GB2126810A

GB2126810A - Electronic drive circuit for discharge lamps

Info

Publication number: GB2126810A
Application number: GB08225340A
Authority: GB
Inventors: David John Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-09-06
Filing date: 1982-09-06
Publication date: 1984-03-28

Abstract

A fluorescent lamp is started and subsequently driven by a circuit having an inductor and a transistor connected in series with one another across a D.C. supply , the transistor being turned on and off at a high rate (15-35 kHz) such that current flows from the inductor through the lamp when the transistor is off. The transistor may be part of a two transistor astable circuit (Fig 2) or may be driven by an R-C relaxation oscillator including a disc (Figs. 4, 7). The inductor may be coupled to a feedback winding (Figs. 3, 7, 10), and a resistor RF and capacitor CF may be connected in series with the lamp to allow current flow therethrough when the transistor is on whereby the lamp is not subjected to unidirectional current. The inductor may form part of an autotransformer (Fig. 9) or may be the primary of a transformer having a secondary connected in parallel with the lamp (Fig. 3). The lamp may be dimmable (Fig. 10). A ceiling mounted lighting unit incorporating the circuit is described (Fig. 13). <IMAGE>

Description

SPECIFICATION Electronic drive circuits for energising fluorescent lamps This invention relates to electronic means of energizing fluorescent lamps as an alternative to the widely used 50Hz series reactive element (generally an inductor or choke) combined with an automatic starter switch.

An established property of fluorescent lamps is that due to the higher ionisation activity at high frequencies of operation (say 1 5 to 35kHz) the light output efficiency is greater than that at 50Hz. Also the starting arrangements can be simplified. For operation at high frequencies it is normally unnecessary to preheat the tube electrodes prior to starting the gas discharge with a high applied voltage.

The object of this invention is to provide particular means of generating the high frequency drive current and initial high voltage using a small number of components chosen to minimise the total cost.

Fig. 1 shows a basic high voltage transistor drive to a gas discharge tube or fluorescent lamp. When the transistor is ON current builds up in the inductor. When the transistor is switched to OFF at time t1 the inductive property produces a voltage such as to keep the current flowing in accordance with the basic formula dl Voltage (E)=L----.

dt A voltage high enough to strike the tube is thereby produced. Once the arc in the tube is established it is maintained by energy supplied during each OFF period of the transistor, from the inductor. A typical value of inductance using a switching frequency of 20kHz is 40mH. Such an inductor has a physical size and weight very much smaller than a conventional 50Hz ballast inductor.

Also no further arrangements for starting the lamp are necessary. (Using readily available high voltage transistors an initial voltage of 1 .2kV can be produced).

For a self-contained lamp drive the switching waveform shown at the base of the transistor in Fig. 1 must be produced by additional circuitry.

There are many ways this can be done of which three are listed.

1) Use of two transistors configured as a free multivibrator, as shown in fig. 2 in which the two time constants R1C1 and R2C2 define the oscillation times.

2) One timing period can be defined by an RC circuit to the base, the other by the time taken for the current to build up in the inductor until its core reaches saturation.

This gives the very simple circuit shown in Fig. 3 but running the core into saturation gives high peak currents and high core losses which means that this circuit is best suited to low voltage and low wattage use or in applications where efficiency requirements are not critical.

3) Use of a relaxation oscillator using a diac element as shown in Fig. 4. (Diacs with a break-over voltage of approximately 30V are readily available and are inexpensive).

The diac characteristic is shown in Fig. 5 and a diac oscillator is shown in Fig. 6.

Two additional features have been added to the circuit using method 3. The first overcomes a problem which can arise when the total current in resistor R1 Fig. 4 is required to be higher than the level at which the diac element will remain conducting (beyond point A on the diac characteristic in Fig. 5). The current in R1 is in turn determined by the need to supply enough current to turn on fully the transistor. As the less expensive high voltage transistors usually have a low current gain the incompatibility with the diac characteristic is common. The method used to overcome this problem is to feedback a small voltage from the collector waveform in series with the diac base drive in such a way as to aid the diac drive action.

Approximately 5 to 10 Volts is sufficient to ensure that the timing capacitor is sufficiently discharged to ensure that the diac stops conducting altogether once the transistor turns off again, and this voltage can be derived from a small secondary winding on the inductor. Once the diac has stopped conducting it can only do so again when the break-over voltage is reached.

The second possible problem is dependent on the type of tube being driven. It will be noted that the circuit of Fig. 4 produces uni-directional interrupted current. With many tubes the unidirectional field causes a migration of the vapour atoms in the tube and this eventually gives rise to reduced brightness at one end of the tube. To overcome the problem, current in the opposite direction can be added by means of a forward path through the lamp when the transistor is conducting. Current in this direction can be controlled by a resistance or a combination of resistance and capacitance. The drive circuit with the addition of a feedback winding and forward current path is shown in Fig. 7.

As an extension of the drive circuit described the use of two tubes in series each having a lower start and run voltage than the single tubes gives a reduction in the maximum starting voltage. This is explained with reference to Fig. 8. First tube A is subject to the full starting voltage via resistor Ps, then tube B has the full starting voltage less the initial gas discharge voltage of tube A. Thus two short tubes demand a smaller starting voltage capability than one long one.

The use of two medium voltage tubes in series can mean that the combined run voltage is nearly equal to the transistor supply voltage Vc and thus minimising the value of RF in Fig. 7. In fact correct choice of tubes and supply voltage can lead to the point where RF can be zero thus obviating the need for both RF and the diode, leaving either a direct connection to the tube or a series capacitor only (dotted alternative in Fig. 7).

For high voltage tubes (or a number of tubes in series) it is possible to increase the voltage available from the drive circuit by an extra winding on the inductor as shown in Fig. 9.

