WO1995022880A1

WO1995022880A1 - Gas discharge lamp dimmer circuit

Info

Publication number: WO1995022880A1
Application number: PCT/GB1994/000349
Authority: WO
Inventors: James Alan Percival
Original assignee: Axiomatic Design Limited
Priority date: 1992-07-23
Filing date: 1994-02-22
Publication date: 1995-08-24

Abstract

An energy saving dimming system for driving one or more gas discharge lamps (14) together with their ballasts (13) and starters (15), from an A.C. supply (16) whereby the flow of current from the A.C. supply to the lamp and starter assemblies is interrupted for example by the opening of an A.C. switch (1) and a low impedance flywheel path substituted, for example by the closing of a second A.C. switch (12). The control ensures that one such interruption period commences when the polarity of the A.C. SUPPLY (16) changes, but ends when the tube currents signaled by (58 and 59) fall below a threshold. Further interruptions may be added to limit initial current build up for a short period before allowing a longer period of normal conduction prior to the next reversal of the A.C. Supply (16). The A.C. switches may be protected from high voltage spikes by steering diodes (2, 3, 4 and 5) which route current to and from capacitors (9 and 10) connected in parallel with suppressors (8 and 11).

Description

GAS DISCHARGE LAMP DIMMER CIRCUIT

TECHNICAL FIELD

This invention relates to energy saving and output control of gas discharge lamps. This includes the dimming of fluorescent tubes and concerns both new installations and retrofits to existing installations. The invention also relates to means of reducing the current taken by gas discharge lamps, improving their power factor, reducing the heat and other losses in such lamps, increasing their efficiency and their life.

BACKGROUND ART

Much of the background material relevant to the present invention is concerned with the problems of either conserving energy or proλiding a variable degree of illumination. The first approach to these problems, works well with ordinary filament bulbs but not with gas discharge lamps. These conventional dimmers are. typically simple low cost two terminal devices comprising a triac and a means of adjusting the time at which the triac is triggered into conduction. Such a dimmer will allow part of a cycle of alternating current to flow through itself and the lamp until the voltage reverses and the current falls to zero. Because the triac is connected in series with the lamp it can then preλent the build up of current which would otherwise begin flowing in the reverse direction. This effective disconnection of the lamp from the supply continues for a time depending on the setting of the dimmer control. The voltage applied to the lamp and the current through it is thus reduced by an amount dependant on this blocking time.

It is inherent in this first approach that the current remains near zero for a significant portion of the mains cycle. When applied to a gas discharge lamp this extended zero current period tends to extinguish the lamps, makes them flicker and reduces lamp life. The current invention embodies measures to avoid periods of zero lamp current.

A second approach which attempts to improve some aspects of the performance of the first approach involves the replacement of the triac by the A.C. terminals of a bridge rectifier while the D.C. terminals are connected to a D.C. switch. This is a method of allowing a D.C. switch to interrupt an A.C. supply as an alternative to using a triac. The advantage of this over the triac is that the A.C. switch can be turned off at any time, whereas a triac will continue to conduct until the current through it falls naturally to near zero. This configuration uses one form of what is referred to here as an A.C. switch. By using the flexibility of the A.C. switch this device can apply and remove the mains voltage at those points in the mains cycle when least disturbance to the supply and least stress to the components involved will be caused.

Since the second approach contains only one A.C. switch, when this switch is in its blocking state little current can flow through it to maintain the lamp current. This approach needs snubbing components to maintain lamp current, hoλvever the degree to which this is effective depends on the relative size of the lamp load and the snubbing component values. For example to maintain the same lamp current through both of two parallel connected lamps the values of the snubbing components would have to be approximately half those appropriate to driving a single lamp. In addition to this, if used with a high total load, an undesirable waste of power would occur by dissipation in the snubbers resistive elements. The present invention avoids the need to adjust component values to match the size of the load since the maintenance of lamp current does not depend a snubber. For this reason also, the dissipation of power in any snubber does not increase dramatically with increases in load.

A third approach which overcomes the limitations of the first and second approaches uses a regulated D.C. current. D.C. has unfortunate effects on for instance common types of fluorescent lamps. These involve the migration of mercury to one end of the tube and the consequent dimming of the depleted end. This has typically to be overcome by proλiding a mechanism for periodic reversal of the current.

Use of the third approach also makes it difficult to supply more than one parallel connected lamp from the same D.C. supply since the negative resistance characteristic of most discharge lamps causes current to tend to increase in the lamp which is already earning the largest current and decrease in the others. In this way some of the lamps may go into overload while others will be extinguished. The current invention avoids this by using an A.C. supply.

A fourth approach attempts to regulate the A.C. mains current flowing through the lamp by replacing the inductive ballast by semiconductor switches. This approach overcomes the problem of mercury migration by using A.C. The fourth approach cannot however be easily employed drive multiple parallel connected lamps because of the negative resistance characteristic of the lamps. It might also be expected that if an attempt is made to run the circuit at low currents, then because there is little inductance to store energy, a current zero will occur near the point at which the supply voltage reverses polarity. Since the A.C. supply voltage reduces in value as the point of reversal is approached, this raises the possibility that there will be insufficient voltage at that time to re-establish lamp current, resulting in the lamp extinguishing. The current invention retains the lamp ballasts and ensures a high lamp current in the region of A.C. supply reversal.

