LIGHTING SYSTEM

Abstract: A lighting system comprises a set of parallel connected flasher lamps (24, 26, 30, 34) driven by an astable multivibrator (14). A bridge circuit detects the current flowing in the lamps and comprises a first resistor (R7) connected in series with the lamps, second and third resistors (R9, R11) connected in series across the lamps, and a transistor (T3) having its base-emitter path connected between the junction of the first resistor and the lamps and the junction of the second and third resistors. The transistor (T3) control the frequency of the multivibrator (14) so as to provide an indication of lamp failure. In order to compensate for the variations of the resistance of the lamps with supply voltage, a zener diode (ZD1) and a fourth resistor (R13) are connected in series across the second and third resistors, and a fifth resistor (R10) is connected between the junction of the second and third resistors and the junction of the fourth resistor and the zener diode.

Description: DESCRIPTION "LIGHTING SYSTEM" The present invention relates to a lighting system including a plurality of parallel connected lamps in which an indication is provided in the event of lamp failure. In one known system comprising a number of lamps connected in parallel, the current flowing in the lamps is detected by a bridge circuit comprising a first resistor connected in series with the lamps, and a second and third resistor connected in series with each other and in parallel with the first resistor and the lamps. The bridge circuit controls a pilot lamp which provides an indication when one of the lamps has failed. This system operates satisfactorily with a constant supply voltage but there is a danger of malfunction if the supply voltage fluctuates as the resistance of the lamps varies with supply voltage. For example-at a low supply voltage, there is a danger that no indication will be provided when one of the lamps fails whilst at a high supply voltage there is a danger that an indication of failure will be provided when none of the lamps has failed. In another known system, in order to compensate for the variation of the resistance of the lamps with supply voltage, the first resistor comprises a resistor in which the resistance varies with temperature. However, the provision of such a resistor is relatively costly, and also such a resistor cannot be formed as part of an integrated circuit. It is accordingly an object of the present invention to provide a new or improved lighting system in which an indication is provided when one of the lamps has failed and in which the above mentioned disadvantages are overcome or reduced. According to the present invention there is provided a lighting system comprising a plurality of parallel connected lamps and a bridge circuit for detecting the current flowing in the lamps, the bridge circuit including a voltage bias means to compensate for the variation of the resistance of the lamps with the supply voltage, and the bridge circuit being used to provide an indication of lamp failure. The voltage bias means, which may be provided cheaply, reduces the danger of no indication being provided when one of the lamps fails at a low supply voltage or of an indication of failure when none of the lamps has failed at a high supply voltage. The present invention will now be described in more detail by way of example with reference to the accompanying drawings in which Figure 1 is a circuit diagram of a vehicle flasher lamp system embodying the present invention; and Figure 2 is a circuit diagram of a vehicle headlamp and sidelamp system embodying the present invention. Referring now to Figure 1, the vehicle flasher lamp system comprises negative and positive supply lines 10, 12 and a drive means in the form of an astable rmrltivibrator, indicated by a dashed line 14, connected between the lines 10, 12 and also connected to the common terminal 16 of a direction indicator switch 18. The indicator switch 18 further comprises a left hand terminal 20 and a right hand terminal 22, a pair of left hand vehicle flasher lamps 24 being connected in parallel between the terminal 20 and line 10 and a pair of right hand vehicle flasher lamps 26 being connected in parallel between the terminal 22 and line 10. A first pilot lamp 31 is connected between terminals 20 and 22. The system shown in the drawing is intended for use with a vehicle and detachable trailer combination and includes left and right hand trailer flasher lamp 30, 34 which may be connected to the terminals 20 and 22 through sockets 32, 36. The multivibrator 14 comprises a p-n-p transistor TI, the emitter of which is connected to line 12 and the collector of which is connected