An additional facility to the circuits described is a simple modification shown in Fig. 10 to permit dimming of the lamp driven. By varying the on to off times of the transistor with a variable resistance element in position X or Y, (Fig. 1 0) the light output from the lamp can be varied approximately 5:1. This range can be further extended if heater windings are added to the inductor as shown dotted in Fig. 1 0.

Two embodiments of the diac transistor circuits into lighting units have been made-one with one tube the other with two tubes. The circuits are shown in Figs. 11 and 1 2 respectively.

The lighting units are very light compared with conventional fluorescent units (about 1/4 the weight) thus making it feasible to mount the units directly into ceiling board as shown in Fig. 13.

Claims (Filed on 30.8.83) 1. A circuit for energising a discharge tube, comprising first and second circuit points between which a discharge tube is connectable, an inductor connected in parallel with said circuit points, a switching device connected in or to an oscillator circuit to be alternately turned ON and OFF by the oscillations, the switching device being connected in series with the inductor for connection across a direct current supply such that current is drawn through the inductor when the switching device is ON and a voltage for striking the tube is developed across said circuit points when the switching device is turned OFF.

2. A circuit as claimed in Claim 1 in which a unidirectionally-conductive device is connected in series with the path through said circuit points to allow circulating current from the inductor to flow through a tube connected to said circuit points when the switching device is turned OFF.

3. A circuit as claimed in Claim 2 in which the switching device has a controllable load current path in series with said inductor and a control electrode for controlling the flow of current in the load current path, and in which the inductor has a secondary winding connected to said control electrode in a positive feedback sense.

4. A circuit as claimed in Claim 1 or 2 in which said switching device has a controllable load current path in series with said inductor and a control electode for controlling current flow in said load current path, and the switching device being connected in a relaxation oscillator circuit having a timing capacitor, means for charging the capacitor, and a threshold voltage device connecting the capacitor to said control electrode such that the switching device is turned ON upon the capacitor voltage exceeding the threshold voltage.

5. A circuit as claimed in Claim 4 in which the inductor has a secondary winding connected to said control electrode in a positive feedback sense.

6. A circuit as claimed in Claim 5 in which said secondary winding is connected in series with the threshold device and the control electrode.

7. A circuit as claimed in any one of Claims 3 to 6 in which the timing capacitor has an adjustable impedance connected thereto to vary the duty cycle of the switching device.

8. A circuit as claimed in any preceding claim in which the inductor has secondary windings connectable to respective heater windings of a discharge lamp to energise same.

9. A circuit as claimed in any preceding claim in which the inductor is a portion of an autotransformer connected across said circuit points.

10. A circuit as claimed in Claim 1 or 2 in which the switching device is one of a pair of active devices connected in an astable multivibrator circuit.

1 A circuit as claimed in Claim 1 or 2 in which the switching device is one of a pair of active devices connected in an astable multivibrator circuit, and the other device of the pair has a further inductor in series with the load current path through said other device, said first circuit point being intermediate one switching device and its series inductor and said second circuit point being intermediate the other switching device and its series inductor.

12. A circuit as claimed in any one of Claims 1 to 9 in which the inductor comprises a transformer having a primary winding in a direct current series circuit with the switching device and a secondary winding connection to said first and second circuit points.

13. A circuit as claimed in any preceding claim in which the oscillations are in the range 1 5 to 35kHz.

14. A lamp assembly comprising support means securable to a ceiling or the like and including a fitment for receiving a discharge tube and a circuit assembly extending generally in a direction normal to the ceiling when so secured so as to pass through a hole in the ceiling adjacent the fitment, said circuit assembly comprising a circuit as claimed in any one of Claims 1 to 1 3 whose first and second circuit points are connected to said tube receiving fitment.

15. A lamp assembly as claimed in Claim 14 in which said circuit assembly is supported on a circuit board whose plane is normal to the ceiling when secured as aforesaid.

1 6. A lamp assembly as claimed in Claim 14 or 1 5 further comprising a housing to pass through the hole in the ceiling and in which said circuit assembly is disposed.

1 7. A circuit for energising a discharge tube substantially as hereinbefore described with reference to Figs. 1, 2 or 3; or Figs. 4 to 6; or any one of Figs. 7 to 12 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. direct connection to the tube or a series capacitor only (dotted alternative in Fig. 7). For high voltage tubes (or a number of tubes in series) it is possible to increase the voltage available from the drive circuit by an extra winding on the inductor as shown in Fig. 9. An additional facility to the circuits described is a simple modification shown in Fig. 10 to permit dimming of the lamp driven. By varying the on to off times of the transistor with a variable resistance element in position X or Y, (Fig. 1 0) the light output from the lamp can be varied approximately 5:1. This range can be further extended if heater windings are added to the inductor as shown dotted in Fig. 1 0. Two embodiments of the diac transistor circuits into lighting units have been made-one with one tube the other with two tubes. The circuits are shown in Figs. 11 and 1 2 respectively. The lighting units are very light compared with conventional fluorescent units (about 1/4 the weight) thus making it feasible to mount the units directly into ceiling board as shown in Fig. 13. Claims (Filed on 30.8.83)

1. A circuit for energising a discharge tube, comprising first and second circuit points between which a discharge tube is connectable, an inductor connected in parallel with said circuit points, a switching device connected in or to an oscillator circuit to be alternately turned ON and OFF by the oscillations, the switching device being connected in series with the inductor for connection across a direct current supply such that current is drawn through the inductor when the switching device is ON and a voltage for striking the tube is developed across said circuit points when the switching device is turned OFF.

18. A lamp assembly for receiving a discharge

tube substantiaily as hereinbefore described with reference to Fig. 1 3 of the accompanying drawings.