EP0011508B1

EP0011508B1 - A method for determining the values of components for a control circuit for a gas discharge lamp

Info

Publication number: EP0011508B1
Application number: EP79302625A
Authority: EP
Inventors: Stephen Domville Mckie
Original assignee: PRACDES Pty Ltd
Current assignee: PRACDES Pty Ltd
Priority date: 1978-11-20
Filing date: 1979-11-19
Publication date: 1984-08-08
Also published as: IE49213B1; IN153361B; IE792212L; FI793608A; HU182982B; JPS55105993A; NZ192162A; ZA796163B; DE2967166D1; EP0011508A1; PT70469A; NO793708L

Description

The present invention relates to a method for determining the values of components for constructing a control circuit for a gas discharge lamp.
In the prior art, the most common type of gas discharge lamps and circuitry therefor are found in fluorescent lighting. Almost invariably in this type of circuit an inductive ballast is used in conjunction with a starter switch or circuit, these components in combination being connected to a supply voltage to give a momentary surge voltage to ionise the fluorescent lamp, with the ballast then acting, after striking of the lamp, as a choke to limit or stabilise the current.
These prior art systems suffer from the following disadvantages:-

1. A need for power factor correction. 2. Humming of the ballast.
3. The weight of fittings.
4. The size of lamp fitting to house the control equipment.
5. Heat generated in the ballast.
6. A comparatively high cost.

