Fluorescent lamp ballast combined with electronic starter and resonant capacitor for reducing input current

Abstract

 

1. An electronic power supply circuit provided for supplying electric power to a fluorescent lamp having a first (F1) and a second (F2) filament, said circuit comprising a ballast having a first and a second coil, said first coil being connected between a first power input pole of said circuit and a first distribution line connecting said first coil to a first connection point for said first filament said ballast comprises a second coil having a first coil pole connected to said first distribution line and wherein a series connection of a capacitor and a bi-directional AC switch is connected between a second coil pole of said second coil and a second distribution line connecting a second power input pole of said circuit to a second connection point for said second connection filament.

Description

[0001] The present invention relates to an electronic power supply circuit provided for supplying electric power to a fluorescent lamp having a first (F1) and a second (F2) filament, said circuit comprising a ballast having a first coil being connected between a first power input pole of said circuit and a first distribution line connecting said first coil to a first connection point for said first filament.

[0002] Such an electronic power supply circuit is used for supplying electric power to a fluorescent lamp. The ballast is used in combination with a conventional starter, which is generally a mechanical type of electrical switch sensitive to heat induced by a neon gas. Upon sensing the heat, the mechanical members of the switch will bend in order to close the circuit thereby preheating the two filaments of the fluorescent lamp. After closing the heat inside the starter is reduced causing an instantaneous opening of the mechanical members. This sudden opening induces a very high voltage at the ballast which will kick-off the starting current inside the fluorescent lamp.

[0003] A draw back of the known power supply circuits is that the life-span of the starter is limited due to mechanical wear and other degrading effects caused by electrical sparks produced during operation of the mechanical members. This problem on his turn leads to a pour starting of the fluorescent lamp. Moreover the mechanical members could remain in contact for a long period of time intermittently, causing a melt of the filaments and consequently to break open the continuity of the whole circuit. Hence the fluorescent lamp can not be used up to its full burning-hours span indicated by the manufacturer.

[0004] Another problem of the use of conventional ballast is that they need at least 190 V AC to start a fluorescent lamp. It can however occur, for example in small rural areas or during peak consumption, that the voltage decrease to 160 V AC. At the latter voltage the ballast cannot induce the required voltage to start the fluorescent lamp.

[0005] An object of the present invention is to realise an electronic power supply circuit that is less sensitive to voltage drops on the main and which offers a solution to the cited starter problem.

[0006] To this purpose an electronic power supply circuit according to the present invention is characterized in that said ballast comprises a second coil having a first coil pole connected to said first distribution line and wherein a series connection of a capacitor and a bi-directional AC switch is connected between a second coil pole of said second coil and a second distribution line connecting a second power input pole of said circuit to a second connection point for said second connection filament The use of a capacitor connected in series with a bi-directional AC switch enables to form together with the ballast a resonant circuit which resonantly discharge over the filaments of the fluorescent lamp causing the latter to ignite. The ballast is not anymore solely used as a high voltage inducer component but to control the current flow during a first half period of the AC sine wave when the switch is switched on. During a second half period , when the switch is switched off, the ballast is connected in series with the capacitor, which discharges, causing to increase the voltage supplied to the lamp and thus to reach the necessary voltage to start the fluorescent lamp. The switch thus serves when switched on, to charge the capacitor, and when switched off, to enable a discharge of the capacitor over the filaments of the lamp. Once the fluorescent lamp has started the switch remains switched-off due to the voltage drop over the fluorescent lamp.

[0007] A first preferred embodiment of an electronic circuit according to the invention is characterized in that said bi-directional AC switch comprises a trigger gate connected via a resistance to said second power input pole. The use of a switch with a trigger gate connected via the resistance to the second power input pole enables the automatic switch off once the lamp is ignited.

[0008] A second preferred embodiment of an electronic power supply circuit is characterized in that a second capacitor is connected in parallel with said first and second power input poles. The use of this second capacitor enables to reduce the power consumption by the lamp.

[0009] The invention will now be described in more detail with reference to the drawings. In the drawings.

Figure 1 shows a preferred embodiment of an electronic power supply circuit according to the present invention;

Figure 2 illustrates schematically by means of an example the impedance of the total circuit, including the lamp and the second capacitor.

[0010] In the drawings a same reference sign has been assigned to a same or analogous element.

[0011] The electronic power supply circuit illustrated in figure 1 comprises a first A1 and a second A2 power input pole to which an AC voltage, for example the 220 Volt of the mains, is applicable. A first coil L1 is connected between the first power input pole A1 and, via a first distribution line DL1, a first filament F1 of a fluorescent lamp FL. The presence of lamp FL in figure 1 is only shown for the sake of clarity, but it should be noted that the lamp is removably connected to the circuit and is thus not necessarily an integral part of the circuit. A second filament F2 of the fluorescent lamp FL is connected via a second distribution line DL2 to the second power input pole A2.

