GB2180418A - Fluorescent lamp supply circuit - Google Patents

Abstract

A circuit supplying arrays of fluorescent lamps comprises a mains to D.C. converter 18 the output of which drives at least one power oscillator 39 operating in the kilohertz range. Oscillator 39 drives L-C resonant circuits 40A, 40A, the C-components of which are each connected in parallel with a respective fluorescent lamp. A voltage control circuit controls the voltage across the C component of the resonant circuit and is effective to limit this voltage in the event of the pertaining fluorescent lamp failing. The control circuit may be a comparator (22), (Fig. 2) which monitors the voltage level at the junction of the L and C components of each resonant circuit and compares this against a reference level. The output of the comparator activates a circuit (21) which inhibits operation of the corresponding oscillator when a fluorescent lamp fails. Alternatively, the Q-factor of the resonant circuit may be reduced by a series- connected PTC resistor (44), (Fig. 4), or the resonant circuit may be detuned by various neons, such as a detuning capacitor (46) associated with a varistor or NTC resistor (45), (Fig. 5), clamping diodes (43) associated with a triac or detuning capacitor (47), (Fig. 6), a triac (48) connected across the C-component, (Fig. 7), or a magnetic amplifier in series with or forming part of the L-component. The lamp array may be in a UV sun bed or a refrigeration cabinet. <IMAGE>

Description

SPECIFICATION Lamp supply circuit This invention relates to a lamp supply circuit for supplying arrays of fluorescent lamps such as are found for example in ultra violet sun beds and canopies and in refrigeration cabinets and cold stores and in a variety of other situations, e.g. multiple lamp lighting fittings.

According to the present invention there is provided a lamp supply circuit for supplying arrays of fluorescent lamps, said circuit comprising a mains frequency to d.c. converter connected to drive at least one power oscillator operating in the kilohertz range, the output of said at least one power oscillator being connected to drive a plurality of parallel-connected L-C resonant circuits each such resonant circuit having terminals for connection to a fluorescent lamp in parallel with the capacitive component thereof, and control means for each resonant circuit for controlling the voltage level across the C component of the pertaining resonant circuit, said control means being effective to limit said voltage level at least upon failure of the pertaining fluorescent lamp.

The control means may comprise a voltage comparator arranged to monitor and compare the voltage level across the C component with a reference voltage level, and an inhibit circuit operable by the comparator and arranged to inhibit operation of the power oscillator driving the pertaining resonant circuit in the event that the monitored voltage level exceeds the reference voltage level.

Alternatively the control means may comprise a detuning circuit connected to said resonant circuit, the detuning circuit including either a positive temperature coefficient resistor or a negative temperature coefficient resistor.

In a further alternative the control means comprises a diode clamp circuit connected to said resonant circuit.

In one embodiment each power oscillator is connected to drive a pair of said resonant circuits and if the voltage level across the C component of one of said resonant circuits becomes excessive the pertaining power oscillator is inhibited, so that said pair of resonant circuits is effectively disabled. In another embodiment each power oscillator is connected to drive many resonant circuits and if the voltage level across the C component of one of said resonant circuits becomes excessive only that resonant circuit is effectively disabled.

By virtue of the present invention because each power oscillator operates in the kilohertz range the weight and bulk of the circuit is very substantially less than with a plurality of conventional fluorescent lamp ballast circuits; hum from mains frequency magnetostriction is eliminated as is perceptible flicker in the light output from the fluorescent lamps; the heat loss from the circuit is substantially less than hitherto; increased light output of the order of at least 10% is achieved; the fluorescent lamps start up substantially instantly without any requirement for starting switches; lamp life is increased; blackening of the ends of the lamps is reduced; running costs and cooling costs are greatly reduced; mains current is reduced resulting in less bulky and less costly cabling and protective circuitry; and the power factor of the circuit is increased nearly to unity without added power factor compensation.

Each power oscillator conveniently operates at 25 KHz or greater and in fact 20-80 KHz is preferred. When banks of fluorescent lamps are supplied, for example, over a large area of lighting or where multiple sunbeds with many lamps are supplied, as in a sun tanning commercial salon, and the circuit is supplied through a sine-wave current convertor or similar circuit to give a very high harmonic filter.

This reduces the possibility of harmonic content being fed into the supply lines where, although the interference from individual lamp drives may be small, the cumulative effect may be unacceptable with conventional filtering.

