EP0020390A1 - Improvements in lamp drive circuits, and cine film projectors or cameras incorporating the same - Google Patents

Abstract

A lamp motor circuit for driving a compact source metal halide discharge lamp (10) includes a DC power supply (18-28) coupled to switching means (32-36). The switching means supply alternating current to the lamp at a switching frequency of about 100 to 250 herz. The DC power supply comprises an AC stage (20 to 25) operating at a frequency of 30 to 40 Hz. The AC stage determines the internal inductive impedance of the DC power supply. It can be seen that the operating frequency of the alternating current stage is greater than the frequency of the alternating current supplied to the lamp. Conveniently the DC power supply comprises a direct-acting converter and includes an "R.F" radio frequency filter (16) as well as a rectifier and planar circuit (17). The ignition circuit (31) is provided to start the lamp (10). The switching means can comprise two transistors (32, 33) operated in the open or closed position alternately by a motor circuit of the lamp and used with a cinematographic film projector or a camera, the switching circuit (36). When the lamp motor circuit is used with a motion picture film projector or camera, the switching circuit can be synchronized with the downward pulling movement of the film transport mechanism by a detector and a control unit (39 ).

Description

IMPROVEMENTS IN LAMP DRIVE CIRCUITS, AND CINE F ILM PROJECTORS OR CAMERAS INCORPORATING THE SAME

This invention relates to lamp drive circuits, and especially to electronic lamp drive circuits for compact source metal halide discharge lamps.

Compact source metal halide discharge lamps are described in Section 15.1.4,"Lamps and Lighting", Second Edition, 1972, published by Edward Arnold. Such lamps can be operated either with alternating current or with direct current. However, alternating current operation of a compact source metal halide discharge lamp can provide improved colour rendering, higher luminous efficiency and longer life, as compared with direct current operation. Nevertheless, direct current operation of such a lamp gives the characteristic of a steady light output whereas the light output of an alternating current operated lamp closely follows the current wave form and with a simple choke in series with a mains supply the light output is strongly modulated at twice the mains frequency. This modulation causes a serious problem when a compact source metal halide discharge lamp is used as the lamp in a cine-projector, since the modulation can beat with the modulations of the light caused by the cine-projector shutter, the result being an obtrusive fluctuation in the light on the screen receiving the light projected by the projector. The beat fluctuation has a frequency typically of a few cycles per second. Yarious methods are known for avoiding the problem of beat fluctuation of cine-projector light. For example, it can be arranged that the time during which the shutter is open is equal to a whole number of cycles of the modulated light output of the lamp. This involves having a different shutter angle for each projection speed and for each electrical mains supply frequency, and is therefore not always considered desirable. In another method, the projector is arranged to operate in synchronism with the lamp supply frequency and the shutter is arranged to be open at the peak of the light output wave form and to be closed at its trough, thereby effectively increasing the transmission of the shutter and hence the quantity of light reach ing the screen. The disadvantages of this method are that the design of the projector is made more complicated and, if synchronisation is achieved by adjusting the projector speed, the range of possible projection speeds is severely limited.

It is also known to operate the lamp at a high frequency compared with the projection speed. This results in the elimination of the obtrusive beat frequency, and the lamp can be used in any projector with any shutter angle operated from any mains frequency at any projection speed without modification.

A further known, method is to supply the lamp with, current in square wave form instead of in sine wave form. The result of the use of a square wave current is that the magnitude of the fluctuation in the light output wave form is much reduced compared with that of the fluctuation with sine wave drive, and in fact the fluctuation would vanish entirely if a true square wave current drive could be achieved It is also thought that square wave operation provides easier starting of the lamp and a longer life for the lamp. A similar problem of beats occurs when an alternating current driven lamp is used for illuminating a scene during the taking of a cine-film.

The known circuits developed hitherto for driving compact source metal halide discharge lamps with current which has a frequency high compared with cine-projector speeds and/or has a square wave form have been large and expensive. It is accordingly an object of the present invention to provide a lamp drive circuit for. a compact source metal halide discharge lamp, the drive circuit being smaller and less expensive than hitherto. According to the present invention there is provided a lamp drive circuit including regulated direct current power supply means so coupled to switching means as to be capable of supplying an alternating current betv/een a pair of output terminals of the drive circuit when a discharge lamp couples the said output terminals to one another, the direct current power supply means including an alternating current stage which determines the internal inductive impedance of the direct current power supply means and is such as to operate at a frequency which is substantially greater than and preferably at least ten times the frequency of the alternating current supplied to the lamp when present in operation.