[0012] A series connection of a second coil L2, a first capacitor C1 and a bi-directional AC switch S is connected in parallel with the lamp FL, in such a manner that a first coil pole of the second coil L2 is connected to the first distribution line DL1 whereas a second gate MT2 of the switch S is connected to the second distribution line DL2. The capacitor C1 is connected between a second coil pole of coil L2 and a first gate MT1 of the switch S. A trigger gate T of the switch is connected via a resistance R to the second power input pole A2. The bi-directional AC switch is preferably a triac including a diac trigger such as for example a Quadrac (registered trademark) manufactured by Teccor electronics.

[0013] Preferably a second capacitor C2 is connected in parallel with the first A1 and second A2 power input pole. The purpose of this second capacitor C2 is to enable power saving such as will be described hereinafter.

[0014] The first L1 and second L2 coil form a ballast for the lamp FL. The ballast together with the first capacitor and the switch S form a primary circuit. The secondary circuit is formed by the second capacitor C2. The two coils of the ballast have different uses. Coil L1 is mainly for the control of current passing through the fluorescent lamp FL, but primarily for helping in creating the high voltage across the first capacitor C1. The second coil L2, which is in series with the first coil L1, is far an additional inductances. In series with the capacitor C1, to ensure that the required open-circuit-voltage across the fluorescent lamp is attained.

[0015] The ballast has three terminals, they are B1, B2 and B3. The first terminal, B1, is directly connected to the power input pole A1, and the first terminal of capacitor C2, the second terminal of the ballast B2, is directly connected to the first distribution line DL1, the third terminal, B3, of the ballast is directly connected to the first terminal of capacitor C1.

[0016] Suppose now that an AC power is applied on the power input poles A1 and A2. Suppose also that during a first half cycle of the applied AC since wave, the polarity is negative. The supplied current will pass through the two coils of the ballast towards the first capacitor C1, charging his first plate C1a negatively. As pole A1 is negative, pole A2 is positive, causing a triggering of the bi-directional AC switch S via resistance R and trigger gate T. Since the switch S is triggered, the latter is rendered conductive, causing the second plate C1b of capacitor C1 to be charged positively. The resistance R which is used in triggering the switch S will sense the current flowing into it and instantaneously trigger the bi-directional component which is incorporated inside the semiconductor switch, thereby switching on the latter and so charging the plate of capacitor connected via the first terminal, MT1, of switch positively.

[0017] During that first half cycle the distribution lines DL1 an DL2 connected to the filaments, F1 and F2 still have an open circuit while the capacitor and inductors are building up their voltages to attain the required open-circuit-voltage required to start the fluorescent lamp.

[0018] The next half cycle of the AC sine wave going into the circuit will be positive at the power input pole A1 and negative at the power input pole A2.

[0019] This time the reaction between the inductances and the capacitor becomes eminent due to the reversal flow of current thus creating a high voltage situation across the distribution lines, DL1 and DL2.

[0020] Since input pole A2 is negative, the switch S is no longer triggered causing the latter to become non-conductive. The first capacitor C1 will now discharge over the lamp FL. Since the switch is non-conductive, the only way for the first capacitor to discharge is via the second coil L2 of the ballast. Since the first capacitor C1 and the second coil L2 are chosen in such a manner as to form a resonant circuit, the capacitor C1 and coil L2 will resonantly discharge. Due to the branching of the first L1 and second coil L2 within the ballast, the resonant discharge current supplied during discharging of the first capacitor C1, will be added to the current supplied by the power source via the first coil L1. In such a manner a sufficient high voltage is created to start the fluorescent lamp FL. At the instant the fluorescent lamp has started, it develops a run-away high current and this is because the resistance inside the lamp reduces drastically thereby causing a voltage drop situation across the fluorescent lamp. This voltage drop situation decouples the connection of capacitor, C1 through the switch S since the current through the resistor R could not anymore trigger the bi-directional diode inside the bi-directional switch. During this critical moment, the coil, C1, will on its turn control the flow of the run-away current inside the fluorescent lamp.

[0021] The electronic power supply circuit according to the present invention enables a quick start of the fluorescent lamp connected thereto. Once the semi-conductor switch S is triggered on the charging of current into the first capacitor it will react with the inductances in series and the result is the creation of a high voltage across the capacitor as well as subsequently across the load itself which is the fluorescent lamp.

