The present invention relates to a lamp, incorporating a magnetron powered light source.

In European Patent No EP 1307899, granted in our name there is claimed a light source a waveguide configured to be connected to an energy source and for receiving electromagnetic energy, and a bulb coupled to the waveguide and containing a gas-fill that emits light when receiving the electromagnetic energy from the waveguide, characterised in that:

(a) the waveguide comprises a body consisting essentially of a dielectric material having a dielectric constant greater than 2, a loss tangent less than 0.01 , and a DC breakdown threshold greater than 200 kilovolts/inch, linch being 2.54cm,

(b) the wave guide is of a size and shape capable of supporting at least one electric field maximum within the wave guide body at at least one operating frequency within the range of 0.5 to 30GHz,

(c) a cavity depends from a first side of the waveguide,

(d) the bulb is positioned in the cavity at a location where there is an electric field maximum during operation, the gas-fill forming a light emitting plasma when receiving microwave energy from the resonating waveguide body, and

(e) a microwave feed positioned within the waveguide body is adapted to receive microwave energy from the energy source and is in intimate contact with the waveguide body.

In our International Application No PCT/GB2010/00091 1, applied for on 6 ^th May 2010, ("Our 1 ^st Light Source and Starter Application") we have described and claimed a light source to be powered by microwave energy, the source having:

• a solid plasma crucible of material which is lucent for exit of light therefrom, the plasma crucible having a closed void in the plasma crucible,

• a Faraday cage surrounding the plasma crucible, the cage being at least

partially light transmitting for light exit from the plasma crucible, whilst being microwave enclosing, • a fill in the closed void of material excitable by microwave energy to form a light emitting plasma therein, and

• an antenna arranged within the plasma crucible for transmitting plasma- inducing microwave energy to the fill, the antenna having:

• a connection extending outside the plasma crucible for coupling to a

source of microwave energy;

the light source also including:

• a controllable source of microwaves coupled to the antenna connection;

• a starter for starting a plasma in the fill in the closed void,

• a detector for detecting starting of the plasma and

• a control circuit for powering the source at low power initially and

simultaneously with the starter and switching off the starter and increasing power of the microwave source after detection of starting of the plasma.

In Our 1 ^st Light Source and Starter Application and in the present application, we use the following definitions:

• "microwave" is not intended to refer to a precise frequency range. We use

"microwave" to mean the three order of magnitude range from around 300MHz to around 300GHz;

• "lucent" means that the material, of which an item described as lucent is

comprised, is transparent or translucent;

• "plasma crucible" means a closed body enclosing a plasma, the latter being in the void when the void's fill is excited by microwave energy from the antenna;

• "Faraday cage" means an electrically conductive enclosure of electromagnetic radiation, which is at least substantially impermeable to electromagnetic waves at the operating, i.e. microwave, frequencies.

EPl 307899 and Our 1 ^st Light Source and Starter Application have in common that they are in respect of:

A microwave plasma light source having:

• a Faraday cage delimiting a waveguide;

• a body of solid-dielectric material at least substantially embodying the

waveguide within the Faraday cage; • a closed void in the waveguide containing microwave excitable material; and

• provision for introducing plasma exciting microwaves into the waveguide;

• the arrangement being such that on introduction of microwaves of a

determined frequency a plasma is established in the void and light is emitted.

Such a light source is referred to herein as a "Microwave Plasma Light Source" or MPLS.

We also refer below to the Microwave Plasma Light Source of Our 1 ^st Light Source and Starter Application as a Light Emitting Resonator or LER.

In our International Application No PCT/GB2011/000920, filed on 17 ^th June 201 1 ("Our Magnetron Power Supply Application"), we have described and claimed a power supply for a magnetron comprising:

• a DC voltage source;

• a converter for raising the output voltage of the DC voltage source, the

converter having:

• a capacitative-inductive resonant circuit,

• a switching circuit adapted to drive the resonant circuit at a variable frequency above the resonant frequency of the resonant circuit, the variable frequency being controlled by a control signal input to provide an alternating voltage,

• a transformer connected to the resonant circuit for raising the alternating

voltage,

• a rectifier for rectifying the raised alternating voltage to a raised DC voltage for application to the magnetron;

• means for measuring the current from the DC voltage source passing through the converter;

• a microprocessor programmed to produce a control signal indicative of a desired output power of the magnetron; and

• an integrated circuit arranged in a feed back loop and adapted to apply a

control signal to the converter switching circuit in accordance with a comparison of a signal from the current measuring means with the signal from the microprocessor for controlling the power of the magnetron to the desired power. This power supply (i.e. the one of Our Magnetron Power Supply Application) is an improvement on an earlier power supply utilising a differently arranged operational amplifier and a differently arranged microprocessor.

Again in this application, we use the further additional definition:

"Magnetron, Switched Converter Power Circuit" or MSCPC means the following components of the power supply:

• the converter adapted to be driven by a DC voltage source and produce an alternating current output, the converter having:

• the resonant circuit including an inductance and a capacitance ("LC

circuit") exhibiting a resonant frequency and

• the switching circuit adapted to switch the inductance and the capacitance to generate a switched alternating current having a frequency greater than that of the resonance of the LC circuit;

• the output transformer for increasing the voltage of the output alternating

current; and

• the rectifier and smoothing circuit connected to the secondary circuit of the output transformer for supplying increased voltage to the magnetron;

The object of the present invention is to provide an improved lamp utilising a MSCPC and a starter improved from that disclosed in Our 1 ^st Light Source and Starter Application.