By "regulated" is meant that such direct current supplied in operation by the direct current power supply means is substantially constant or varies so that the power supplied to the lamp in operation is substantially constant throughout operation of the lamp, irrespective of differences in lamp voltage from one lamp to another and of changes in lamp voltage during the lifetime of a lamp, or such that the direct current supplied lies between the values obtained by these two conditions, i.e. the condition of substantially constant current and the condition of substantially constant power. The fact that the frequency of operation of the alternating current stage of the direct current power supply means is at least ten times the frequency of the operating current for the lamp enables the direct current power supply means to be constructed so as to have a short transient response time and consequently enables the switching means to be operated with fast commutation. Furthermore, the need for a large and expensive storage element to prevent deionisation of the lamp during commutation is avoided. The commutation effected by the switching means is preferably so fast that the alternating current supplied in operation to the lamp when present is a square wave.

In a preferred embodiment of the invention, an input stage is provided in the form of a radio frequency interference filter coupled to a full wave rectifying and smoothing circuit, the filter being such as to be connect able to an alternating current mains supply point. The direct current power supply means includes a forward converter which operates at a frequency of, for example, 30 or 40 Kilohertz and receives the rectified and smoothed output of the input stage. The switching means includes two transistors connected in series with one another and so controlled as to be switched on and off at the intended lamp operating frequency, each of the two transistors being in its conducting state when the other is in its non-conducting state and vice versa. The two transistors may be driven at a fixed frequency, preferably in the range 100 Hertz to 250 Hertz, or may be made to operate synchronously with a part of a film transport mechanism, for example, a shutter member, where the lamp to be driven by the drive circuit is the lamp in a cine-projector.

Embodiments of the invention can be used for operating alternating current lamps of up to 750 Watts at frequencies in the range up to 5 Kilohertz.

In some embodiments of the invention, the alternating current stage of the direct current power supply means may be in the form of fly-back converter. In other embodiments, the direct current power supply means may be in the form of any other suitable current control circuit operating at a sufficiently high frequency and having a low internal inductive impedance and a short transient response time.

An advantage of an embodiment of the invention in which the switching means operate at a fixed frequency and enable the direct current power supply means to supply a square wave current to the lamp when present in operation, is that the lamp light output is substantially constant with time so that the lamp gives all the advantages of alternating current operation combined with the advantage of steady light output. The lamp can then be used in any projector using any shutter angle operated from any mains frequency without the need to restrict the shutter angle or the projector speed.

The choice of a lamp operating frequency in the range 100 Hertz to 250 Hertz has the advantage that the lamp light output frequency is high enough to avoid beat effects with standard projector speeds of 18 frames per second to 24 frames per second, and the generation of noise by the lamp circuitry is not a serious problem. Furthermore, cathode spot flicker effects are also avoided.

Embodiments can furthermore be constructed in which the switching means is controlled by means arranged to sense the occurrence of each pull-down phase in the cyclic operation of a projector film transport mechanism and to so inhibit the switching means during each pull-down phase that the lamp when present in operation is extinguished for the duration of each pull-down phase. Such embodiments have the advantage that shutterless projection is possible, and there is a significant increase in the effective working screen lumens as compared with conventional projectors.

Similarly, embodiments may have means so controlling the switching means that the square wave current driving the lamp when present in operation is synchronised with a cyclically moving part of a projector film transport, so that synchronous projection can be effected at any projection speed. Other situations in which an embodiment of the invention may be used to advantage are in the driving of a lamp for film scene lighting and in the driving of the lamp of a portable searchlight.

The invention will now be described in more detail by way of example with reference to the drawings, in which :-

Fig. 1 is a block diagram of an embodiment of the invention;

Fig. 2 is a detailed circuit diagram of part of a preferred embodiment of the invention;

Fig. 3 is a circuit diagram of a radio interference filter for use with the embodiment of Fig. 2; and

Fig. 4 is a circuit diagram of part of the switching means of the embodiment of Fig. 2.

In Fig. 1, a compact source iodide (CSI) lamp 10, which may be as described in Sec.15.1.4 of "Lamps and Lighting", Second Edition 1972 published by Edward Arnold, or as described in British Patent application No. 7921217 (also published as German Offengungsschrift 29 24463), has its electrodes connected to the output terminals 11 and 12 of a lamp drive circuit embodying the present invention. In this embodiment, the lamp drive circuit has three input terminals 13, 14 and 15 which, in use, are connected respectively to the live, neutral and earth contacts of for example a 50 Hz A.C. mains supply point (not shown). A radio frequency interference filter 16, for preventing the passage of radio frequency oscillations into the mains supply, couples the input terminals 13 and 14 to a rectifying and smoothing circuit 17 having a positive output terminal 18 and a negative output terminal 19.