It will be understood that the mains frequency to d.c. converter functions as a common input for the various power oscillators etc. and as a result the cost of the lamp supply circuit is comparable with a collection of conventional fluorescent lamp ballasts despite the relative expensiveness of the various resonant circuits etc. The greater the number of lamps in the array the greater is the cost reduction and at the same time the weight and bulk improvement is greatly enhanced both of which factors are particularly important when the array forms part of a portable equipment such as a sun bed or sun canopy.

The mains frequency to d.c. converter is preferably formed by a low-pass filter and surge suppression circuit series connected with a full-wave rectifier series connected with a d.c. smoothing circuit or energy buffer.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 illustrates a sun bed incorporating a lamp supply circuit shown in block diagram form in Fig. 2; Fig. 3 illustrates another embodiment of the lamp supply circuit, and Figs. 4, 5, 6 and 7 illustrate different modifications applicable to a detail of the Fig. 3 circuit.

As shown in Fig. 1 of the drawings a sun bed 10 incorporates a bench 11 fitted with a set of fluorescent lamps 12 and surmounting the bench 11 is a canopy 13 fitted with a set of fluorescent lamps 14. In the interests of clarity only a limited number of lamps 12, 14, have been illustrated but it will be understood that typical sun beds incorporate up to thirty or forty such lamps. The sun bed 10 is powered from a mains supply circuit located within bed 10 and which operates the entire array of lamps 12, 14.

As is shown in Fig. 2, cable 16 supplies mains frequency voltage (either 50 Hz or 60 Hz) to a d.c. converter 18 the output of which drives a power oscillator 19 operating at a pre-set frequency in the region of 20 to 80 KHz. Oscillator 19 drives a pair of resonant circuits 20A, 20B, connected in parallel each circuit 20A, 20B, comprising inductive (L) and capacitive (C) components connected in series, the C components being connected in parallel with terminals for receiving a UV emitting fluorescent lamp. The component values of circuits 20A, 20B, are selected so that the natural resonant frequency of these circuits is the same as the pre-set frequency of oscillator 19.

Oscillator 19 is arranged to drive circuits 20A, 20B, following connection to mains supply 15, and in the event that a fluorescent lamp fails to ignite the oscillator 19 is inhibited from further operation by means of inhibit circuit 21 which is controlled by comparator circuit 22 the latter monitoring the voltage levels at the junction of the L and C components of circuits 20A, 20B, and comparing each of these monitored voltage levels with a pre-set reference voltage level. Inhibit circuit 21 is of the latching type so that when enabled by comparator 22 it latches into the status which inhibits operation of oscillator 19.

Converter 18 drives a plurality of power oscillators 19', 19", etc. each with respective resonant circuits all connected in parallel with oscillator 19 whereby a complete array of fluorescent lamps 12, 14, may be energised from a single converter 18. Only additional oscillator 19' is illustrated. Converter 18 itself comprises a low-pass filter and surge suppression circuit series connected with a full wave rectifier series connected with a d.c. smoothing circuit or energy buffer. The harmonic filter may also consist of a sine wave current converter circuit.

By way of example, a lamp supply circuit as described for thirty-two fluorescent lamps and operating at 45 KHz weighs about 3.2 kilograms, which is less than one-tenth the weight of convention ballast arrangements and gives rise to a heat loss of only 120W which is about one-fifth the heat loss of the conventional ballast arrangement. In addition the heat generated in the lamps is reduced by the high frequency operation giving greater efficiency and more stable lamp temperatures.

The lamp supply circuit which is illustrated in Fig. 3 is generally similar to that of Fig. 2 in that it comprises an AC/DC converter 18 formed by a filter, a rectifier and a smoothing circuit the output of which drives power oscillator 39. In the Fig. 3 circuit only one oscillator 39 is illustrated and this drives a plurality of resonant circuits 40A, 40B, etc. all of which may be connected in parallel, all being connected to a common decoupling capacitor 41 for return to the zero level or the mid supply voltage level.As previously explained each resonant circuit 40A, 40B, etc. comprises L and C components connected in series and terminals are provided for connection to a fluorescence lamp in parallel with the C (capacitive) component and the voltage across the component is monitored and controlled in this instance by a protective network 42A, 42B, etc. and which operates to prevent excess voltages occurring. The networks 42A, 42B, etc. may function as detuning devices for the pertaining resonant circuits or voltage clamping devices to clamp the maximum levels of the monitored voltages at pre-set values.

Fig. 4 illustrates two alternative forms of the network 42A which comprises a positive temperature coefficient (PTC) resistor 44 connected in series with the L and C components of resonant circuit 40A. In these arrangements the PTC resistor 44 reduces the 0 factor of resonant circuit 40A when failure of the fluorescent lamp to conduct could cause the voltage across the resonant C component to increase to dangerous levels. These arrangements of PTC resistors may also be used in combination with the forms of control network 42A described with reference to Figs. 5, 6, 7 and 8.

