GB2151090A - Electronic ballast - Google Patents

Abstract

An electronic ballast for instant starting of fluorescent tubes H has a high frequency push-pull oscillator Q1, Q2, T1 powered from an AC source via a rectifier B, high frequency filter L1, C2 and regulator T3, Z1, Z2, the output windings of a saturating transformer T1 of the oscillator being connected to a dummy load transformer T2. Secondary windings on the transformer T1 provide feedback to transistor Q1, Q2 and to a regulating transformer T3. Each of the two fluorescent tubes H has one filament energised by the output windings of transformer T1 and its other filament energised by the secondary winding of the transformer T2 which provides a dummy load for the oscillator if the tubes H are disconnected. <IMAGE>

Description

SPECIFICATION Electronic ballast The present invention relates to an electronic ballast, the preferred embodiment of which comprises a rectifier to convert alternating currents into direct current, a high frequency filtering and protecting winding to filter the high frequency noises which would otherwise interfer with other appliances and to protect transistors, an electromagnetic oscillating circuit, comprising two transistors and a transformer in which the two transistors form a pushpull type amplifying circuit to produce an oscillation to convert direct currents into a high frequency alternating current with high voltage, a dummy load transformer coupled to the secondary winding of the transformer of the electromagnetic circuit to apply a high voltage of high frequency to both ends of a discharging tube (or two tubes in series) and meanwhile apply a false load by means of the alternating current of high frequency and high voltage to heat the filaments at both ends of the tube to achieve instant starting and instant light emission. After lighting up, the voltage applied to the filaments of the tube is only about 2 volts to continue the discharge and the corresponding current is very low. Hence it has the advantage of long duration of life, low power consumption, high stability in use and low operating temperature of the tube.

Conventional florescent tubes are generally coupled with a ballast and starter. In use, an AC source voltage is aplied to both ends of the starter through the ballast and the filaments at both ends of the discharging tube.

Since the ballast's output voltage is higher than the triggering voltage of the starter, glow discharging occurs in the two electrodes of the starter, and the small current as a result of the glow discharging causes a bimetal movable electrode to heat up, and gradually expand to reach a fixed electrode, thus completing the circuit.

When the two electrodes of the starter meet each other to cause a short circuit, the source is connected, and the current passes through the ballast, one of the filaments of the tube, the starter, and the other of the filaments of the tube to make a preheating loop circuit.

After the two electrodes contact each other, the glow discharge stops so that the bimetal movable electrode is no longer electrically heated and therefore gradually cools down.

On the other hand, the two filaments of the tube gradually increase in temperature due to the preheating current. Thus, when the movable electrode gradually cools and retracts, the instant current in the preheating circuit just leaving the fixed electrode suddenly falls to zero. At this moment, the abrupt change of current causes the ballast to produce a high tension on the end of the preheated tube, hence leading to the emission of the preheated electrons on the filaments to light the tube.

Accordingly, it takes several seconds to light up the tube. Moreover, the tube may flicker several times before completely lit up due to the effect of the starter. After the tube is lit up, almost all the source voltage is applied to the filaments, and results in excessive heat. Furthermore, the fluctuation of the source voltages directly affect the brightness of the tube. All these shorten the life of the tube, and cause its flickering. The working ballast may even produce a high frequency noise that may interfere with the normai reception of radio.

Accordingly it is the chief object of this invention to provide a ballast whereby the disadvantages of conventional means are obviated.

Numerous other features and advantages of this invention will become apparent when read in association with the accompanying drawing, in which: Figure 1 is a wiring diagram of this invention.

