GB2103033A - Operating circuit for fluorescent lamps - Google Patents

Abstract

A circuit for electronically igniting and operating fluorescent lamps (F) of either the one-pole ("cold"), or the incandescent ("hot") electrodes type is disclosed. In the first case, the circuit comprises a capacitor (C1) and a rectifier device (B) connected in series with the cold electrodes (G). The rectifier device satisfies the conditions of having an avalanche breakdown voltage level less, than twice the peak value of the supply voltage and more than the peak voltage across the lamp during its operation. In the case of hot electrodes (G'), a diode (D2) is connected across the capacitor (C1). <IMAGE>

Description

SPECIFICATION Operating circuit for flourescent lamps The present invention relates to gas discharge tubes for lighting purposes, particularly to fluorescent lamps or, as they are commonly called, "Neon" lamps, of the type widely used for domestic and commercial purposes.

Fluorescent lamps of this type are in the form of an elongated glass tube containing mercury or other suitable gas vapor at extremely low pressure. The inner surface of the glass tube is coated with a phosphorescent metallic salt composition serving as the light-emitting substance. A pair of incandescent electrodes ("filaments" or "hot cathodes") are provided at the two extreme ends of the tube, connected across the alternating voltage supply through a relatively large inductive reactance device known as the "choke" coil. A device called the "starter" is provided for activating or igniting the lamp, the function of which being to break the circuit of the choke, thereby causing it to produce a voltage surge which is high enough to initiate the discharge in the fluorescent lamp itself after an initial thermionic discharge has occurred in the vicinity of the hot cathodes.

There is aiso known what is called "cold starting" ignition method of fluorescent lamps, namely, without the provision of incandescent electrodes or filaments. However, these installations, require very large transformers and coils for producing the high voltage surge required for the ionization of the gas.

The common, hot cathodes type ignition circuits for fluorescent lamps suffer from numerous disadvantages, the main ones being the need for a special, external starter device which frequently goes out of order and its replacement causes a nuisance to the user; and that the life of the tube is in fact determined by the durability of the filament electrodes which, after a certain number of working hours, burn out while the more expensive tube as such is still in good condition.

It is therefore the object of the present invention to provide a fluorescent lamp starting and operating circuit which will overcome the above-mentioned and other disadvantages.

It is a further object of the invention to dispense with the starter device and to provide a fully electronically-operated circuit for the purposes in question.

According to one aspect of the invention there is provided a fluorescent tube lamp ignition and operating circuit including a fluorescent tube provided with one cathode electrode at one end, and a second cathode electrode at the other end thereof, and an alternating supply voltage source connected across the electrodes in series with a choke, characterized in that the circuit comprises, connected across the electrodes, a capacitor and a rectifier device having an avalanche breakdown voltage level less than twice the peak value of the voltage source and more than the peak voltage across the tube during operation thereof.

According to another aspect of the invention, the said cathode electrodes are of the filament type, connected in series with the alternating supply voltage source, a diode being connected across said capacitor.

According to a preferred embodiment of the invention the rectifier device comprises, connected in parallel, a diode and an SCR device; the gate of the SCR may be short-circuited so that the avalanche breakdown voltage of the SCR will be maximum, or, alternatively, it may be connected to a voltage divider for controlling the said breakdown voltage.

Alternatively, the said rectifier device may comprise other electronic circuitry and components as described in more detail below.

These and further constructional details and advantages of the invention will become more fully understood in the light of the following description, of a number of preferred embodiments of the invention, given by way of example only with reference to the accompanying drawings, wherein: Fig. 1 is a general diagram of the circuit according to the invention employing single-pole electrodes; Figs. 2a-2d are graphical representations of the operation of the various components of the circuit of Fig. 1; Fig. 3 is a first preferred embodiment of the invention; Fig. 4 is a modified embodiment of the circuit shown in Fig. 3; Fig. 5 is a second embodiment of the invention; Fig. 6 is a third embodiment of the invention; Fig. 7 is a fourth embodiment of the invention; Fig. 8 is a fifth embodiment of the invention;; Fig. 9 is a diagram of the circuit according to the invention, employing incandescent-type electrodes; and Figs. 10a-10d are graphical representations of the operation of the various components of the circuit of Fig. 9.

As is shown in Fig. 1, there is provided a fluorescent lamp F with electrodes G at both its ends. It will be noted that the electrodes are single-pole electrodes rather than of the filament or incandescent type electrodes which need to be connected in series with a voltage source for the flow of current therethrough (see Fig. 9). The electrodes are connected across an alternating voltage supply, V9, via coil L. Capacitor C2 may be provided, as shown in broken lines, for preventing disturbances to the network, as known in the art.