A fifth approach applies additional filament heating to increase filament temperature and make it easier for the lamps to resume conduction after a period at low or zero current. Disadvantages of the fifth approach not shared by the current invention include the cost of the more complex transformer ballasts or heater transformer, and in retrofit applications the need to replace existing inductive ballasts and rewire the luminairs with more complex wiring.

A sixth approach uses existing transformer ballasts to provide the additional filament heating required when dimming. This adds a high frequency component to the mains supply which because of the action of the transformer powers the heaters rather than adding to the lamp power. It does this because of the characteristics of the transformers.

In parts of the world where the supply voltage is higher and transformer fed lamps are less common, this sixth approach cannot be widely applied. In contrast the current invention does not require transformer ballasts.

A seventh approach is used in what are generally known as high frequency ballasts. Instead of applying a switched mains voltage to the load giving a supply whose predominant frequency is still the same as that of the supply, these devices synthesize their own frequency. To do this requires complex circuitry. Typically units include an A.C. to D.C. convenor or power supply. The smoothed output from this supplies an inverter using expensive high frequency switching components. They may use frequency variation to control the lamp current and achieve dimming with the help of special high frequency ballasts, or they may allow the current to reduce by remo\ing the applied voltage for some of the time.

Unfortunately deλices using the seventh approach are not in general suitable for retrofit as it is difficult to route the high frequency power involved across typical lighting installations without large power losses and unacceptable radio frequency interference. In addition the approach typically requires removal of the existing ballasts and replacement of these by high frequency inductors. Because of this High Frequency units generally incorporate the high frequency inductors within the unit. Thus not only is the electronics complicated and expensive but high frequency ballast inductors have to be provided. The current invention avoids the need to supply high frequencies to the tubes. Apart from in the retained inductive ballasts, the current invention does not require reservoir capacitors or other storage to power the tubes during parts of the A.C. supply cycle when the voltage is low.

An eighth approach reduces the current through the tubes by imposing one or more notches in the voltage waveform. It has been known for some time that the presence of such notches reduces the current through the tubes, however the optimum number and position of such notches has not been known. It is found that the number, position and duration of the notches dramatically affects the tendency of the tubes to extinguish themselves even if the level of dimming achieved is unchanged. They also affect the power factor of the system. In contrast to previous approaches, the current invention ensures that for a desired level of dimming, the number, position and duration of the notches is determined in such a way as to proλide a maximum tube stability and an optimum power factor. One unique characteristic of the current invention is that if the amount of dimming required is small, the system acts merely to prevent, for a short period of time, power being fed back from the load to the supply. As the amount of dimming required is increased, this reverse power flow is prevented for a longer period of time until no significant reverse power flow is allowed. DISCLOSURE OF THE INVENTION

The current invention is designed to drive gas discharge lamps including common European switch start fluorescent tube installations with inductive ballasts and glow starters.

The requirements for successfully driving gas discharge lamps as they concern the present invention, can be expressed as follows...

Firstly: The lamp should either be fed with A.C or the lamp current periodically reversed.

Secondly: A certain minimum current through the lamp should be maintained for as greater portion of the operating cycle as possible. Thirdly: Any zero current period should last for as short a time as possible.

Fourthly: Any current zero or change of direction of current should be followed by imposing a high voltage across the lamp and ballast assemblies in order to re-establish the tube arc.

It is interesting to note that a conventional inductive ballast achieves these requirements by virtue of the fact that the current lags the voltage to a considerable degree (Producing waveforms similar to those shown in FIGURE 1) so that the current zero corresponds to a supply voltage maximum. This means that any hesitation in re-establishing lamp current presents the lamp with potentially the full mains voltage.

As can be seen from the background given in the previous section, existing low frequency dimmers have in general failed to meet the requirements. There is therefore a requirement for a low frequency dimmer which does fulfil the requirements and so minimises the likelihood of the extinction of the lamp arc while retaining the possibility of retrofit to existing switchstart installations.

The current invention satisfies these requirements by controlling the power fed to the tubes in such a way that an A.C. mains supply to them is maintained in the period immediately before a mains voltage reversal, and then sλvitched off in the period immediately following mains voltage reversal. In this way the tube currents are built up and supported by the mains voltage for as long as possible. When the mains voltage reverses and would otherwise act on the tubes and ballasts in such a direction as to reduce the current flowing through them, the mains supply is switched off. With the mains supply switched off. the tube currents can be maintained by other means such as the inductance of the tube ballasts. By allowing the tube currents to maintain themselves for as long as possible following mains voltage reversal, time is allowed for the mains voltage to build up in the opposite direction. At some time before the tube current has decreased to a level that might endanger the tubes continued conduction, the mains voltage will have built up to a value near its peak. The resumption in the supply of mains power to the tube and ballast assemblies at this point results in the tube current reversing under the influence of a high applied voltage. The tube arc thus rapidly re-establishes and the chance of the tube extinguishing is minimised. In order to prevent the tube current falling to and remaining at zero, the current is measured. If the tube current falls below a threshold, power is restored to the tubes. This can be used merely to limit the period normally set by other means during which the mains supply is switched off. Alternatively, the current threshold may be made the normal means of restoring the power to the tubes, in which case the amount of dimming can be varied by varying the current threshold.

