GB2038572A - Control circuits for fluorescent lamps - Google Patents

Abstract

In a fluorescent lamp starting circuit wherein current is supplied to the lamp heaters (H1 and H2) during starting via a thyristor (T), the thyristor gating circuit includes a positive temperature coefficient resistor (PTC). If the lamp (FL) fails to ignite the positive temperature coefficient resistor value increases so that the thyristor (T) fires only occasionally and overheating of components of the starter circuit is avoided. The positive temperature coefficient resistor may be arranged to be responsive to the heat from the thyristor or other circuit component or to the current flowing through itself. <IMAGE>

Description

SPECIFICATION Fluorescent lamp circuit arrangements This invention relates to fluorescent lamp circuit arrangements.

The invention relates particularly to fluorescent lamp circuit arragements comprising a controllable electronic switching device con nected across the lamp to provide a path for the passage of current from an alternating current power supply through an electrode heating means of the lamp when the switching device is conducting; and a gating circuit connected across the lamp and to the switching device to control the conduction of the switching device in dependence on the value of the voltage developed across the lamp when the alternating current supply is connected to the circuit, whereby the switching device is switched on during an initial period by the gating circuit so as to produce conditions for ignition in the lamp during the initial period, and is held nonconductive when the lamp has ignited.Such a circuit arrangement is hereinafter referred to as a circuit arrangement of the kind specified.

In operation of a circuit arrangement of the kind specified, if the lamp fails to ignite, for example, due to a faulty lamp, current will continue to flow through the heating means giving rise to the possibility of overheating of components of the circuit arrangement. The ballast choke normally connected in series with the lamp across the supply is particularly vulnerable in this connection.

It is an object of the present invention to provide a circuit arrangement of the kind specified wherein this difficulty is overcome.

According to the present invention in a circuit arrangement of the kind specified the gating circuit includes a resistance which is heated in operation of the arrangement by current passing through itself and/or by heat produced in one or more other components of the arrangement, has a positive temperature coefficient, and is connected in a path for the supply of current to the control electrode of the switching device so that, if the lamp fails to ignite, the value of said resistance assumes a sufficiently high value to cause unreliable operation of said switching device and thereby to maintain the mean current supplied to said heating means at a reduced value.

The resistance is suitably arranged to be heated by heat produced in a component of the circuit, for example, the switching device.

The resistance is suitably connected in series with a voltage breakdown device.

Preferably said resistance is connected between the control electrode of the switching device and the tapping point of a potential divider connected across said lamp.

The potential divider preferably comprises a further resistance connected in series with a capacitance.

The switching device is suitably a thyristor.

One arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of the arrangement; and Figure 2 shows waveforms illustrating the operation of the arrangement.

Referring to Fig. 1, the arrangement includes a pair of input terminals 11, 12 between which an alternating current supply is connected in operation, and two pairs of output terminals 01, 02 and 03, 04 between which electrode heaters H1 and H2 of a fluorescent lamp FL are respectively connected in operation.

A ballast inductor L is connected between the input terminal 11 and the output terminal 01, and a power factor correction capacitor C1 is connected between the input terminals 11 and 12.

The input terminal 12 is directly connected to the output terminal 04.

To provide a path for the supply of current from the supply to heaters H1 and H2 during starting of the lamp a thyristor T is connected between output terminals 02 and 03. The thyristor T is provided with a gating circuit comprising a resistor R1, and a capacitor C2 connected in series in the order stated between output terminals 02 and 03.

The gating circuit further includes a zener diode Z, a resistor R2 and a positive temperature coefficient thermistor PTC connected in series between the junction of resistor R1 and capacitor C2 and the gate electrode of the thyristor T. The thermistor PTC is mounted in good thermal contact with the thyristor T.

A capacitor C3 is connected between the output terminals 02 and 03.

In operation, when the supply is first connected to the terminals Ii and 12, the thyristor T fires during each positive half cycle of the supply at a firing angle mainly determined by the values of the components R 1, C2, and Z of the gating circuit. As a result, a pulse of current flows through the heaters H1 and H2 during each positive half cycle of the supply, as indicated at 1 in Fig. 2b which shows the waveform of the heater current in operation, Fig. 2a showing the waveform of the supply voltage.

During each negative half cycle, the thyristor T initially passes a current pulse indicated by 2 in Fig. 2b, due to its finite turn-off time.

