[0001] This invention relates to a starter circuit for a fluorescent tube lamp and a lamp
fitting incorporating the circuit.
[0002] Fluorescent tubes are lamps which produce light by means of an electrical discharge
in a gas which excites a phosphor coating on the tube. When in operation, the impedance
of the tube is negative and therefore requires an added series impedance so that the
operation is stable. For AC circuits the series impedance is chosen to be inductive
so as to reduce power losses and to produce the pulses for striking the tube.
[0003] Once the tube has been struck the "running" voltage is between 20 and 60 per cent
of the nominal AC supply voltage, the remainder of that voltage being dropped across
the added series impedance. The purpose of a starting circuit is to strike the discharge
in the tube and the voltage required to achieve this is higher than the running voltage
and depends on the age of the tube, its operational environment and the length of
time for which striking voltage is applied. Tubes have heated cathodes which provide
a source of ions and electrons for the discharge and reduce the magnitude of the voltage
required to strike the tube. It is possible to strike a tube when the cathodes are
cold but the striking voltage/time requirement is usually beyond the capability of
conventional starting circuits; and, in any case, the cold striking of a tube with
high voltage pulses tends to shorten its life. Moreover the high voltages required
for the larger tube sizes would call for correspondingly high voltage components,
so that cold striking is not normally used for such tubes. It is therefore a function
of starting circuits to provide a period for which current is applied to the cathodes
to heat them and a well-known circuit for achieving this includes a glow tube switch
which is used to complete a series circuit including the ballast series impedance
and the two cathode heaters, the switch itself being connected between the two cathode
heaters. When power is first supplied to the circuit, the full AC supply voltage is
applied to the glow tube in which a discharge is set up and the heat of this discharge
heats up a bimetallic strip. When sufficiently hot, this strip closes some switch
contacts which short-circuit the glow tube and cause the cathode heaters to be heated
by the supply current. After a certain period of time the bimetallic strip cools allowing
the switch to open again which interrupts the heater current and causes the ballast
impedance, which is usually an inductor, to produce an e.m.f. in addition to the supply
voltage which is usually sufficient to strike the tube. If the tube does not strike,
the glow tube will repeat its attempt to strike it as described above. After the tube
has struck insufficient voltage is applied to the glow tube for the discharge to be
set up in it and the starter remains quiescent.
[0004] The main problems with the glow tube switch as described above are that the actual
time of opening of the switch is random relative to the supply voltage so that the
actual e.m.f. applied to the tube in an attempt to strike it is frequently insufficient
so that the striking of the tube is delayed and it is preceded by an unpleasant series
of flashes at intervals of about 1 second. Furthermore, the performances of the glow
tube starters are very variable which can result in unreliable operation in some instances.
In addition, the life expectancy of the glow tube switch is unpredictable. Moreover,
the continued attempts to strike a faulty tube by such a switch can be very annoying.
[0005] In order to overcome the above disadvantages of a glow tube starter switch certain
semiconductor solutions have been proposed, but in many instances the circuits operate
in a similar manner to a glow tube switch and produce similar delays in striking the
tubes and the preliminary flashes.
[0006] It is an object of the present invention to provide an improved form of electronic
circuit for starting a fluorescent tube lamp.
[0007] According to the present invention there is provided a starter circuit for an AC
energised fluorescent tube lamp having cathodes with heaters and an inductive ballast
impedance connected in series with the lamp across an AC supply, and in which in use
the starter circuit is connected between the cathodes of the lamp itself, wherein
the starter circuit includes a switch and a control circuit for the switch which is
responsive to the presence of voltage across the tube to cause the switch to execute
a cycle of connecting together the cathodes of the tube through a low resistance path
and then breaking the connection between them several times in each half cycle of
the AC supply, whereby the starter circuit when in use causes the ballast impedance
to apply a corresponding succession of voltage pulses to the lamp.
[0008] The circuit may include a full wave rectifier producing a DC supply for the other
parts of the circuit from the alternating voltage across the lamp. The switch may
include a thyristor constructed to require a high holding current to maintain conduction
or otherwise being capable of being turned off by a signal applied to its gate and
preferably also has a zener diode connected so as to limit the voltage of the pulses
produced by the ballast impedance when the circuit is in use. The control circuit
may be responsive to the current through the low resistance path to cause the switch
to break the connection whenever the current reaches a particular value. Alternatively,
the control circuit may include an oscillator producing a rectangular wave for controlling
the switch and having a frequency and a mark to space ratio matched to the charge
and discharge characteristics of an inductive ballast impedance with which the starter
circuit is intended to be used.