The tube current can be measured by inserting a low value resistor preferably non inductive, in the path of the current. Alternatively a hall effect device or current sampling switching devices may be used.

Using the system described above, nearly 50% current reduction can be provided while enhancing the conditions under which the tubes are operated. For example, the voltage available to re-establish tube current following a current reversal tends to be increased and the power factor of the current taken from the supply tends to improve. Beyond this level however, it is necessary to allow the tube currents to fall to zero, making it difficult to re-establish tube current. In order to allow further current reduction while still obtaining the advantages of the invention described above, one preferred embodiment incorporates a further feature into the control. Shortly after the re-establishment of tube current, at the start of the tube current waveform, the supply is switched off for a brief period. During interruption the tube currents reduce at a time when with no interruption they would have increased. This reduction in tube current is largely maintained during the remainder of the subsequent current waveform having a considerable effect on the average current through the tubes.

The onset and duration of these additional interruptions can be of fixed, timed duration, or. they can be controlled by setting current thresholds. In either case, it is important that the interruptions occur early in the current waveform for the following reasons...

The current reduction has effect over a larger portion of the current waveform resulting in a greater effect on the average current: and the supply voltage is high at the start of the current waveform resulting in little danger that the tubes will extinguish, whereas towards the end of the current waveform the voltage drops away to zero: and the effect of the interruption on the tube current reduces slightly with time. The current reduction at the time of supply voltage reversal is therefore less if as great an amount of time as possible is allowed to elapse between the interruption and supply voltage reversal. In this way the advantages of the current invention are largely maintained.

There is a limit to the duration of the interruption of the supply, since at the start of the current waveform, the tube currents are low and quickly fall to zero. If very low current levels are required it may become desirable to place a subsequent interruption after the first. For the reasons above this should also take place as early as possible in the current waveform. Likewise any third or subsequent interruptions. It will however be appreciated that the generation and transmission of high frequencies is not desirable nor with this invention is it necessary.

It is preferred that the impedance presented to the load be low at all times in order to make the operation of the circuit independent of the current demand or the size of the load attached to it. The impedance presented to the load can be composed of resistive elements or reactive elements such as that provided by capacitors or inductors. It is however preferred that any such elements are low in value or not placed in the current path from the supply to the load.

For this reason it is preferred that the supply is connected to the load via a switching device containing at least one active semiconductor component. This component or components are configured as an A.C. switch. Suitable A.C. switches can be based on the following techniques...

The first technique uses a pair of D.C. switches wired in anti-parallel so that one D.C. switch is used to carry current in one direction and the other D.C. switch is used to cany current in the other direction. These D.C. switches may be Bipolar Transistors. MOSFETS. Insulated Gate Bipolar Transistors,

Gate Turn off de\ices. or at higher powers MOS Controlled Thyristors. The application of these devices would be known to persons skilled in the art.

The second technique uses a bridge rectifier as described later using a single MOSFET. The

MOSFET could be replaced with Bipolar Transistors. Insulated Gate Bipolar Transistors. Gate Turn off devices, or at higher powers MOS Controlled Thyristors with slight modifications to the driving circuitry that would be known to persons skilled in the art. The economic choice will be dependant on the required current handling capacity required, reliability requirements, component cost, and a variety of other factors.

Although it ma}' be convenient to construct both A.C. Switches in the same way for production reasons, since the A.C. Flywheel switch tends to earn- less current than the A.C. Supply switch, there will occasionally be advantage in constructing them differently.

When the A.C. switch switches off. the cessation of current through the switch can cause a build up of voltage across the switch. This is catered for by a suppression circuit which is capable of taking, for a brief period of time, the current previously taken by the A.C. switch.

When the discharge tubes are switched on from cold, the starting mechanism tends to take a current greater than the normal running current of the tubes alone. This is catered for in one preferred implementation by contacts which are arranged to remain closed during tube start up and open only when it is desired to dim. It is not therefore necessary to provide semiconductors capable of handling the tube starting current. It also means that the starters retain their full effectiveness since full mains voltage is available. It may be convenient if these contacts are provided in the form of a relay which can in turn be controlled automatically bv a timer or microcontroller. Contacts or relay connected such as described above can be used to ensure that the presence of the dimmer does not reduce the availability of the lighting system to unacceptably low levels. By providing an alternative path for current, a failure to supply power to the tubes can be corrected. Means can then be included for the detection of a failure to power the tubes, and arrangements made to close the contacts if such a failure is detected.

In the case of luminairs with inductive ballasts, the tendency of the ballasts will be to maintain tube current, even at those times when the A.C. supply is interrupted.

In the case of luminairs with power factor correction capacitors, there will always be a path for the conduction of tube current even when the A.C. supply is interrupted. With luminairs with inductive ballasts and power factor correction capacitors, tube current will therefore tend to continue regardless of further provisions.

