GB2140991A - Emergency supply for fluorescent tube - Google Patents

Abstract

The tube LR is normally powered from the mains supply 2, 3 via a reactor RE, and in the event of mains failure the tube is supplied from a battery powered high frequency oscillator OSC coupled via a winding L3 of a transformer TR1 to a winding L5 connected between a starter GL and a filament G1 of the tube. A capacitor C5 has a high impedance at mains frequency and a lower impedance at the oscillator frequency, the oscillator producing voltage transients so that the tube LR will start even with cold filaments G1, G2. An electromechanical or electronic relay turns the oscillator OSC off if the mains supply is present. Alternatively, the battery charging current may saturate a transformer (TR2), (Fig. 2), in oscillator OSC and also establish a voltage drop to hold a transistor (T1) in the oscillator OSC blocked when the mains supply is present. The transformer TR1 has a further winding L4 having the same number of turns as winding L5 so that when the oscillator is operating points 4 and 5 are at the same potential, whereby if the switch S is closed there is no potential drop across the mains terminals 2, 3 so that the oscillator will not be loaded by any other load connected across 2, 3. The oscillator OSC includes a circuit (D2, C4, T2) to turn it off if excess voltage is produced, eg. if the tube LR is disconnected. <IMAGE>

Description

SPECIFICATION Power device for fluorescent tubes Technical field The present invention relates to a power device for a fluorescent tube, the power device being designed in such a manner as to permit operation of the fluorescent tube from two alternative power sources, in which one preferably consists of the mains supply power and the other of an electric battery. Furthermore, the power device is designed in such a manner that the tube is automatically started if the normal power supply fails, irrespective of whether the tube was connected to or disconnected from this mains supply at the time of the power failure.

The state of the art As regards lighting installations for combined normal lighting and back-up lighting in the event of a power failure, use has been made of different types of accumulators which have provided the back-up power supply when the normal mains supply has failed. The current from such accumulators has, as a rule, been converted to high tension A.C. voltage in a special converter and this high tension A.C. voltage has then been fed to the fluorescent tube for its operation. This conventional solution has entailed that complex switches, relays or the like have been required to switch from normal mains supply operation to operation from the back-up power source and to prevent the total or partial loss of the power emitted by the back-up power source, through, for example, a bulb which was coupled in parallel with the fluorescent tube circuit when the power failure occurred.

One further drawback in conventional solutions in this art entails that a fluorescent tube fitting of the standard type cannot be readily supplemented by power means for back-up lighting. Thus, this has meant that the fluorescent tube fittings which could operate from both a back-up power source and from the standard mains power supply must be specially constructed for this purpose.

Object of the invention The object of the present invention is to realise a power device for a fluorescent tube, the power device being readily integrated in of to a fluorescent tube fitting and being designed so that, moreover, the tube is automatically connected-in, in the event of a mains power supply failure, at the same time as the risk is eliminated that the power supplied by the back-up lighting power source is fed to loadings connected to the mains if the fluorescent tube was connected to the mains supply when the power failure there occurred.

Solution This object is achieved according to the present invention by means of a power device which is operative for running the fluorescent tube from a first or second power source, of which the second comprises an electric battery and an oscillator, the tube being connected to the first power source by the intermediary of a reactor and having a starter between its filaments, the device being characterised in that a transformer with a first winding is connected to the second power source and with a second winding between the starter and one end of the first filament of the fluorescent tube, while the second end of this filament is in electrical communication, via the reactor, with the first power source which is also in communication with the second filament; and that electric power emitted by the second power source is at a frequency which is so adapted to a capacitor included in the starter that this capacitor has low-ohmic reactance.

One particularly advantageous embodiment of the present invention further includes the facility that the second power source is in electrical communication with the first and includes a blocking device which, on function of the first power source, is operative to block the oscillator; that the transformer further comprises a third winding which is connected between the reactor and the second end of the first filament of the fluorescent tube; and that the second and third windings of the transformer are arranged to give potential difference between their ends facing away from the first filament which is substantially zero.

So as to avoid that the transformer winding coupled in series with the reactor affects the operation of the fluorescent tube to any appreciable degree when operation is effected from the mains supply, it further applies according to the present invention that the reactance of this transformer winding at the frequency of the first power supply is slight in relation to the reactance in the reactor at this frequency.