A transformer 20 has a primary winding one end of which is connected directly to the positive terminal 18 and the other of which is connected to the collector of an NPN transistor 21 the emitter of which is coupled firstly by a resistor 22 to the negative terminal 19 and secondly by a current amplifier 23 of adjustable gain and a control circuit 24 to the base of the transistor 21. The control circuit 24 comprises any circuitry suitable for causing the transistor 21 to be switched alternately between its conducting and non-conducting states at a frequency of about 30 kilohertz, the relative durations of the conducting and non-conducting states varying in such a way as to control the current or the power supplied to the lamp to be sub stantially constant throughout operation of the lamp irrespective of differences in the lamp voltage from one lamp to another or changes during the lifetime of a lamp. Many suitable control, circuits will be known to those skilled in the art for this purpose. If desired, the current may be controlled to be between the constant current value and the value for the constant power.

The transformer 20 has two equal secondary windings 25 and 26 The ends of each of these secondary windings are connected to a respective rectifying and smoothing circuit 27 or 28. The circuits 27 and 28 have a common terminal 29 which is the negative output terminal of the circuit 27 and the positive output terminal of the circuit 28. The common terminal 29 is coupled through the secondary winding of a pulse transformer 30 to the output terminal 11 of the whole lamp drive circuit. The primary winding of the transformer 30 is connected to be supplied with pulses by a striker circuit 31, which is connected across the terminals 29 and 12. The output terminal 12 of the whole lamp drive circuit is connected to the emitter of an NPN transistor 32, whose collector is connected to the positive output terminal of the circuit 27, and to the collector of an NPN transistor 33 whose emitter is connected to the negative output terminal of the circuit 28.

The respectiva bases of the transistors 32 and 33 are connected to respective output terminals 34 and 35 of a switching circuit 36. In some circumstances, the circuit 36 may be such as to have a control input 38 from a control unit 39 which operates in response to a sensor 40, as will be explained hereinafter.

In the case where the switching circuit 36 does not have a control input 38, the switching circuit 36 provides output signals at its output terminals 34 and 35 which alternately hold the transistor 32 in its conducting state and the transistor 33 in its non-conducting state, and vice versa, so that in operation at any particular instant either the transistor 32 is conducting and the transistor 33 is not conducting, or the transistor 33 is conducting and the transistor 32 is not conducting. The preferred rate of switching effected by the circuit 36 is such that a substantially square wave of current with a repetition rate in the range 100 Hz to 250 Hz is passed through the lamp 10, the power being drawn via the rectifying and smoothing circuits 27 and 28. The striker circuit 31 comprises any suitable circuitry responsive to the initial portion of the square wave pulses for supplying pulses to the primary winding of the pulse transformer 30, sufficient to establish conduction of the lamp 10. Such circuitry will be well known to those skilled in the art.

The switching circuit 36 may comprise basically a multivibrator circuit, oscillating at the frequency which is required as that of the substantially square wave of current passed through the lamp 10. The range 100 Hz to 250 Hz is preferred since the use of a frequency of 100 Hz or higher ensures that flickering of the cathode spot in the lamp IO will not be noticeable, and the use of a frequency of 250 Hz or lower ensures that any audible effect of the circuitry operating at this frequency will not be objectionable at the power levels involved.

The lamp drive circuit as described so far is suitable for driving a lamp in a film projector. A drive circuit constructed as shown in Fig. 1 can be operated directly without modification from several different power supplies, namely, alternating current power supplies of between 180 volts r.m.s. and 265 volts r.m.s. of frequencies from 50 Hertz to 400 Hertz, and direct current power supplies of 250 volts to 400 volts irrespective of polarity, the live and neutral input terminals 13 and 14 being respectively connected to the supply terminals in the case of a two wire direct current supply, and the rectifying and smoothing circuit 17 being of the kind that passes direct current. With simple change in connection points in the circuit 17, the alternating current supply voltage range can be changed to 90 volts r.m.s. to 135 volts r.m.s.