through series connected resistors R1 and R2 to the line 10. The base of transistor T1 is connected through a resistor R3 to the line 12 and through a resistor R4 to the terminal 16. The junction of resistors R1 and R2 is connected to the base of an n-p-n-transistor T2 the emitter of which is connected to line 10 and the collector of which is connected through the coil 38a of a relay 38 to the line 12. The terminal 16 is connected through the normally open contacts 38b of relay 38 and a resistor R7 to the line 12. The collector of the transistor T2 is connected to one end of a potential divider chain comprising a pair of resistors R5 and R6 connected in series, and the other end of the potential divider chain is connected to the collector of a p-n-p transistor T3. The junction of resistors R5 and R6 is connected through a capacitor C1 to the base of transistor T1 and the collector of transistor T2 is connected to its base by a capacitor 02. The flasher system further includes a first and second bridge circuit. The first bridge circuit includes the resistor R7 and the transistor T3 and further includes a diode D1, the anode of which is connected to the line 12 and the cathode of which is connected through a preset resistor R9 and a fixed resistor R11, connected in series, to the line 10. The junction of resistors Rg and R11 is connected to the base of transistor T 3 and the emitter of transistor T3 is connected to the junction of resistor R7 and the contacts 38b. The junction of resistors R9 and R77 is also connected through a resistor R10 and a resistor R13 connected in series to the line 10. The first bridge circuit also has a zener diode ZD1 having its anode connected to the junction of resistor R10 and R13 and its cathode connected to line 12. The second bridge circuit includes the diode Dl, the zener diode ZD1 and the resistor R13 and further comprises a preset resistor R14 having one end connected to the cathode of the diode D1 and the other end connected through a resistor R13 to the line 10. The junction of resistors R14 and R13is connected through a resistor R12 to the anode of zener diode ZD1. The junction of resistor R14 and R15 is also connected to the base of a p-n-p transistor T4, the emitter of which is connected to the junction of resistor R7 and contacts 38b The system further includes a p-n-p transistor T5, the emitter of which is connected to the emitter of transistor T4, and the collector of which is connected througt a resistor R17 and a second pilot lamp 40, connected in series to the line 10. The base of transistor T3 is connected to the collector of transistor T4 and also to the anode of diode D2. The cathode of diode D2 is connected through a resistor R16 to the line 10 and through a capacitor C3 and a resistor R8, connected in series, to the terminal 16. In the first bridge circuit, the transistor T3 compares the voltage drop across resistor R7 which depends on the current flowing in the flasher lamps, with the voltage drop across the resistor R9 and the value of the resistor R9 is adjusted so that the transistor T3 is conductive at all times except when two or more lamps are working and energised. The zener diode ZD1 and resistors R10 and R13 are provided to compensate for the non-linear resistance characteristics of the flashing lamp as will now be explained. In the range of normal working voltage the voltage current curve for the flashing lamps is approximately a straight line originating at a voltage-V . In a 12 volt system, V has a value of approximately 10 0 volts, and, in a 24 volt system, V0 has a value of approximately 23w volts. In order to compensate for this non-linear characteristic, the resistor R10, zener diode ZD1 and resistor R13 are arranged as a voltage bias means to simulate one end of the resistor R11 being connected to a voltage -V with reference to the 0 line 10. If R11 were connected to -V0 then the current in resistor R9, iR9, would be given approxiamtely by: iR9 = (V5 + V0)/ R11 where Vss is the supply voltage. However, in the circuit shown, the current 9 is given approximately by: iRg = V + ¯ R11 R10 where Vz is the breakdown voltage of the zener diode ZD1. Thus in order to compensate for the non-linear resistance characteristics of the flasher lamps, it is only necessary to arrange that: V V z o R11 In the case of the second bridge circuit, the resistor R14 is adjusted so that the transistor T4 is conductive at all times except when three lamps are working and energised. In order to compensate for the non-linear resistance characteristics of the flasher lamps, the value of the resistors R12 and R15 are chosen so that: V Y = 0 R12 R15 The operation of the system will now be described firstly with trailer lamps 