EP0011410B1

EP0011410B1 - Electronic starter circuits for discharge lamps

Info

Publication number: EP0011410B1
Application number: EP79302368A
Authority: EP
Inventors: Mark Weinberg; Mendel Krichevsky
Original assignee: Ben Gurion University of the Negev Research and Development Authority Ltd
Current assignee: Ben Gurion University of the Negev Research and Development Authority Ltd
Priority date: 1978-11-06
Filing date: 1979-10-30
Publication date: 1983-06-01
Also published as: DE2965584D1; IL55875A; IL55875A0; JPS5566897A; EP0011410A1

Description

The present invention relates to electronic starter circuits for igniting discharge lamps.
In FR-patent application No. 2041024 there is proposed an electronic starter circuit for a discharge lamp provided with pre-heatable electrodes, each of which has a first terminal connectable to a respective input terminal of an AC voltage source and one of the first terminals being connected to the AC voltage source through an inductance. The circuit comprises a thyristor switch connected in series to a pair of second terminals of the lamp electrodes, and an ignition circuit which repeatedly renders the thyristor switch conductive during a period which does not exceed a given limit if the lamp fails to ignite.
The starter circuit proposed in the above application further comprises a first capacitor having first and second plates respectively connected to the anode of the thyristor through a diode and to the gate of the thyristor to a Shockley diode or diac and a second capacitor and appropriate discharge resistors. When the circuit is operating, the first capacitor receives during each positive half-cycle a charge which is greater than that which it loses through the discharge resistor during the following negative half-cycle, thereby progressively increasing its charge and consequently decreasing the charge which is taken by the second capacitor during successive positive half-cycles. The interval of time during which the thyristor is open is progressively reduced and finally reaches a zero.
This previously proposed electronic starter circuit therefore operates for a predetermined number of cyclces irrespective of whether the lamp ignites. If the lamp does ignite during the operation of the circuit it will be repeatedly turned off and on and will therefore create an annoying flicker until the predetermined number of cycles is reached. Also wastage of energy will occur due to the operation of the starter circuit when this is no longer required.
According to the present invention, there is provided an electronic starter circuit for a discharge lamp having pre-heatable electrodes provided with respective first terminals connectable to a pair of input terminals of an AC voltage source and respective second terminals between which a thyristor switch is connected, said starter circuit further comprising an ignition circuit connected to the switch to render it repeatedly conductive during a number of cycles which does not exceed a given limit if the lamp fails to ignite and including a capacitor which is connected to the anode of the switch through a diode which is conductive when said anode is positive with respect to the cathode of the switch and to the gate of the switch through a threshold element, the number of cycles for which the ignition circuit repeatedly renders the switch conductive being dependent on the capacity of the capacitor, characterised in that the gate-connected side of the threshold element is connected to the cathode of the switch through an impedance and in that the threshold element of the ignition circuit has its threshold level set so that it is inhibited from conducting when the voltage across the switch falls to the level it adopts when the lamp is ignited.
In contradistinction to previously proposed starters operating on the principle of the voltage level which is applied across the lamp, the starter of the present invention operates on the principle of ignition time, resulting in a safe, flickerless and a more rapid fluorescent-lamp starting system.
Basically, the electronic starter of the present invention includes a controlled switch which is constituted by a thyristor and an ignition circuit. The term thyristor used herein is meant to refer to the electronic solid state components, such as, a silicon controlled rectifier (SCR), a Diac, a Triac and their equivalents.
The actuation of the controlled switch, i.e. the rendering of said thyristor into its conduction state in order to pass current therethrough for igniting the lamp, is repeatedly effected for a predetermined period of time by means of properly chosen values of the components of the ignition circuit. This predetermined period of time is chosen or adjusted to be of a duration which is always at least equal to, and preferably greater than the longest duration required for a positive sure ignition of a lamp under the commonly acceptable conditions for igniting the kind of lamps for which the proposed starter is intended. With the starter according to the present invention, in case when the lamp did not ignite during said predetermined period of time due to a malfunction, the starter circuit ceases to conduct after said period of time as the thyristor is switched back to its nonconductive state.
This independence of the ignition time of the proposed starter from the actual starting process, as opposed to the case in many previously proposed electronic starters, the operation of which is based on the change of the voltage level applied to the lamp during the ignition period, facilitates the provision of a novel universal starter suitable for igniting a wide range of discharge lamps of various operating potentials and output powers.
Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a circuit diagram of a starter according to the invention:
Figure 2 are waveforms of voltages and currents appearing in the circuit of Figure 1 plotted against time; and
Figure 3 shows a circuit diagram of another embodiment of a starter according to the invention.