In case these capacitors are not present or if present are not adequate, one preferred implementation provides a flywheel path through which most of the tube current flows during periods when the mains supply to the tubes is switched off. The presence of the flywheel path reduces the voltage across the tube and ballast assemblies. Since such a voltage would act in a direction to oppose the flow of current, the presence of the flywheel path acts to reduce the rate at which the tube current falls. Such a system can achieve a lower tube current and a greater degree of dimming without the danger of the tube current falling to the point where the tube arc could extinguish.

The preferred method of implementing the system described above uses a very versatile A.C. switch connected across the lamp and ballast assembly to act as a Awheel element.

The A.C. switches are not only capable of conducting current in both directions but can be switched on or off at any time. There is no necessity to wait for the current to fall to zero before breaking the circuit as in a triac or to wait until the voltage is zero before making the circuit as in a zero crossing switch. If full output is required then the A.C. supply switch can be controlled to conduct continuously and the A.C. flywheel switch need never close. The full A.C. Supply voltage is then impressed on the lamps and their ballasts. The voltage across the tube and resulting current through the tube and ballasts can be seen in FIGURE 1.

If it is desired to reduce the current through the lamps, then the two A.C. switches are controlled so as to conduct alternately. This is done with the aim of ensuring that firstly there is always a low- impedance path for lamp current and secondly that there is never a low impedance path conducting current from one side of the supply to the other without passing through the lamps. Thus during the energisation part of the cycle, the A.C. Supply switch allows mains power to flow from the A.C. supply to the lamp and ballast assemblies. The energy stored in the inductance of the ballasts and current through the lamps may increase during this period. During the Awheel part of the cycle, the A.C. supply switch opens and the A.C. Awheel switch closes. The voltage across the lamp and ballast assemblies is then near zero. The lamp currents drop under the influence of the lamp voltages and the ballast energies drop as they feed power into the lamps. The rate of fall of the lamp currents is determined by the lamp voltages and the inductance of the ballasts.

The preferred time for the energisation state to give way to the Awheel state is when the A.C. supply changes polarity. Before this time the A.C. supply voltage acts in such a direction as to maintain lamp current. After an A.C. supply voltage reversal however, this voltage would if applied to the lamps, have the effect of hastening the rate of reduction of lamp current. Given that the lamp current may already be lower than normal as a result of a reduced energisation period, it is desirable that the lamp current be kept as near constant as possible. This is achieved by switching the circuit from the energisation state to the Awheel state, effectively disconnecting the A.C. supply and replacing it by a Awheel action in which current is maintained by the inductance of the lamp ballasts acting against the lamp voltages themselves but without haλing to act against the supply voltage as well. This can be seen in FIGURE 2. In order to reduce the average voltage impressed on the lamp and ballast assembly and so dim the lamps further, the length of the Awheel state is extended at the expense of the energisation state. The Awheel state may be extended up to the point when one or more lamp currents have fallen to zero.

When using a 240 or 220 volt supply, this minimum current, minimum power condition is achieved when the duration of the Awheel state is approximately equal to the duration of the energisation state. The effective voltage impressed on the lamp and ballast assemblies is then about half of the normal supply voltage and the lamp currents, power consumption and lamp outputs similarly reduced. This can be seen in FIGURE 3.

If it is desired to reduce the average voltage impressed on the lamp and ballast assemblies still further it is desirable to replace the single period of conduction of the A.C. supply switch by several periods of conduction interspersed by non conduction or interruption of the A.C. supply switch. As before, when the A.C. supply switch is not conducting the A.C. Awheel switch is brought into conduction to provide for the continuing, but reducing. Aow of lamp current. In this way the lamp currents can be prevented from building up. A larger final period of conduction of the A..C. supply switch before supply reversal is then used to proλide sufficient tube current to last until the voltage of the A.C. supply builds up to the point when it can ensure a swift reliable reversal of the tube current. It is important to note that the duration of this current and therefore the voltage available to ensure rapid

of the tube current are increased by switching to the Awheel state when the supply voltage reverses as revealed above. Additional interruptions are arranged to occur well before the next expected change of polarity of the supply. Interruptions should not be so numerous or so large in magnitude that high frequencies appear at the output of the device. FIGURE 4 shows an example where one additional Awheel period has been provided.

When the system switches between the supply and Awheel states, it is desirable to avoid periods when both switches are conducting at the same time. If allowed, these would put a short across the A.C. supply resulting in large currents flowing. To guard against this a short period when neither A.C. switch is conducting may be allowed. To provide a path for tube current during such periods a suppressor circuit can then be employed. Numerous suppressor circuits have been documented but a preferred circuit uses steering diodes to halve the number of suppressors that might be needed and allow the suppressors to be augmented or replaced by suppression capacitors. This is further described below. A preferred method of determining when to switch the mains supply back on is to establish a current threshold. The tube currents are then monitored and when they fall to or below the threshold, the mains supply is switched back to power the tubes. The threshold has a considerable effect on the average current taken be the tubes. The greater the threshold the greater the tube currents. If the current required is increased so that it is greater than the minimum current that the system could achieve, then the preferred method ensures that the tube currents prior to current reversal are also increased. Any safety factor built in to prevent the tube currents extinguishing is therefore also increased.

To cater for variations in the total value of the connected load, a mechanism for estimating the load and adjusting the tube current threshold accordingly, may be desirable.

