An electrical control circuit for controlling a variable AC load

Abstract

 A control circuit is provided for controlling the operation of a variable AC load (1), such as a metal halide lamp as used in photolithographic printing machines. The control circuit has a converter (3) connected with the load to receive the AC current arranged to pass through the load and to provide a DC output voltage representative of the amplitude of the load current. A magnetic amplifier (4) is connected to receive the DC output of the converter (3) and also connected between an AC voltage mains supply and the load to control the AC current through the load (1) in dependence upon the DC voltage applied to the magnetic amplifier (4). The AC load can alternatively be a tungsten or other incandescent lamp.

Description

[0001] This invention relates to an electrical control circuit for controlling a variable AC load, particularly a load such as a metal halide discharge lamp which inherently has unstable operating characteristics.

[0002] Metal halide lamps having a Mercury vapour therein and suitably doped to emit ultra-violet light are known to be used for photographic exposure in photolithographic printing machines, however, because the operating characteristics of the lamp are very unstable, the output power of the lamp varies rendering the exposure of photographic plates unreliable. The operation of metal halide lamps is dependent upon lamp pressure and hence operating temperature which would normally be of the order of 850-900°C. The mininum operating tenperature is 560°C whilst the maximum operating is of the order of 950-1000°C being limited by the maximum operating temperature of the quartz envelope of the lanp. It is known that under thermal runaway conditions the output power of the lanp would be increased and the lamp would burst. If an insufficient voltage is maintained across the lanp then the lamp will simply go out.

[0003] Such lanps require a voltage of 5kv + to ignite the lamp and with such a requirement the lamp does not readily lend itself to control by solid state circuitry since at the instant of switching on the control circuitry would be presented with an effective dead short being the condition of the lanp at that instant.

[0004] These problems have to a very great extent been overcome by the provision of cooler units in association with the metal halide lanps so that the lamp is maintained at a reasonably stable temperature environment during operation. However, because of the large power/ voltage level involved and the lack of isolation techniques in utilising solid state circuitry, manufacturers of such photolithographic printing machines have utilised the well known inductive/capacitive ballast type circuits. However, such ballast circuits are not control circuits but are effectively limit circuits and within these limits a wide variation in output power by as much as 20% can be obtained resulting from temperature changes in spite of the use of cooler units.

[0005] Therefore it is desirable to provide a control circuit for a variable load device such as a metal halide lamp in which the operating characteristics of the load device and consequently the output power thereof remains in a substantially stable state.

[0006] According to one aspect of the present invention there is provided a control circuit for controlling the operation of a variable AC load, comprising converter means connectable with the load to receive the AC current therethrough and to provide a DC output voltage representative of the amplitude of the load current, and a magnetic amplifier connected with the DC output of the converter means and arranged to be connected between an AC voltage mains supply and the load for controlling the AC current through the load in dependence upon the DC voltage applied to the magnetic anplifier.

[0007] In one particular embodiment there is provided a sensor and amplifier circuit arranged to provide a control signal for controlling the operation of the converter means in dependence upon the output power of the load.

[0008] Preferably an AC signal is applied to an input of the sensor and amplifier circuit, the AC signal being controlable by a light sensitive resistor. The AC signal can be supplied through a Schmidt trigger or by a square-wave oscillator.

[0009] Advantageously there is provided a selectable voltage arrangement by which one of the series of predetermined voltages is selectable and applied as a control signal to the converter means, said selectable voltage arrangement comprising a series of potential divider circuits.

[0010] Conveniently the converter means includes an AC filter circuit coupled with a bridge rectifier circuit arranged to operate for selected periods of an AC cycle. Preferably the bridge rectifier circuit is provided with a thyristor in each of two arms of the bridge with the firing of the gate electrodes of the thyristors controlled in dependence upon the output of the sensor and amplifier circuit and a selected one of the selectable preset voltages of the selectable voltage arrangement.

[0011] Preferably, there is provided a pratection overload circuit for determining the operation or not of the control circuit in dependence upon such factors as voltage overload or temperature runaway. It is preferred the overload circuit be provided with an opto-isolator to naintain isolation between part of the overload circuit coupled with the variable load and part of the overload circuit controlling the connection of the AC voltage mains supply.