To facilitate starting of the fluorescent tube when it is to be operated from the second power source, it further applies according to the present invention that the second power source is designed to emit to the first winding of the transformer voltage components which are sufficiently high to start the fluorescent tube even with cold filaments.

Brief description of the accompanying drawings The invention will now be described in greater detail below, with reference to the accompanying Drawings.

In the accompanying Drawings: Figure 1 shows how a conventional fluorescent tube fitting is supplemented with certain auxiliary equipment, primarily a transformer and a back-up lighting assembly connected thereto; and Figure 2 shows the design of the back-up lighting assembly connected to the transformer of Figure 1.

Description ofpreferred embodiment Referring to the Drawings, it is apparent from Figure 1, that the subject matter of the present invention comprises a fluorescent tube LR, a starter GL and a reactor RE, these components being connected to an A.C. power supply, at the connections 2 and 3 by the intermediary of a switch S. Both the reactor RE and the tube LR, as well as the starter GL are of completely conventional construction, which int. al. entails that the tube has both filaments G1 and G2, while the starter contains a capacitor C5 and a bimetal spring in a gas-filled globe. Furthermore, there is connected, between the reactor and the tube, a transformer TR1 whose detailed function will be described below.

In normal operation of the tube, the switch S is made, which results in a voltage being impressed on the starter GL, this voltage being fed from the switch S, via the reactor RE, the two windings L4 and L5 in the transformer TRl ,the filament G1 and, on the other side ofthe starter GL, also the filament G2 in the tube. The capacitor C5 in the starter GL has such capacitance that it only permits a very slight current passage at normal mains frequency, as a rule 50 Hz.

This entails that the current through both of the filaments will be insufficient to heat them.

At the commencement of the start-up cycle, the bimetal spring in the starter is open, which means that a voltage will be impressed on the contact pair in the starter, ionisation taking place which, in its turn results in the generation of heat in the starter.

This generation of heat will have as a consequence that the bimetal spring bends, so that the contact is made, this in turn entailing that the two filaments G1 and G2 will be interconnected so that current sufficient for heating these filaments will flow through them. Hereby, electron emission will take place at both of the filaments. When the bimetal contact in the starter makes, this also entails that ionisation and generation of heat ceases in the starter, as a result of which the bimetal spring rapidly cools and breaks the contact once again. At this moment of breaking, both of the filaments G1 and G2 have been sufficiently heated so that, at this point in time, the voltage prevailing across the tube will occasion a discharge in the tube, i.e. the tube lights up.

The capacitor C5 has no real function at the starting of the tube or during its continued operation as long as such takes place from the mains power supply. The sole task of the capacitor in the conventional starter is to damp radio disturbance from the tube.

In order that the winding L4 in the transformer TR1 should not influence the operation of the tube LR under normal operating conditions, i.e. on operation from the mains power supply, the winding L4 has a reactance which amounts approximately to from 2 to 8% of the reactance in the standard reactor RE.

It will thus be apparent from the above that the tube and its operation are not influenced by the transformer TR1 and the operating circuit OSC, as long as the tube is run from the mains supply. It will also be apparent from the Diagram that a conventional fluorescent tube fitting may very simply be supplemented with the operating circuit and transformer quite solely in that the connections between the reactor and the starter and the one filament of the tube are disconnected and that the transformer TR1 is then connected therebetween. No other alterations apart from this need be carried out.

According to the present invention, the purpose of the operating circuit OSC is to run the fluorescent tube LR on such occasions when the mains power supply fails. For this reason, the operating circuit OSC is provided with a battery B, preferably an accumulator, and an oscillator which, by the intermediary of the connections 11 and 12 to the transformer TR1, emits a pulsating voltage which is of high frequency in relation to the mains frequency.

In practical embodiments of the invention, the frequency for this pulsating voltage may lie in the order of magnitude of 75 kHz, but, naturally, may vary within broad limits in the framework of that which will be apparent below. Furthermore, a blocking device is included in the operating circuit, and holds the oscillator blocked or stopped during such periods of operation when full mains power supply is available, but, in the event of a mains power failure, immediately starts the oscillator. Finally, the operating circuit also includes a device for charging the battery included in the system. A detailed description of the operating circuit will be given below.

