GB2226463A - Control of fluorescent lights - Google Patents

Abstract

A fluorescent lamp 10 is connected across a capacitor 30 of a series-resonant L-C circuit 28, 30 and is operated by a high frequency a.c. supply 40, 42 which during lamp starting has a frequency gradually changing between two frequencies respectively lower and higher than the resonant frequency of the L-C circuit. The supply may ramp down from the higher to the lower frequency, or may ramp up from the lower to higher frequency over a one second period, dwell at the higher frequency for half a second and then fall to a normal operating frequency. Lamp brightness may be varied by adjusting the frequency and/or width of the h.f. energising pulses and may be controlled in response to ambient light. A common h.f. supply may power a plurality of L-C/lamp circuits and may have an overcurrent sensor (160), (Fig 6), which causes a control circuit (140) to turn off output MOSFETS (102), (104) for 5 millisecs. <IMAGE>

Description

CONTROL OF FLUORESCENT LIGHTS ETC.

This invention relates to electronic control of electrical powered loads normally powered by mains alternating electrical current suply and requiring higher electric potentials or presenting greater electrical resistance initially for bringing them into operative states than for maintaining those operative states, such as applies to fluorescent lights.

Fluorescent lights are well known and require establishment of plasma and gas discharge operative to activate a fluorescent coating usually interior of a containing tube. Customary starting/control arrangements for operation from normal mains a.c.

electrical supplies, which are at about 240 volts and 50 Hz in the United Kingdom, tend to be rather crude and inefficient. Large iron-cored chokes are used in combination with electro-mechanical switching devices using bimetallic strips. Heater elements at each end of the fluorescent tube serve to warm up the tube contents to get lower strike voltage requirements, while other heating means readies the electro mechanical device for switching to give fast release of high voltage electrical energy from the choke into the tube. It is quite usual for several switching cycles to be required before successful starting occurs, when switching is at or near peak a.c. voltage. Fluorescent tube flickering is commonplace and failure of such starter/control provisions is a common fault.It is believed that these systems are not only heavy and intrinsically rather unreliable in service, but that they lead to inefficiencies and undue stress that shortens fluorescent tube life unnecessarily.

It is an object of this invention to provide lightweight and otherwise improved starting/control systems for fluorescent tubes or, it is envisaged, loads of a similar but not necessarily identical nature.

According to one aspect of this invention, there is provided electronic circuitry comprising a source of relatively high frequency alternating supply well above normal mains frequency, and load-associated means affording both of inductance (L) and capacitance (C) that cooperate together and with the output of said source in bringing the or each load into its operative state utilising LC resonance or near-resonance effects but takes no more than a small proportion of operating current during subsequent maintenance of such operative state.

In achieving gas discharge or similar effects for fluorescent lights, required inductance is much lower than hitherto. Subsequent description is largely in terms of fluorescent lights as said loads.

In one embodiment, a capacitor is connected across heater elements at ends of a fluorescent tube with a light-weight choke, say of ferrite core type, in series with those heater elements. Required or desired initial driving of the load, say for heating the tube contents, usefully precedes actual achievement of operative resonance effects, specifically to strike gas discharge for fluorescent lights conveniently through a prescribed rise or fall of a.c supply frequency towards LC resonance.

According to another aspect of this invention, there is provided electronic circuitry comprising a source of alternating electrical supply at variable frequency and means for controlling rise or fall of that frequency towards resonance of inductance and capacitance associated with a load for achieving conditions required by that load in order to change its state, usually resistive, as applies to lowered resistance at striking of gas discharge in a fluorescent tube.

For a fluorescent tube with a capacitor in parallel across its heater elements and an inductor in series with those elements, striking of gas discharge drastically lowers resistance of the tube and effectively shorts out the capacitor. The result can go beyond lower voltage requirements to lowered current consumption consistent with maintaining gas discharge, in practice with much lower continuing heating effects at end caps of the tube.

A further aspect of this invention arises from utilising frequency change after the desired or required load change, i.e. after striking of a fluorescent tube, within limits not adversely affecting that load change, i.e. not losing discharge or extinguishing of a fluorescent tube, in order to vary light output and/or efficiency of a lit fluorescent tube.

Implementing this further aspect leads to a controlled frequency source operative to raise its frequency to a prescribed or required level, hold it for a prescribed or required time, then again vary it within prescribed or required limits, say to a prescribed or required higher or lower value. Interestingly, it is found that light output and efficiency of a fluorescent tube have opposite characteristics relative to frequency, the former falling and the latter rising with frequency increase. Moreover, striking can be made at or close to mid-regions of both such characteristics, and is also seen as an aspect of this invention.

Alternative implementation of that further aspect can be by use of a controlled frequency source operative to lower its frequency towards a prescribed or required minimum level required for striking gas discharge, then holding (from strike if preferred) and changing as desired.