[0009] The control circuit may include a circuit for delaying the start of the switching
cycles for a period of time after switching on the supply to the lamp, and therefore
to the starter circuit, to permit the cathodes of the lamp to be preheated before
the pulses are applied to the lamp.
[0010] The control circuit may be responsive to a characteristic of the voltage across
the lamp to detect when it strikes and then terminate the switching cycles for at
least the remainder of the particular half cycle of the supply. A timing circuit may
be provided to terminate the switching cycles if the lamp does not strike within a
predetermined time of being switched on.
[0011] In order that the present invention may be fully understood and readily carried
into effect an example of it will now be described with reference to the accompanying
drawings, of which:
FIGURE 1 is a block diagram of the complete energisation circuit for a fluorescent
tube lamp incorporating a starter circuit according to the present invention;
FIGURES 2A to 2D represent waveforms occurring in parts of the circuits shown in Figure
1 which will be used to explain the operation of the circuit; and
FIGURE 3 is a detailed circuit diagram of a starter circuit according to the invention.
[0012] The present invention relates to a starter circuit for a fluorescent tube lamp and
to a lamp incorporating such a circuit. Although the operation of the circuit could
be described without reference to the other components used in the energisation circuit
for the lamp, the point of the various functions of the starter circuit would only
become apparent from a consideration of their effects in the overall energisation
circuit with a lamp and therefore for convenience the entire energisation circuit
and its manner of operation will be described.
[0013] Referring now to Figure 1, an alernating mains electricity supply is connected to
terminals 1 and 2, the supply typically having a voltage of 240 volts. The terminal
1 is connected via an on/off switch 3 and a ballast inductance 4 to a cathode 5 of
a fluorescent tube lamp 6. The lamp 6 has a second cathode 7 which is connected directly
to the terminal 2. The cathodes 5 and 7 incorporate heaters and these take the form
of electrical resistances through which current is caused to flow from the supply
terminals 1 and 2 to the starter circuit 8 connected between the ends of the cathodes
5 and 7 remote from the terminals 1 and 2. A capacitor 9 is connected at nodes A and
B in parallel with the starter circuit 8 for the purpose of suppressing radio frequency
interference due to the action of the starter circuit in conjunction with the inductance
4.
[0014] The starter circuit 8 includes a full wave bridge rectifier 10 producing a DC supply
across conductors 11 and 12. The conductors 11 and 12 are joined by a zener diode
13 and a controllable switch 14. A control circuit 15 also receives the DC supply
and produces on a conductor 16 control signals for the switch 14. Although the switch
14 is shown as a mechanical switch, it would in practice either be a thyristor able
to be turned off by a signal applied to its gate or a device known as a fluoractor
as described in European Patent Application No. 0118309. In the case where switch
14 is a fluoractor, the zener diode 13 is incorporated into it. The control circuit
15 may include means connected in series with the switch 14 for responding to the
current through it.
[0015] In the operation of the circuit shown in Figure 1, the control circuit 15 is arranged
to open and close the switch 14 at about 300 to 400 times per second so that in each
half cycle of the AC supply, assumed to be of 50 Hz, there are three to four pulses
applied to the lamp 6 when the switch 14 is open-circuited and the energy stored in
the inductance 4 is discharged to produce the pulses. The zener diode 13 operates
to limit the voltage of the pulses produced by the inductance 4 and in doing so extends
their duration, it having been found that a pulse of slightly lower voltage but of
longer duration is more effective to cause a fluorescent tube lamp to strike.
[0016] Ignoring for the present the matter of providing a preheating time for cathodes of
the lamp 6, since it has been found that a starter circuit according to the invention
can strike a fluorescent tube lamp when its cathodes are cold without causing significant
damage to the lamp, the circuit has the advantage that on switching on the lamp produces
a progressively increasing amount of light without apparent flashing until the lamp
strikes. If a preheating time is provided, for example by being built into the control
circuit 15, then the progressive brightening of the lamp will follow after, say, half
a second delay normally provided for the preheating of the cathodes.
