GB2057795A - Improvements in or relating to operating circuits for electric discharge lamps - Google Patents

Abstract

An operating circuit for a low pressure sodium vapour lamp or a cold cathode fluorescent lamp comprising a tapped choke whose winding is divided into two small coils L1, L3 and a main larger coil L2 includes a booster with a semiconductor switch Tr1 controlled by the condition of the lamp. The booster circuit includes a capacitor C2 which forms an oscillatory circuit with inductive components, such as the coil L3, and generates lamp striking voltages under the control of the semi-conductor switch Tr1. The switch Tr1 is controlled by the lamp voltage, or in other examples by the lamp current, through a control circuit which introduces a delaying phase shift when the lamp is running up to ensure that the energy of the oscillatory lamp striking voltage is available at the time required in each half cycle of lamp operation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to operating circuits for electric discharge lamps This invention relates to operating circuits for discharge lamps, and particularly but not exclusively low pressure sodium or cold cathode fluorescent lamps.

Low pressure sodium lamps and cold cathode fluorescent lamps in the standard range of these lamps having high striking and re-striking voltages, and cannot be normally operated with an ordinary series choke ballast circuit from the normal public supply voltages. In general, the 240 volt mains supply voltage is not sufficiently high to initiate the discharge or to re-strike that discharge after the lamp has been ignited, and moreover, only those lamps with short arc tubes (low voltage) can be properly ignited and maintained with a series choke and separate igniter.

The control gear for a low pressure sodium lamp must provide a high voltage to initiate an arc discharge in the starting gases before the sodium vaporises, while at the same time preventing the current in the lamp from reaching too high a value, and finally so control the lamp current when the sodium has vaporised as to operate the lamp at its correct running wattage. A transformer with a leakage reactance path in the iron circuit has been in common use for this purpose but it has a number of disadvantages.

The operating characteristics of these lamps with standard length arc tubes are such that striking voltages from 450 volts to 700 volts r.m.s., are required, while the average lamp voltages are in the order of 100 to 1 70 volts once the lamp has stabilised. The transformer therefore has to cope with a very wide regulation and has in consequence a large iron and winding content, and a large power loss in most commercially made units. The high inductance of the step up winding and the leakage inductances necessitate the use of a large power factor correction capacitor, and the saturation of parts of the transformer iron circuit by the leakage flux increases power losses and increases lamp circuit distortion.

Method of employing a series choke ballast with a simple igniter are generally only suitable for short arc length lamps, for although some low pressure sodium vapour lamps may appear to have a relatively low average running voltage, the complexity of the gas filling is such that ionisation and deionisation of the different constituents of the discharge gases may take place at different intervals in the cyclic re-ignition period during each half cyle. In consequence the restriking voltage at each reversal of the lamp current is usually a lot higher for these lamps than other types of discharge lamp, such as mercury vapour electric discharge lamps.

The present invention provides means of circumventing some of these drawbacks.

The present invention is defined in claim 1 hereinafter, and preferred features of embodiments of the invention are defined in the claims dependent to claim 1.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a conventional lamp operating circuit for a low pressure sodium lamp; Figure 2 is a circuit diagram of an embodiment of the present invention; Figures 3, 4 and 5 are circuit diagrams of parts of other embodiments similar to that of Fig.

2; Figure 6 is a circuit diagram of another embodiment of the invention; Figure 7 is a graphical representation of voltage waveforms associated with operation of the circuit of Fig. 2; Figure 8 is a circuit diagram of an embodiment of the invention in which a controlled semiconductor switch is connected in series with the path for lamp current; Figures 9 and 10, are circuit diagrams of further embodiments in which a semi-conductor switch is in series with the path for lamp current; Figures ii to 14 are circuit diagrams of embodiments which include transformers; Figure 15 is a circuit diagram of an embodiment employing a series choke and capacitor arrangement; and Figures 16 and 17 are circuit diagrams of parts embodiments similar to that of Fig. 15.

In the drawings, items having the same or similar functions in the different circuits are given the same references.