It is considered important to appreciate advantages arising from the frequency source being independent of the load in the sense that its available output including in terms of frequency of operation is substantially independent of and unaffected by change of load characteristics, such as occurs at striking of gas discharge in a fluorescent tube, i.e. electrical parameters of the load contribute no significant part of frequency-determining components of the supply, and this also constitutes an aspect of this invention.

Moreover, a particular practical advantage arises from systems hereof that are of a modular nature, normally where a single frequency source services a plurality of loads, whether emplacing choke/capacitor pairs together with the source on a single board for banks of fluorescent lights, e.g. as used in sunbeds, or with just the source on one board and other components (coil;capacitor combinations) separate therefrom and local to the loads.

A preferred form of frequency source produces a pulsed output at the frequency concerned with time-spacing intervals between the pulses, and is a yet further aspect of this invention. That is readily achieved using a frequency-defining switching control signal for a suitable push-pull operating output stage, for example employed a pair of switching field-effect transistors (FETs). Varying the ratio between pulsewidths and intervals between the pulses can be employed to control power output, effectively by varying pulse widths. In fact, pulsed driving of fluorescent tubes is both advantageous in itself, say as a means for ensuring minimum heating effects at tube end caps, and affords a means of varying brightness, i.e., light output. Such provision and use can result in particularly advantageous results alone and/or in conjunction with the afore-mentioned frequency variation and noted effects concerning light output and efficiency.

Practical implementation of this invention in its various aspects will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is an outline circuit diagram of a fluorescent light with prior startZcontrol means; Fig. 2 us an outline circuits diagram for a fluorescent light with start/control hereof; Fig. 3 is a block circuit diagram for one suitable frequency supply; Fig. 4 shows graphical representation of light output:'efficiency relative to frequency; Fig. 5 shows circuit diagrams of a line filter and AC DC converter; Fig. 6 is a circuit diagram of an overall electronic starter/control system; and Fig. 7 shows pulse-type switching signals used in Figure 6.

In the drawings, referring first to Figure 1, a fluorescent tube light 10 has end caps 12A, 12B with heater elements 14A, 14B offering resistive loads RA, RB to a.c electrical mains supply, indicated at nominal 240 voltsi50 Hertz, from live at 16 through a large iron-cored choke 18 to heater element 14A, and from heater element 14B to neutral at 20. Other ends of the heater elements 14A, 14B are connected through a starter 22 including a bimetallic switch 24. The tube contents are heated by the heater elements 14A, 14B as the bimetallic switch 22 heats up to open its contacts, whereupon the large iron-cored choke 18 applies a large back emf for striking gas discharge in the tube 10.

After successful striking, often requiring several operations of the bimetallic switch 24 until start occurs at or near peak mains voltage and/or gas temperature becomes sufficiently increased, virtually all current flow is through the tube 10 and the starter 22 is effectively shorted out.

Investigations reveal that striking voltage for conventional fluorescent tube lights is of the order of 1 Kvolt (depending on gas temperature), but that a.c.

supply at about 150 volt peak will maintain gas discharge.

Turning to Figure 2, the same references are repeated where they apply, and it will be appreciated that a.c.

electrical supply at 150 volts peak and 50 KHertz is indicated. Also, the large iron-cored choke of Figure 1 is replaced by a lightweight small ferrite core choke 28 and the starter 22 simply by a capacitor 30. In terms of equivalent circuits when the tube has struck and gas discharge is taking place, there is little difference between the equivalent circuit at the tube 10 for Figures 1 and 2, i.e the gas discharge takes most of the supply current previously going through the heater elements 14A, 14B, and 150 volts rms (i.e. with a margin on actual requirement) is adequate safely and reliably to maintain gas discharge, with reduced possibility of flicker effects at frequencies much higher than mains, indicated as 40 KHz.

Prior to striking, of course the electric supply "sees" a series connection of inductance L of the choke 28, resistance RA of the heater element 14A, capacitance of the capacitor 30 and resistance RB of the heater element 14B. For a frequency of electric supply at or near the resonant frequency of the inductance/capacitance of the choke 28 and capacitor 30, the load approaches pure resistance and there is a large increase in voltage across the capacitor 30, also a large increase in current through the heating elements 14A, 14B. This effect can produce striking of gas discharge at frequencies of the order of 30 KHz and above, the heating effect of the increased current to the heater elements 14A, 14B bringing down the strike voltage required for gas discharge until that is reached by the capacitor 30.

For gas discharge, most of the supply current goes through the tube 10, rather than through the heater elements 14A, 14B and the capacitor 30. A residual latter current is however, readily maintained to keep the heater elemnts 14A, 14B warm enough to ensure continuity of the gas discharge.