As was mentioned above, the operating circuit is activated as soon as the mains power supply fails at the connections 2 and 3. This applies irrespective of whether the switch is made or broken.

When the operating circuit OSC is activated, this entails that the above-mentioned high-frequency pulsating voltage will be impressed on the winding L3 in the transformer TR1 across the connections 11 and 12. This voltage induces, via the core K2 in the winding L5, a voltage which on the one hand is impressed on the filament G1, and, on the other hand, on the starter GL.Since the frequency of this voltage is high, the capacitor C5 will be low-ohmic, which entails that substantially all of the voltage induced in the winding L5 will lie across the tube LR between its two filaments G1 and G2. Furthermore, the operating circuit OSC is designed in such a manner that the emitted, high-frequency pulsating voltage contains voltage transients which have been adapted such that these voltage transients in the winding L5 will give rise to such high voltage peaks that the tube LR will be capable of starting even with cold filaments. Once a discharge has taken place in the tube, the filaments will be heated by the discharge current and thereby reduce the starting voltage level for the fluorescent tube.

In a situation in which the switch S is broken, the transformer TR1 could very well do without the winding L4, since, in actual fact, it is only the winding L5 which powers the fluorescent tube LR. If, however, the switch S is made, this would entail that the voltage which rests on the fluorescent tube LR would also rest across the mains supply connection 2 and 3, via the reactor RE, which would have the result that for example a bulb connected to mains supply, a bulb which, thus, is in principle connected in parallel (via the reactor) to the fluorescent tube LR, would operate as a short-circuit or parallel loading to the tube. Naturally, this would result in an unintentional waste of the limited supply of power which the operating circuit can contain and, moreover, that the voltage available for starting the tube would be reduced.

To solve the above-outlinsd problem, there is disposed, as is apparent from Figure 1, a further winding L4 in the transformer TRi, this winding being in the same direction as the winding L5 and having also essentially the same number of windings. Hereby, the voltages induced in the windings L4 and L5, i.e. between the connections 4 and 6; and 5 and 7, respectively, will be of equal magnitude, with the result that, in principle, there can be no potential difference between the two connections 4 and 5 to the windings L4 and L5, respectively. Since the capacitor C5, at the frequencies now under consideration, functions as a low-ohmic reactance, this will entail that the potential at the connection 5 will also prevail at the connection 8 to the filament G2 of the tube and, naturally, also at the mains connection 3.Correspondingly the potential at the connection 4 of the winding L5 will, via the reactor RE, be transmitted to the switch S which is assumed to be made and thence further to the mains connection 2. Since the potentials at the connection 2 and 3 are identical, this means that no current can flow through an outer loading which, in the mains network, is connected in between the connections 2 and 3.

The above-discussed operating circuit is shown in detail in Figure 2. As was mentioned above, the operating circuit OSC has an output via the connections 11 and 12, this output being connected to the primary winding L3 of the transformer TR1. Furthermore, the operating circuit is connected to the mains by the intermediary of the connections 9 and 10 of the operating circuit, which, as is apparent from Figure 1, are directly connected to the mains connections 2 and 3.

A bridge rectifier BR is disposed between the connections 9 and 10 included in the operating circuit, and a capacitor C1 is connected in series with the bridge rectifier and functions as a currentrestricting capacitor. The positive pole of the bridge rectifier is connected to the positive pole of a battery B whose negative pole, via a diode D1, the winding L1 in the transformer TR2 and the resistor R1, is in communication with the negative pole of the bridge rectifier. Furthermore, a resistor R2 is connected in parallel across the battery B and may be considered as relatively high-ohmic, as well as a capacitor C3.

The positive pole of the battery is, further, in direct communication with the connection 11 of the operating circuit, while the negative pole via the emittercollector on the transistor T1, and the winding L2 in the transformer TR2 is connected to the output connection 12 of the operating circuit. The transistor T1 has a switch function and forms part of the oscillator which emits the high frequency and timevariable voltage to the transformer TRi.