Control by means of current threshold alone allows a D.C. current to build up through the tubes. It is therefore preferred that the switching times on consecutive positive and negative half cycles be equalised by some means. One preferred example of such a scheme involves deriving a maximum flywheel time and ensuring that no Awheel time exceeds this. If current measurement indicates that the Awheel period should end before the maximum Awheel time then, the Awheel period ends and the maximum Awheel time is reduced to the actual Awheel time. If current measurement would allow a Awheel time greater than the maximum Awheel time then the maximum Awheel time is increased slightly and this increased time used to override the current information, thus ending the Awheel period.

Many other such schemes will now occur to those skilled in the art.

As is generally recognised, as the ambient temperature falls it becomes harder to start and maintain tube current. The preferred system allows for some variation in the condition of the tubes by switching the supply back on earlier if the monitored tube current falls faster. Under some circumstances however it may be necessary to enhance this by making the tube current threshold vary with temperature. In this way the tubes can be prevented from failing to conduct by attempting too low a current level when the temperature is low. while still enabling increased amount of dimming when temperatures are higher. Ambient temperature, temperature in the vicinity of the tubes, or the temperature of the tubes themselves mav be used. It is also recognised that the light output of for instance Auorescent tubes falls with age. This, and changes in external light sources can be compensated for by incorporating a radiation detector and using it in such a way that a decrease in detected radiation results in a reduction in the amount of dimming demanded. The control for a scheme such as described here can be implemented by using one of a number of available microcontrollers programmed in accordance with these instructions. One embodiment of the current invention employs a PIC16C54 from Arizona Microchip Technology of Chandler Arizona USA.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURES 1. 2. 3 and 4 show typical Voltages applied by the invention to tube and ballast assemblies, together with associated tube currents. FIGURE 1 describes the undimmed state with normal mains voltage and tube current. Each of FIGURES 2. 3 and 4 show typical responses of the invention to a progressively greater demand for dimming.

Figs 5. 6. 7. 8 and 9 show in the form of typical signals the control information that is used to implement the invention. Figs 5. 6 and 7 correspond to the voltage and current waveforms described in FIGURE 3. Figs 5. 8 and 9 show the control signals corresponding to the voltage and current waveforms described in FIGURE 4.

FIGURE 10 shows a block diagram for the current invention including novel protection circuitry. FIGURE 11 shows a preferred implementation of the control circuitry represented by a box in FIGURE 10 and featuring a microcontroller with input and output circuitry suitable for implementing the invention.

FIGURE 12 shows an implementation of the A.C. Switches represented by boxes in FIGURE 10.

BEST MODE FOR CARRYING OUT THE INVENTION

An A.C. supply (or terminals for connection to an A.C. supply) 16 in FIGURE 10 is connected through A.C. switch 1 and low value non inductive resistor 7 to a load consisting of a co entional ballast and lamp assembly 13. 14. The load may comprise any number of such lamp and ballast assemblies connected in parallel.

When the A.C. switch 1 opens, a low impedance path for lamp current is provided by the closing of a second A.C. switch 12 connected in parallel with the load.

The A.C. SWITCHES are controlled by signals 51 - 52 indicating the supply voltage. 58 - 59 giλing lamp current information, and 54 - 56 indicating the position of contacts 6. This information together with an indication of the amount of dimming required is used by the control to derive signal 53 controlling the A.C. supply sλλitch 1. signal 57 controlling the A.C. Awheel switch 12. and signal 55 controlling contacts 6. This is accomplished by the box marked CONTROL in FIGURE 10. During lamp starting or other full povrer operation, the capacitors 9 and 10 will charge to nearly the peak A.C. supply voltage by means of diodes 4 and 5. Very little further current will flow. The A.C. switches do not therefore need to charge these capacitors. However, any λoltage spike across either A.C. switch which tends to exceed the peak mains voltage will cause conduction of one of the diodes 2. 3. 4 or 5 the connected capacitors 9 or 10 together with the resistive suppressor components 8 and 11 to prevent the spike from reaching damaging magnitude.

When the unit is poλvered off, contacts 6 close so as to connect the A.C. supply 16 directly to the lamp and ballast assemblies 13 and 14. This alloλvs starters 15 to light lamps 14 as normal. The control senses the condition of contacts 6 and ensures that the A.C. supply switch 1 is closed and the A.C. flywheel switch 12 remains open. After the elapse of a suitable period of time contacts 6 are allowed to change. When this is sensed by the control then the control is enabled to operate the A.C. SWITCHES as described aboλ-e in order dim the lamps.

If a failure is indicated either automatically or by manually intervention, then contacts 6 can be closed. Before these contacts close, the control senses the opening of points 54 and 56. switches and holds the A.C. Awheel switch 12 off and switches the A.C. supply switch 1 on.

FIGURE 11 shoλvs microcontroller 30 receiλing and monitoring signals 51 and 52 containing supply voltage information, signals 54 and 56 containing oλerride switch position information, and signals 58 and 59 containing tube current information, and using them to control the Oλ'erride switch by means of signal 55 and the A.C.Sλλitches by means of signals 53 and 57. Microcontroller 30 is informed of Sλλitch on and ensures that the oλerride switch remains de- energised λλhile the tubes start. After elapse of a suitable time interval, the microcontroller makes signal 55 active, energising the sλvitch. When signals 54 and 56 indicate that the override sλλitch has energised, microcontroller 30 is permitted to commence dimming as described below.