[0012] Conveniently, a voltage overload circuit is provided comprising an AC filter circuit, preferably having a delta capacitor network, a bridge rectifier connected to the filter circuit and a smoothing circuit connected to receive the output of the bridge rectifier. Preferably, a potentiometer forms part of the smoothing circuit and feeds a light emitting diode constituting part of the opto-isolator.

[0013] Advantageously the tenperature overload condition is controlled by at least one transistor switch circuit each connected with a temperature dependent switch such as a bimetallic strip.

[0014] A Schnidt trigger circuit is conveniently provided to receive the outputs of the voltage and temperature overload circuits. The output of the Schmidt trigger is conveniently connected to a switching circuit for energising a relay and disconnecting the AC voltage mains supply from the load.

[0015] In the embodiments the load is a metal halide lamp.

[0016] Preferably there is provided an ignition circuit comprising rectifier means for receiving an AC voltage and producing a DC voltage output, energy storage means connected in series with a primary winding of an ignition transformer across the output of the rectifier means and switch means connected in parallel with the energy storage means and primary winding for effecting discharge of the energy storage means through the primary winding to produce an ignition voltage across a secondary winding of the ignition transformer.

[0017] Preferably, the ignition circuit includes a timer circuit which includes a delay relay for controlling the time during which the AC voltage is applied to the rectifier means.

[0018] Preferably, the energy storage means is a capacitor and the primary winding is a Tesla-coil and the switch means comprises a thyristor the gate of which is controlled by the charge across a capacitor ccnnected with the gate via a zenor diode. Conveniently, the energy storage means and primary winding are connected in parallel with a diode for discharging the peak reverse voltage across the energy storage means.

[0019] An embodiment of a control circuit according to the present invention will now be described by way of example with reference to the accompanying electrical circuit diagram illustrating schematically the control circuitry for controlling the operation of a metal halide lamp.

[0020] The control circuit disclosed in the acoarpanying drawing includes a sensor and amplifier circuit 2 which monitors the output power of a metal halide lamp 1 in which the metal elements are suitably doped with iron and indium to provide an ultra-violet light output. The sensor and amplifier circuit 2 operates to provide a low voltage DC output signal to a half-controlled thyristor bridge and firing circuit 3 which in turn derives, fran AC current passing through the lamp 1, a DC output which is applied to the control winding of a magnetic amplifier 4. The AC supply to drive the lanp 1 is provided through the magnetic amplifier 4 and therefore the voltage applied across the lamp is dependent upon the DC signal applied to the control winding of the magnetic amplifier. Accordingly, any change in the output power of the lamp resulting from a change in pressure or temerature of the lamp, is detected by the sensor and amplifier circuit 2 which in turn controls the operating period of the half controlled thyristor bridge and firing circuit 3 to provide a change in the DC output signal applied to the magnetic amplifier 4 which in turn provides a corresponding change in the AC supply to the lanp 1 to maintain the lamp operating at a constant output power.

[0021] The control circuit further comprises an ignition circuit 5 for initially igniting the lamp 1 and an overload or protection circuit 6 for switching the lamp 1 off in the event of a voltage overload occurring across the lanp 1 or temperature runaway of the lamp.

[0022] The circuit so far described operates in a closed loop condition in which the output power of the lamp is directly monitored by the sensor and anplifier circuit 2. However, the control circuit can be operated in an open loop condition by connecting into the circuit a selectable potential divider network 7 which provides selectable fixed voltage outputs to the DC input of the half-controlled thyristor bridge and firing circuit 3.

[0023] The metal halide lamp 1, as is known, is operable in the ultra-violet and visible light spectrums depending upon the doping of the electrodes which are essentially mercury based halides. To obtain ultra-violet light the doping elements are mainly iron and indium whilst the doping elements for visible light are gallium and lead. The envelope defining the lamp is made of quartz and the melting point of this material determines the upper operating temperature of the lamp which is 950-1000°C. Otherwise the quartz softens and the lamp would disintegrate. The minimum operating tenperature is 560°C whilst normally operation is at 850-900°C.

