GB2219896A

GB2219896A - Operating incandescent light bulbs to prevent surge currents

Info

Publication number: GB2219896A
Application number: GB8814043A
Authority: GB
Inventors: Thomas Riehemann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1988-06-14
Filing date: 1988-06-14
Publication date: 1989-12-20
Also published as: GB8814043D0

Abstract

A small current is supplied to an incandescent light bulb before it is fully switched on. This avoids the very high currents which arise with completely cold filaments and thus extends the life of the bulb. The small current may be a permanent steady current, Figure 1, or a current which rises from zero or other small value at switch-on to full value shortly thereafter Figures 2, 3. These methods of control may be implemented by applying current pulses to the bulb with the pulse width and/or amplitude being varied, (Figures 4 to 6). The methods may be applied to lamps in a vehicle lighting system which may incorporate a multiplexed control system with microcontroller (20), (Figure 8), adjacent the headlight units. <IMAGE>

Description

CIRCUIT FOR AND METHOD OF OPERATING INCANDESCENT LIGHT BULBS The present invention relates to a circuit for and a method of supplying current to incandescent light bulbs.

When an incandescent light bulb is switched on when it is cold, it has an electrical resistance which may be only a tenth or a twentieth of its resistance when warm.

The high initial current which results drastically reduces the operating life of the bulb and is detrimental to the operation of electronic components through which the current flows.

The present invention seeks to overcome or reduce the above problem.

According to a first aspect of the present invention, there is provided a method of operating an incandescent light bulb characterised in that, before supplying full current, a smaller current is supplied to the bulb.

An advantage of this method, which can be designated as "soft start", is that the working life of the bulb is considerably increased. This is of great importance when account is taken of the vast number of incandescent bulbs used in commercial and industrial applications.

In practice, extension of the working life means that bulb replacement programmes can be planned with longer intervals between bulb inspections or automatic replacements.

The smaller current may have a substantially constant value and may be supplied to the bulb whenever it is not lit. An advantage of this arrangement is that, when the bulb is subsequently switched on, it reaches full brightness more quickly. The smaller current may be a d.c. or a.c. current with a substantially constant amplitude which may have any level between a value just below that required to cause the light to glow, and relatively small value. Alternatively the current may be pulsed, with pulses of constant width which may be just narrow enough not to cause the light to glow, or narrower. In certain cases, a dim glowing of the light may be permitted, in which case the continuous current may have a higher average value.

In an alternative method the smaller current is only applied to the bulb immediately before switching on full current. The current may be supplied continuously in a manner gradually increasing from zero to full current.

Alternatively current pulses of constant amplitude and gradually increasing width from zero to full value may be supplied to the bulb. At full value the pulses may merge to form a continuous signal.

Whether the continuous current or pulsed current option is employed, the two methods may be combined is that the current is initially at a steady intermediate value but at switch-on is gradually raised to the full value.

According to a second aspect of the present there is provided a method of extending the life of incondescent light bulb by employing any of the methods of operation and set out in the preceding paragraphs.

According to a third aspect of the present invention there is provided a circuit for operating an incandescent light bulb comprising current supply means and switch means, characterised in that the current supply means is responsive to activation of said switch means to supply a full current to the bulb to light it and is arranged to supply a current with a steady and smaller value or average value at other times.

A second switch means may be provided for switching off the current supply means. This has the advantage of conserving energy when it is known that the bulb is not to be operated for an extended period of time.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figs. 1 to 6 illustrate six current diagrams of six respective methods of operating an incandescent light bulb; and Figs 7 and 8 show vehicle lighting systems employing the present invention.

In Fig. 1 a continuous steady current 1o is supplied to the lamp when it is not lit. This maintains the resistance of the filament of the bulb at a relatively high level. At time to when it is desired to light the bulb, the current is substantially instantaneously switched to its full level IF.