30 and 34 disconnected. If either the left hand or right hand vehicle flasher lamps 24 or 26 are connected to the multivibrator 14 through the switch 18 and both lamps are working, then during the on periods when the lamps are energised, both transistors T1 and T2 will be on, the coil 38a will be energised and the contacts 38b will be closed. Also, as explained above, the transistor T3 will be off. The capacitor C1 will be charged with its positive plate held at a voltage.which is less than the supply line 12 by an amount V be(T1)' the base-emitter satuation voltage drop of transistor T1, and with its negative plate falling towards a voltage which is greater than the supply line 10 by an amount Vce(T2) the collector-emitter saturation drop of transistor T2, until the base current of transistor T1 falls below the level required to keep this transistor turned on. At this stage transistor T1 will turn off, thereby turning off transistor T2, de-energising coil 38a, opening contacts 38b, extinguishing the lamps) and the off period will commence and, as explained above, when the lamps are extinguished, the transistor T3 will be conductive. The negative plate of capacitor C1 will now be connected to the positive supply line 12 through resistor R5 and transistor T3 as well as through resistor R6 and coil 38a with the result that, at the beginning of the off period, the positive plate of capacitor C1 will be at a voltage slightly greater than the voltage of the positive supply line 12 thereby holding transistor T1 off. However, during the off period, capacitor C1 will discharge with the voltage of the positive plate of capacitor C1 falling towards the voltage Vs. R3 below the supply line 12, where (R3 + R4) V6 is the supply voltage and R3 and R4 are the resistance values of resistors R3 and R4. When the voltage of the positive plate of capacitor C7 falls below the voltage of the supply line 12 by an amount Vbe(T1), the transistor T1 and hence transistor T2 will turn on and the on period will commence again and the cycle will be repeated. If instead, one of the sets of lamps 24 and 26 is connected to the multivibrator 14 through switch 18 and one of the lamps is not working, the system will operate as follows. During the on period, both transistors T1 and T2 will be on and the capacitor C1 will charge with the positive plate again held at a voltage which is less than the voltage of the supply line 12 by an amount Vbe(T1). However, as explained above, transistor T3 will be conductive and so the resistors R5 and R6 will function as a potential divider and the negative plate of the capacitor C1 will fall towards a voltage which is greater than the supply line 10 by an amount approximately equal to V . R6 (R5 + R6) As a result, the base current of transistor T1 will fall below the level required to keep this transistor turned on more quickly than in the case when both lamps are operating and so the on period will be shorter and the capacitor C1 will be charged to a lower voltage. At the beginning of the off period, the negative plate of capacitor C1 will again be connected to the positive line 19 through resistor R5 and transistor T3 as well as through resistor R6 and coil 38a, the positive plate of capacitor C1 will be at a voltage greater than the supply line 12, though by a smaller amount than when both lamps are working, and it will then fall towards the voltage Vs. R3below supply line 12 as before. R3 + R4 However, as capacitor C1 was charged to a lower voltage during the on period, the positive plate of capacitor C1 will fall below the voltage of the supply line 12 more quickly than is the case when both lamps are working and so the off period will be shorter. Thus, when one of the lamps 24 or 26 is not working, the flashing frequency will increase thereby providing an indication to the driver of a vehicle in which the system is installed that one of the lamps has failed, since the pilot lamp 31 will flash with the flasher lamps. The frequency of flashing will be mainly determined by the resistance values of resistor R3, R4, R5, R6 and the capacitance value of capacitor C1. Thus, these elements form part of the timing circuit of the multivibrator 14 and, conveniently, their values are chosen so that the frequency of flashing approximately doubles when one lamp has failed. Capacitor C2 performs the dual purpose of limiting the voltage which appears across the collectoremitter junction of transistor T2 at the beginning of the off period, and also provides a direct path for positive going transient voltages which may appear on supply line 12 such that both transistors T1 and T2 turn on. When on, the collector-emitter voltage of these transistors is held at a low value