US 3781594 A
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United States Patent [191 Roberts Dec. 25, 1973 LAMP CIRCUITS  Inventor: Frank B. Roberts, Gateshead,
England  Assignee: Victor Products (Wallsend) Limited,
Wallsend, Northumberland, England  Filed: Nov. 9, 1972  Appl. No.: 304,895
 Foreign Application Priority Data Nov. 18, I97] Great'Britain 53,633/71  US. Cl 315/98, 315/105, 315/283, 3l5/DIG. 5  Int. Cl. H05b 41/14  Field of Search 3l5/DlG. 2, DIG. 5, 3 15/98, I05, 283
 I References Cited FOREIGN PATENTS OR APPLICATIONS 138,941 8/1946 Australia 3l5/DIG. 5 709,l00 8/l94l 'Germany 3l5/DIG. 5
Primary Examiner-Rudolph V. Rolinec Assistant ExaminerWm. H. Punter Attorney-Solon B. Kemon et al.
[ 5 7 ABSTRACT A circuit for operating a fluorescent lamp, particularly in hazardous conditions, comprises two parallel legs including inductive and capacitive reactances, respectively, and a third leg in series with the other legs and including a reactance which forms, with one of the parallel legs, a series circuit which is substantially tuned to the supply frequency. A heater of the lamp is connected across a relatively low value resistance in that parallel leg which forms part of the series tuned circuit, and the lamp electrodes are connected across a relatively high value resistance in the other parallel leg. The tuned circuit provides the necessary voltage for firing the lamp, the reduced impedance of the lamp when fired then damping the tuned circuit. The low resistance of the lamp heater normally shunts the resistance in the tuned circuit, but if the lamp is removed, or the heater becomes open-circuited, that resistance clamps the tuned circuit. A second lamp heater may be energised from a transformer in one of the parallel legs.
5 Claims, 2 Drawing Figures LAMP CIRCUITS This invention relates to lamp circuits for operating fluorescent lamps in atmospheres which are hazardous because they contain flammable mixtures of gases, vapours and/or dusts.
Where electrical apparatus is to be used in situations where such atmospheres are present or may be present,
it is essential that the apparatus is designed so that the gases, etc., cannot be ignited outside the apparatus by sparks or high temperatures caused by either normal or faulty operation of the apparatus. One design concept which conforms with this requirement is that of intrinsic safety. The apparatus is designed to operate at such low energy levels that any sparks which may occur The circuit should operate safely even if the lamp is broken or removed and no component or combination of components in the lamp control circuit should store more than a given amount of energy.
-In the past, it has beennecessary, in hazardous situations, to use cold-cathode fluorescent lamps because the heater of a heated-cathodelamp would present a very considerable danger if the lamp envelope were broken. Consequently, the required initial voltage to cause the lamp to strokehas been high, thereby considerably hampering the design of circuits which conform with intrinsic safety requirements.
It is an object of the present invention to provide a fluorescent lamp operating circuit which operates safely even if the lamp is removed or broken.
According to the invention, an electric circuit for supplying power from a supply to a fluorescent lamp includes two parallel legs containing inductive and capacitive reactances, respectively; a third leg connected in series with said parallel legs and containing a reactancewhich forms, with the reactance in one of the parallel legs, a circuit which is at least approximately tuned to the supply frequency; high resistance circuit means connected in the other parallel leg, across which means the lamp electrodes canbe connected; and, in said one parallel leg, low resistance circuit means across which heater means forthe lamp can be connected.
Preferably each of'said legs also contains an element having a very low resistance value, connected in series with the respective reactance.
The power supply frequency is preferably at or around the lower end of the KHz range, e.g. up to about Referring now to FIG. 1 of the drawing, a circuit for supplying electrical power from a supply, for operating a heated-cathode fluorescent lamp, comprises two parallel legs 12 and 13, the leg 12 comprising a capacitor 14 connected in series with a resistor 15 of low resistance value, and a high resistance 16 across which the electrodes of a fluorescent tube 17 can be connected.
The leg 13 comprises an inductive reactance 18 connected in series with a low resistance 19 and a resistance 20 having a value which is higher than the impedance of the remainder of the leg at the supply frequency, but considerably lower than the resistance of the resistor 16. The legs 12 and Bare connected in series with a leg 21 comprising a capacitor 22 in series with a low resistance 23. The capacitor 22 and the inductance 18 are arranged to form a series resonant circuit at the frequency of the supply from a source 24 which is connected across the circuit.
Although,in the past, cold-cathode lamps have been used for the reasons specified above, lamps are available having heaters and electrodes of such low thermal capacity that the heat would be quickly dissipated, if the lamp envelope were broken and thereby allowed the hazardous atmosphere to come into contact with the heaters and electrodes. The present embodiment makes use of such a lamp having a single heater 25 connected across the resistor 20.
For a source 24 supplying a voltage of volts at a frequency of 26 KHz, suitable component values are as follows: capacitors l4 and 22, 0.0047 pf; resistors l5, l9 and 23, 150 Q; resistor 16, k0; inductance l8, 8 mH; resistor 20,7 kfl.