A former system that has been used is a series resistor operating in conjunction with a low-voltage striking tube. In this case, the peak supply voltage is sufficient to strike the lamp. If the lamp is 20W type operating at, say, 100v running voltage on a 250v supply circuit, the resistor has to drop 150v after lamp striking. This will dissipate approximately 20W at the lamp and 30W in the resistor, so that the arrangement is consuming approximately 50W for the 20W of illuminating power. This excessive power consumption is, of course, undesirable.
Numerous attempts have been made to minimise the waste of power in the ballast and to avoid at least some of the disadvantages listed above. The use of a resistor as the principal component for ballasting is far from ideal as these components are noted for their high watts loss. Inductors have been preferred due to their reduced watts loss compared to resistors, despite their relative bulk and increasing cost. Hence, the ballast circuit for a gas discharge lamp as presently adopted almost invariably has the form of the circuits disclosed in U.S. Patent Specifications Nos. 2 575 001 and 3 857 063. In the former specification, the operating circuit for the lamp includes a series connection from the applied power through an inductor and a capacitor having a capacitance of about 13,uF and a reactance at the line frequency of about twice that of the inductor. The principal function of the capacitor is to increase the starting potential on the lamp. In the latter specification, the operating circuit comprises only an inductor for stabilising the lamp operating current.
French Patent Specification No. 1 557 851 discloses a circuit for a gas discharge lamp in which the lamp is connected across input supply terminals, a series connection of an inductor and a capacitor being connected in series with the lamp between one of the input terminals and a terminal of the lamp. A starting circuit comprising a series connection of a diode and a resistor is connected across the lamp. The inductor and the capacitor are provided to ensure correct operation of the lamp after starting, but no teaching is given as to what values they should have.
United States Patent Specification No. 3 983 449 discloses a ballast circuit for a gas discharge lamp, comprising a series connection of an indicator and a capacitor connected in series with the lamp between an input supply terminal and a terminal of the lamp. A starter switch is connected across the lamp. The reactance of the capacitor is between 24 and 55 times the reactance of the inductor, for the purpose of reducing third harmonic distortion. However, this means that the impedance of the inductor is low, and barely sufficient to ensure adequate limitation of the peak current through the lamp.
In United States Patent Specification No. 2 297 257, a gas discharge lamp is disclosed which is in series across input supply terminals via a resistor and a capacitor, the resistor being connected between one input terminal and a lamp terminal and the capacitor being connected between the other input terminal and the other lamp terminal. From the information included in this United States patent specification, the calculated peak currents are such that it is highly improbable that the system disclosed therein would be permitted to be connected to any electricity supply in any substantial numbers. In lamps according to the specification, the crest factor would be well in excess of 2. This would adversely affect other connected equipment. Also, the component values listed in the specification (namely a current limiting resistor of the order of 3 ohms for a 250W lamp using a 36_JLF capacitor with a gas discharge tube in series) would mean an unacceptable waveform for most power supplies. No explanation is given as to how to control the crest factor. Although United States patent Specification No. 2 297 257 acknowledges the efficiency of operation when a capacitor is used as a current control element, it does not address the resulting problems of a high crest factor and the influence on the control circuit of the starting circuit. An effective gas discharge lamp which uses a capacitive control element and which is suitable for both domestic and commercial use cannot be achieved unless these problems are solved.
As another example of a circuit for a gas discharge lamp using a capacitor and an inductor there can be mentioned United States Patent Specification No. 2 134 439, the design following the assumption that the capacitor and inductor should be regarded as a smoothing filter to eliminate excessive peaks in the operating
current. A series capacitor of 30pF and a series choke of 2 to 3H function as the filter, with the values of both being kept as large as possible if normally undesirable variation in the light intensity from the lamp is to be avoided.
As other pieces of prior art there can be mentioned: Swiss Patent Specification No. 492 379 which discloses a gas discharge lamp in which a ballast impedance and a protective case for elements of a starting circuit are housed inside the vessel of the lamp; and French Patent Specification No. 963 793 which discloses a gas discharge lamp fitting in which opposite terminals of the lamp are connected to respective auxiliary electrodes spaced more than half-way down the length of the lamp from their connecting input terminals, the connection including resistors.
However, it has been generally accepted in the industry that a capacitor is not readily adaptable as the principal component for ballasting in the operating circuit of a gas discharge lamp, regardless of the fact that a capacitor generates in itself very little heat. The basis of this reasoning is the large peak currents readily passed by a capacitor relative to those passed by an inductor.
The object of the present invention is to provide for the use of a capacitor as a control element in a control circuit for a gas discharge lamp in series with an attenuator in the control circuit, in which the values of the capacitor and attenuator are optimised.
According to the present invention there is provided a method for determining the values of the capacitance of a capacitor and the impedance of an attenuator for a control circuit for a gas discharge lamp, said control circuit comprising input terminals for connection to an A.C. power supply source and output terminals connected to the terminals of the lamp as well as components of a starting circuit for the lamp connected across the output terminals, the method comprising connecting a variable capacitor box and an attenuator box in one of respective ones of series circuits between each of said input terminals and a respective one of said output terminals and effectively in series with each other when said control circuit is operating and supplying the said input terminals with A.C., characterised in that a larger than anticipated capacitance value of said capacitor and impedance of said attenuator are introduced by said boxes, the capacitance value being then progressively reduced until light output from said lamp falls suddenly and then marginally increased, so as to restore the earlier light output, and said impedance is then progressively reduced until a readily discernible flicker in the light output occurs and then marginally increased so as to eliminate flicker in the light.
The present invention, therefore, lies in the suitable selection of a capacitor, i.e. one with a capacitance barely sufficient to ensure adequate lamp operating power, together with an attenuator having an impedance barely sufficient to ensure adequate limiting of the peak current through the lamp. This attenuator is required to have as low a value as practical otherwise the watts loss in this unit will approach that of a conventional inductive ballast. If the impedance value is too low, permitting excess peak current in operation, a fall in the light output and damage to the lamp will result. If the capacitor is of too low a value, there will be insufficient operating power delivered to the lamp. To have minimum losses, the capacitor reactance has to be as high as practical and the attenuator have as small an impedance as possible.
The relationship of values of the series capacitor and attenuator is contrary to the conventional design approach. It has hitherto been considered necessary to choose the value of capacitance as large as possible (see for example the above-mentioned United States Patent Specification No. 2 134 439). By approaching the design in the fashion of the present invention, considerable simplification and miniaturisation can be achieved.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of an example of a gas discharge lamp and a control circuit according to the present invention;
Figure 2 is a graph showing typical operating conditions of an 8W gas discharge lamp plotted against changes in capacitive and resistive values;
Figure 3 is a diagrammatic representation of an arrangement to enhance starting in larger lamps associated wiht a control circuit according to the invention; and
Figure 4 shows an assembly in actual size of the components of the circuit of Figure 1, except resistor R1, accommodated within a housing.