Satisfactory operation has been achieved where the frequency of the electric supply is variable from well below resonance of inductance/capacitance (L-C) involved increasing gradually through striking of gas discharge in the tube 10 to above the resonant frequency nominally required for such striking, but well below a maximum, found to be about 80 KHz, at which gas discharge is lost. That has beneficial effects both in assuring adequate heating by the heater elements 14A, 14B and in assuring striking does take place for a variety of fluorescent tube lights from different manufacturers and having different tolerances, and further taking account of tolerances attaching to inductance of the ferrite core choke 28 and the capacitance of the capacitor 30. A highly reliable system results for frequency variation from 40 KHz to 65 KHz (depending on inductance/capacitance values chosen).

Frequency rise over a period of the order of one second is found to be satisfactory, but that is not to be taken as indicative that one second rise is necessary, nor that start and maximum frequency need to be so high (nor could not be higher). A dwell at maximum frequency of about half-second is adequate to assure first-struck gas discharge is not lost, and similarly not to be taken as necessary.

If preferred, initial frequency variation can be downwards from above the resonant frequency gradually down through striking of gas discharge.

Frequency variation after striking can usefully control brightness of the operating fluorescent tube.

Thus, a predetermined frequency rise time, dwell time, then change up or down, is highly advantageous, see variable rate switching signal generator 40 in Figure 3 shown used to control a switch 42 of an a.c. output stage 44 operative relative to nominal +10 volt and -160 volt d.c. inputs at 46 and 48. Such d.c. signals are readily derived from typical 240 volt/50 KHz mains by way of an a.c.-d.c. converter 50. When deriving from normal mains, it is preferred to utilise a line filter 52 both to protect the circuitry itself from mains transients and to reduce noise from that circuitry reaching the mains supply.

It is, of course, to be appreciated that any convenient form of frequency source can be employed, and any form of frequency varying control. A built-in start sequence of frequency rise, then maximum frequency dwell is also readily achieved, for example as will be described for Figure 6. Thereafter, brightness control may be by a manually variable component or components, or further be built-in or determined by some reference arrangement. Another reason for frequency change after discharge is established is that efficiency of operation is found to vary with frequency, but usually inversely to brightness, see Figure 4.

A particular feature of preferred a.c. electric supplies hereof is that their operation is substantially independent of electrical parameters of the fluorescent light tube itself. Indeed, it is highly advantageous, as aforementioned, for the supply to have sufficient power to supply a plurality of flourescent tubes, whether as a bank, say for a sunbed, or otherwise.

Figure 5 shows more detail of one suitable line filter 52, of fused or neon tell-tale type (see 54), and also utilising a one-to-one ferrite core transformer 56 with capacitors 58, 60, 62 and 64 and voltage-dependent resistor 66 connected in conventional configuration as illustrated, and conveniently connected to a heat sink at 68. A suitable heat sink comprises an aluminium channel containing a circuit board carrying at least the whole of the frequency source electronics.

Figure 5 further shows more detail of one suitable acdc converter 50 using a diode bridge 70, capacitors 72 and 74 on each output, and resistor 76 and capacitor 78 across its positive and negative outputs 80, 82 as shown.

Turning to Figure 6, a.c. output is indicated as being provided by alternate conduction of a pair of field effect transistors, actually MOSFETs, 102 and 104 caused to conduct alternately, i.e. operate in push-pull by switching signals to their gate electrodes at 106 and 108. Those switching signals are preferably derived from pulse signals, see Figure 7, that are so spaced in time and of such widths t as to leave quiescent intervals td between switching either transistor 'off' and the other 'on'. Those pulsed switching signals are shown supplied from lines 110 and 111 to the primary 112 of another ferrite core transformer 114 having oppositely poled equal secondaries 116,118 going to the transistor control electrodes over resistors 120, 122 and with resistor-capacitor connections 124R,C and 126 R,C across the secondaries 116,118 and resistors 120,122.Using MOSFETs 102, 104 protection against inductive load effects is advisable, and is shown provided by Schottky diodes 128, 130 connected similarly poled between the MOSFETs 102, 104 and affording low forward voltage characteristics, and two further diodes 134, 136 of fast high-voltage type connected across each OSFET/Schottky diode pair and poled oppositely to the Schottky diodes 128, 132 with nodes between each pair of diodes connected together and to the live a.c. supply line. Control signals at the gates of the MOSFETs 102, 104 are also shown in the bottom two lines of Figure 7.

The basic switching control signals of Figure 7, even signals direct to the MOSFETs 192, 104 or equivalent circuitry, can be realised in any convenient way. Figure 6 shows use of an available proprietary integrated circuit (SG 3525A) indicated at 140 that permits a facility for varying the width of its output pulses, hence average power to the fluorescent tube and thus its brightness (whether alternatively or additionally to frequency control). Usually, not all facilities of a proprietary chip need to be used, and some may be modififed to the purposes hereof (both of which apply to SG 3525A).