which protects thaL during the positive voltage transient. The operation or the system will now be described with the trailer lamps 30 and 34 connected to switch 18 through sockets 32 and 36. If either the left hand or right hand lamps are connected to the rultivibrator 14 through switch 18 and all three lamps connected are working, then during each on period the transistor T4 will be non-conductive as explained above. Consequently, transistor T5 will be rendered conductive and pilot lamp 40 will be energised. During each off period, the transistor T4 will be conductive, thereby rendering transistors T5 non-conductive and preventing energisation of pilot lamp 40. Thus, with all three lamps working, the pilot lamp 40 will flash with the flasher lamps thereby providing an indication that all three lamps including the trailer lamp are working. However, if one of the flasher lamps is not working, then the transistor T4 will remain conductive during the on periods, thereby preventing energi6ation of the pilot lamp 40. The function of capacitor C3 will now be explained. When the flasher lamps are first energised at the beginning of each on period, the current rises rapidly to a high value for approximately 50 milliseconds. In the absence of capacitor C3, and with only two lamps working, this could result in transistor T5 being rendered conductive for approximately 50 milliseconds at the beginning of each an period and pilot lamp 40 providing a false indication that all three flasher lamps are working. However capacitor C3 is charged during each off period through transistor T4 and the flasher lamps, and the value of capacitor C3 and resistor R16 are chosen so as to hold transistor T5 non-conductive for approximately 50 milliseconds at the beginning of each on period. The system shown in Figure 1 may be modified by providing a switch to connect all the lamps 24, 26, 30 and 34 to the astable multivibrator 14 to provide a hazard warning, and then the lamps will flash at the normal flashing frequency if two or more lamps are working, and the pilot lamp 40 will flash if three or more lamps are working. Referring now to Figure 2, the vehicle headlamp and side lamp system comprises a positive supply terminal 50 and a negative supply terminal 52 connected to a negative supply line 54. The positive supply terminal 50 is connected through a sidelamp switch 56 to a supply rail 58. The supply 56 is connected to one end of a resistor R107, the other end of which is connected through four parallel connected sidelamps 60 to the negative supply line 54. The supply rail 58 is also connected through a headlamp switch 64 to a supply rail 66. The supply rail 66 is connected to one end of a resistor R207, the other end of which is connected through two parallel connected headlamps 68 to the negative supply line 54. The system further includes a first and a second bridge circuit. The first bridge circuit includes the resistor R107 and further includes a diode D101 the anode of which is connected to the supply rail 38 and the cathode of which is connected through a preset resistor R109 and a resistor R111, connected in series, to the negative supply line 54, and a zener diode ZDlO1, the cathode of which is connected to the supply rail 58 and the anode of which is connected through a resistor R110 to the Junction of resistors R109 and R111 and through a resistor R113 to the negative supply line 34. The junction of the resistors R109 and Rill is connected to the base of a p-n-p transistor T6, the emitter of which is connected to the junction of the resistor R107 and the sidelamps 60 and the collector of which is connected through a resistor R117 and a pilot lamp 62, connected in series, to the negative supply line 54. The second bridge circuit includes the resistor R207 and further includes a diode D201, the anode of which is connected to the supply rail 66 and the cathode of which is connected through a preset resistor R209 and a resistor R2t1 connected in series to the negative supply line 54, and a zener diode ZD201 the cathode of which is connected to the supply rail 66 and the anode of which is connected through a resistor R210 to the Junction of resistors R209 and R211 and through a resistor R213 to the negative supply line 54. The junction of resistors R209 and R211 i8 connected to the base of a transistor T7, the emitter of which is connected to the junction of resistor R207 and the headlamps 68 and the collector of which is connected through a resistor R217 and a pilot lamp 74 connected in series, to the negative supply line 54. In the first bridge circuit, the transistor T6 compares the voltage drop across the resistor R107, which depends on the current flowing in the sidelamps 