In a first embodiment shown in Fig. 2, the terminals of a low pressure sodium lamp are arranged to be connected across a.c. mains input terminals I and I' via a tapped choke whose winding is divided into a main large coil L2 and two small coils L, and L3. The first part of the choke, i.e. the coil L1 is tapped by a capacitor C1. This capacitor C1 normally forms part of the power factor correction in a conventional circuit. In Fig. 2 however, it is connected into the choke at the first small coil L1 so capacitive current flows in that coil L,. An electronic booster circuit is connected on the other small coil L3 and has a special control circuit which monitors the lamp condition.

Initially, when the a.c. mains is connected to the mains input terminals I and I' of the circuit, the booster circuit, which includes a semiconductor switch, preferably a triac Tr1 as shown, operates, generating the lamp striking voltage required to ignite the starting gases of the lamp.

The electronic booster circuit has, in addition to the triac Tr1, a capacitor C2 and a resistor R which are connected in series with the triac Tr1 and the coil L3 across the lamp. The special control circuit of the booster circuit includes a diac Tr2 and a capacitor C4 connected in series between the trigger of the triac Tr1 and its adjacent main electrode as shown, and a capacitor C5 connected in series with two resistors R2 and R3 and the resistor R1 to form a resistancecapacitance series combination connected across the output terminals of the lamp circuit i.e.

between the lamp terminal end of the coil L3 and the common input/output terminal I, as shown. The resistor R3 is connected in parallel with the capacitor C4 so tha the capacitor can be coupled to the trigger of the triac Tr1 through the resistor R2 and the diac Tr2.

When the a.c. mains supply is initially applied to the lamp circuit input terminals I and 1', the capacitor C5 is charged through the coils L1, L2 and L3 in series and the resistors R,, R3 and R2 in series until the diac Tr2 breaks down and the triac Tr1 triggers, thereby discharging the capacitor C4 and partially discharging the capacitor C5. Triggering of the triac Tr1 enables the capacitor C2 to the charged through triac Tr, and the resistor R, by the voltage between the tap between the coils L2 and L3 and the common input/output terminal I. While the triac Tr1 is conducting, the capacitor C2 forms a semi-resonant circuit with the choke and the other components of the lamp circuit and the oscillatory voltage which results across the lamp serves as the lamp striking voltage.This oscillatory voltage has a frequency of a few hundred hertz and is dissipated by the lamp and resistance in the lamp circuit within a half cycle at mains frequency. Also, the triac Tr has ceased to conduct by the end of such a half cycle. The triac Tr1 continues switching in successive half cycles to re-ignite the arc and permit a continuous flow of current through the lamp. The switching action produces resonant voltages in windings L1 and L3 and a transformer action, so the lamp terminal voltage is substantially above that of the supply voltage entering at the input terminals I-I'.

An additional capacitor C3 is connected across the lamp to form part of the resonant circuit.

During running up of the lamp, i.e. after the lamp has struck but before it has stabilised, the arc has a tendency to extinguish towards the end of each mains frequency half cycle of lamp voltage, and the driving voltage produced by the tapped choke without assistance from the booster circuit is not sufficient to prevent extinction. Furthermore, the resonant voltage produced by the booster circuit would be dissipated before the time at which extinction is possible if the resonant voltage were started only shortly after the beginning of each half cycle of lamp voltage.

Because of the complexity of the lamp voltage waveform, particularly while the lamp is running up, its average value, which is relatively constant, is difficult to monitor. However, the present control circuit operates at the beginning of the mains current cycle, until the lamp is struck, and then shifts in phase as the lamp runs up.

The shift in phase is produced in relation to the voltage at the tap between the coils L2 and L3 by the capacitor C5 and the resistor R2 in cooperation with the other components of the control circuit coupled across the lamp as the lamp warms up, and as the current stabilises the circuit fires at about 1/3 of the period of the lamp voltage 'on' duration during each half cycle thus responding to the flat portion of the lamp voltage waveform, Fig. 7 shows the lamp waveform when the lamp is in stable operation having been fully run up. Adjustment of the time constants of the circuit C5, R2, R3 and C4 can be used to monitor the lamp at different points in the half cycle. In Fig. 7, a monitor point is indicated in each half cycle of lamp voltage. The monitor point is the instant at which the capacitor C4 causes the triac Tr1 to begin to fire.

The resistor R1 and inductor L3 act as buffers and respectively damp the resonant voltage generated by the booster circuit and prolong its duration over each mains half cycle.

When the lamp has stabilised, the triac Tr1 ceases to operate.