[0024] When switching on the lamp 1 from a condition in which thelamp is at ambient temperature the potential divider circuit 7 is connected into the circuit in place of the sensor and amplifier circuit 2 to provide a fixed DC signal to the half-controlled thyristor bridge and firing circuit 3 depending upon the particular circuit selected. The potential divider network comprises two potential divider circuits connected in parallel across a 5 volt DC supply with the centre point of each potential divider circuit being selectable by a switch Sl. The selected output voltage is applied through a further switch S2 to the half-controlled thyristor bridge and firing circuit 3. The switch S2 comprises the switch contacts of a reed relay (not shown) and serves to connect or disconnect the potential divider network 7 with the half-controlled thyristor bridge and firing circuit 3. The resistors Rl, R2 and R3, R4 of each potential divider circuit are arranged such that resistors Rl, R2 produce an output voltage to the bridge and firing circuit 3 to produce half the output power across the lamp 1 than would be produced if the potential divider circuit of resistors R3, R4 was used. Although only two such potential divider circuits are described any number of such circuits can be utilised.

[0025] Simultaneously with the switching of the switch S2 to connect the potential divider network7 to the half-controlled thyristor bridge and firing circuit 3, the ignition circuit 5 is connected to a mains supply (not shown) via a timer T capacitively coupled by a capacitor C₁ to a full wave bridge rectifier circuit 10 having a 3 amp rating. The timer T includes a delay relay for which the contacts are closed to connect the mains to the rectifier 10 for a predetermined period of 2-4 seconds. The output of the bridge rectifier 10 charges up energy storage capacity C2 connected in series with a Tesla coil TP1 of an ignition transformer IT₁ across the output of the bridge rectifier circuit. A capacitor C3 charges at a rate determined by series connected resistors R5, R6 to switch on zener diode ZD1 to trigger the gate of a thyristor THl connected directly in parallel with the capacitor C2 and coil TP1. When the voltage across capacitor C3 is sufficient the zener diode ZD1 is rendered conductive discharging the capacitror C3 through the gate cathode cormection of the thyristor TH1. This causes the thyristor TH1 to conduct discharging capacitor C2 through the thyristor. The discharging capacitor C2 reverses its polarity during discharge and the peak reverse voltage is discharged through a diode Dl with any unwanted peaks being filtered out via the series connected RC network R7C4. The resonant danped oscillations through the coil TP1 are transformed up by the inductive coupling between the winding TP1 and TP2 and the voltage generated in the winding TP2 is applied across the lamp 1 to ignite the lamp. Following discharge of the capacitors C2, C3 the thyristor TH1 is rendered unconductive and allows the capacitors C2, C3 to recharge so that the circuit repeats itself continuously over the period determined by the timer T.

[0026] Utilising this particular ignition circuit the lamp 1 usually ignites within half a second but because of the inherently unstable characteristics of the lamp and the minimum operating temperature of 550°C, the circuit repeats itself over the time period of 2-4 seconds to ensure the lamp 1 remains in its ignited state and begins to warm up to its minimum operating tenperature. Once ignited the lamp reaches its minimum operating tenperature in some 2 minutes.

[0027] At the end of the timer period, the timer switches off disconnecting the AC mains supply from the bridge rectifier circuit 10. Simultaneously, the switch S2 is open circuited disconnecting the potential divider network 7 from the half-controlled thyristor bridge and firing circuit 3. A switch S3 is closed to connect the sensor and amplifier circuit 2 to the half-controlled thyristor bridge and firing circuit 3.

[0028] The sensor and amplifier circuit 2 comprises an input 11 to which a low voltage AC signal is applied (3-4 volts AC at lkHz). The input signal is applied via resistors R8, R9 to a diode D2 which provides half wave rectification of the signal and inputs it to a Schmidt trigger 12 which effectively acts as a noise gate or filter. The output of the Schmidt trigger is connected via a series resistance R10 to a light dependent resistor 13 of standard type ORP 12. The amount of light incident upon the resistor 13 and the setting of a potentiometer 14 in parallel with the resistor determines the amplitude of the AC signal which is applied to the base of a transistor Tl which is effectively an AC amplifier having base biasing resistors Rll and R12, a collector resistance R13 and an emitter circuit comprising resistor R14 and capacitor C6. The input signal to the amplifier T1 is coupled with the light dependent resistor 13 via coupling capacitor C5 and resistor R10 and is anplified and outputted via capacitor C7 to rectifying diode D3 connected to a potentiometer 15. The rectified output is taken via the potentiometer 15 through switch S3 and applied as a control signal to the half-controlled thyristor bridge and firing circuit 3.