For operation with direct current supplies only, the circuit in Fig. 1 may be modified by omitting the rectifying and smoothing circuit 17, the filter 16 being retained to prevent high frequencies entering the leads of the input terminals 13 and 14. A small addition to this modification, in the form of a D.C.-to-D.C. converter, enables the circuit to be used with any particular direct current supply voltage. Alternatively, for example, instead of adding a

D.C.-to-D.C. converter, the turns ratio of the transformer 20 may be designed to suit the intended direct current supply voltage. With a D.C. supply in portable or transportable form, the drive circuit can serve as a drive circuit for a film scene lighting lamp or a portable searchlight.

The use of the drive circuit to drive a film scene lighting lamp, gives the advantage that the light does not beat with the shutter of a cine camera.

In the form modified for use with a direct current power supply, the circuit can be operated by batteries. A portable generator of alternating or direct current can be used to supply the unmodified form of the circuit, which may also be battery powered. The low weight of the circuit makes it easy to carry, and when operated by batteries it has the advantage that more or larger batteries can be used. The battery operated circuit is thus particularly suitable for use in driving film scene lighting lamps for reporting and documentary film work, since it results in more light being provided from a given weight of lighting equipment, it being possible to use a compact source metal halide discharge lamp instead of a filament lamp. Another instance of the advantage of low weight is in use of the battery driven circuit to drive the lamp of a porta le searchlight of the kind used by police at night at the scene of a road accident.

In using the drive circuit in a film projector, it may be desirable to synchronise the operating frequencies of the lamp 10 with the periodic motion of part of the film transport mechanism, e.g. a shutter. In such an arrangement, the periodic motion of the shutter or other part of the transport mechanism is sensed by the sensor 40 which may be any suitable device, for example a proximity probe sensing the position of the shutter, and which produces a periodic signal whose phase is representative of the periodic movement of the sensed part. This periodic signal is supplied to the control unit 39 which, in any suitable manner, produces a synchronising signal to supply to the input 38 of the switching circuit 36 and thereby lock the switching of the transistors 32 and 33 to the periodic motion of the sensed part of the film transport. As a further development of this last arrangement, or as an alternative thereto, it can advantageously be arranged that the control unit 39 inhibits the switching circuit 36 during the time occupied by the pull-down motion of the film transport mechanism, and hence that the shutter can be dispensed with. Thus, shutterless projection can be achieved by electronic control of the energisation of the lamp 10. Since the lamp 10 is only operated during the time that each film frame is being projected, the lamp power during this time can be increased without increasing the average lamp power. This increase in lamp power gives a significant increase in the effective working screen lumens. Suitable circuitry for the control unit 39 may comprise bistable circuits and gating circuits actuated by the periodic signal produced by the sensor 4θ. The circuit 36 may basically comprise two bistable circuits driven by the control unit 39 and inhibited during pull-down times by an inhibiting signal from the control unit 39.

It should be noted that with the embodiment shown, the open circuit voltage at the output terminals 11 and 12 is high, i.e. sufficiently large for re-ignition of the lamp at each half cycle of the square wave to be achieved easily.

Because the transistor 21 of the alternating current portion of the direct current power supply, constituted by the circuitry between the terminals 18 and 19 on the one hand and the output terminals of the circuits 27 and 28 on the other hand, is switched at a frequency high compared with that of the lamp drive current, the internal inductive impedance of this direct current power supply is accordingly lower by a factor approximately equal to the ratio of the lamp operating current frequency to the switching frequency of the transistor 21 than would be the case if this switching frequency were equal to the lamp operating current frequency determined by the switching circuit 36. Furthermore, the higher the switching frequency of the transistor 21, the smaller and hence less costly the inductive and capacitive elements required in the said alternating current portion to ensure that a controlled current and/or power which is substantially independent of the lamp voltage is supplied to the lamp.

In one embodiment constructed as shown in Fig. 1, but omitting the input 38 and the sensor 40 and unit 39, the turns ratio of the primary winding of the transformer 20 to each of its secondary windings is 1 to 1. The voltage pulses provided in this particular embodiment at the output terminals of the striker circuit 31 are 8 kilovolt pulses. Since the lamp 10 is driven by a substantially square wave of current there is an improvement in the waveform factor and in the luminous efficacy. The rapid commutation results in easier lamp reignition on each half cycle and longer lamp life. In Figs. 2, 3 and 4, which constitute the circuit diagram of a forward converter embodying the present invention, those components or groups of components which correspond to parts of the block diagram of Fig. 1 are given the same reference numerals as in Fig. 1. Furthermore, to simplify the diagrams, some points in the circuitry which are actually directly connected together are shown in Figs. 2 and 4 as terminals indicated by reference characters. These reference characters are a set of pairs of identical characters, the two members of each pair representing a direct connection. The pairs are: 'P' and 'P' in Fig. 2, 'Q' in Fig. 2 and 'Q' in Fig. 4, 'R' in Fig. 2 and 'R' in Fig. 4, and the four pairs formed by 'S', 'T' , 'U' and 'V' in Fig. 2 and 'S' , 'T' , 'U' and 'V' in Fig. 4. Conventional electrical and electronics- symbols are used in Figs. 2, 3 and 4.