[0017] Figure 2A represents the AC supply and Figure 2B is the supply after full wave rectification
by the rectifier 10. Figure 2C represents the current pulses in the inductance 4 in
each half cycle of the mains supply, the half cycles being marked by the lines 17.
Figure 2D represents the voltage across the lamp 6. As shown in Figures 2C and 2D,
in the first half cycle the switch 14 is opened three times and three pulses of voltage
V determined by the zener diode 13 are applied to the lamp 6. It is assumed that the
lamp does not strike during this half cycle, but the gas in the lamp will be heated
and the lamp caused to produce a small amount of light output which will not, however,
appear as flashing because the frequency of the pulsing is too high. During the second
half cycle the lamp is assumed to strike on the second pulse which therefore appears
shortened in Figure 2D, so that the voltage across the tube falls to V1 as determined
by the conduction of the lamp. In this half cycle the lamp produces a greater amount
of light because it is struck for about half the time. In the third half cycle the
lamp is assumed to strike on the first pulse and produces even more light by having
been struck for about three-quarters of the half cycle. After this the lamp will probably
remain struck because the gas inside will have become sufficiently heated and the
control circuit 15 is arranged not only to detect the striking of the lamp and to
terminate the pulses in the rest of the half cycle in which the striking occurred
but also to discontinue the application of pulses to the switch 14 after detecting
the striking of the lamp on the first pulse in preceding half cycles or after a predetermined
period of time.
[0018] The striking of the lamp can be detected in several ways, for example by measuring
the duration of the voltage pulse across the lamp and noting when it is greatly reduced
as a result of the striking of the lamp, measuring the voltage across the lamp and
noting the rapid reduction in that voltage as a result of the lamp striking, or monitoring
the current through the zener diode 13 and noting when it is interrupted. The ends
of half cycles of the AC supply could be detected by a zero voltage condition between
the conductors 11 and 12.
[0019] The generation of the pulses used to open-circuit the switch 14 may be achieved by
monitoring the current through the switch which will be dependent on the current through
the inductance 4 and when the current through the switch reaches a predetermined value
corresponding to a required energy charge in the inductance 4, the switch 14 is open-circuited.
This will result in the pulses applied to the lamp 6 being of substantially the same
energy which can therefore be adjusted to provide optimum starting conditions for
the lamp without causing damage to its cathodes. If preheating of the cathode is not
employed, it is desirable to restrict the energy of the starting pulses so as to warm
up the gas in the lamp without disrupting the material from the cathodes.
[0020] Figure 3 shows in detail the circuit diagram of the starter circuit 8 of Figure 1.
The nodes A and B of the circuit of Figure 1 are marked in Figure 3 connected to the
rectifier bridge 10 which produces a DC voltage across conductors 11 and 12. The switch
14 and the zener diode 13 are combined in a fluoractor 20 connected from the conductor
11 through a series of four diodes D5, D6, D7 and D8 to the conductor 12. The junction
of the fluoractor 20 and the diode 5 is connected through a diode D9 to a conductor
21 on which a steady voltage is maintained relative to the conductor 12 smoothed by
a capacitor C1. The voltage on the conductor 11 is not smoothed and retains the form
shown in Figure 2B.
[0021] Initially, the fluoractor 20 is conducting as a result of the current through a resistor
R2 from the conductor 11, the transistor TR1 bein non-conducting at this time. The
remaining parts of Figure 3 constitute the control circuit 15 of Figure 1 and will
now be described. A comparator IC1A compares the voltage across a resistor R1 shunting
the diode D8 with the steady voltage established at the junction of resistors R3 and
R4 connected in series between conductor 21 and conductor 12, and the output of the
comparator IC1A is appllied to the set input of a D-type flip-flop IC2 which causes
the Q-output of IC2 to go high when the voltage across the resistor R1 due to the
current flowing through fluoractor 20 and down the diode chain reaches a predetermined
value. This causes the transistor TR1 to become conducting which switches off the
fluoractor causing a pulse to be applied to the lamp. If the lamp does not strike
the decaying of the voltage pulse across the lamp 6 to below 400 volts will be detected
by a comparator IC1C which compares a proportion of the voltage between conductors
11 and12 determined by resistors R15 and R16 in series with a voltage at the junction
of resistors R12 and R13 of the series chain R12, R13 and R14 connected from conductor
21 and conductor 12. The output of the comparator IC1C clocks the flip-flop IC2 which,
as its D-input is low for reasons to be explained, causes its Q-output of IC2 to
become low again switching off TR1 and allowing the fluoractor 20 to turn on again
for another cycle as just described.
