GB2095488A - Improvements in or relating to electronic ballasts - Google Patents

Abstract

An electronic ballast for a fluorescent tube 6 comprises a rectifier 1 whose input is connected via a capacitor 2 to the mains L,N. The output of the rectifier is connected via a switch 3 to output terminals L1L2 for the tube, across which is connected an auto-transformer AT. A control circuit 5 repetitively switches the switch 3 on and off. The switch and auto-transformer generate a high voltage for starting the fluorescent tube and the capacitor 2 limits the discharge current. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electronic ballasts The present invention relates to electronic ballasts. Such ballasts may be used between a power supply and a discharge type lamp, such as a fluorescent lamp.

According to the invention, there is provided an electronic ballast comprising mains supply input terminals connected via a capacitor to a rectifier, output terminals connected via switch means to the output of the rectifier, an inductor connected between the output terminals, and a control circuit arranged to switch the switch means on and off repetitively.

Preferably, the inductor is an auto-transformer having taps for supplying current to the heaters of a fluorescent lamp.

Preferably, a reservour capacitor is connected across the output of the rectifier. The control circuit may comprise an astable multivibrator whose supply inputs are connected via a voltage divider across the reservoir capacitor. Preferably, the frequency of oscillation of the multivibrator is between 50 and 200 kHz. The switch means may comprise a transistor whose emitter-collector path is in series with the rectifier output and the output terminals, and whose base is connected to the output of the multivibrator. The transistor may comprise a darlington pair of transistors.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an electronic ballast constituting a preferred embodiment of the invention; and Figure 2 is a circuit diagram of the electronic ballast of Fig. 1.

The electronic ballast shown in Fig. 1 comprises an on/off switch SW connected to a power input terminal. A rectifier 1 is connected via voltage attenuator 2 to the switch SW. The output of the rectifier 1 is connected to a power amplifier 3 and, via a voltage divider 4, to the power supply rails of an astable multivibrator 5. The output of the multivibrator 5 is connected to a control input of the power amplifier 3, which functions as an electronic switch. The output of the power amplifier 3 is connected to a lamp 6, such as a fluorescent lamp.

The electronic ballast shown in Fig. 1 is illustrated in more detail in the circuit diagram of Fig. 2. The voltage attenuator 2 is constituted by the capacitor C1, one of whose plates is connected via the one/off switch SW to the live input terminal L of a mains supply input.

The rectifier 1 comprises a bridge rectifier whose input terminals are connected to the other plate of the capacitor C, and to the neutral mains input terminal N respectively.

The negative output of the bridge rectifier 1 is connected to a common line C whereas the positive terminal of the rectifier is connected to one plate of a reservoir capacitor C2 whose other plate is connected to the common line C. The positive terminal of the rectifier is also connected to one end of a resistor R, and to a first output terminal L, for connection to the fluorescent lamp 6. The resistor R, together with a resistor R2 forms a voltage divider connected across the reservoir capacitor C2.

The output of the voltage divider is connected to the positive supply line P of the multivibrator 5, which comprises transistors Q, and Q2, resistors R3, R4, R5 and R6 and capatitors C2 and C5. The multivibrator is of the well known type in which the emitters of the transistors Q1 and Q2 are connected to the common line C, the collectors of the transistors Q, and Q2 are connected via the resistors R3 and R6, respectively, to the positive supply line P, the bases of the transistors Q, and Q2 are connected via the resistors R5 and R4 to the positive supply line P, the base of the transistor Q, is connected via the capacitor C5 to the collector of the transistor Q2, and the base of the transistor Q2 is connected via the capacitor C3 to the collector of the transistor Q,.

The output of the multivibrator 5 at the collector of the transistor Q2 is connected to the input of the power amplifier 3, which comprises transistors Q3 and Q4 arranged in darlington or "super-alpha" configuration and a resistor R7. The output of the power amplifier 3 is connected to a second output terminal L2 so that the collector-emitter path of the transistor Q4 is connected in series between the common line and the terminal L4. The power amplifier 3 thus functions as an electronic switch controlled by the multivibrator 5.