Fig. 3 is a modification of the circuit which permits switching of more than one capacitor, in this case three capacitors which are capacitors C2, C3 and a capacitor C6, to produce resonant voltages in unison, but each having a different time constant generating higher and/or prolonged voltages at the lamp terminal.

Fig. 4 is a further modification which produces a slightly different resonating voltage effect compared with the circuit of Fig. 2. The main difference is that if a large value of C3 is required whereas a current pulse will occur in the lamp in the Fig. 2 circuit during each half cycle, this effect will not occur in the Fig. 4 circuit.

Typical practical values of some of the components of the circuits shown for the operation of a 55 watt low pressure sodium lamp, and a 90 watt low pressure sodium lamp from the 240 volt 50 Hz mains alternating current supply are as follows: L1/L3 Inductor 358.5 ohms at 0.6 Amps Tapped C1 51l Fd C2 0.331l Fd C3 0.022p Fd C4 0.01L Fd C5 0.02p Fd R, 3.3 ohms R2 67 K ohms R3 10K ohms Tr1 Triode a.c. semiconductor type: 228M supplied by SSC Tr2 Diode a.c. semiconductor type: DB3 supplied by SSC.

Lamp 55 watt SOX supplied by Phillips.

L,/L3 Inductor 233.5 ohms at 0.9 Amps Tapped C, 7 Fd C2 0.47 Fd C3 0.02y Fd C4 0.01y Fd C5 0.02y Fd R, 3.3 ohms R2 67 K ohms R3 10K ohms Tr1 Triode a.c. semiconductor type: 228M Tr2 Diode a.c. semiconductor type: DB3 supplied by SSC Lamp 90 watt SOX supplied by Phillips.

Instead of the specific components Tr1 and Tr2 described other equivalent components can, of course, be used. For example other diacs or bi directional break over diodes or "Shockley" diodes can be used.

A further modification of the design is shown in Fig. 5. In this arrangement L, forms a separate winding comprising a small number of turns wound over L3, and connected at the tap of L2/L3 to the common neutral via C3.

This provides an additional resonant boost voltage stepped up by the specific transformer action of L,-L3 when Tr, switches.

Where the circuit of Fig. 2 is used for operating a larger lamp such as the 90 watt SOX lamp, it is advantageous to alter the connection of the capacitor C3. If the capacitor C3 is connected as shown in Fig. 2 or is omitted, the lamp current in a 90 watt SOX lamp increases when the mains supply voltage increases and decreases when the mains supply voltage falls. To reduce such fluctuations in lamp current, the capacitor C3 is disconnected from the junction of the resistors R, and R3 and is connected in series with a parallel combination of a 3.3 ohm resistor and a non-linear resistor with a positive temperature coefficient, for example an E22022/04.

The capacitor C3 can then be 0.33 microfarads, the series capacitance-resistance combination of the capacitor C3 and the parallel resistors being connected directly across the lamp terminals.

The value of the resistor R, in series with the triac Tr, is then increased to 39 ohms, and the capacitor C5 and the resistor R2 can be replaced by a single resistor of 87 kilohms. It may also be desirable to include a 57 ohm resistor in series between the diac Tr2 and the capacitor C4.

Other components can have the values listed hereinbefore for use with the 90 watt SOX Phillips lamp.

Fig. 6 shows a further modified control circuit similar to those shown in Figs. 2 to 5 but operating in a somewhat different manner. The control circuit of Fig. 6 responds to the voltage across the coils L, and L2 relative to the common input/output terminal I. Initially a large voltage appears across three series resistors R2, R 2a and R3. Short circuiting of the resistor R2a by a diac Tr3 then occurs causing the triac Tr, to generate the maximum resonant voltage in order to run the lamp up quickly. When the lamp nears its target current and wattage, the voltage across the diac Tr3 falls and it ceases to short circuit the resistor R2a. Both resistors R2 and R2a are then in circuit and the triac Tr, fires with a much lower voltage across its main terminals and gives a more gentle boost until its switching ceases and the control circuit drops out altogether.

Should the lamp current fall and the lamp voltage rise, due for example to the supply voltage dropping sharply, the lower boost will switch on again maintaining the lamp within specific wattage tolerances. Instead of diacs, break-over diodes or back-to-back zena diodes may be used in the circuit of Fig. 6.