This pulse decays rapidly as the thyristor T turns off producing a ringing voltage across the capacitor C3, the capacitor C3 forming with the ballast inductor L a resonant circuit damped by the resistance of the heaters H1 and H2. A high voltage pulse is thus applied between the lamp electrodes, as indicated by 3 in Fig. 2c which shows the waveform of the voltage across the lamp electrodes. By suitable choice of component values a voltage of optimum value to reduce end-blackening of the lamp is provided across the lamp electrodes H1, H2. If the lamp fails to strike, a further current pulse is applied to the heaters during each succeeding positive half cycle of the supply, thereby progressively raising the lamp electrode temperature until sufficient electron emission occurs for the lamp to strike.

When the lamp strikes the peak voltage between the lamp electrodes falls in the usual manner, and the potential at the junction of resistor R1 and capacitor C2 no longer rises to a sufficietly high value to breakdown the zener diode Z and fire the thyristor T. Hence the thyristor T remains non-conducting and no further current is supplied to the heaters H1 and H2.

If for any reason the lamp FL fails to strike the temperature of the thermistor PTC, due to its being heated by the thyristor T, becomes sufficiently high to cause the resistance of the thermistor PTC to rise appreciably. As a result, the firing current supplied to the thyristor T is reduced below the value at which reliable firing of the thyristor T occurs. Thereafter, the thyristor T fires only occasionally keeping the thermistor PTC sufficiently hot to prevent reliable firing of the thyristor T and reducing the mean current flowing through the heaters H 1, H2 and inductor L to a sufficiently low value to avoid overheating of the inductor.

The thermistor PTC preferably exhibits a characteristic having a sharp 'knee' so that its resistance increases very rapidly when its temperature exceeds a predetermind value. The need for accurate tolerancing between the characteristics of the thermistor PTC and thyristor T is then avoided.

A further advantage of the circuit arrangement is that if the termistor, PTC fails, the circuit becomes inoperative, i.e. it 'fails safe'.

In one particular embodiment of the arrangement for operating a 1 500 millimetre long 65 watt fluorescent lamp, details of the circuit components relevant to the invention are as follows: Capacitor C2 0.47 microfarads Capacitor C3 0.022 microfarads Resistor R1 22-30 kilohms Resistor R2 1-5.6 kilohms Thermistor PTC Type YC 1 20 supplied by I.T.T. Industries Inc.

Zener diode 39-56 volts breakdown Inductor L 0.86 henries With these components a ringing voltage of peak amplitude of between 500 and 600 volts is applied across the lamp electrodes and the R.M.S. value of the current supplied to the heaters is initially about 1.4 amps. The thermistor is suitably heat-sinked to the thyristor T with a thermal time constant of about 50 seconds. This results in the heater current being reduced after one or two minutes.

It will be appreciated that in alternative arrangements in accordance with the invention the thermistor PTC may be raised in temperature other than by way of heat derived from the electronic switching device. For example it may be arrnaged to derive heat alternatively or additionally from another component in the circuit e.g. the inductor L, or alternativeiy or additionally be directly heated by the passage of current through itself.

Claims (8)

1. A fluorescent lamp circuit arrangement comprising: a controllable electronic switching device connected across the lamp to provide a path for the passage of current from an alternating current power supply through an electrode heating means of the lamp when the switching device is conducting; and a gating circuit connected across the lamp and to the switching device to control the conduction of the switching device in dependence on the value of the voltage developed across the lamp when the alternating current supply is connected to the lamp circuit whereby the switching device is switched on during an initial period by the gating circuit so as to produce conditions for ignition in the lamp during the initial period, and is held nonconductive when the lamp has ignited, the gating circuit including a resistance which is heated in operation of the lamp circuit by current passing through itself and/or by heat produced in one or more components of the arrangement, said resistance having a positive temperature coefficient and being connected in a path for the supply of current to a control electrode of the switching device so that, if the lamp fails to ignite, the value of said resistance assumes a sufficiently high value to cause unreliable operation of the switching device and thereby to maintain the mean current supplied to said heating means at a reduced value.

2. A fluorescent lamp circuit arrangement according to Claim 1, wherein, in operation, the resistance is heated by heat produced in a component of the circuit.

3. A fluorescent lamp circuit arrangement according to Claim 2, wherein said component is the switching device.

4. A fluorescent lamp circuit arrangement according to any preceding claim, wherein said resistance is connected in series with a voltage breakdown device.

5. A fluorescent lamp circuit arrangement according to any preceding claim, wherein said resistance is connected between the control electrode of the switching device and a tapping point of a potential divider connected across the lamp.

6. A fluorescent lamp circuit arrangement according to Claim 5, wherein the potential divider comprises a further resistance connected in series with a capacitance.

7. A fluorescent lamp circuit arrangement according to any preceding claim, wherein said switching device is a thyristor.

8. A fluorescent lamp circuit arrangement substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.