An auto-transformer AT is connected between the output terminals L, and L2 and has first and second taps T, and T2 connected to third and fourth output terminals L3 and L4. A conventional fluorescent lamp or tube may thus be driven from the electronic ballast. For this purpose, the output terminals L, and L2 are connected to the respective main electrodes at opposite ends of the fluorescent tube. The tube has a pair of heaters disposed at its respective opposite ends, a first of the heaters being connected between the terminals L, and L5 and the other heater being connected across the terminals L2 and L4.

In use, the electronic ballast shown in Fig.

2 is connected with its terminals L and N to the live and nuetral mains supply terminals respectively, and a fluorescent lamp is connected to the output terminal Lr-L4 described above. When the switch SW in its ON position, mains voltage is supplied to the bridge rectifier 1 via the capacitor C1 and a direct voltage is provided across the reservoir capacitor C2 from the output of the bridge rectifier.

This direct voltage is also supplied across the series combination of the auto transformer AT and the collector-emitter path of the transistor Q4. The voltage divider 4 supplies a suitable working voltage to the multivibrator 5 which oscillates freely to produce a square-wave output to the power amplifier 3. The two levels of the output signal from the multivibrator drive the transistors Q3 and Q4 alternatively into saturation and cut/off so that the transistor Q4 repetitively interrupts the power of supply to the auto-transformer AT and to the lamp. The auto-transformer AT provides between the output terminals L1 L3 and between the output terminals L2 and L4 a voltage suitable for driving a heating current through the heaters of the fluorescent tube so as to pre-heat the tube for starting purposes.

When Ike swi:er SW is initially switched on, the current consumption of the whole circuit is very small, so that the voltage drop across the c3pacitor C1 is negligible. The input voltage to the rectifier 1 is thus substantially equal to the mains voltage and the output voltage across the capacitor C2 is substantially 1.4 times the RMS mains voltage. The multivibrator oscillates at a frequency much higher than that of the mains, preferably between 50 and 200 kHz, and the rapid switching by the transistor Q4 of the circuit through the transformer AT causes a high voltage to be induced in the transformer which starts the fluorescent tube. When the tube has started, the current through the lamp increases so that the voltage drop across the capacitor C, also increases. Thus, the input voltage to the rectifier 1 decreases and the output voltage for the tube and the transistor Q4 decreases. Eventually, the circuit stabilizes so that the voltage across and the current through the tube are appropriate for normal operation of the tube, the value of the capacitor essentially determining the current through the tube.

The use of the relatively high switching frequency allows the size and cost of the transformer AT to be reduced, thus permitting the cost of the ballast to be reduced.

Claims (9)

1. An electronic ballast comprising mains supply input terminals connected via a capacitor to a rectifier, output terminals connected via switch means to the output of the rectifier, an inductor connected between the output terminals, and a control circuit arranged to switch the switch means on and off repetitively.

2. An electronic ballast as claimed in claim 1, in which the inductor is an autotransformer having taps for supplying current to the heaters of a fluorescent lamp.

3. An electronic ballast as claimed in claim 1 or 2, in which a reservoir capacitor is connected across the output of the rectifier.

4. An electronic ballast as claimed in claim 3, in which the control circuit comprises an astable multivibrator whose supply lines are connected via a voltage divider across the reservoir capacitor.

5. An electronic ballast as claimed in claim 4, in which the frequency of oscillation of the multivibrator is between 50 and 200 kHz.

6. An electronic ballast as claimed in claim 4 or 5, in which the switch means comprises a transistor whose emitter-collector path is in series with the rectifier output and the output terminals, and whose base is connected to the output of the multivibrator.

7. An electronic ballast as claimed in claim 6, in which the transistor comprises a darlington pair of transistors.

8. An electronic ballast substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. A fluorescent tube arrangement including an electronic ballast as claimed in any one of the preceding claims.