[0029] The half-controlled thyristor bridge and firing circuit 3 caiprises a filter circuit 16 having a capacitor C8 connected in series with the lamp 1 with two magnetically controlled inductors Ll, L2 each connected to one side of the capacitor C8 and connected at their opposite ends to the input of a bridge rectifier circuit 17 comprising oapacitors C9, C10, diodes D4, D5, and thyristors TH2, TH3. Each of the thyristors TH2, TH3 has a series connected capacitor/resistor circuit R15/C11 and Rl6/C12 respectively connected in parallel with it to act as a filter or snubber circuit. The gate electrodes of each thyristor are connected to a standard phase angle trigger circuit 18 which serves to control the conducting period of the thyristors to provide a DC output which is dependent upon the AC current flowing through the lamp 1. The phase angle trigger circuit 18 is itself controlled by the amplitude of the output signal from the sensor and anplifier circuit 2.

[0030] The DC output signal produced by the half-controlled thyristor bridge and firing circuit 3 is applied directly to the control windings of a magnetic anplifier 4 which comprises two series connected saturable reactors SR1, SR2 in which the primary control windings Wl are coupled together and connected across the output of the bridge circuit 17. The secondary winding W2 of SR2 is connected to the secondary winding TP2 of the ignition transformer IT1 and then lamp 1. One end of winding W2 of SR1 is connected to timer T. The DC voltage applied to the windings Wl controls the reactance of the windings and hence the AC current through the windings W2 to the lamp 1. Accordingly, the AC current applied to the metal halide lanp 1 from the magnetic amplifier is directly deperident upon the amplitude of the DC current or output signal from the bridge rectifier 17 which is in turn dependent upon the firing period of the thyristors TH2, TH3 controlled by the output signal of the sensor and amplifier circuit 2.

[0031] To summarise the operation of the control circuit described above, the output power of the ultra-violet light emitted from the metal halide lanp 1 is monitored by the light dependent resistor 13 which determines the amplitude of an AC signal applied to the base input of the AC anplifier Tl. The output of the anplifier is rectified by diode D3 and applied to the control input of the phase angle trigger circuit 18 to control the firing periods of the thyristors TH2, TH3 of the bridge circuit 17. The input to the bridge circuit 17 receives an AC signal via an inductive filter 16 which is oonnected directly with one terminal of the metal halide lanp 1. The DC output signal fran the bridge rectifier circuit 17 is applied to the control windings Wl of the magnetic amplifier 4 and controls the amplitude of the AC voltage across the secondary windings W2, which voltage is applied to the metal halide lamp 1 to reduce or increase the output power of the lamp as necessary.

[0032] Although the circuit described provides an accurately controlled output power for the metal halide lamp 1, the unstable operating characteristics of the lamp nevertheless require some protection system and the overload of protection circuit 6 is provided for this purpose. The overload circuit comprises a solid state logic NAND gate arrangement connected as a Schmidt trigger 19 and formed by a standard CM3S4023B chip. The Schmidt trigger 19 has two inputs to which are applied signals representing different operating conditions for example electrical voltage overload or thermal overload.

[0033] The voltage overload condition circuitry uses the voltage across the metal halide lamp 1 and applies this to a filter circuit comprising resistors R17, R18, a delta capacitor circuit 20 and zenamic varistors 21 having a 275v AC rating. The filter circuit is connected to a 1 amp rated full wave bridge rectifier 22, the output of which is connected to a smoothing circuit comprising capacitor Cl3 and a potentiometer 23. The capacitor/zenamic varistor network has the effect of filtering out voltage spikes and increasing the time constant of the circuit to give high noise immunity. The potentiometer 23 is set to determine the tripping level of the Schmidt trigger circuit 19, the output of the potentiometer being applied to a light emitting diode (LED) LD1 in a 10kv opto-isolator 24 to give high tension isolation between the metal halide lamp circuit and the overload cut out circuit. LED LDl is biassed by a zener diode ZD2. The signal received by the photo-transistor PT1 of the opto-isolator 24 is connected to one input of the logic NAND gate 19. This particular type of NAND gate connected as the Schmidt trigger provides low power, good propogation and good noise immunity.