In this embodiment, the power supply, whose output stage is the transformer 20, is a simple 'switch mode' converter operating at approximately 30 KHz. The forward converter provides two D.C. power rails via transformer 20 which are essential for the operation of an A.C. lamp. A half bridge inverter formed partly by transistors T6 and T7 provides the symmetrical A.C. waveform for the lamp.

Operation of the input circuits is now described. Mains powe is fed in via radio interference filter 16 (Fig. 3) comprising capacitors C26, C27, C28, C29, inductors L3, L4, a symmetric choke 60, and resistors R52, R53 and R54. Inductors L3, L4, and capacitors C28 and C29 attenuate series mode noise, whilst the choke 60 and the capacitor C26 and C27 attenuate common mode noise. A fuse FS1 is included. Mains power is then fed via a diode full wave bridge rectifier 17 comprising diodes Dl, D2, D3 and D4 to reservoir capacitors Cl, CIA, C2 and C2A. For 115V operation the neutral line 61' is connected by a link 63 to the junction of the capacitors Cl and C2 to provide a voltage doubling network, otherwise it is connected to junction 62. At nominal mains voltage, a 350 volts D.C. rail 18 is the only supply to the converter. This contains a fuse FS2. All other auxiliary rails for operation of other circuits are provided from the main inverter transformer 20.

The forward converter operates on a blocking oscillator principle with the exception that 'base drive! to the transistor 21 (T5 in Fig. 2) is depleted by means other than gain limiting. The transistor T5 is the main switching transistor which has primary T₂P of transformer 20 for its collector load. Base drive for T5 is provided from T P by a secondary winding IS . Base drive current flows via R21 and D10. Consider T5 turned on momentarily. T5 collector goes low and a voltage is induced in T S of the order of 25 volts. T5 then becomes latched. A timing capacitor C6 which was in the discharged state, charges up via R14 and R17 until conduction starts through D8. When sufficient current flows through D8, T3 turns on which also turns on T4. T4 robs base current from T5 and as a result, T5 turns off. The energy stored in the transformer 20 due to a magnetising current and leakage reactance causes T5 collector voltage to rise above the nominal 350 volts D.C. line until the 'core' of the transformer 20 has reset. As the voltage of T5 collector rises above nominal D.C. line voltage, the voltage at point 'Q' falls negative. R22 (which is large compared with R21) then guarantees that negative bias is applied to T5 during fly-back, keeping the transistor in a safe operating area. T3 and T4 are maintained in the 'on' state during T5 turn off due to their base charge storage effects and a storage network R12, C5 and R13. Current flowing through T5 is limited by a current transformer having a secondary winding T₃S. The burden for this current transformer is mainly R23, and when sufficient current flows through R23, ZD2 conducts turning transistor T13 on. T13 discharges C6 very quickly, hence limiting the conduction angle of T5. Power flow through the converter is thus controlled.

When T5 turns off, a snubber network D15, R26 and C9 maintain T5 in a safe operating area. However, in open circuit (when no load applied), C9 prevents a rapid collector voltage rise time due to small leakage reactance energy components being present. As a result, the magnetising current rises rapidly and dangerously high collector voltages can occur. This is prevented by Zener diodes ZD3 , ZD4, ZD5, ZD6 and ZD7, a capacitor Cl8 and a feed forward network R27 and D22. When the Zener diodes conduct, Cl8 charges up in the order of a few volts. D22 conducts and T13 turns on, thereby reducing the conduction angle and the peak collector voltage. This network also protects the circuit against input voltage transients and load transients. When T5 is in the off condition, it will not turn on again without an initial signal. A pulse generator formed by T2 and a transformer with windings T₁P, T₁S₁ and T₁S₂ provides an initial pulse every 33 micro-seconds to start the conduction of T5. Once T5 has started to conduct base drive is maintained from the secondary T₂S₃ as explained herein before.