With reference to Figure 1, the circuit shown is designed for a low wattage fluorescent lamp and has its input terminals A and B connectable to a power supply S which in this case is 220/260 A.C. volts at 50Hz. Output terminals C and D are connected to the filaments of a fluorescent lamp L and in series therewith is a capacitor C1 which, as will be explained hereafter, is chosen to determine the mean operating current supplied from the source S to the lamp L to ensure adequate lamp illumination, and is assigned a value commensurate with that task. An attenuator, in this instance a resistor R1, is also serially connected between the input terminals A-B and chosen to limit the operating current peaks supplied to the lamp L to protect it against damage, and is assigned a value according to this task. The resistor R1 may be positioned in any part of the circuit providing it is in one of the series arms connecting the lamp L to the supply S. As shown in Figure 1, the resistor R is positioned between the output terminal D and the starting circuit. Although many different forms of ignition, or starting, circuits may be employed with the control circuit, such as an oscillating circuit, for the sake of circuit simplification a conventional series network of a diode D1 and resistor R2 is preferably used. The diode D1 supplies a charge to the capacitor C1 on each cycle of the supplied power. By appropriate selection of the resistance of resistor R2, the positive charge stored in capacitor C1 during the positive half cycles of the applied A.C. power from supply S is additive to the voltage at the supply S on appropriate half-cycles, thus effectively increasing, and even doubling with the correct choice of resistor R2, the voltage supplied to the lamp L. Thus, it will be seen that when capacitor C1 is connected in series with both the starting circuit and the output terminals C, D it serves a dual function. The value of resistor R2 may be in the range from 8,000 ohms to 30,000 ohms and will ensure such a voltate and additionally eliminate flicker from the lamp L, due to the shunt connection of diode D1, by effectively disconnecting the shunt circuit of network D1 and R2 when the lamp L fires. Upon firing, a damaging peak current would flow through the lamp L from capacitor C1 in the absence of the current limiter resistor R1.
In those instances where a large wattage fluorescent lamp, say a 40W lamp, is in use, the series attenuator may be composed entirely of an inductor X instead of resistor R1 or additionally thereto. However, due to the value assigned thereto, the watts loss, and physical size, of the inductor is considerably less than that of a conventional ballast inductor.
A typical capacitance for capacitor C1 for the operation of 4W, 6W, 8W and 13W conventional fluorescent lamps is 1_;uF. Because the lamp characteristics are not identical for each of the above lamps, the value of the peak current limiting resistor R1 has to be varied and therefore the watts loss therein varies. Typical values for the current limiter resistor R1 when associated with a 1.0µF capacitor C1 for a 4W and 6W lamp are of the order of 300 ohms, 2 watts. For an 8W lamp, there values are approximately 500 ohms, 3 watts and in the case of a 13W lamp, 500 ohms, 5 watts. If the capacitor C1 is reduced to 0.8 µF, the current limiting resistor R1 can be reduced and the watts loss therein is reduced because the average power through the lamp is reduced. However, an 8W lamp would not have sufficient operating means current applied to it if an 0.8µF capacitor was in series with the supply S. Therefore, a 1.0µF capacitor having a reactance of approximately 3,200 ohms at 50 Hz would be used in series with a 300 ohms peak current limiter. Therefore, the peak current limiter R1 has a value about 10% of the value of the capacitor C1 reactance at 50 Hz.
In the case of 20W lamp operating at 240V, 50Hz, capacitor C1 is typically a 4.0µF capacitor which represents a reactance of about 800 ohms and this is used in association with an 80 ohms current limiting resistor R1, to produce a ratio of the capacitive reactance of capacitor C1 to the value of resistor R1 1 of about 10 to 1. However, it has been found that this ratio does not hold in all cases. If it were so, the following calculation would provide the optimum values:-Assume a supply of 250V at 50 Hz, a drop of 100V across the lamp after firing, a drop of 150V across a series resistor, 20W dissipated at the lamp, and 30W dissipated in the resistor.
If a capacitor C1 and a current limiting resistor R1 having a 10 to 1 impedance ratio be used, 10 volts will be dropped across the capacitor C1 and one volt across the resistor R1 for every 11 volts, so that the 30W loss by this method of calculation would give about 3W loss in the current limiter R1. However, this does not apply because the current limiter R1 is required to handle excessive peak current and in fact it has been found to dissipate 5 to 6 watts and this could vary between one manufactured lamp and another.
On the other hand, it has been found that there appears to be consistency when selecting the value of capacitor C1 in miniature 15 mm diameter lamps (4W, 6W, 8W and 13W with varying lengths) as they all operate satisfactorily with a capacitor between 0.8,uF and 1.25,uF. This is shown by the graphs of Figure 2, where capacitive values of 1.25µF, 1.1µF and 1µF are plotted against values of resistor R1 and wattage loss in that resistor. It will be seen that the value of resistor R1 will be chosen to be within the area defined by the plotted co-ordinates for high wattage loss and light flicker. The specific part of this area chosen is preferably determined by the light output of the lamp. All fluorescent lamps possess a light output plateau with changing operating current and it has been found that if the value of total impedance to the current flow be chosen at or near the lower end of that plateau (i.e. with decreased current) simplification of circuit component construction and longer lamp life are achieved. The light plateau of an 8W lamp may be plotted on the same co-ordinates as the wattage on the graph of Figure 2. Therefore, by reference to Figure 2 it will be seen that the optimum values for an 8W lamp would be 1_JLF for capacitor C1 and 280 to 300 ohms, 2 watts for resistor R1.
Consideration has been given to the development of a formula for calculation of these values but, whereas values can be obtained through calculation in respect of the lamps within a limited range of classes and ratings, it does not appear possible to derive as a general rule these values by calculation. However, it has been found that very little variation occurs in the optimum values required between lamps of the same rating and class produced by different manufacturers. Therefore, component values may be advised for specific lamps independently of manufacturing source.
The method presently adopted, which has proved to be the most reliable, is by empirical selection. After all, there is a limited variety of fluorescent lamps, or other gas discharge lamps, and values once determined for a class and rating of lamp will not require subsequent change. The following procedure has been employed successfully with laboratory test equipment employing appropriate voltmeters, ammeters, a variable capacitance box for C1 values, a variable attenuator (being inductive and/or resistive) for R1 values, a variable resistor for R2 values, a diode, variable power supply means, and a lightmeter, connected in the manner of Figure 1 with a monitoring facility.

