficient level to fire the
triac 122 a certain length of time after the zero-crossing
of the mains voltage. Therefore, firing of the ~riac 122
will be shifted in the negative half cycles too. The circuit
path through resistor 123, capacitor 126 and either diode
127 or 128, continuing through capacitor 130 and 132 provides
protection against cold ignition of the tubes and protection
against hlgh frequency signals acting on the triac 122. The
network comprising resistor 133, capacitor 135 and parallel
connected vol~age dependent resistor 134 provides overvoltage
protection for the triac~
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A preferred embodime~t has the following values:-
Resistors Capacitors
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12368 kilohms 1261 uF
.:
12~1 megohm 13068 nF
125-2.2 ~egohms 132150 nF
1311 kilohm 135100 nF
133 33 kilohms
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