Alternative lamp operating circuit arrangements embodying the invention are shown in Figs. 8 to 1 0. These circuits employ a semiconductor switch in series with the choke and lamp so that this switch switches the full lamp current.

In these cases the semiconductor switch is so controlled that during lamp starting when the lamp is cold, the non-conducting period of each half of the supply cycle is relatively long-producing a high resonant voltage across the lamp for striking the arc. As the lamp runs up and stabilises, the non-conducting period of the switch becomes progressively smaller, and the superimposed resonant voltage at the lamp terminals also reduces progressively. When the lamp has run up and stabilises reaching its temperature equilibrium, the semiconductor switch is arranged to be fully conducting (operating at very near zero change during each half cycle) or switching with a very short 'off period', thus keeping any disturbance to a minimum during lamp operation when the lamp is drawing mains current.

The latter mode of operation is used to generate some resonant voltage at each half cycle of lamp re-ignition to keep the lamp operating in a stable condition, if that particular lamp cannot be maintained by the supply voltage alone. In these circuits there is a fairly long 'off period' during starting, the voltage is high at the lamp terminal, and thus the lamp current is higher during the 'on period' of the semi-conductor switch. When the lamp has run up however, the lower resonant voltage is compensated by the increase of lamp current because of the long conduction period of the semiconductor. Thus the RMS value of lamp current tends to be relatively constant.

The lamp stable position is determined by the ability of the lamp to strike on the available voltage at each re-ignition half cycle and is set by the time constants of the control circuit.

Fig. 8 shows a charging circuit with a capacitor C4 and a trigger diode Tr2 which is a diac. A resistor R2 and a thermistor Th form the charging and time constant circuit with the capacitor C4.

When the supply is switched on the triac Tr1 fires due to the voltage appearing via the capacitor C3 or C2 (depending upon the position of the triac Tr, in the circuit). The control circuit charging the capacitor C4 is largely resistive and fires at a fairly high voltage. The circuit is also lightly loaded with no lamp current and generates the maximum duration and peak values of resonant voltage.

When the lamp strikes, the phase change in current causes a substantial voltage to appear across the triac Tr1 when the current next passes through zero, thus the charging time of the capacitor C4 is reduced. As the lamp runs up the thermistor Th falls in resistance thereby increasing the conduction angle further. If a non-linear resistor VR, is included it tends to by pass current from the resistor R2 and the thermistor Th in the initial stage so that current through R2 and Th is built up at a slower rate and the conduction angle change from maximum to minimum is increased.

A further arrangement is shown in Fig. 9, in which, as a result of the presence of a capacitor C5, a phase shift occurs progressively as the average lamp voltage falls, thereby decreasing the non-conducting angle of the triac Tr,.

In the circuit of Fig. 10, the additional booster winding L, wound over coil L3.

Figs. 11 and 1 2 show examples of a circuit having a series choke and a small step-up transformer in combination. The choke is used in the manner previously described as a series inductor with an electronic booster circuit. The transformer is used to step up the supply voltage either to: (a) just keep the lamp stable once it has fully run up when the electronic booster circuit is switched off, or (b) maintain a low level output voltage supplemented by the electronic booster circuit if the lamp is still unstable at the boosted supply voltage.

One particular aspect of the design is that a power factor correction capacitor C, is connected across the whole of the transformer windings Fig. 11 and 12, so that only the corrected current, and not the full lamp current flows in the step-up windings.

Thus the transformer need only be a small auto-winding with small diameter winding wire as it does not carry the full lamp current.

In Fig. 11 the step up winding T2 is connected to the series inductor and series semiconductor switch which is a triac Tr,.

When the mains supply is connected to the circuit, The triac Tr, operates with a fairly long non-conducting period to generate the maximum resonant voltage in addition to the mains boosted voltage. As the lamp runs up, the control circuit (not shown) reduces the nonconducting period during each half cycle of the triac Tr, until it is almost fully switched on over each complete cycle or has a small non-conducting period to generate some resonant boost voltage.

In the alternative circuit shown in Fig. 12, the semiconductor switch (triac Tr1) is connected across the lamp and operates from the average lamp voltage.

Another arrangement employing a transformer choke combination employs the series choke and a separate small transformer which is used to boost the resonant voltages and not the main supply voltage. The transformer in this case is wound on a small ferrite core. Fig. 1 3 and 14 show two examples of such circuits with the series choke arrangements as previously described but with a small additional extra booster transformer T,.