1. What are known to be excessive values for R1 and C1 are introduced and power applied to start the lamp.
2. As a coarse adjustment, progressively reduce the value of capacitance for C1, thus increasing its reactance and reducing the operating mean current to the lamp, until the output of light falls below the end of the lamp's light plateau and then marginally increase the capacitance value above the plateau end. Note: this plateau is readily discernible, as beyond either end of the light output from the lamp decreases rapidly compared with variations on the plateau.
3. As a coarse adjustment, progressively reduce the value of resistance R1, if it be assumed to be a resistor instead of an inductor, until a discernible flicker from the lamp illumination occurs, and then marginally increase this resistance value. A peak current meter will provide an accurate indication of the degree of flicker and with experienced use will ensure that excessive peak currents are not applied to the lamp under test.
4. Repeat steps 2 and 3 as often as necessary with progressively finer adjustment of values until optimum values are determined.
5. Switch off power and then restore to check whether the lamp readily restarts. If it does not start, reduce the resistance of R2 until the D.C. voltage across C1 rises to effect lamp starting. A compromise in value can be struck between ready starting of the lamp and commencement of flicker in the lamp illumination. Occurrence of the latter under these circumstances is an indication that some operating current is being by-passed through the starter circuit.
6. Vary the supply voltage between pre-set limits simulating possible fluctuations to be experienced in service. If the light output flickers at any of these voltages, increas marginally the value of R1 to reduce the flicker to tolerable limits and then repeat steps 4 and 5 above for compensating alteration of the value of C1 and correct consequential adjustment of the value of R2.