In Figure 6, starting frequency sequencing is achieved using the +160 volt supply, which will be established shortly (say about 20 mseconds) after switch on.

Capacitor 142 charges slowly over resistor 144 and raises base voltage of transistor 146 so that more current passes through its collector thereby increasing current source setting for the chip 140, and hence its operating frequency. That continues until transistor 146 is saturated (corresponding to maximum frequency), but the voltage on the capacitor 142 continues to rise until (say half-second later) transistor 148 begins to conduct. Then the base voltage of the transistor 146 begins to fall until it switches off and frequency becomes determined solely by the resistor 150 (corresponding to final frequency).

If there was to be a fall in input mains voltage, frequency from the chip 140 would actually rise, thus ensuring continuing gas discharge in the fluorescent tube despite the fall of mains voltage. If a few cycles of mains voltage were lost, the capacitor 142 would discharge through diode 152 and the start-up sequence would repeat.

The high frequency a.c. supply can be monitored at its live line using a current limiter 160 having a current transformer 162 and rectifier 164 operative for scaling down so that driving the maximum number of fluorescent tubes does not result in reaching a threshold for the chip 140. Should a large current be detected, the output MOSFET will be switched off within a few microseconds for a period of about five milliseconds.

Brightness control by varying pulse width of outputs of the chip 140 using variable resistor 166, and can represent improvement on frequency variation for brightness control, because increasing frequency to reduce brightness results in more current flowing through the starter capacitor(s), hence the heater elements, and with reduction in current flow through the fluorescent tube 10.

It is feasible for the variable resistor 166 to be replaced or additional control effected using a light sensitive resistor, then allowing operation according to a pre-set level of lighting and taking account of ambient light.

Figure 6 also shows a transformer 170/rectifier 172/ capacitor 174 arrangement for deviation of low voltage supply for the chip 140.

Claims (16)

1. Control system for loads normally driven by mains alternating current electrical supply, such loads requiring higher electric potentials and presenting greater electrical resistance initially for bringing them into operative states than for maintaining those operative states, such as applies to fluorescent lights, the system comprising electronic circuitry affording an alternating voltage source of substantially higher frequency than normal electrical supply to the or each load, and load-associated means affording both of inductance (L) and capacitance (C) that cooperate together and with output of said source in bringing the or each load into its operative state utilising LC resonance or near resonance effects but take no more than a small proportion of operating current during subsequent maintenance of such operative state.

2. Control system according to claim 1, wherein the alternating voltage source is substantially independent of electrical parameters of the or each load so that the frequency is substantially unaffected by the state of the or each load.

3. Control system according to claim 1 or claim 2, comprising a single said alternating voltage source servicing plural loads by way of plural said loadassociated means, one for each load.

4. Control system according to claim 3, wherein said single alternating voltage source and said plural loadassociated means are physically and electrically located in one equipment unit.

5. Control system according to any preceding claim, for fluorescent light tube loads, comprising a capacitor connected across heater elements at ends of the or each said tube and a light-duty choke in series with those heater elements, the load serving effectively to short circuit the capacitor in the operative state of the load.

6. Control system according to any preceding claim, wherein the inductance is afforded by a ferrite core type of choke.

7. Control system according to any preceding claim, wherein the alternating voltage source of the electronic circuitry is of variable frequency type and that circuitry includes means for varying rise or fall of that frequency towards said LC resonance.

8. Control system according to claim 7, wherein the means for varying frequency is operative to change the frequency after bringing the or each load into its operative state so as to vary maintained load operation.

9. Control system according to claim 8, wherein the means for varying frequency serves to raise frequency applied to a prescribed load, halt it for a prescribed time, then further vary it within prescribed limits.

10. Control system according to claim 9, wherein the further varying is to a prescribed higher or lower value

11. Control system according to claim 9 or claim 10 for fluorescent light tubes, wherein the frequency at striking is at or near mid regions of both of light output and efficiency characteristics relative to frequency.

12. Control system according to any preceding claim, wherein the alternating voltage source is operative to produce a pulsed output at the frequency concerned with frequency dependent time spacing intervals between its pulses.

13. Control system according to claim 12, wherein the alternating voltage supply comprises a push-pull operating output stage.

14. Control system according to claim 13, wherein the push-pull operating output stage comprises a pair of switching field-effect transistors.

15. Control system according to claim 12, 13 or 14, comprising means for varying mark-space ratio of said pulses to control electrical power available and supplied to the or each load.

16. Control system for fluorescent light tubes or similar loads arranged and adapted to operate substantially as herein described with reference to and as shown in the accompanying drawings.