The control compares the λ'oltage on lines 51 and 52 by means of comparator 25 of FIGURE 11. protected by filtering potential diλiders 21 and 22. The output of comparator 25 is shown as the Voltage signal of Fig 5. signaling to microcontroller 30 the transition points marked A and E.

The Current signals 58 and 59 in FIGURE 10 are fed to two comparators 27 and 28 of FIGURE 11. both of which have an equal but variable offset such that they require their positive input to be significantly more positive than their negative input in order to produce a high output. When the current through the lamps is zero, the λ-oltage across 58 and 59 is zero, comparators 27 and 28 give a loλv output into nor gate 29 λvhich in turn gives a high output into microcontroller 30 informing it that the current is loλv as shoλvn by the current pulses in Fig 6 and Fig 7. If sufficient current is established through the lamps in such a way as to make 58 more positive than 59 then comparator 27 puts a high into nor gate 29 λvhich gives a loλv into microcontroller 30. If sufficient current flows through the lamps in the other direction. 59 becomes more positive than 58 and comparator 28 puts a high into nor gate 29 and ensures that microcontroller 30 receives a signal indicating that the current is no longer loλv. The thresholds of comparators 27 and 28 can be simultaneously varied by a dimming control λλired in such a way that if little dimming is demanded, the thresholds of comparators 27 and 28 are raised causing switching points B and F to occur earlier. When set to demand more dimming, the thresholds of comparators 27 and 28 are dropped so that sλλitching occurs at low tube currents.

Microcontroller 30 responds to the signal from comparator 25 by switching outputs 53 and 57 to the Awheel state as shown at points A and E of Fig 7. The sλλitching of nor gate 29 causes microcontroller 30 to change outputs 53 and 57 to the energisation state as shoλλTi at point B and F of Fig 7.

When greater dimming levels are demanded than can be achieλ-ed by the above mechanism, microcontroller 30 introduces an additional short duration Awheel state. This starts shortly after the first flywheel state as shoλvn at points C and G of Fig 9. This second Awheel state is not alloλλed to last long as is shoλλn at points D and and H in Fig 9.

Various designs of A.C. sλλitch are known. There are production advantages in making both the A.C. supply Sλλitch and the A.C. flywheel switches similar. The preferred implementation shoλvn in FIGURE 12 may therefore be used for both blocks 1 and 12 in FIGURE 10.

FIGURE 12 shows hoλv the requirements for an A.C. sλλitch may be implemented. There must be two terminals as draλλTi using circles in the bottom right of FIGURE 12. The A.C. sλλitch must be capable of earning current in both directions from these terminals. In this example of a preferred implementation this is achieλed by connecting the terminals to the A.C. terminals of a bridge rectifyer formed from four BYT03-400 diodes. The diodes steer the current so that it ahvavs passes through the D.C. terminals of the bridge in the same direction. The problem is therefore reduced to that of proλiding a D.C. Sλλitch.

In the preferred implementation the D.C. terminals of the bridge have an 1RF740-CF MOSFET connected across them. The FET gate is protected by two back to back 12 λOlt zeners connected between the source and gate and the stability of the MOSFET is enhanced by a 100 Ohm resistor connected in series λλith the gate. The gate of the MOSFET is still very sensitive to spikes and feedback. To avoid this the source of the MOSFET forms the star point for the poλver supply so that λvhether the gate voltage is dπven to the positive supply rail or the negative supply rail the gate λ-oltage remains referenced to the source.

The ideal drive signal for the MOSFET is fast and clean λλith both a positive and negative sλλing. In the preferred implementation this is achieved by the use of a 7667 driver chip. The negative sλving is ensured by connecting the negative pin of the 7667 driver chip to a negative line labelled F-5V. The positive supply to the 7667 driver chip is taken from a positive line labelled F+10V.

The input of the 7667 driver chip is referenced to the F-5V rail and thence to the MOSFET source. Partly because of the action of the BYT03-400 diodes, there is no other control signal λvhich is referenced to this point in the circuit. In particular the inputs of the 7667 driver chips for the A.C. flywheel switch and A.C. supply sλvitch are referenced to different voltages. In the preferred implementation this problem is overcome by isolating both A.C. switches from each other and from the control by means of an opto isolator. Although the output of the 7667 is able to Sλλing through nearly 15 volts, there is no problem in controlling it by means of the 5 λOlt output of an opto isolator. In order to do this the output half of the isolator is given a supply SOV and S+5V. such that SOV is derived from F-5V and S+5V is derived from FOV. Thus the ground reference is shifted from the source pin of the MOSFET to the negat e rail as λve go back in the circuit The output gate of the isolator is driven by a photo diode. Tλλo possible connections of the photo diode are shown both suitable for connection to the output of a buffered CMOS gate. The top one labelled SUPPLY de-energises the photo diode and opens the sλλitch on being driven by a High signal. The bottom one labelled FLYWHEEL energises the photo diode and closes the sλλitch on being driven by a high signal. In the preferred implementation the FLYWHEEL and SUPPLY Sλvitches have their photo diodes wired as shoλλii in the respective parts of the diagram and their inputs are both connected to the same FLYWHEEL signal. In this λλay λvhen one sλλitch is closed the other λλill be open.