The above procedure will also be followed in those instances where gas discharge lamps other than fluorescent lamps are to be used.
A lamp and a control circuit according to the invention enable miniaturisation of components so that the components can be an integral part of a lamp, and also a considerable reduction in the cost of components. Lamps controlled by such a circuit are preferably operated near the lower end of their light output plateau and although the illumination is reduced, but not to any marked extent, they, and especially lamps up to 20W rating, operate much more efficiently. The watts loss per lumen output is better than that of a conventional ballast, particularly with the small reduction in light output referred to above. The heat generated hy the control circuit is so low as to permit miniaturisation and lamp fittings do not require the contemporary large space for a ballast. If the components of the circuit are in an upright lamp fitting, a reduction in the size of the base thereof to about 13 mm thick is possible, as compared with the 50 mm for a conventional ballast. Experimental units have been operating satisfactorily for long periods using such a control circuit without deterioration.
This invention permits the majority of the components, i.e. the capacitor and charging circuit, to be placed in the lamp base. In an integral configuration, the components may be formed in an end cap of the lamp, or alternatively be a plug-in attachment, or if a lamp manufacturer desires he may incorporate the components actually in the glass envelope permitting the lamp merely to be plugged directly into the supply. Also, another possibility is that if the resistor R1 is internally connected to the opposite end of the lamp, a lamp could have all its terminals at one end, the other end of the lamp merely having a glass dome, or being otherwise sealed off.
Figure 4 shows the actual size of a thermoplastic housing H and components C1, R2 and D1 housed therein which are used for fluorescent lamps of 4W, 6W, 8W and 13W rating whereby the housing H may be secured to the body of lamp fitting to serve as a protective cover for its housed components. The connecting leads Y will be connected to terminals in the lamp fitting. The resistor R1 will be connected externally of the housing H for better heat dissipation and will be in this instance 300 ohms, 2 watts.
Some gas discharge lamps by virtue of their shape are more difficult to start than others and also as they age some tend to become harder to start. Furthermore, starting is more difficult with lamps of higher rating than 13. The higher voltages that would be needed for starting in these cases impose further requirements on the value of C1. So as not to detract from its principal function of controlling means operating current to the lamp, an exciting arrangement has been devised for the lamp to avoid the requirement of higher starting voltages.
Figure 3 shows the exciting arrangement comprising a piece of electrically conducting material 1 placed in contact with the outer glass envelope of the lamp L at a position approximately 80% of the way along the length of the lamp and electrically connected to the more remote lamp terminal socket D. This arrangement may be duplicated by having a second contact 1A also positioned 80% of the way along the lamp L in relation to the other lamp terminal socket C and electrically connected thereto. That is to say, the contacts 1 and 1A are spaced more than half way down the length of the lamp from their connecting input terminals C and D. As the conducting contacts 1 and 1 A are accessible, that is not enclosed, it is necessary to meet local and/or International standards by inserting one or more resistors 2, in the connections between the contacts 1 and 1 A and the end terminals D and C. This arrangement does not require an earth and its starting efficiency is such that full preheating of the lamp filaments is not necessary.
An alternative arrangement (not shown) is applicable where the base support or socket holder of the gas discharge lamp is earthed. The alternative arrangement comprises a generally dome-shaped conducting rubber grommet, or the like, which is attached to the earthed metal lamp support with the top of the dome just touching the outer surface of the discharge lamp. However, in this case it is necessary to place a conducting rubber contact approximately 20% of the way from each end of the lamp as there is a possibility that the supply voltage polarity may not be known. Both these arrangements are effective for use with those lamps that are known to be more difficult to strike, such as a 13 watt long lamp and a 40 watt, 12.2 metre lamp, and for prolonging the useful life of older lamps.

Claims

A method for determining the values of the capacitance of a capacitor (C1) and the impedance of an attenuator (R1) for a control circuit for a gas discharge lamp (L), said control circuit comprising input terminals (A, B) for connection to an A.C. power supply source (S) and output terminals (C, D) connected to the terminals of the lamp as well as components (D1, R2) of a starting circuit for the lamp connected across the output terminals, the method comprising connecting a variable capacitor box and a variable attenuator box in one or respective ones of series circuits between each of said input terminals and a respective one of said output terminals and effectively in series with each other when said control circuit is operating and supplying the said input terminals with A.C., characterised in that a larger than anticipated capacitance value of said capacitor (C1) and impedance of said attenuator (R1) are introduced by said boxes, the capacitance value being then progressively reduced until light output from said lamp falls suddenly and the marginally increased, so as to restore the earilier light output, and said impedance is then progressively reduced until a readily discernible flicker in the light output occurs and then marginally increased so as to eliminate flicker in the light.