GB2269948A - Gas discharge lamp dimmer circuit - Google Patents

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 has a series switch 1, which is on immediately before zero crossings of the voltage waveform of the AC supply 16 and turns off substantially at such zero crossings, and a short flywheel switch 12 turned on and off in antiphase relative to switch 1. Dimming is achieved by increasing the duration of the ON period of switch 12 in each AC supply voltage half cycle and reducing the ON period of switch 1 until such periods are equal. Further dimming may be achieved by opening switch 1 one or more times towards the start of each half cycle of the lamp current waveform, (Fig 4). Switch 1 may be controlled to turn on when the lamp current falls to a threshold. This threshold may be dependent on lamp load and ambient temperature. A bypass switch 6 may be closed during lamp starting and in the event of a failure. Switch 1 is also held closed and switch 12 open during starting. A control system for switches 1, 6, 12 may respond to supply voltage, lamp current, and lamp light output. Switches 1, 12 may use gate turn off devices, MOS controlled thyristors, or a combination of a MOSFET and bridge rectifier, (Fig 6). Diodes 2, 3, 4, 5, resistors 8, 11 and capacitors 9, 10 provide voltage spike suppression. <IMAGE>

Description

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 voltage applied to and the current taken by gas discharge lamps, improving their power factor, reducing the heat and other losses in such lamps, increasing their efficiency, and increasing their life.

BACKGROUND OF THE INVENTION Much of the background material relevant to the present invention is concerned with the problems of either conserving energy or providing 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 iength of time the triac is allowed to conduct. 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 prevent 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 avoids 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 rectifyer 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 altemative 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, however 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 providing 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 carrying 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. convertor 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 removing the applied voltage for some of the time.

Unfortunately devices 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 provide 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.

SPECIFICATION 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 switched 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.

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 reestablishment 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. It is important that the interruption occurs 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 and is further described later in the specification.

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 relay 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.

The same relay can also 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 altemative 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 flywheel element. It is important therefore to chose the right switching elements. The economic choice will be dependant on the required current handling capacity required, reliability requirements, component cost, and a variety of other factors.

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 altemately. 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 flywheel part of the cycle, the A.C. supply switch opens and the A.C. flywheel 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 flywheel 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 flywheel state, effectively disconnecting the A.C. supply and replacing it by a flywheel action in which current is maintained by the inductance of the lamp ballasts acting against the lamp voltages themselves but without having 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 flywheel state is extended at the expense of the energisation state. The flywheel 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 flywheel 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 of the A.C. supply switch. As before, when the A.C. supply switch is not conducting the A.C. flywheel switch is brought into conduction to provide for the continuing, but reducing, flow 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 can then be used to provide 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.FIGURE 4 shows an example where one additional flywheel period has been provided.

When the system switches between the supply and flywheel 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 one 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.

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.

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 may be used.

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 remembering the previous flywheel time and ensuring that the immediately following flywheel time is not more than incrementally greater. Many other such schemes will now occur to those skilled in the art.

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.

DETAILED DESCRIPTION OF DRAWINGS See the above text for a description of FIGURES 1 - 4.

An A.C. supply 16 in FIGURE 5 is connected through an A.C. switch 1 and low value non inductive resistor 7 to a load consisting of a conventional ballast and lamp assembly 13 and 14. The load may comprise any number of such lamp and ballast assemblies.

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 giving 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 switch 1, signal 57 controlling the A.C. flywheel switch 12, and signal 55 controlling contacts 6.

During lamp starting or other full power 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 voltage 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 allowing 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 powered off, contacts 6 close so as to connect the A.C. supply 16 directly to the lamp and ballast assemblies 13 and 14. This allows 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 above 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. flywheel switch 12 off and switches the A.C. supply switch 1 on.

Various designs of A.C. switch are known.

There are advantages in making both the A.C. supply switch and the A.C. flywheel switches similar. The preferred implementation may therefore be used for both blocks 1 and 12 in FIGURE 5.

FIGURE 6 shows how the requirements for an A.C. switch may be implemented. There must be two terminals as drawn using circles in the bottom right of FIGURE 6. The A.C.

switch must be capable of carrying current in both directions from these terminals. In this example of a preferred implementation this is achieved 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 always passes through the D.C. terminals of the bridge in the same direction. The problem is therefore reduced to that of providing a D.C.

switch.

In the preferred implementation the D.C.

terminals of the bridge have an IRF740-CF MOSFET connected across them. The FET gate is protected by two back to back 12 volt zeners connected between the source and gate and the stability of the MOSFET is enhanced by a 100 Ohm resistor connected in series with 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 power supply so that whether the gate voltage is driven to the positive supply rail or the negative supply rail the gate voltage remains referenced to the source.

The ideal drive signal for the MOSFET is fast and clean with both a positive and negative swing. In the preferred implementation this is achieved by the use of a 7667 driver chip. The negative swing 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 BYT03-40t diodes, there is no other control signal which 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 switch 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 swing through nearly 15 volts, there is no problem in controlling it by means of the 5 volt output of an opto isolator. In order to do this the output half of the isoiator is given a supply S0V 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 negative rail as we go back in the circuit The output gate of the isolator is driven by a photo diode. Two 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 switch on being driven by a High signal. The bottom one labelled FLYWHEEL energises the photo diode and closes the switch on being driven by a high signal.In the preferred implementation the FLYWHEEL and SUPPLY switches have their photo diodes wired as shown in the respective parts of the diagram and their inputs are both connected to the same FLYWHEEL signal. In this way when one switch is closed the other will 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 5 is connected to the 240v tapping of an isolating step down transformer in FIGURE 6. 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 volt transformer secondary outputs are taken to the A.C. connections of a bridge rectifyer formed from four 1 N4001 diodes.