For the same reasons that the control is passed through an opto isolator, so the supply has to be isolated. In the preferred implementation this is done by a transformer fed from the main incoming A.C. supply. Thus the A.C. SUPPLY 16 in FIGURE 10 is connected to the 240v tapping of an isolating step doλλT. transformer in FIGURE 12. This transformer can be obtained from RS Components as part number 177-191. The center tap of the secondary is taken to a star point located on or near the source leg of the IRF740-CF FET. All points labelled FOV are connected to this star point. The two 6 λolt transformer secondary outputs are taken to the A.C. connections of a bridge rectϋyer formed from four 1N4001 diodes.

The D.C. outputs of the bridge are smoothed by tλvo 1500uF 16v electrolytic capacitors. The positive output designated F+10V. The negative output is not used direcfly but fed through a -5v 7905 regulator.

The 7667 driver is poλλered from the F+10V positive line and the F-5V negative line. The isolator feeding this chip has as its 0 λOlt connection the same F-5V line but labelled SOV. The isolator positive supply is derived from the FOV line but is labelled S+5V. While preferred embodiments of the present invention haλe been shoλλn and described herein, it λλill be obvious that such embodiments are proλided by λλay of example only. Numerous λ-ariations, changes, departures, substitutions and partial and full equivalents will now occur to those skilled in the art without departing from the im-ention herein. INDUSTRIAL APPLICATION

The current im'ention has applicability to installations of gas discharge tubes including those for Auorescent and other lighting. The invention may be applied to groups of tubes λλired in parallel and can be retrofitted to suitable existing installations. It has application for energy saλing and poλver factor improvement, and could be used to decrease the stresses on installations λλhere tubes do not have to be permanently energised at full power. Further applications may occur λvhere a reduction in tube output or dimming is desired.

Many further applications λλill now occur to those skilled in the art. Accordingly, it is intended that the im'ention be limited only by the spirit and scope of the folloλλing claims.

Claims

1. A system for reducing or controlling the electrical poλλer proλided to a load containing a plurality of gas discharge lamps, said system comprising: an A.C. supply in the form of either A.C. supply means, or means of connection to an A.C. supply; supply voltage monitoring means capable of determining the time at which the polarity of the voltage of said A.C. supply changes; sλλitching means capable of switching between at least tλλo states, a first state in which substantial electrical poλver from said A.C. supply is fed to said load and a second state in λvhich said switching means acts to substantially reduce the amount of electrical poλver fed to said load from said supply; a control means capable of ensuring that said sλλitching means is in said first state before changes in the polarity of the A.C. supply voltage regularly occur, and that said sλvitching means changes from said first state to said second state at approximately the time that the polarity of the A.C. supply voltage changes.

2. A system according to claim 1 in λλhich said control means also comprises a means for controlling said sλλitching means so that it changes from said first state to said second state and back to said first state again for periods of time during λvhich changes in the polarity of the A.C. supply λ'oltage do not regularly occur.

3. A system according to claim 2 λvherein the frequency of sλλitching betλveen said first state and said second state is less than 1KHZ.

4. A system according to any one of claims 1 to 3 λvherein the impedance presented to said load during said first state, λvhen measured at the frequency of said A.C. supply is low in comparison λλith the impedance of said load at that frequency.

5. A system according to claim 4 λvherein the load is effectively supplied via an A.C. supply sλλitch connected betλveen said A.C. supply and said load, and controlled so as to have a significantly loλλer impedance when the system is in said first state than when the system is in said second state.

6. A system according to claim 5 λvherein said A.C. supply sλλitch contains Uvo anti-parallel D.C. sλλitches.

7. A system according to claim 5 λλherein said A.C. supply sλλitch contains a bridge rectifier λ hose A.C. terminals are used as the connections of said A.C. supply sλλitch and whose D.C. connections are coimected to the terminals of a D.C. supply sλλitch.

8. A system according to any one of claims 6 or 7 λvherein said D.C. supply sλλitches contain MOSFETs.

9. A system according to any one of claims 6 or 7 λvherein said D.C. supply switches contain Bipolar Transistors.

10. A system according to any one of claims 6 or 7 λvherein said D.C. supply sλλitches contain Insulated Gate Bipolar Transistors.

11. A system according to any one of claims 6 or 7 λvherein said D.C. supply switches contain Gate Turn Off deλices.

12. A system according to any one of claims 6 or 7 λλherein said D.C. supply switches contain MOS Controlled Thyristors.

13. A system according to any one of claims 5 to 12 λλherein said A.C. supply Sλλitch is partially protected by a suppressor circuit including a positive suppressor and a negative suppressor, both positive and negative suppressors being referenced at one end by effect e connection to the side of said A.C. supply sλλitch connected to said load, λλith the unreferenced end of said positive suppressor effectively connected to the positiλ'e end of a diode, the negatiλe end of λλhich is effectively connected to the supply side of said A.C. supply sλλitch. and the unreferenced end of said negative suppressor effectively connected to the negative end of a diode, the other end of λvhich is also effectively connected to the supply side of said A.C. supply sλλitch.