As was mentioned above, the operating circuit is to be kept blocked or stopped during such periods of time as when full mains power supply is available at the mains connections 2 and 3, and, then, naturally also across the connections 9 and 10 to the operating circuit. According to the invention, this is realised in such a manner that the charging current from the bridge rectifier BR, through the battery B, passes through the diode D1 which causes a voltage drop giving a potential difference in the blocking direction between base and emitter in the transistor T1.The charging current is further adapted in such a manner that it, via the winding L1 in the transformer TR2, realises magnetic saturation in the core K1,whereby the transformer TR1 is put out of operation and no coupiing between the two windings L1 and L2 can exist.

Since the charging current also flows through the resistor R1, the voltage at the bridge rectifier may be adapted in such a manner that it can charge the capacitor C2 to such a high voltage that the capacitor C2 may emit sufficient current to keep the core K1 saturated even during such periods as when the mains power supply is insufficient. Hereby, the oscillator is effectively held blocked as long as full mains power supply prevails between the input connections 9 and 10.

As was mentioned above, the oscillator shall start automatically as soon as mains supply fails at the input connections to the operating circuit. If, thus, the mains voltage fails, the charging current will cease to flow through the bridge rectifier BR and the battery B. Instead the battery B will, via the highohmic resistor R2, charge the capacitor C3 to a voltage which is sufficient to supply operating current to the transistor T1 via the connection point 15, the winding Ll,the connection point 13 and the lead 14. This entails that the transistor T1 will be live, so that a current can flow from the battery B, via the connection 11, the winding L3 in the transformer TR1 (see Figure 1), the connection 12, the winding L2 in the transformer TR2 and finally through the transistor T1 and back to the battery.The current thus flowing through the winding L2 induces, in the winding L1, a voltage which lies with the positive side facing downwardly in Figure 1 and which, thus, increases the operating current to the transistor T1.

The current from the winding L1 will rapidlydomin- ate over the current which the battery forces through the resistor R2, for which reason the capacitor C3 will be recharged to opposite polarity. At the same time as this takes place, the magnetic flux in the core K1 will increase so greatly that the core reaches magnetic saturation, which entails that the induced voltage in L1 will disappear as good as instantaneously.

Hereby, the operating current to the transistor T1 will also disappear, which entails that no current can flow between the collector-emitter and also the winding L2 in the transformer TR2 will be dead.

The magnetic force which is present, at the moment of saturation in the core of the transformer TR2 gives rise to a counter-directed induced voltage in the winding L1. This voltage will lie in series with the charge voltage across the capacitor C3 and will, moreover, be rectified in the blocking direction of the transistor T1,for which reason the transistor will remain dead. An oscillation phase is started, whose frequency is essentially determined by the capacitor C3 and the winding L1. During this oscillation phase, the core K1 will switch between saturated and unsaturated state at the same time as there will be periodic feeding of operating current to the transis- tor T1 so that this makes and breaks the current from the battery B through the windings L3 and L2 in the transformers TR1 and TR2, respectively.

On magnetic saturation in the core K1 to the transformer TR2, an extremely rapid breakage of the current flowing through the transistor will be realised by means of the transistor T1. This entails a generation of voltage transients in the winding L3, these voltage transients being transferred to the fluorescent tube LR via the winding L5 in the transformer TR1. The transformer and the rest of the circuit are designed in such a manner that these voltage transients will be fully sufficient to start the tube LR even with cold filaments.

In such situations when the tube LR is disconnected, the voltage transients which occur in the transistorT1 on breakage will have such high values that damage might easily occur. For this reason, the operating circuit is fitted with a safety circuit which substantially consists of a Zener diode D2, a capacitor C4, a resistor R3 and a transistor T2.

That voltage which lies across the Zener diode D2 and the capacitor C4 consists of the battery voltage plus the voltage transients which may occur as described above. The Zener diode D2 has been selected such that its threshold voltage is greater than the voltage which lie across the Zener diode on normal operational conditions, i.e. when the tube LR is connected in. If, on the other hand, the fluorescent tube LR is disconnected, the voltage transients increase considerably, which entails that the voltage lying across the Zener diode is greater than its threshold voltage, for which reason the Zener diode will become live and the capacitor C4 will be charged. As is apparent from Figure 2, the voltage in the capacitor C4, via the resistor R3, will lie between base-emitter on the transistor T2.This means that when the capacitor has been charged to a certain voltage, the transistor T2 will receive operating current, with the result that the transistor T2 shorts the transistor T1 across base-emitter. It follows from this that the transistor T1 will be blocked and dead, for which reason the oscillator will be blocked.