According to the invention there is provided a magnetron powered lamp, the lamp comprising:

• a Microwave Plasma Light Source;

• a magnetron arranged to power the MPLS;

• a Magnetron, Switched Converter Power Circuit arranged to power the

magnetron;

• a microprocessor arranged to control the MSCPC;

• a starter for starting a plasma in the fill in the closed void of the MPLS, the starter comprising: • a starter electrode arranged to apply starter voltage to the closed void,

• a starter circuit including:

• a capacitor,

• means for selectively charging the capacitor from a switched point in the MSCPC,

• means for discharging the capacitor,

• a transformer having:

• a primary winding arranged to receive discharge current from the capacitor and

• a secondary winding arranged to generate the starter voltage, the secondary winding being connected to the starter electrode for application of starter voltage to the closed void and

• a detector for detecting starting of the plasma;

wherein:

• the microprocessor is arranged to select charging of the capacitor for starting of the plasma until the detector detect that the plasma has started.

Whilst it is envisaged that the selective charging means could be an electronic switch normally isolating the discharging means from the switched point of the power circuit, in the preferred embodiment, the selective charging means is a electronic switch normally grounding the discharging means. In either instance, the state of the switch is changed for starter operation.

Also in the preferred embodiment, the means for discharging the capacitor is a gas discharge unit. Alternatively trigger diode could be employed.

Further in the preferred embodiment, the microprocessor controls the MSCPC via an integrated circuit arranged in a feed back loop and adapted to apply a control signal to the converter switching circuit in accordance with a comparison of a signal from means for measuring MSCPC with a signal from the microprocessor for controlling the power of the magnetron to a desired power. To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a magnetron powered lamp of the invention;

Figure 2 is a more detailed circuit diagram of a Magnetron, Switched

Converter Power Circuit similar to that described in Our Magnetron Power Supply Application and incorporating a starter of this invention; and

Figure 3 is a scrap view of a variati on of the diagram of Figure 1.

Referring to Figure 1 , the LER lamp is shown diagrammatically as having a quartz crucible 1 with a central closed void 2 containing material 3 excitable by microwaves as a plasma. The crucible is enclosed in a Faraday cage 4 defining a waveguide, in which microwaves resonate in operation of the lamp. An antenna 5, having a coaxial connection 6 extending from a matching circuit wave guide 7, passes into the crucible adjacent to the fill. Remote from the crucible a magnetron 8 is arranged to transmit microwaves into the wave guide for onwards transmission to the crucible.

Extending close to the end of the void is a starter electrode 11 and adjacent to this is mounted a photodiode 12 for detecting whether the plasma has been lit and is emitting light.

A power supply 21 for the magnetron 8 is connected to a voltage source 22 and a microprocessor 23. As shown in Figure 2, the power supply comprises a quasi- resonant converter 101 having MOSFET field effect switching transistors T1,T2. These are switched by an integrated circuit IC1. An inductance LI and primary coil of a transformer TRl are connected in series to the common point C of the transistors and capacitors C3,C4 connected beyond the primary coil back to the remote contact of the transistors. The inductances and the capacitors have a resonant frequency, above which the converter is operated, whereby it appears to be primarily an inductive circuit as regards the down-stream magnetron circuit. This comprises four half bridge diodes D3,D4,D5,D6 and smoothing capacitors C5,C6, connected to the secondary winding of the transformer and providing DC current to the magnetron 8. The windings ratio of the transformer is 10: 1, whereby voltage of the order of 4000 volts is applied to the magnetron, the augmented mains DC voltage on line 105 being 400 volts (at least in Europe).

To the common point C of the transistors is connected a coupling capacitor CI 1 which provides input to a starter circuit 24. A transistor switch 25 is in series with the capacitor CI 1 and a diode Dl. When the switch is off no current flows in Dl 1. When the switch is made, Dl 1 conducts during alternate halves of cycles present at C. A second diode D12 also conducts and allows current to pass through discharge capacitor CI 2. This progressively charges until the voltage across it reaches the breakdown voltage of a gas discharge tube GTD. Whereupon the capacitor discharges through the primary winding of transformer TR2. The secondary winding has many more turns and a starter voltage is induced in the starter electrode 1 1. This is isolated from the Faraday cage 4 and terminates adjacent the crucible, close to the void 2.

Every time the discharge capacitor discharges, the void is pulsed. The magnetron is being driven - the starter being able to operate only as a result of the converter operating. Once a plasma in the void establishes, this is detected by a photodiode 12 adjacent the starter electrode 1 1. Presence of plasma is signalled to the microprocessor which opens the transistor switch 25.

For completeness, a current measurement resistor Rl, an operational amplifier EA1 and associated components are shown for operation of the converter in accordance with Our Magnetron Power Supply Application. A further transistor switch 26 is also shown. With this the microprocessor can immediately close down the power supply, either under human control or automatically, for instance in the event of the magnetron current exceeding a limit such as when its magnets degrade.

In practical operation, with the lamp not on, the voltage source (not shown above) and the microprocessor are switched on. The microprocessor is instructed to power up the lamp in accordance with one or more protocols. The microprocessor controls the power supply to apply a low power to the magnetron and the starter to apply a starter pulse stream of a determined duration to the starter. If the plasma does not start, the pulse stream is repeated after a delay. The process is repeated until the plasma lights. Should this fails the operator is alerted. Once the plasma has lit, power to the magnetron is increased to a desired level, commensurate with desired light output from the plasma crucible.

Turning to the variant of Figure 3, the arrangement of the discharge capacitor CI 1 and the gas discharge tube GTD is interchanged. They operate in an analogous way to that in which they operate in Figure 2. The variant also includes a voltage doubler stage comprising diodes D14, D15 and capacitors CI 4, CI 5. With this arrangement, including an appropriate value GDT, doubled primary voltage is applied to the transformer TR2.