[0034] Another of the inputs to the Schmidt trigger circuit 19 is connected to a series of parallel connected transistor circuits T2 each having a light emitting diode LD2 connected in its emitter circuitry. The base of each of the transitors T2 is connected to a bi-metallic strip constituting switches S4 which at 150°C are arranged to open to switch on the respective transistor to illuminate the light emitting diode LD2 and to feed a signal into the Schmidt trigger circuit 19. The bi-metallic strips are located at convenient points, such as on the shutter of an exposure deviceor close to the lanp 1 to monitor lamp temperature. The output from the Schmidt trigger circuit is fed via resistor R19 to the base of a transistor T3 having a biassing resistor R20 and connected by its emitter to the gate circuit of a thyristor TH4. The output signal applied to the base of the transistor T3 renders the transistor conductive causing the gate of the thyristor TH4 to fire energising a relay coil 25 connected in series with the thyristor TH4, to open a set of contacts 26 and thereby disconnect the main power circuit from the metal halide lanp 1. The collector of transistor T3 is connected between thyristor TH4 and relay coil 25 and to a source of potential via a regular 30 and a diode D10.

[0035] The slider of the potentiometer 23 is also connected to ground through relay contacts 27, 28. The contacts 27 and 28 are closed to provide a short circuit for the overload protection circuit when the control circuit is in the ignition cycle and the short circuit prevents excess reverse voltage damaging the opto-isolator 24. Once the ignition cycle has been completed the switches 27, 28 are opened automatically.

[0036] The metal halide lamp 1 is conveniently used for photographic exposure in photolithographic printing machines where the output power of the metal halide lamp must be accurately controlled to obtain the correct exposure of photographic plates. The photolithographic printing machines have the necessary logic interface to control the switching of the various switches in the control circuitry described herein whilst the control circuitry of applicants invention advantageously fixes the light output of the metal halide lanp to a very fine degree and for the same energy consumption as conventional techniques and a greater output power is provided with increases in efficiency of 25% being obtained. Indeed, efficiencies of 95% have been noted when utilising the control circuit of applicants invention whilst in the conventional choke systems overall efficiency of only 70-75% is obtainable. Furthermore, conventional inductive/capacitive ballast circuits rely on physically large inductive chckes or banks of capacitors producing considerable disadvantages as to weight and size. However, with applicants circuitry utilising solid state techniques without the large inductive chokes or capacitors the physical size is very much reduced and considerable savings in costs are , obtainable.

[0037] Because of the greater control over the output power of the lanp with the circuitry of applicants invention faster exposure times are obtainable in the photographic processes and the lanp has a much more consistent operating level throughout its life than has been hitherto known. Furthermore, the need for cooler units to maintain the lamp 1 at substantially constant pressure and tenperature such that a reasonably constant output power is achieved for the lamp, is substantially eliminated thus producing a further saving in costs for many applications.

[0038] However, other applications are envisaged such as the control of the metal halide lamps when used to illuminate football pitches or sports grounds, in solariums, UV sterilising of air, water and hospital instruments, erasing of E PRCMS and UV curing of ink, paint or adhesive.

[0039] Furthermore, whilst the lamp has been described as being flourescent lamp of the metal halide type the lamp can take the form of a tungstan or other incandescent lanp.

[0040] In addition the sensor and amplifier circuit 2 can be modified by replacing the AC input and Schmidt trigger by a square-wave oscillator at lkHz.

[0041] The control circuit of applicants invention can be further modified by using a magnetic amplifier in which the control windings are connected in parallel. In another construction, the windings of the magnetic amplifier can be wound on a single core, whilst in yet another form the windings are presented as toroidal windings.