The following example illustrates the performance of embodiments of the invention; LAMP TESTS ON EXPERIMENTAL 90 WATT SOX CONTROL GEAR Comparison tests made between a commercially available control unit and a low unit according to the invention.

Phillips 90 watt SOX lamp A NEW LOW COMMERCIALLY LOSS UNIT AVAILABLE UNIT Supply Volts: 240 240 Supply Amps: 0.48 0.63 Supply Watts: 102 131 Power Factor: 0.88 0.87 Pf Capacitor: 8 Fd 25y Fd Lamp Volts: 114 115 Lamp Amps: 0.89 0.88 Lamp Watts: 90 90 Gear Losses: 12W 41W Osram 90 watt SOX lamp A Supply Volts: 240 240 Supply Amps: 0.46 0.82 Supply Watts: 103 1 33 Power Factor: 0.93 0.89 Lamp Volts: 118 118 Lamp Amps: 0.86 0.87 Lamp Watts: 91 92 Gear Losses: 12W 41W Fig. 1 5 shows a lamp operating circuit which has a choke W, and a series capacitor C and embodies the invention.A small additional inductor W2 is connected in parallel with the series combination of the series capacitor C and the lamp and corrects the total power factor of the current drawn from the mains, which would otherwise be a leading capacitive current. The semiresonant characteristic of the choke W, with the series capacitor C enables lamps with higher arc voltages to be maintained with normal mains supply voltages. The inductor W2 and the booster circuit which induces the triac Tr, enable linear and SOX low pressure sodium vapour lamps to be operated.Regulation of the lamp is improved as, in addition to the control of current by the choke W, and the series capacitor C, magnetisation current drawn by the inductor W2 increases and decreases with increases and decreases respectively in the r.m.s. mains voltage so that fluctuation of the average lamp current with r.m.s. mains voltage variation is reduced. A circuit such as that of Fig. 1 6 can be used to operate a linear or SOX low pressure sodium lamp at 135/140 watts or 1 80 watts. In a practical example for a 1 35 watt SOX Phillips lamp the inductor W, has an impedance of 257 ohms at 0.9 amps, and the inductor W2 consists of 1 900 turns wound on a 2.54 cm stack of no. 35 laminations.

Figs. 1 6 and 1 7 show details of two further booster circuits for lamp operating circuits using a series choke and capacitor combination (not shown).

In Fig. 16, a triac Tr, is triggered by breakdown of a diac between resistors R, and R2.

Conduction of the triac Tr, results on triggering of a triac Tr3 in a parallel resonant circuit in which the reactive components are a capacitor CA and an inductor Lc. The oscillatory voltage produced by this parallel resonant circuit has a frequency of several kilohertz and an amplitude of between 1 kilovolt and 1.5 kilovolts. Before the lamp strikes, the parallel resonant circuit generates the high frequency voltage continuously once the triac Tr2 is triggered since the triac Tr2 is repeatedly switched on and off by, respectively, rising voltage on the inductor Lc and the falling portion of the reverse voltage, these voltages being coupled to the trigger of the triac Tr2 by the capacitor C5 in Fig. 1 6.

When the lamp strikes, the lamp voltage, which is between points 'a' and 'b', is such that, through the resistor R1, the capacitor C4 and the resistor R2, the resultant voltage at the trigger of the triac Tr, causes a reduction of the flow of current in the triac Tr, and as a result the triac Tr2 ceases to allow continual oscillation of the parallel resonant circuit throughout each supply cycle but allows oscillation in part of each supply cycle to assist in running the lamp up. At a predetermined voltage between the points 'a' and 'b', which may be a voltage which occurs when the lamp has completely warmed up, the triac Tr, entirely ceases to conduct and the parallel resonant circuit ceases to generate any oscillations.

The current drawn from the series capacitor is a moderate steady current. Mains current drawn by the parallel resonant circuit is limited by a capacitor Cc, which acts a low impedance path to the high frequency voltages of the parallel resonant circuit.