In Figure 1 there are shown input terminals 2 and 4 which are intended to be connected to a standard AC supply source. The terminals 2 and 4 are shunted by a series arrangement of an inductance (choke) 6 and a discharge lamp 8. The lamp 8 is provided with preheatable electrodes 10 and 12. The terminals of the electrodes which are remote from the terminals 2 and 4 are respectively connected across a starter circuit including an ignition circuit, a capacitor 14 and a SCR 16, which capacitor is adapted to protect the SCR against pulses of high voltage from the mains.
The components of the ignition circuit include a threshold element in the form of a zener diode 18, connected between the gate of the SCR 16 and a first capacitor 22, a diode 20 connected between the anode of the SCR 16 and the plate of the capacitor 22, remote from the zener diode 18, and a discharge resistor 24 in parallel with the diode 20. There is also provided a capacitor 28 which protects the gate of the SCR 16 against impulses of high voltage which may appear in the circuit and which also acts as an impedance to establish a potential between the gate of SCR 16 and the cathode of the SCR 16.
The operation of the starter will now be described with reference also to Figure 2.
Upon the actuation of the starter, a voltage Vst of an AC voltage source is applied across the terminals 2 and 4. As long as the potential across the zener diode 18 does not exceed its threshold value, capacitor 22 does not charge and there is no current flow through the capacitor (see V_C22 and l_c22 in Figure 2).
When the potential across the zener diode 18 reaches the diodes threshold value V_z18, capacitor 22 starts to charge and the charging current passes through the diode 20. The gate of the thyristor 16 will tender the latter conductive and the thyristor will fire, when the potential across it reaches its firing voltage V_th' At the current zero-crossing point (which is after the voltage zero-crossing due to the phase shift introduced by the choke 6), the thyristor will close and there will be formed an instant high voltage pulse for igniting the lamp. The capacitor 14 and the choke 6 effectively act to widen the igniting pulse.
If the lamp 8 is not ignited, in the following cycle capacitor 22 receives an additional charge V as is seen in Figure 2. When the capacitor 22 becomes fully charged, after the predetermined number of cycles, the thyristor 16 will no longer fire and the heating of the lamps electrodes will cease. On the other hand, if the lamp 8 does fire, the voltage across the lamp will become lower and the zener diode 18 in combination with the capacitor 22 will no longer be capable of firing the thyristor, the threshold value of the diode being set to enable this effect to be achieved.
Only when the starter is deactuated, will the potential accumulated on capacitor 22 be discharged through the resistor 24.
As can be further seen in Figure 1, advantageously, there may be provided a second capacitor 26, connected in parallel with the capacitor 22 between the cathode of the SCR 16 and the plate of the capacitor 22 remote from the zener diode 18. In this case the combined values of the two capacitors 22 and 26 will determine the period of time (or the number of cycles) in which the ignition circuit repeatedly renders the thyristor conductive to ignite the lamp. The second capacitor 26 also facilitates a substantial decrease in the values of the capacitor 22 and of the discharge resistor 24, and this second capacitor will cause the current which passes through the zener diode 18 and the thyristor 16, to be advantageously smaller.
When the second capacitor is in circuit, upon the actuation of the starter, it is first charged through the diode 20. Until the potential across the capacitor 26 reaches the breakdown voltage of the zener diode 18, there will be no current flow through capacitor 22. During the heating of the lamp's electrodes, capacitor 26 discharges through the resistor 24 and since the potential across the capacitor 26 is higher than the potential across the capacitor 22, the latter capacitor does not discharge.
A typical example of the values chosen for the starter of Figure 1 adapted to ignite a 40 W lamp operating from a 220 V AC source, is as follows:

SCR 16 - 400 V, 1A
Zener diode 18 - 170-200 V
Capacitor 14 - 5000-15,000 pF
Capacitor 22 - 0.2-1 µF
Capacitor 26 - 0.04-0.2 µF
Capacitor 28 - 0.1-0.3 pF
Resistor 24 - 560 KQ

In Figure 3 there is shown a modification of the circuit of Figure 1 in which the discharging resistor 24 is connected in parallel with the second capacitor 26. Also, instead of the capacitor 28 of Figure 1 there is provided a resistor which, together with the zener diode 18, determines the potential at which the thyristor is ignited or conducts. The added diode 32 serves to provide additional protection to the circuit from high voltage impulses originating at the AC source to form impulses which are formed during the ignition process. Otherwise, the operation of this circuit is similar to the operation of the circuit of Figure 1.
Finally, it should be mentioned that if it is desired to increase the heating time of the lamp's electrodes without changing the values of the capacitors 22 and/or 26, it is possible to connect between said capacitors (as shown by the hatched lines in Figure 3) a diode 34, instead of the lead 36 connecting said capacitors. This increase in the heating time will facilitate the use of the same starter for a larger range of lamps having different values of heating times.

Claims

1. An electronic starter circuit for a discharge lamp (8) having pre-heatable electrodes (10, 12) provided with respective first terminals connectable to a pair of input terminals (2, 4) of an AC voltage source and respective second terminals between which a thyristor switch (16) is connected, said starter circuit further comprising an ignition circuit connected to the switch (16) to render it repeatedly conductive during a number of cycles which does not exceed a given limit if the lamp fails to ignite and including a capacitor (22) which is connected to the anode of the switch (16) through a diode (20) which is conductive when said anode is positive with respect to the cathode of the switch (16) and to the gate of the switch through a threshold element (18), the number of cycles for which the ignition circuit repeatedly renders the switch (16) conductive being dependent on the capacity of the capacitor (22), characterised in that the gate connected side of the threshold element (18) is connected to the cathode of the switch (16) through an impedance (28 or 30) and in that the threshold element (18) of the ignition circuit has its threshold level set so that it is inhibited from conducting when the voltage across the switch (16) falls to the level it adopts when the lamp is ignited.

2. A circuit as claimed in claim 1, characterised in that a second capacitor (26) is connected between the cathode of the thyristor switch (16) and the plate of the first capacitor (22) which is remote from the threshold element (18).

3. A circuit as claimed in claim 1 or claim 2, characterised in that a discharge resistor (24) is connected in parallel with the diode (20).

4. A circuit as claimed in claim 2, characterised in that a discharge resistor (24) is connected in parallel with the second capacitor (26).

5. A starter as claimed in claim 4, further comprising a diode (34) connected in series between said first and second capacitors (22, 26).

6. A starter as claimed in any one of claims 1 to 5, characterised in that the threshold element is a zener diode (18).

7. A starter as claimed in any one of claims 1 to 5, further comprising a diode (32) connected between the lamp and both the anode of the thyristor switch (16) and the ignition circuit to protect the circuit against high voltage impulses.