Now, according to the present invention, there are further connected across the lamp F a capacitor C1 connected in series with a circuit generally represented by the block B, which circuit has to satisfy the following conditions: (a) It has to act as a full-wave rectifier, i.e. as a low resistance conductor during one half-wave of the applied voltage, and as a cut-out during the other half-wave of the voltage cycle; (b) It has to have an avalanche breakdown voltage level (EB), namely, that in the non-conducting half-wave phase, if loaded by a potential difference greater than a certain specified value, it will abruptly resume its former conducting properties; (c) The avalanche voltage E9 should be less than twice the peak value of the voltage source VB; and (d) It must be more than the peak voltage appearing across the tube during its operation, i.e., in its ignited state (VF).

The operation of the circuit will now be analyzed with the aid of the graphs shown in Fig.

2. Fig. 2a shows a typical sinusoidal variation of the supply voltage Vs of the mains (normally 220 volts at a frequency of 50 cycles), showing the peak value (positive and negative) at the points a and b. As known, the said peak value V9p equals axVs (about 310 V).

Fig. 2b shows the voltage variation across the capacitor C2. At the beginning of the half-wave conducting phase of the circuit B, the capacitor will be charged up to the peak value V9p. At the point c, the circuit B becomes blocked, and remains so for as long as the potential difference across it stays below the critical specified value designated in Fig. 2b as EB. Since a polarity reversal of the voltage V9 has occurred, the overall or absolute voltage difference as seen in the circuit B(VB) is the total of V9p which is maintained by the capacitor C1, and the gradually increasing voltage of the negative cycle.At the point d, the absolute voltage difference approaches the critical breakdown voltage of the circuit B, which therefore becomes conductive, causing an abrupt (t2-t1) discharge of the capacitor C1, as shown in Figs. 2b and 2c; at the times2 the voltage VB across the circuit B becomes zero.

The choke current variations ICH, shown in Fig.

2d, represent the phenomenon known as Lenz' Law, which states, as known, than an induced emf in a conductor is always in such a direction that the produced current would oppose the charge which causes the induced emf. Therefore, the abrupt change of current flow through the coil L will cause a pulse of high voltage (in the above example, in the order of 1,000 to 1,200 volts) thereacross. Such voltage surges will occur at every cycle, namely, 50 times per second. It has been found that after a few such voltage surges the lamp F will ignite, i.e. an ionization of the gas will occur, and the lamp will start to conduct.

A voltage drop VF will be developed across the lamp F and the remaining voltage drop, namely V9V, will be dropped across the coil L, whereas the capacitor C1 and circuit B no longer take part in the operation of the circuit.

It will be thus readily understood that the voltage surges developed by the choke L in the circuit according to the present invention are much higher in absolute value, since they are induced by voltage difference approaching twice the peak value of the voltage source V9, rather than by the effective (RMS) voltage of V9 employed in the conventional, starter-type ignition circuits. This is why such a series of, say 1 2 voltage surges (occurring during about 1/4 second) can stimulate the ionization of the tube without filament-type electrodes on the one hand, and without recourse to the especially high inductance coils required for the presently-known "cold" ignition installations, on the other hand.

In view of the foregoing description, persons skilled in the art will readily understand that the circuit B may take various forms, using many types of commercially-available electronic components. A few such circuits will be now only briefly described and explained.

In Fig. 3, there are shown, connected in parallel with respect to each other and in series with respect to the capacitor C1, a diode D1 and an SCR component (silicon controlled rectifier) having a gate G1 which is short-circuited to the cathode side of the SCR. During the positive halfcycle of V9, the diode D1 will conduct to enable the charging of the capacitor C1 to the maximum peak value of V9. The SCR does not conduct. After a while, during the negative half-cycle and upon reaching the specified breakdown value EB of the SCR, the SCR starts to conduct while its resistance drops to practically zero.The specified breakdown voltage of the SCR should be, as aforesaid, less than twice the peak value of V9, namely about 500-550 volts. On the other hand, E B should exceed the normal operating voltage drop across the lamp to avoid its extinguishing after the ignition thereof.

The breakdown voltage value of the SCR can be controlled by connecting its gate G1 between resistors R1 and R2, forming a voltage divider, as shown in Fig. 4.

In Fig. 5 the SCR and diode components are replaced by a commercially-available integrated thyristor rectifier unit connected as shown.

In Fig. 6 a P-N-P transistorTR1 is employed connected to an N-P-N transistorTR2 as shown, essentially fulfilling the same function as the formerly described SCR component. Hence, in the negative half-cycle when the voltage over the resistance R2 is larger than a predetermined value the bias of TR 1 is high and it starts to conduct. At this point the bias of TR2 is enough to drive the transistor into conductance. By properly selecting their characteristics, the conducting state of the transistors can be preset to the required EB value.