[0022] If the lamp does strike, the voltage pulse across it will be shortened and there
will be a rapid decrease in the voltage of the pulse. This rapid decrease is detected
by the transistor TR2 in conjunction with the differentiating circuit formed by capacitor
C3 and resistor R11. This causes the collector of the transistor TR2 to go low which
in turn via the comparator IC1B causes the D-input of the flip-flop IC2 to go high.
This rapid decrease occurs before the pulse decays below 400 volts as mentioned above
so that under these circumstances the D-input is high before the flip-flop IC2 is
clocked by comparator IC1C. As a result the Q-output of the flip-flop IC2 remains
high and the transistor TR1 is kept conducting so that the fluoractor 20 is maintained
in the nonconducting condition and the lamp can continue its conduction, at least
until the next half cycle. If the lamp does not strike the D-input of IC2 will remain
low as mentioned above.
[0023] A comparator IC1D detects when the voltage on the conductor 11 falls below 30 volts
positive with respect to conductor 12 and through the NAND-gate IC3 resets the flip-flop
IC2, so that its Q-output becomes low again and the transistor TR1 is turned off so
that the fluoractor 20 can be triggered into conduction at the beginning of the next
half cycle. The NAND-gate IC3 serves to eliminate simultaneous application of a low
condition to both the R and S inputs of the flip-flop IC2 because of the control of
the NAND-gate IC3 by the Q-output of the flip-flop.
[0024] As thus far described, the circuit turns off the fluoractor 20 repeatedly in each
half cycle until the lamp strikes when the fluoractor 20 is held in the off condition
up to the end of the half cycle. In order to terminate the striking pulses once the
tube has been struck, a timing circuit formed by resistor R19 and capacitor C5 produces
a rising voltage which is applied to a comparator IC4A the output of which is connected
through diode Dll to the set input of the flip-flop IC2 to prevent it being reset
again until the mains supply is turned off and then on again.
[0025] When the mains supply is turned off, the ripple on the conductor 11 is no longer
conveyed by capacitor C4 to the comparator IC4B which permits the capacitor C6 to
be charged up by current through resistor R22 thereby turning on the transistor TR3
and discharging the capacitor C5 through it. Whilst the mains supply is maintained,
negative-going pulses from the comparator IC4B keep the capacitor C6 discharged.
[0026] Although, as mentioned above, it is not necessary to preheat the cathodes of the
lamp before applying pulses to it, it may be desirable to do so to avoid any possibility
of damage to the cathodes resulting from the application of the pulses to the lamp
before the cathodes are heated. A preheating delay can be provided in the circuit
by connecting a capacitor C2 in parallel with resistor R3 which thereby causes the
voltage applied to the non-inverting input of the comparator IC1A to start substantially
at the potential of the conductor 21 and to fall gradually to the normal operating
value as the capacitor C2 is charged up by current through resistor R4. This means
that the output of the comparator IC1A is prevented from going low in response to
the voltage developed across the resistor R1 for a short period of time. This permits
the cathodes of the lamp to heat up before fluoractor 20 is turned off for the first
time and pulses start to be applied to the lamp.
[0027] From the above description, it will be appreciated that the circuit of Figure 3 includes
certain of the alternatives mentioned above in the description of the control circuit
15. The circuit of Figure 3 could be modified in a number of ways. For example, a
capacitor could be connected across the resistor R4 instead of resistor R3 with the
result that the width of the pulses applied to the lamp would start at a low value
and increase gradually to the normal value because the comparator IC1A would be caused
to trigger the flip-flop IC2 at lower levels of current through the fluoractor 20
than normal. This latter feature could be combined with a pre heating delay by the
use of an additional comparator to block the start of the gradually increasing pulses
until after the preheating delay, which would be useful for any lamps which are susceptible
to "end blackening" when pulsed before the cathodes are sufficiently heated.