Fig. 1 7 shows circuitry similar to that of Fig. 1 6 but differing in having a resistor Rs connected in parallel with the capacitor C4, and the capacitor C5 replaced by a resistor R6. A resistor R7 may be connected in parallel with or replace the capacitor Cc. The effects of these modifications are to alter the resonant frequency and waveform of the high frequency oscillations generated by the parallel resonant circuit which again has the capacitor CA and the inductor Lc as its reactive components.

In a further modification of Fig. 17, the resistor R5 is replaced by a non-linear resistor or a semi-conductor device controlled by the lamp current. For example, the non-linear resistor may be a heat sensitive semi-conductor device, such as a VA 1 056S Mullard semiconductor placed in thermal contact with a low value resistor, such as a wire wound 0.5 ohm resistor, connected in series with the lamp between the series capacitor (not shown) and the point 'a'. Lamp current flowing through this small resistor heats the latter and thereby causes the nonlinear resistor to reduce its resistance and thereby switch off the triac Tr. Similarly, the non-linear resistor may be a Hall effect semiconductor device arranged to be controlled by the magnetic flux field of the ain, i.e. series, choke.

In a practical example of Fig. 1 7, the following components can be used for operating a 135 Watt Osram Lamp.

Item Value R1 91 kilohms R2 39 ohms R3 18 kilohms R4 1 kilohm R5 16 kilohms R5 100 kilohms R7 222 ohms Ct 6ss1 Fd 450 v WKg C2 0.47y Fd C3 0.068p Fd C4 0.1y Fd Lc 400 millihenrys wound on a miniature Ferrite Ecore by Inductive Devices Ltd.

Tr1 SC 116 Tr2 SC 116 Diac ST2 The series inductor (not shown) in Figs. 16 and 1 7 is an untapped inductor.

Comparing Fig. 1 5 with Figs. 16 6 and 1 7 it will be seen that the oscillatory booster voltage is generated in Fig. 1 6 by series resonant circuitry in which the capacitor C2 serves as the capacitive element, and in Figs. 1 6 and 17 7 the oscillatory booster voltage is generated by parallel resonant circuitry in which the capacitor CA serves as the capacitive element.

In all three circuits, the generation of oscillatory voltage is controlled by the respective triac Trt which is controlled by the lamp voltage or, in the case of the modification described for Fig.

17, the lamp current. A capacitor corresponding to the capacitor C5 of Fig. 1 5 may be included in series with the resistor R1 in Figs. 1 6 and 1 7 if it is necessary to introduce a delaying phase shift between the lamp voltage and the voltage applied to the trigger of the triac Tr1 in Figs. 16 and 17.

Claims (7)

1. An operating circuit for a discharge lamp, the circuit comprising a pair of input terminals for connection to an a.c. supply, a pair of output terminals for connection to a discharge lamp, at least one inductive circuit member so connected between one of the input terminals and one af the output terminals that lamp current passes therethrough in operation, voltage booster means including a capacitor arranged to form part of a resonant circuit which, in operation, supplies oscillatory voltage to the output terminals, and a controlled semi-conductor device arranged to control the occurrence of the oscillatory voltage and having a control terminal connected to control means including a resistancecapacitance arrangement adapted to respond, in operation, to the lamp voltage or to the lamp current.

2. An operating circuit according to claim 1, wherein the resonant circuit is series resonant.

3. An operating circuit according to claim 1 or 2, wherein a capacitive circuit member is connected in series with the said at least one inductive circuit member between the said inductive circuit member and the said one output terminal, and a further inductive circuit member is connected across the said capacitive circuit member and the output terminals.

4. An operating circuit according to claim 1 or 2 or 3, wherein the said resistancecapacitance arrangement includes a series combination of capacitors and resistors connected across the said output terminals.

5. An operating circuit according to claim 4, wherein the semi-conductor device is a triac and the said series combination of capcitors and resistors includes a capacitor coupled through a diac to the trigger terminal of the triac.

6. An operating circuit according to claim 1, wherein the resonant circuit is parallel resonant and is connected in series with the semi-conductor device, the booster means being connected across the said output terminals, and the resistance-capacitance arrangement comprises a series combination of a resistor and a capacitor, the resistance-capacitance arrangement being connected across the said output terminals, and the semi-conductor device having a trigger terminal coupled to the resistance-capacitance arrangement.

7. An operating circuit according to claim 1 and substantially as described hereinbefore with reference to any one of Figs. 2 to 6 and 8 to 1 7 of the accompanying drawings.