During the positive half-cycle, both transistors are blocked.

The embodiment of Fig. 7 uses a Zener diode of the Sidac type, again acting as a rectifier device with an avalanche breakdown preset voltage value, the same as the Triac component employed in the embodiment of Fig. 3.

It has been thus established that the circuits based on the principles of the present invention achieve the goals of simply and effectively replacing the expensive and inconvenient use of thermionic or hot cathodes together with the starter devices according to the conventional installations.

It should be further noted that the hot electrodes are usually coated with ionization stimulating substance, which substance, when consumed, causes the malfunction of the conventional tubes. While in use of hot electrodes the consumption rate of this substance is relatively high, it may last much longer when applied to cold electrodes as suggested according to the present invention, thereby extending by far the useful life of the tube.

While devices featuring the principles of the invention as heretofore described proved to operate satisfactorily, it has been found that by introducing only a minor change in the circuitrythe connection of a diode across the capacitor Cl-the operation mode of the circuit changes in such a manner that it becomes readily applicable to normal, hot cathode tubes, as explained in more detail below.

Among the outstanding advantages, of the modified circuit are the following: First, most of the existing installations are based on the conventional, filament-type tubes and it would be a big waste to dispose of them only for the purposes of using the basic circuit of Fig. 1.

(Incidentally, once a conventional tube system becomes inoperative, due to filament burn-out, it could be restored by the "cold" electrode circuit as above described).

Secondly, the modified circuit assures a tubes useful life much longer than conventionally known before, besides the saving of the costs and inconvenience of employing the "starter" devices.

This achievement is mainly attained because the newly operated filaments become intermittently heated, and only to the minimum extent required for the effective ignition of the tube.

As noted from Fig. 9, the only change with respect to the circuit of Fig. 1 is the addition of a diode D2 across the capacitor C1, while the electrodes G' are of the filament or incandescent type, adapted to initially be heated by the current of the source V9 in the conventional manner.

The operational characteristics of this circuit, represented in Figs. 1 0a-1 Od, are again similar to those described in connection with Figs. 2a2d, except that during the discharge of the capacitor Cl (t2-t1), the diode D2 becomes conductive and supply the full negative half-cycle current to the filaments of the electrodes G'.

Since such capacitor discharges occur at 50 cycles per second, intermittent heating-up current pulses flow through the electrodes (about every 0.02 sec.) until the required, ignition glowing temperature is reached, in practice after 3 to 5 cycles (about 0.1 sec). This mode of filament heating is completely different from the conventional, starter-type systems wherein the incandescent current between ignition intervals is continuous, which results overheating of the filaments and usually lasts for over 2-3 seconds.

It may thus be said that the uniqueness of the above described operation resides in that the high voltage pulses and the filaments heating current pulses occur alternately during every cycle, however in a practically concurrent manner.

It will be readily understood that the various examples of circuits of the rectifier device B shown in Figs. 3-8 are also suitable for the purposes of the modified form of the invention shown in Fig. 9.

Claims (11)

Claims

1. A fluorescent tube lamp ignition and operating circuit including a fluorescent tube provided with one cathode electrode at one end, and a second cathode electrode at the other end thereof, and an alternating supply voltage source connected across the electrodes in series with a choke, characterized in that the circuit comprises, connected across the electrodes, a capacitor and a rectifier device having an avalanche breakdown voltage level less than twice the peak value of the voltage source and more than the peak voltage across the tube during operation thereof.

2. The circuit as claimed in Claim 1 wherein the electrodes are one-pole electrodes.

3. The circuit as claimed in Claim 1 wherein the electrodes are incandescent electrodes, a diode being connected across the capacitor.

4. The circuit as claimed in Claim 1, wherein the rectifier device comprises, in parallel, a diode and an SCR device.

5. The circuit as claimed in Claim 4, wherein the gate of the SCR is short-circuited so that the avalache breakdown voltage of the SCR is maximum.

6. The circuit as claimed in Claim 5, wherein the gate of the SCR is connected to a voltage divider for controlling the avalanche breakdown voltage thereof.

7. The circuit as claimed in Claim 1 , wherein the rectifier device comprises an ITR and a voltage divider.

8. The circuit as claimed in Claim 1, wherein the rectifier device comprises P-N-P and N-P-N transistors connected to a voltage divider.

9. The circuit as claimed in Claim 1, wherein the rectifier device comprises a SIDAK device.

10. The circuit as claimed in Claim 1, wherein the rectifier device comprises a TRIAK device.

1 0. Fluorescent tube lamp ignition and operating circuits substantially as hereinbefore described with reference to the accompanying drawings.

11. The features herein described, or their equivalents, in any novel selection.