[0022] The creation of high voltage is instantaneous. Unlike the conventional starter which will pass a series of stages before kicking-up an induced high voltage into the fluorescent lamp.

[0023] Once a filament is cut-off or the two filaments are cut-off, the circuit becomes incomplete and the fluorescent lamp should need replacement.

[0024] It is impossible to short-out the two terminals of each filament with the conventional starter otherwise a ballast overheats in a short period of time and the coils become grounded to the negative core and will render useless.

[0025] The circuit according to the invention can extend the life-span of a fluorescent lamp to its maximum specified burning hours. It doesn't need pre-heating of the filaments because the capacitor itself across the load creates the required high voltage to kick-off electrons inside to start the fluorescent lamp.

[0026] Since the ballast is not anymore used as the sole source of high voltage in starting the fluorescent lamp, variations of voltages with a minimum of 160 Volt cannot anymore hinder in starting the fluorescent lamp because the creation of high-voltage across the capacitor C1 is enough to start the fluorescent lamp.

[0027] Additional inductances in series with the capacitor coming from the uniquely designed ballast ensures the starting capability for the latest "TLD" fluorescent tubes.

[0028] Furthermore, as already mentioned another capacitor, C2, is incorporated in the circuit for safety purposes and as well for power saving by reducing the current, without degrading the fluorescent lamps illumination.

[0029] During operation of the fluorescent lamp, once started, capacitor C1 will no longer be operational and only capacitor C2, is operating in parallel with the coil L1, of the ballast which is in series with the fluorescent lamp.

[0030] Referring to figure 2 where the impedance of the present circuit is illustrated by way of example, suppose:

Vac = 220 V the voltage applied to the circuit

ZB = 570 Ω the impedance of the ballast

ZL= 50 Ω the impedance of the lamp

ZF = 166 Ω the impedance of the filaments

Without second capacitor the current flowing into the circuit is:

I=0.357 A

Suppose now that the second capacitor C2 has a value C = 3.5 µF the frequency of the mains being supposed to be 60 Hz. Then

I = 0,163 amp.

Thus the presence of the second capacitor C2 enables to reduce the current and consequently the consumed power.

[0031] This phenomenon can be explained with the theory of the "resonant effect" between the capacitor and the inductor using LENZ LAW.

[0032] LENZ LAW states that "in all cases of electromagnetic induction, the induced voltages have a direction such that the currents which they produce opposed the effect which produces them."

[0033] Firstly, the coil L1 opposes the current going in through, thus it controls the current passing into the fluorescent lamp. From the peak voltage and decreasing to zero, the capacitor discharges to the coil trying to hold the voltage at the coil from decreasing.

[0034] Upon reaching zero voltage, the coil still have a stored magnetic field from the previous magnetic field direction. Hence the direction of electric current is still the same as with the first cycle.

[0035] This time the next half cycle is increasing from zero to peak voltage in the other direction. Since the previous current is opposite in direction and coming from the stored magnetic field, the current in the next half cycle is opposed so that the current going into the system is drastically reduced into half if the resonant frequency is equal to the line frequency which is 60 Hertz.

[0036] Therefore, the inductive reactance is primarily controlling the current into the fluorescent lamp by LENZ LAW. But the addition of current coming from the capacitor into the coil by its leading current explains that the capacitive reactance is virtually in series with the inductive reactance of the coil.

Claims

1. An electronic power supply circuit provided for supplying electric power to a fluorescent lamp having a first (F1) and a second (F2) filament said circuit comprising a ballast having a first and a second coil, said first coil being connected between a first power input pole of said circuit and a first distribution line connecting said first coil to a first connection point for said first filament characterized in that said ballast comprises a second coil having a first coil pole connected to said first distribution line and wherein a series connection of a capacitor and a bi-directional AC switch is connected between a second coil pole of said second coil and a second distribution line connecting a second power input pole of said circuit to a second connection point for said second connection filament.

2. An electronic power supply circuit as claimed in claim 1, characterized in that said bi-directional AC switch comprises a trigger gate connected via a resistance to said second power input pole.

3. An electronic power supply circuit as claimed in claim 1 or 2, characterized in that said bi-directional AC switch is provided for enabling a charge of said capacitor during a first half cycle of an AC power supply to said first and second pole and for enabling a discharge over said first and second filament during a second half cycle of said AC power supply.

4. An electronic power supply circuit as claimed in claim 3, characterized in that said bi-directional AC switch is a triac including a diac trigger.
An electronic power supply circuit as claimed in anyone of the claims 1-4, characterized in that a second capacitor is connected in parallel with said first and second power input poles.

Drawing