With reference to Fig. 1, in which two discharging tubes (H) are employed, the rectifier (B) comprises: a bridge rectifying circuit consisting of four diodes and capacitor (C,) to convert the AC input into DC output; a high frequency filter and protecting winding constituted by coil (L,), capacitor (C2), transformer (T2) and two Zener diodes (Z,) (Z2), wherein the coil and capacitor serves to filter high frequency noises which would otherwise interfere with the normal operation of other appliances such as a radio set, while the transformer (T3) and the two zener diodes (Z1) (Z2) supply a safety source to the transistors (Q,) (Q2) of the next stage and protect them against the destructive noises of high voltage impulses. The electromagnetic amplifier comprises two transistors (Q1), (Q2) and saturating transformer (T1).The transistors (Q1), (Q2) and the primary winding (L,1) and secondary winding (L13) of transformer (T1) are connected to form a push-pull amplifying circuit. When a direct current flows from Zener diodes (Z1) (Z2) to the primary winding (L,1) of transformer, the instant potential difference of the DC source rising from zero to a fixed valve firstly conducts one of transistors (which was chosen by (L12)) (Q1) or (Q2).When a DC current enters the central portion of the 1/2 primary winding (L11), the instant potential difference causes the instant current to flow upwardly to transistor (Q1), and causes secondary winding (L13) to induce a voltage which provides positive feedback to the bases of the conducts transistor (Q1). Because the voltage across the transformer is constant and approximately equal to the source voltage so the current is initially small (high inductance), and increases rapidly as the transformer near saturation, when the transister (Q1) can no longer supply the rising current needed to maintain the voltage across the transformer, this voltage fails and winding (it3) no more provide positive feedback voltage for (Q1) causes (Q1) to be cut off.The rapid reduction of current in the sunt inductance causes an overshoot voltage to appear across the transformer (T1) and this energy is applied to the other transister (Q2) (which was previously cut off) causing it to conduct and provide the other positive feedback voltage for (Q2) and then the oscillation occurs, thus converting a DC source into a high frequency oscillating AC source.

The other end of transformer (T1) and the two ends of secondary winding (L14) (Lrs) are connected to a dummy load transformer (T2).

The middle portion and one end of secondary windings (L14) (Las) are respectively connected to a filament at one end of each of the two tubes (H) for heating purpose, whilst both ends of the secondary winding (L22) of dummy load transformer (T2) are connected to both ends of the filament to the other end of each tube. In so doing, a high voltage with a high frequency, say 25 KHz can be instantly applied across the tube (H), to instantly light it up.

Apart from helping the tube (H) to light up during the starting, the dummy load transformer (T2) also serves as dummy load when the -circuit is not connected to a real load (fluorescent tube) to avoid the damage of the circuit, since the transformer (T1) of electromagnetic amplifier can work with high voltage of high frequency. After the tube is lit up, since the filament on each end of each tube (H) is connected to the secondary winding (L14) (L1s) (L22) of transformers (T1) (T2), only a very low (about 2 volts or so) voltage is applied to the filament, thus the consumption of electricity is also rather low. Taking two tubes of 40W for example, the consumption is only about 0.58 amps, which would otherwise be as high as 1.8 amps using conventional ballasts. This saves power on the one hand, and reduce the working temperature of the tube on ghe other hand. The ends of the tube are less prone to turn "black", and the life of the tube is considerably prolonged. At the same brightness, the temperature of the invention is only 50"F above ambient temperature, which would be 90'F in conventional means. Moreover, since the AC source is firstly converted into DC, which is, in turn, oscillated to give a high frequency voltage, the power supply is well stabilized, because there is provided means to stabilize the DC throughput, and the oscillation is done by electromagnetic amplifier, controlling the amplification ratio by the feedback of the external load. Hence, the fluctuation of the AC source does not affect the voltage in the filament, so that the light intensity can be stabilized.

Claims (3)

1. An electronic ballast for a fluorescent tube comprising an AC to DC rectifier, a high frequency filtering circuit, a voltage regulator, an electromagnetic amplifier comprising a push-pull oscillating circuit for providing a high frequency voltage output, and a dummy load transformer connected to the amplifier.

2. An electronic ballast comprising: a rectifier; a high frequency filtering and protecting circuit, wherein high frequency noises are filtered by an inductor and a capacitor, and high frequency impulses are barriered by a transformer and provide steady bias to magnetic AMP; a Zener diode serving to protect the transistor from damage by surge voltage; an electromagnetic amplifier consisting of two transistors and a transformer, wherein said two transistors and the primary and secondary windings of said transformer define a push-pull oscillating circuit, while the middle portion and one end of the other two secondary winding are connectible to the filament on one end of a discharging tube; the feedback winding being connected to a transformer for regulating the amplifier; a dummy load transformer connected to the output of the secondary winding of said electromagnetic amplifier, the secondary winding of said load transformer being connectible to the filament of the other end of said discharging tube; characterised in that an AC current is firstly converted into DC current while filtering the high frequency noises, then the DC current is oscillated to give a high frequency voltage by means of said pull-push amplifying circuit, and then transformed by a coupled dummy load transformer to instantly light up said discharging tube.

3. An electronic ballast for a fluorescent tube, substantially as herein described with reference to the accompanying drawings.