Fig. 5 illustrates the network 42A in the form of a negative temperature coefficient (NTC) resistor 45 in series with a common detuning capacitor 46 and operates so that voltage rises cause the resistor 45 to become less resistive thereby introducing the capacitor 46 into the resonant circuit 40A and limiting the voltage across the C component of circuit 40A. A varistor may be used in place of the NTC resistor 45 by means of which the monitored voltage level is additionally clamped at the varistor cut-off level.

Fig. 6 illustrates the network 42A in the form of a conventional diode-clamp device 43, the diodes of which are connected to the supply voltage rail and to the zero voltage rail, coupled to the monitored voltage level by a detuning capacitor 47. Capacitor 47 may be replaced by a controllable device such as a triac the gate circuit of which is controlled by a sensor monitoring the L-C junction voltage of the resonant circuit 40A. As is shown in Fig. 7 triac 48 and sensor 49 may be used in the absence of the diode-clamp device 43 to clamp the C-component voltage to zero when the triac conducts.

Various other forms of the network 42A may be envisaged whereby the resonant circuit 40A is detuned; for example by modifying the inductive (L component) of the resonant circuit. This can be achieved by a magnetic amplifier in series with or as part of the L component the impedance of which is altered by a DC control winding operated by a voltage level sensor connected to the L-C junction of circuit 40A. This arrangement increases the impedance of the circuit and changes the inductance value.

A particular advantage of the Fig. 3 circuit is that the single power oscillator can be subjected to a dimmer controller which is then effective on the entire array of the fluorescent lamps. The dimmer controller may take any known form for manual or automatic operation. The lamp supply circuits which have been described either singly or in combination control against the undesirable effects caused by start-up voltage pulses and lamp failures.

A particular advantage of the lamp supply circuit according to the present invention when used in sun beds and canopies is that because the same light output can be obtained for less energy input than hitherto the lamp array requires less cooling and provides a more consistent light output along the length of the lamp because temperature is more uniform along the lamp and in turn this gives rise to more even skin tanning along the length of the sun bed or canopy. Furthermore in sun beds and canopies there is a need to energise the ultra violet emitting fluorescent lamps for a predetermined duration to avoid skin burning and the present lamp circuit containing a plurality of oscillators provides a convenient electronic clock source for a digital timer.

A particular advantage of the lamp supply circuit according to the present invention when used in refrigeration cabinets or cold stores is that lamp start is practically instantaneous at temperatures down to as low as -25"C, and no starting switches are required.

Claims (9)

1. A lamp supply circuit for supplying arrays of fluorescent lamps, said circuit comprising a mains frequency to d.c. converter connected to drive at least one power oscillator operating in the kilohertz range, the output of said at least one power oscillator being connected to drive a plurality of parallel-connected L-C resonant circuits each such resonant circuit having terminals for connection to a fluorescent lamp in parallel with the capacitive component thereof, and control means for each resonant circuit for controlling the voltage level across the C component of the pertaining resonant circuit, said control means being effective to limit said voltage level at least upon failure of the pertaining fluorescent lamp.

2. A lamp supply circuit as claimed in claim 1, wherein said control means comprises a voltage comparator arranged to monitor and compare the voltage level across the C component with a reference voltage level, and an inhibit circuit operable by the comparator and arranged to modify operation of the power oscillator driving the pertaining resonant circuit in the event that the monitored voltage level exceeds the reference voltage level.

3. A lamp supply circuit as claimed in claim 2, wherein each power oscillator is connected to drive a pair of said resonant circuits and if the monitored voltage level in one of the resonant circuits becomes excessive the pertaining power oscillator is inhibited so that said pair of resonant circuits is effectively disabled.

4. A lamp supply circuit as claimed in claim 1, wherein said control means comprises a detuning circuit connected to said resonant circuit.

5. A lamp supply circuit as claimed in claim 1, wherein said control means includes a voltage clamp circuit.

6. A lamp supply circuit as claimed in claim 1, wherein each power oscillator operates at a frequency in the range 20-80 KHz.

7. A lamp supply circuit as claimed in claim 6, wherein each power oscillator operates at a frequency of at least 25 KHz.

8. A lamp supply circuit as claimed in claim 1, wherein the mains frequency to D.C. converter is formed by a low-pass filter and surge suppression circuit series connected with a full wave rectifier series connected with a d.c.

smoothing circuit.

9. A lamp supply circuit as claimed in claim 1, and substantially as hereinbefore described with reference to the accompanying drawings.