14. A system according to any one of claims 5 to 13 wherein a pair of auxiliary contacts are effectively connected across said A.C. supply switch.

15. A system according to claim 14 λλherein said pair of auxiliary contacts are the contacts of an electromechanical relay.

16. A system according to claim 15 λvherebλ said auxiliary relay is controlled so that its contacts proλide a loλv impedance path during lamp starting.

17. A system according to any one of claims 14 to 16 λvhereby said aaxiliary relay is controlled so as to proλide a loλv impedance path to maintain the lamp supply in the event of parts of the system failing.

18. A system according to any one of claims 1 to 17 λvherein significant current continues to floλv through said lamps during the majority of the time λvhen said sλλitch is in said second state.

19. A system according to claim 18 λλherein the gas discharge lamps are ballasted by components haλing significant effecth'e inductance.

20. A system according to any one of claims 18 to 19 wherein the current passing through said lamps λvhile said sλλitch is in said second state is substantially independent of said A.C. supply.

21. A system according to claim 20 λvherein a Awheel means is provided λvhereby λvhile said sλλitch is in said second state, current is alloλved to Aoλv through from said load, through said Ayλvheel means and back to said load. The impedance of the Awheel means, λvhen the system is in said second state, when measured at the frequency of said A.C. supply, is km in comparison with the impedance of said load at that frequency:

22. A system according to claim 21 λvherein said Awheel means substantially consists of an A.C. flywheel Sλλitch connected across said load and controlled so as to have a significantly lower impedance λvhen the system is in said first state than λλhen the system is in said second state.

23. A system according to claim 22 λvherein said Awheel means contain ttvo anti-parallel D.C. sλλitches.

24. A system according to claim 22 λvherein said A.C. Awheel Sλλitch contains a bridge rectifyer λvhose A.C. terminals are used as the connections of said A.C. Awheel Sλλitch and λλhose D.C. connections are connected to the terminals of a D.C. flyλvheel sλλitch.

25. A system according to any one of claims 23 or 24 λvherein said D.C. flywheel switch contains a MOSFET.

26. A system according to any one of claims 23 or 24 λvherein said D.C. Awheel switch contains a Bipolar Transistor.

27. A system according to any one of claims 23 or 24 λvherein said D.C. flywheel Sλλitch contains an Insulated Gate Bipolar Transistor.

28. A system according to any one of claims 23 or 24 wherein said D.C. flyλvheel switch contains a Gate Turn Off Deλice.

29. A system according to any one of claims 23 or 24 λvherein said D.C. Awheel sλλitch contains a MOS Controlled Thyristor.

30. A system according to any one of claims 22 to 29 λvherein said Awheel sλλitch is partially protected by a suppressor circuit including a positive suppressor and a negative suppressor, both positive and negative suppressors being referenced at one end by effective connection to the side of said A.C. supply sλλitch connected to said load, λλith the unreferenced end of said positive suppressor effectively connected to the positive end of a diode, the negative end of λvhich is effectively connected to the side of said A.C. flywheel sλλitch not effectively connected to said A.C. supply sλvitch and the unreferenced end of said negative suppressor effectrvely connected to the negative end of a diode, the other end of λvhich is effectively connected to the side of said A.C. Awheel sλλitch not effectively connected to said A.C. supply switch.

31. A system according to any of claims 20 to 30 and further including: a mechanism incorporating or deriving a tube current threshold; current monitoring means capable of estimating the point at λvhich the tube currents have fallen to or below said tube » current threshold; and in which said Sλλitching means can be caused to switch from said second state to said first state by the tube currents falling to or beloλv said tube current threshold.

32. A system according to claim 31 wherein said current monitoring means is carried out by measuring the voltage across a low value resistor effectively connected in series with said load.

33. A system according to either claim 31 or claim 32 in λvhich said tube current threshold can be λ-aried by a temperature monitoring deλice.

34. A system according to any one of claims 31 to 33 λvherein said tube current threshold is adjusted in accordance λλith the measured load in such a λvay that the threshold is higher λvhen the load is greater.

35. A system according to any one of claims 31 to 34 wherein a maximum flywheel time is derived by measuring the time after the reversal of the polarity of the A.C. supply and before said tube currents have fallen to said current threshold, said maximum Awheel time being subsequently reduced if in any cycle a similar measurement produces a smaller time, and subsequently increased periodically by a small increment, λvhile being used to provide a maximum Awheel period by forcing the system back to the first state if it has been in the second state for a duration in excess of the maximum flywheel time.

36. A system according to claims 1 to 35 λλherein the time during λvhich the system remains in said second state is adjustable.

37. A system according to claim 36 λvherein radiation is emitted from one or more of the said gas discharge lamps, and this is alloλλed to affect a radiation detector λvhereby the time during λλhich the said sλλitch remains in said second state is adjusted by the output of said radiation detector in such a way that an increase in detected radiation results in a reduction of the radiation from said lamps.

38. A deλice capable of taking poλver from a mains supply and controlling this in such a way as to drive a plurality of lamp and ballast assemblies so as to implement a system defined in any one of claims 1 to 37.

39. A discharge lamp dimmer circuit substantially as hereinbefore described λλith reference to the accompanying draλλings.