The operation of the blocking oscillator is as follows. C4 charges via R3 and R57, until T2 starts to receive base current. As

T2 turns on, secondary T₁S₂ provides positive feedback and T2 latches hard on. Conduction of T2 is maintained until C4 and C30 are discharged. T2 remains off whilst C4 charges through R3 and R57, and then the cycle repeats. Square shaped pulses are consequently coupled through to T5 base via secondary T S and Rll and D6. C30, D27 and D28 enable transistor T2 to have rapid turn off. The operation of the circuit for long periods in open circuit is inhibited by SCR1 and associated circuitry. In the open circuit condition, C3 is allowed to charge via R2 until DIAC-1 conducts and turns SCR1 on. When SCR1 is in the on state, operation of the pulse generator is inhibited and hence also that of the main converter. In the open circuit condition, insufficient current from the current transformer secondary T₃S prevents Tl from turning on. Resistor R55 sets the length of the time delay.

The half bridge inverter whose output terminals are the terminals 11 and 12 operates as follows. T6 and T7 turn on alternatel to provide an approximately square wave to the lamp terminals 11 and 12. Base drive to T6 and T7 is generated in the following manner.

An auxiliary rail (Fig. 4) is provided for T8, T9, TIO, Til and T12 from the feedback winding T₂S₃. D25 and R44 provide a peak charging path into C19 which maintains a reasonably smooth supply.

T8 is a unijunction oscillator providing negative edges to the flip-flop formed by T9 and T10. T8 oscillates at twice the required lamp frequency. T8 and T9 form a divide-by-two circuit which guarantees a 1:1 mark space ratio. T11 and T12 form a push-pril driver circuit which is emitter coupled to the flip-flop. Secondary windings T₅S₁ and T₅S₂ are connected via R42 and R43 to emitter-base circuits of

T6 and T7 respectively. R4l and C24 form a snubber network for T11 and T12. The two power rails for T6 and T7 are derived from secondary windings T₂S₁ and T₂S₂ of transformer 20. Rectification for the positive rail (T6 supply) is obtained via Dl6, rectification for the negative rail (T7 supply) is obtained via D17. Diodes D20 and D21 are "flywheel diodes" which maintain current flow in the circuit when T5 is in the OFF state. D20 and D21 cease conducting when all the energy from smoothing chokes LI and L2 has been transferred to capacitors C11 and C12 respectively. These chokes reduce the 30 KHz current ripple components.

The igniter circuit comprises the pulse transformer 30, a spark gap S.G., resistors R30 and R51, and a capacitor C17. R30 and C17 have a sufficiently short time constant to allow C17 to be charged up to the breakdown voltage of the spark gap S.G. As the applied square wave voltage is symmetrical, breakdown of S.G. occurs at roughly the same part of the cycle. When voltage across C17 reaches the breakdown voltage of S.G., C17 discharges quickly through R51 and primary T₄P. A sufficiently large voltage is induced in secondary T₄S to ionise the gap in the lamp. R51 limits the peak current flowing through the circuit which lengthens the life of the spark gap and capacitor C17 considerably.

Claims

CLAIMS :

1. A lamp drive circuit including regulated (as herein defined) direct current power supply means (18 to 28) so coupled to switching means (32 to 36) as to be capable of supplying an alternating current between a pair of output terminals (11, 12) of the drive circuit when a discharge lamp (10) couples the said output terminals to one another, the direct current power supply means including an alternating current stage (20 to 25) which determines the internal inductive impedance of the direct current power supply means and is such as to operate at a frequency which is substantially greater than and preferably at least ten times the frequency of the alternating current supplied to the lamp when present in operation.

2. A lamp drive circuit according to claim 1, in which the direct current power supply means comprises a forward converter.

3. A lamp drive circuit according to claim 1, in which the alternating current stage operates at a frequency of the order of 30 to 40 KHz.

4. A lamp drive circuit according to claim 1, in which the switching means comprises at least two transistors (32, 33) connected in series with one another and so controlled as to be switched on and off alternatively at the lamp operation frequency.

5. A lamp drive circuit according to claim 1, in which the lamp operating frequency is of the order of 100 to 250 Hz.

6. A lamp drive circuit according to claim 1, having a compact source metal halide discharge lamp connected across the output terminals.

7. A cine film projector or camera provided with a lamp having a lamp drive circuit in accordance with claim 1.

8. Apparatus according to claim 7ι including means for synchronising the operation of the switching means to that of the optical system of the projector or camera.