[0042] In another embodiment of the control circuit according to the present invention the sensor and anplifier circuit 2 is replaced by a transformer/rectifier circuit in which the input winding of the transformer is connected in series with the lamp 1. The AC current through the input winding produces a corresponding voltage across the transformer output winding which is rectified and applied as a DC signal to the input of the phase angle trigger circuit 18 to control the same as previously described.

Claims

1. A control circuit for controlling the operation of a variable AC load (1), conprising converter means (3) connectable with the load to receive the AC current therethrough and to provide a DC output voltage representative of the amplitude of the load current, and a magnetic amplifier (4) connected with the DC output of the converter means and arranged to be connected between an AC voltage mains supply and the load for controlling the AC current through the load in dependence upon the DC voltage applied to the magnetic amplifier.

2. A circuit as claimed in claim 1, characterised in that there is provided a sensor and amplifier circuit (2) arranged to provide a control signal for controlling the operation of the converter means (3) in dependence upon the output power of the load.

3. A circuit as claimed in claim 1 or 2, characterised in that an AC signal is applied to an input of the sensor and amplifier circuit, the AC signal being controlable by a light sensitive resistor (R13).

4. A circuit as claimed in any preceding claim, characterised in that the AC signal is supplied through a Sctmidt trigger (12) or by a square-wave oscillator.

5. A circuit as claimed in any preceding claim, characterised in that there is provided a selectable voltage arrangement by which one of the series of predetermined voltages is selectable and applied as a control signal to the converter means, said selectable voltage arrangement comprising a series of potential divider circuits (7).

6. A circuit as claimed in claim 5, characterised in that the converter means includes an AC filter circuit (16) coupled with a bridge rectifier circuit (17) arranged to operate for selected periods of an AC cycle.

7. A circuit as claimed in claim 6, characterised in that the bridge rectifier circuit is provided with a thyristor (TH2, TH3) in each of two arms of the bridge with the firing of the gate electrodes of the thyristors controlled in dependence upon the output of the sensor and amplifier circuit (2) and a selected one of the selectable preset voltages of the selectable voltage arrangement (7).

8. A circuit as claimed in claim 7, characterised in that a voltage overload circuit is provided comprising an AC filter circuit, having a delta capacitor network (20), a bridge rectifier (22) connected to the filter circuit and a smoothing circuit (Cl3, C23) connected to receive the output of the bridge rectifier.

9. A circuit as claimed in claim 8, characterised in that a potentiometer (23) forms part of the smoothing circuit and feeds a light emitting diode (LD1) constituting part of the opto-isolator (24).

10. A circuit as claimed in claim 9, characterised in that the temperature overload condition is controlled by at least one transistor switch circuit (T2, LD2) each connected with a temperature dependent switch (S4).

11. A circuit as claimed in claim 10, characterised in that a Schmidt trigger circuit (19) is provided to receive the outputs of the voltage and temperature overload circuits.

12. A circuit as claimed in claim 11, characterised in that the output of the Schmidt trigger (19) is connected to a switching circuit (T3, TH4) for energising a relay (25, 26) and disconnecting the AC voltage mains supply from the load (1).

13. A circuit as claimed in claim 1, characterised in that there is provided an ignition circuit (5) comprising rectifier means (10) for receiving an AC voltage and producing a DC voltage output, energy storage means (C2) connected in series with a primary winding (TP1) of an ignition transformer (IT1) across the output of the rectifier means and switch means (TH1) connected in parallel with the energy storage means and primary winding for effecting discharge of the energy storage means through the primary winding to produce an ignition voltage across a secondary winding of the ignition transformer.

14. A circuit as claimed in claim 13, characterised in that the ignition circuit includes a timer circuit (T) which includes a delay relay for controlling the time during which the AC voltage is applied to the rectifier means (10).

15. A circuit as claimed in claim 14, characterised in that the energy storage means is a capacitor (C2) and the primary winding is a Tesla-coil (TP1) and the switch means comprises a thyristor (TH1) the gate of which is controlled by the charge across a capacitor (C3) connected with the gate via a zenor diode (ZD1).

16. A circuit as claimed in claim 15, characterised in that the energy storage means (C2) and primary winding (TP1) are connectedin parallel with a diode (Dl) for discharging the peak reverse voltage across the energy storage means.

Drawing