GB2211038A

GB2211038A - Protective electronic ballast circuit for fluorescent lamps

Info

Publication number: GB2211038A
Application number: GB8724122A
Authority: GB
Inventors: Sum Wing Lau
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-10-14
Filing date: 1987-10-14
Publication date: 1989-06-21
Also published as: GB8724122D0

Abstract

An electronic ballast unit (2-4) for fluorescent lamps (F1, F2) comprises a high frequency oscillator (IC1, TR) for controlling and feeding a power supply from an input (1) to the lamps (F1, F2), current surge detecting means (L2, L4), and a thyristor (SCR) for disabling the oscillator (IC1, TR) when the detecting means (L2, L4) responds to a short in the fluorescent lamp circuit. Preferably, a filter circuit (4) is provided to filter out harmonic interference resulting from the use of a high frequency oscillating voltage to drive the fluorescent lamps. <IMAGE>

Description

ELECTRONIC BALLAST FOR A FLUORESCENT LAMP The present invention relates to an electronic ballast unit for a fluorescent lamp and in particular to the provision of a protection circuit to protect the unit in the event of a short ciruit at the output, and a filter circuit for filtering out harmonic interference resulting from oscillation.

A conventional electronic ballast for a fluorescent lamp utilizes a transistor based oscillator and a transformer to produce a high frequency power supply for driving the fluorescent lamp. The high frequency AC power supply causes the lamp to flicker very rapidly such that the flicker is barely perceptible to the human eye. However, when the fluorescent lamp or the unit output is shorted, a large current or voltage is produced which tends to destroy the whole circuit or at least burn out some of the components.

In addition, whatever type of oscillator circuit is used, harmonic waves will be produced, which result in electrical interference.

A first aspect of the present invention provides an electronic ballast unit for a fluorescent lamp, cqmprising an input for connection to a power supply, an output for connection to a fluorescent lamp circuit, control means for controlling and feeding the power supply from said input to said output, detecting means for detecting a short in said lamp circuit, and disabling means for disabling said control means when said detecting means detects a short in said lamp circuit.

Other preferred features and advantages of the invention will be apparent from the following description and the accompanying claims. The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 shows an electrical circuit of an electronic ballast embodying the invention; and Figure 2 is a flow chart showing the manner of operation of the circuit of Figure 1.

Referring to Figure 1, an electronic ballast unit according to the invention comprises a full-wave rectifier circuit 1, half-bridge circuit 2, a protection circuit 3 and a harmonic filter circuit 4. Two lamps F1,F2 are supplied with power from the half-bridge circuit 2.

The rectifier circuit 1 is conventional and converts an AC mains input to DC which is fed to the halfbridge circuit 2. The half-bridge circuit 2 utilises an integrated circuit IC1 to produce a high frequency oscillating voltage which is fed to transformer TR which controls transistors T1 ,T2. The harmonic wave filter circuit 4 is connected in series between an electrode of each lamp F1,F2 and the half-bridge circuit 2. The filter circuit 4 serves to filter out harmonic interference resulting from the oscillating voltage. The protection circuit 3 detects when there is a current surge in the circuit connected to the output from the half-bridge circuit 2 (i.e. the lamps F1,F2), and supplies a disabling signal to the integrated circuit IC1 to turn off the half-bridge circuit.

The operation of the ballast unit will be described in more detail with reference to Figures 1 and 2 Mains AC power supply is fed to the rectifier circui 1 (step 21) and the lamps F1, F2 are illuminate (step 22). Two circuit loops are formed. Current i one loop including lamp electrodes Ela, E2a i grounded through resistors R1, R2, R3 (step 23). Th integrated circuit IC1 is activated (step 24) an provides a high frequency oscillating voltage which is fed through transformer TR to the lamps F1, F2 (step 25). The filter circuit 4 is in series between lamp electrodes Elb, E2b and comprises at least one inductor coil L3, L3' in series with each lamp F1, F2, the number of coils being directly proportional to the number of lamps. Capacitors C5, C7 are connected in series with each coil to form an oscillator circuit.The coils L3, L3' are wound with opposite polarity on a common core and so act to filter out harmonic interference resulting from the oscillator circuit. Second coils L2, L2' wound with same polarity are provided and these magnetically couple to a coil L4 which supplies a signal to the protection circuit 3 (step 26).

Under normal operating conditions current flows through the filter circuit 4, and only a small current flow is induced in coil L4, producing a voltage less than the breakdown voltage of a zener diode Dz in the protection circuit 3. In this situation IC1 remains operating (step 28).

When there is a short in the lamp circuit, there is a current surge through either one or both of the coils L2, L2', causing L4 to induce a high voltage, above the breakdown voltage of zener diode Dz. This will switch o-n a silicon controlled rectifier SCR, which allows a current flow from the COMP pin of IC1 to the DEAD TIME or DISABLE pin of the integrated circuit, which disables the integrated circuit thus turning off the half bridge circuit 2 to protect the ballast unit (step 27).

Various modifications may be made to the embodiment described and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims

1. An electronic ballast unit for a fluorescent lamp, comprising an input for connection to a power supply, an output for connection to a fluorescent lamp circuit, control means for controlling and feeding the power supply from said input to said output, detecting means for detecting a short in said lamp circuit, and disabling means for disabling said control means when said detecting means detects a short in said lamp circuit.

2. A ballast unit as claimed in claim 1, wherein said control means comprises an oscillator which controls the power supply via a transformer, and said disabling means disables said oscillator.

3. A ballast unit as claimed in claim 1 or 2, wherein said detecting means comprises a first inductor.

4. A ballast unit as claimed in claim 3, wherein said first inductor is coupled to a second inductor connected in circuit with said output.

5. A ballast unit as claimed in claim 2, further comprising a filter for filtering out harmonic waves.

6. A ballast unit as claimed in claim 5, wherein said filter comprises an inductor.

7. A ballast as claimed in claim 6, for supplying power to a plurality of lamps, wherein said filter comprises a separate inductor for each lamp.

8. A ballast unit as claimed in claim 7, wherein a pair of said separate inductors are coupled together with opposite polarity to filter out said harmonic waves.

9. A ballast unit as claimed in claim 7 or 8, wherein an additional inductor is provided for each lamp, and said detector comprises an inductor coupled to said additional inductors.