The energy which gave rise to the dangerously high voltage transients was originally in the form of energy stored in the core K2 in the transformer TRi.

This energy will be transmitted to the capacitor C4, whereafter the energy discharges via the resistor R3 and base-emitter on transistor T2. When the capacitor C4 is almost empty, the voltage across this falls to such a level that the transistor T2 is no longer live and, thus, no longer blocks the operating current to the transistor T1. In this state, the circuit is reset and the oscillator starts once again. If, in this instance, such great voltage transients occur that the voltage lying across the Zener diode D2 once again exceeds its threshold value, the above-described cycle will be repeated.

The length of blocking time caused by the safety circuit is determined essentially by the capacitor C4 and the stored power volume in the core K2. in purely practical terms, it may be suitable to dimen sion the safety circuit in such a manner that it holds the operating circuit blocked during a period of time which is at least from 10 to 50 times longer than the coupling-in time or so that the heat generation in the circuit will not be injurious.

Although the above-described operating circuit is highly advantageous, the operating circuit may, naturally, be designed in many different manners.

Thus, the oscillator section of the operating circuit need not be designed in the above-described manner. It is, therefore, possible to use as good as any oscillator for controlling a breaking device as an alternative to the transistor T1 as long as the frequency out from the operating circuit is placed at a suitable value with reference to the capacitance in the capacitor C5 included in the starter Gi. The reason for this is that the capacitor, at mains supply frequency, shall have a high-ohmic reactance, while the reactance at the frequency of the oscillator must be low-ohmic.

Neither is it necessary, according to the present invention, to use the charging current to the battery B to hold the oscillator blocked. Thus, it is fully possible to allow the mains supply voltage to influence a relay which is held in the broken state as long as there is mains supply voltage, but which couples-in or starts the oscillator circuit as soon as the mains supply voltage fails. Naturally, such a relay may be either electro-mechanic or fully electronic.

The present invention may be modified in a number of ways, without departing from the spirit and scope of the appended claims.

Claims (5)

1. A power device for fluorescent tubes provided for the operation thereof from a first or second electric power source, of which the second (OSC) comprises an electric battery (B) and an oscillator, the fluorescent tube (LR) being connected to the first power source via a reactor (RE) and having a starter (GL) between filaments (G1 and G2), characterised in that a transformer (TR1 ) with a first winding (L3) is connected to the second power source (OSC) and with a second winding (L5) between the starter (GL) and one end of the first filament (G1 ) of the fluorescent tube (LR), while the second end of this filament has, via the reactor (RE) electric communication with the first power source, which is also in communication with the second filament (G2); and that the electric power emitted by the second power source is at a frequency which is so adapted to a capacitor (C5) disposed in the starter that this capacitor is of low-ohmic reactance.

2. The power device as claimed in claim 1, characterised in that the second power source (OSC) is in electrical communication with the first and includes a blocking device which, on function of the first power source, is operative to block the oscillator (T1, L1, K1, C3);thatthetransformer(TR1) includes a third winding (L4) which is connected between the reactor (RE) and the second end of the first filament (G1) of the tube (LR); and that the second (L5) and third windings of the transformer are operative to give a potential difference, between their ends (7, 6, respectively) facing away from the first filament, which is substantially zero.

3. The power device as claimed in claim 2, characterised in that the reactance of the third winding (L4) of the transformer (TRi) at the frequency of the first power source is slight in relation to the reactance of the reactor (RE) at this frequency.

4. The power device as claimed in any one of the preceding claims, characterised in that the second power source (OSC) is operative to emit, to the first winding (L3) of the transformer (TR1 ) voltage components which are sufficient to start the fluorescent tube (LR) with cold filaments (G1, G2).

5. The power device as claimed in any one of the preceding claims, characterised in that there is disposed, in the second power source (OSC), a voltage-sensing device (D2) for sensing the voltage across the first winding (L3) of the first transformer (TR1), the voltage-sensing device being operative, during certain times, to block the oscillator (T1, L1, K1, C3) if the sensed voltage exceeds a predetermined value.