[0028] Instead of the circuit described for turning off the fluoractor 20 a fixed frequency
oscillator could be provided having a frequency and mark to space ratio so chosen
with reference to the inductance 4 to provide the most appropriate pulse energies
for different lamps. Small fluorescent lamps require higher values of inductance,
and if the triggering of the pulses is based on the current through the inductance
as described above, then the smaller tubes would receive larger energy pulses which
they do not require. With the time of the charging current applied to the inductance
being fixed, the higher values of inductance will have a smaller current through them
because of the lower rate of increase of current and thus smaller tubes would receive
more appropriately sized striking pulses.
[0029] The analogue timing circuits used in Figure 3 could be replaced by a pulse generator
and counters which would permit much of the circuit to be realised by a custom-built
gate array which would avoid the need to design an integrated circuit specifically
for it.
1. A starter circuit for an a.c. energised fluorescent tube lamp having cathodes with
heaters and an inductive ballast impedance connected in series with the lamp across
an a.c. supply, and in which in use the starter circuit is connected between the cathodes
of the lamp itself, wherein the starter circuit includes a switch and a control circuit
for the switch which is responsive to the presence of voltage across the lamp to cause
the switch to execute a cycle of connecting together the cathodes of the lamp through
a low resistance path and then breaking the connection between them several times
in a half cycle of the a.c. supply, whereby the starter circuit when in use causes
the ballast impedance to apply a corresponding succession of voltage pulses to the
lamp.
2. A circuit according to claim 1 including means responsive to the occurrence of
a characteristic change in the voltage across the lamp indicative of the lamp striking
to terminate the cycles of operation of the switch for at least the remainder of the
particular half cycle of the a.c. supply in which the characteristic change in the
voltage across the lamp occurs.
3. A circuit according to claim 2 wherein the control circuit includes means responsive
to the voltage across the lamp not exceeding a preset value indicative of the lamp
being struck to prevent the switch from closing the connection across the lamp.
4. A circuit according to claim 2 or 3 wherein the characteristic change in the voltage
across the lamp is a rapid decrease in that voltage which occurs before the voltage
falls below a threshold value.
5. A circuit according to claim 2 or 3 wherein the characteristic change in the voltage
across the lamp is reduction in the duration of the voltage pulse across the lamp.
6. A circuit according to claim 5 including a zener diode connected in parallel with
the switch for limiting the voltage of the pulses produced by the ballast impedance,
wherein the characteristic change in the voltage across the lamp is detected by means
responsive to the current through the zener diode.
7. A circuit according to any one of claims 1 to 5 including a rectifier through which
the switch and the control circuit are connected to the cathodes of the tube, so that
the a.c. voltage across the lamp is applied in rectified form to the switch and the
control circuit.
8. A circuit according to claim 7 wherein the rectifier is a full wave rectifier.
9. A circuit according to claim 7 or 8 wherein the switch is a thyristor constructed
to require a high holding current to maintain conduction or otherwise being capable
of being turned off by a signal applied to its gate and the control circuit is connected
to turn on and off a current fed to the gate of the thyristor.
10. A circuit according to claim 9 including a zener diode connected in parallel with
the controlled current path of the thyristor to limit the voltage of the pulses produced
by the ballast impedance when the thyristor is turned off.
11. A circuit according to claim 10 wherein the zener diode is incorporated into the
thyristor.
12. A circuit according to any one preceding claim wherein the control circuit includes
means responsive to the current applied to it through the ballast impedance tending
to open-circuit the switch whenever that current reaches a particular value.
13. A circuit according to any one of claims 1 to 11 wherein the control circuit includes
an oscillator producing a rectangular wave for controlling the conductivity of the
switch, the rectangular wave having a frequency and a mark to-space ratio matched
to the charge and discharge characteristics of an inductive ballast impedance with
which the starter circuit is intended to be used.
14. A circuit according to any one preceding claim wherein the control circuit includes
means for delaying the start of the switching cycles for a period of time after switching
on the a.c. supply, thereby to permit the cathodes of the lamp to be preheated before
voltage pulses are applied to the lamp.
15. A circuit according to any one preceding claim wherein the control circuit includes
a timing circuit responsive to the period of time for which the a.c. supply has been
switched on, to terminate the switching cycles after a predetermined time period.
16. A circuit according to any one preceding claim constructed using electrical analogue
circuit techniques.
17. A circuit according to any one of claims 1 to 15 constructed using digital circuit
techniques.