The D.C. outputs of the bridge are smoothed by two 1500uF 16v electrolytic capacitors.

The positive output designated F+1 OV. The negative output is not used directly but fed through a -5v 7905 regulator.

The 7667 driver is powered from the F+10V positive line and the F-5V negative line. The isolator feeding this chip has as its 0 volt 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 have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes, departures, substitutions and partial and full equivalents will now occur to those skilled in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the following claims.

Claims (39)

1. A system for reducing or controlling the electrical power provided 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; switching means capable of switching between at least two states, a first state in which substantial electrical power from said A.C. supply is fed to said load and a second state in which said switching means acts to substantially reduce the amount of electrical power fed to said load from said supply; a control means capable of ensuring that said switching means is in said first state before changes in the polarity of the A.C.

supply voltage regularly occur, and that said switching 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 which said control means also comprises a means for controlling said switching 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 which changes in the polarity of the A.C. supply voltage do not regularly occur.

3. A system according to claim 2 wherein the frequency of switching between said first state and said second state is less than 1 KHZ.

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

5. A system according to claim 4 wherein the load is effectively supplied via an A.C. supply switch connected between said A.C. supply and said load, and controlled so as to have a significantly lower 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 wherein said A.C. supply switch contains two anti-parallel D.C. switches.

7. A system according to claim 5 wherein said A.C. supply switch contains a bridge rectifyer whose A.C. terminals are used as the connections of said A.C. supply switch and whose D.C. connections are connected to the terminals of a D.C. supply switch.

8. A system according to any one of claims 6 or 7 wherein said D.C.

supply switches contain MOSFETs.

9. A system according to any one of claims 6 or 7 wherein said D.C.

supply switches contain Bipolar Transistors.

10. A system according to any one of claims 6 or 7 wherein said D.C.

supply switches contain Insulated Gate Bipolar Transistors.

11. A system according to any one of claims 6 or 7 wherein said D.C.

supply switches contain Gate Turn Off devices.

12. A system according to any one of claims 6 or 7 wherein said D.C.

supply switches contain MOS Controlled Thyristors.

13. A system according to any one of claims 5 to 12 wherein said A.C.

supply switch 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 switch connected to said load, with the unreferenced end of said positive suppressor effectively connected to the positive end of a diode, the negative end of which is effectively connected to the supply side of said A.C. supply switch, and the unreferenced end of said negative suppressor effectively connected to the negative end of a diode, the other end of which is also effectively connected to the supply side of said A.C. supply switch.

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 wherein said pair of auxiliary contacts are the contacts of an electromechanical relay.

16. A system according to claim 15 whereby said auxiliary relay is controlled so that its contacts provide a low impedance path during lamp starting.

17. A system according to any one of claims 14 to 16 whereby said auxiliary relay is controlled so as to provide a low 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 wherein significant current continues to flow through said lamps during the majority of the time when said switch is in said second state.

19. A system according to claim 18 wherein the gas discharge lamps are ballasted by components having significant effective inductance.

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

21. A system according to claim 20 wherein a flywheel means is provided whereby while said switch is in said second state, current is allowed to flow through from said load, through said flywheel means and back to said load. The impedance of the flywheel means, when the system is in said second state, when measured at the frequency of said A.C. supply, is low in comparison with the impedance of said load at that frequency:

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

23. A system according to claim 22 wherein said flywheel means contain two anti-parallel D.C. switches.

24. A system according to claim 22 wherein said A.C. flywheel switch contains a bridge rectifyer whose A.C.

terminals are used as the connections of said A.C. flywheel switch and whose D.C.

connections are connected to the terminals of a D.C. flywheel switch.

25. A system according to any one of claims 23 or 24 wherein said D.C.

flywheel switch contains a MOSFET.

26. A system according to any one of claims 23 or 24 wherein said D.C.

flywheel switch contains a Bipolar Transistor.

27. A system according to any one of claims 23 or 24 wherein said D.C.

flywheel switch contains an Insulated Gate Bipolar Transistor.

28. A system according to any one of claims 23 or 24 wherein said D.C.

flywheel switch contains a Gate Turn Off Device.

29. A system according to any one of claims 23 or 24 wherein said D.C.

flywheel switch contains a MOS Controlled Thyristor.

30. A system according to any one of claims 22 to 29 wherein said flywheel switch 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 switch connected to said load, with the unreferenced end of said positive suppressor effectively connected to the positive end of a diode, the negative end of which is effectively connected to the side of said A.C. flywheel switch not effectively connected to said A.C. supply switch and the unreferenced end of said negative suppressor effectively connected to the negative end of a diode, the other end of which is effectively connected to the side of said A.C. flywheel switch 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 which the tube currents have fallen to or below said tube current threshold; and in which said switching means can be caused to switch from said second state to said first state by the tube currents falling to or below 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 which said tube current threshold can be varied by a temperature monitoring device.

34. A system according to any one of claims 31 to 33 wherein said tube current threshold is adjusted in accordance with the measured load in such a way that the threshold is higher when 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 flywheel time being subsequently reduced if in any cycle a similar measurement produces a smaller time, and subsequently increased periodically by a small increment, while being used to provide a maximum flywheel 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 wherein the time during which the system remains in said second state is adjustable.

37. A system according to claim 36 wherein radiation is emitted from one or more of the said gas discharge lamps, and this is allowed to affect a radiation detector whereby the time during which the said switch 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 device capable of taking power 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 with reference to the accompanying drawings.