60, with the voltage across the resistor R109, and the value of the resistor R109 is adjusted so that the transistor T6 is non-conductive at all times except when the switch 56 is closed and one or more of the lamps 60 are not working. In the second bridge circuit, the transistor T7 compares the voltage drop across the resistor R207, which depends on the current flowing in the headlamps 68, with the voltage drop across the resistor R209 and the value of the resistor R209 is adjusted so that the transistor T7 is non-conductive at all times except when the switches 56 and 64 are closed and one or both of the headlamps 68 are not working. As in the case of the flasher lamp described with reference to Figure 1, the voltage-current curve of the sidelamps 60 and of the headlamps 68 is approximately a straight line originating at a voltage -v0. In order to compensate for these non-linear characteristics, in the case of the first bridge circuit, the values of the resistors R110 and R111 are chosen so that: V V z 0 R110 TL111 In the case of the second bridge circuit, the values of the resistors R210 and R211 are chosen so that: Vz = Vo R210 R211 In operation, when the switch 56 is closed, the sidelamps 60 will be energised and, providing all the sidelamps 60 are working, the transistor T6 will be non-conductive and so the pilot lamp 62 will not be energised. However, if one or more of the lamps 60 is not working, the transistor T6 will be rendered conductive thereby energising the pilot lamp 62 and providing an indication that one or more of the lamps 60 is not working. If the switch 64 is also closed, the headlamps 68 will be energised andt providing all the headlamps 68 are working, the transistor T7 will be non-conductive and so the pilot lamp 70 will not be energised. However, if one or both of the headlamps 68 is not working, the transistor T7 will be rendered conductive thereby energising the pilot lamp 70 and providing an indication that one or more of the headlamps 68 have failed. As may be readily appreciated the majority of the components of the various bridge circuits described hereinbefore can be formed as components of an integrated circuit. For example, in Figure 2 the diodes D101 and D201, the zener diodes ZD101 and ZD202, the transistors T6 and T7 and the resistors R109, R110, Rill, R113, R117, R209, R2102 R211, R213, and R217 can be formed as components of an integrated circuit.

Claims: Claims: 1. A lighting system including a plurality of parallel connected lamps (24, 26, 30, 34; 60; 68) and a bridge circuit (R7, R9, R11; R7, R14, R15; R107, R109, Rill; R207, R209, R211) for detecting the current flowing in the lamps, said bridge circuit being used to provide an indication of lamp failure, and characterised in that said bridge circuit includes a voltage bias means (ZD1, R10, R13; zDi, R12, R13t ZD101, R110, R113; ZD201, R210, R213) to compensate for the variation of the resistance of the lamps with supply voltage, 2. A lighting system according to claim 1. in which said bridge circuit includes a first resistor (R7; R107; R207) connected in series with the lamps, and a second resistor (R9; R14; R1O9; R209) and a third resistor (rip; R15; Rill; R211) connected in series with each other and in parallel with the lamps and first resistor, and characterised in that the voltage bias means is connected to the junction of the second and third resistors. 3. A lighting system according to claim 2 characterised in that the voltage bias means includes a zener diode (ZD1; ZD101; ZD201) and a fourth resistor (R13; R113; R213) connected in series with each other and in parallel with the second and third resistors, one terminal of the zener diode being connected to one terminal of the first resistor, and a fifth resistor (R13; R113; R213) connected between the junctions of the second and third resistors and the junction of the fourth resistor and the zener diode. 4. A lighting system according to claim 2 or claim 3 characterised in that the bridge circuit includes a transistor (T3; T4; T6; T7) having its base-emitter path connected between the junction of the first resistor and the lamps and the junction of the second and third resistors. 5. A lighting system according to any one of the proceeding claims characterised in that the lamps are flasher lamps (24, 26, 30, 34) and in that the system includes a drive means (14) for driving the lamps, the drive means including a timirwg circuit (R3, C1), the timing action of which is controlled by the bridge circuit whereby the frequency of flashing varies in the event of lamp failure.

Inventors: NICOL, THOMAS
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1979
Main classification: H05B37
IPC codes: B60Q11, H05B37