Inoperation of the circuit, when the supply is first connected to the circuit, the inductor 18 and the capacitor 22 form a series resonant circuit which isv damped only by the resistors 19 and 23 in the legs 13 and 21. The leg 12 at this stage presents negligible damping because of the high resistance of the resistor 16, the lamp 17 having a high impedance until it strikes. Because the resistance of the heater 25 is very low (say 15 ohms) compared with the resistance of the resistor-20, the latter resistor has negligible effect on the tuned circuit.
A voltage of, say, 220 volts is generated in the circuit and this causes the lamp 17 to fire. The lamp then presents a low impedance across'the resistor 16 and this damps the tuned circuit, causing the voltage to fall. The voltage across the resistor 16 remains at the normal running voltage of the lamp 17,
The capacitor 14 serves two purposes, namely it helps to limit the lamp current and it also acts as a power factor correcting capacitor.
The circuit of the present invention has the following very important advantages over the known lamp circuits. If any one component fails, the circuit is capable of accommodating the fault without the supply being overloaded. Since at least one of the electrodes of the lamp 17 is heated, the initial voltage required to cause the lamp to strike is much reduced compared with cold-cathode tube circuits. This is clearly advantageous if the lamp fails to strike. Furthermore, if the lamp is removed from its fitting, or if the heater 25 becomes open-circuited, the resistance of the heater no longer appears across the resistor 20. This latter resistor therefore causes a large reduction in the voltage which the tuned circuit generates. The circuit is therefore intrinsically safe irrespective of whether it is operating normally or whether the tube is faulty or disconnected.
In conventional lamp circuits, if the leads to the lamp electrodes become shorted together, sparks having sufficiently high energy levels to ignite the surrounding gas, etc. would be produced. In the present circuit, the lamp leads, references A, B and C in the drawing, may be subjected to any of the faults set out in the following table, without those faults producing spark energy beyond a designed magnitude. The supply on these leads is, therefore, intrinsically safe.
Lead A can make and break to lead B.
Lead A can make and break to lead C.
Lead B can make and break to lead C.
Leads A and B shorted together can make and break to lead 0.
Leads A and C shorted together can make and break to lead B.
Leads B and C shorted together can make and break to lead A.
Modifications to the above described circuit can clearly be made without departing from the scope of the present invention. For example, the component values and the frequency mentioned are given merely as examples and clearly any other suitable values may be used. Although the resistor 16 is connected at the lower end of the leg 12, it could be moved to the top of that leg, but it is considered to be advantageous to connect it at the position shown, because neither electrode is then connected directly to the supply. In other words, the lamp electrodes are both kept as far as possible from the supply lines.
The resistor 16 and the lamp could be moved to the leg 13, but then the resistor 20 would have to be moved to the leg 12, and the capacitor 22 would have to be replaced by a corresponding inductance, this inductance and the capacitor 14 then forming the tuned circuit.
FIG. 2 of the drawing shows a circuit which is almost identical to that in FIG. 1, but is slightly modified to supply power to an alternative type of lamp having two heaters 25 and 26, one for each electrode. Power is supplied to the heater 26 by connection of the heater to a secondary winding 27 of a transformer 28,0f which the inductor 18 may form the primary winding.
Although in the above circuits the reactances in the series legs have been described as forming a tuned circuit, in some applications merely approximate tuning of the circuit to the supply frequency may be sufficient The circuit, together with the power supply 24, may be contained within a flameproof enclosure.
1. An electric circuit for supplying power from an alternating current supply to a fluorescent lamp including two parallel legs containing inductive and capacitive reactances, respectively; a third leg connected in series with said parallel legs and containing a reactance which forms, with the reactance in one of the parallel legs, a circuit which is at least approximately tuned to the supply frequency; high resistance circuit means connected in the other parallel leg, across which means the lamp electrodes can be connected; and, in said one parallel leg, low resistance circuit means across which heater means for the lamp can be connected.
2. A circuit as claimed in claim 1, in which at least one of said legs also contains an element having a low resistance value, connected in series with the respective reactance.
3. A circuit as claimd in claim 1, in which the parallel leg containing said low resistance circuit means also includes transformer means for supplying power to a second heater means for the lamp.
4. A circuit as claimed in claim 3, in which the parallel leg containing the low resistance circuit means also contains said inductive reactance, which inductive reactance is provided at least partially by the transformer.
I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION "3,781,594 Dated December 18, 1974 Inventor s) FRANK B ROBERTS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 2', line 53, after "capacitor." insert --Eurthermore, with the low impedance of the fired lamp across the resistor 16, the legs 12 and 13 together form a parallel circuit which is at least approximately tuned to the supply frequency.--
I Column 2, last line', before "tuned" insert series--.-
Column 4, first line, before "tuned" insert ---series---.
Add the following claim """75. A circuit as claimed in Claim 1, in which when said high resistance circuit means is shunted by the low impedance of an operating fluorescent lamp, said parallel legs' together form a circuit which is at least approximately tuned to said supply frequency.-
Signed and sealed this 4th day of February 1975.
McCOY M. GIBSONJR. Attesting Officer C. MARSHALL DANN Commissioner of Patents