In addition, since current 1o is already flowing, transient effects are reduced so that full brightness is achieved more quickly than with a zero initial current. Ihls can be important when the bulb is required to be switched on for a short time of accurate length, e.g. when it is part of a flashing display.

In Fig. 2, no current flows until switch on at time to.

Current limitation then occurs and the current increases at a constant rate until time tl at which full current IF is reached In a modification, the current may increase non-linearly between 0 and IF.

In Fig. 3 the methods of Figs. 1 and 2 are combined, i.e. a stage in accordance with Fig. 1 is followed by a stage in accordance with Fig. 2.

In Figs. 4, 5 and 6 which correspond to Figs. 1, 2 and 3, but have pulsed currents, the pulses have a constant amplitude 1c but their widths vary. In Fig. 4 current pulses of a constant width do are supplied to the lamp when it is not lit. At time to, when it is desired to light the bulb, the pulse width is increased te full width aF. In Fig. 5 no pulses are supplied until switch on at time to immediately after which current limitation occurs, that is the pulse width increases gradually fron.

dB to dF. The pulse width may increase linearly or nonlinearly between these values. In Fig. 6, the methods of Figs. 4 and 5 are shown combined.

In the embodiments of Figs. 4, 5 and 6 the final pulse width dF may be such that a continuous current flows, i.e. there are no spaces or only infinitesimal spaces between the pulses. In another modification the pulse amplitude may vary instead of or in addition to the pulse width.

In all the examples a.c. may be used instead of d.c. In the embodiments of Figs. 1, 3, 4 and 6 the current before switch on may be selected to cause the bulb to glow dimly in certain applications where complete darkness is not essential; this has the advantage of less variation in the resistance of the filament and quicker attainment of full brightness.

A preferred field of use of the invention is in automotive lighting systems. Partial circuit diagrams of two such systems are shown in Figs. 7 and 8. Fig. 7 shows a conventional system in which the front left parking light bulb L1, the left dip light bulb L2 and the left full beam headlight bulb L3 and their righthand side counterparts Lll, L21 and L31 are connected via long conductors C1 to C9 to the vehicle battery voltage Ubatt. The conductors are routed via corresponding switches S1, S2 and S3 incorporated with the operating lever at the steering wheel. By supplementing the switches S1, S2 and S3 with suitable control means, appropriate smaller currents may be supplied to the lights before supplying full current, in accordance with the present investion.

A preferred system employing a multiplexing principle is shown in Fig. 8. At the operating lever, switches S1, S2 and S3 supply an immediately adjacent microcontroller 10 with the driver's commands.

The relevant data is fed over a single signal - current conductor CL to a second microcontroller 20 arranged in or immediately adjacent to the front left headlight unit (with a corresponding arrangerent at the right hand side, not shown). This contrasts with the arrangement of Fig. 7 in which, despite the proximity of lights L1, L2 and L3, separate conductors must be provided from the operating lever to the front bumper, each conductor having to carry a relatively high current.

Microcontroller 20 decodes the received data and converts it into control signals for the higher-current switches LS1 LS2 and LS3 for lights L1, L2 and L3.

These switches may be relays, bipolar- or MOS transistors etc. This arrangement may be used to implement the present invention simply by arranging the steady current supply in the switched-off condition of each light, to be controlled by a suitable program in microcontroller 20. This program can readily be incorporated in microcontrollers of existing systems.

Thus, there is no need for this current to be transmitted over the long conductor CL. This conductor CL, and the other conductors C1 - C3 and C10 - Cll can be controlled by conductors which are cheaper, lighter, occupy less space and last longer than the conductors C1 to C9 of Fig. 7.

At all times the soft-start method can be applied with only a very small current flowing through the conductors and switches at the operating lever, which provides a safer arrangement. In addition a so-called intelligent switching arrangement can be provided which has "softstart", current control and over current protection features at low cost.

Claims

1. A method of operating an incandescent light bulb characterised in that, before supplying full current, a smaller current is supplied to the bulb for a period of time.

2. A method according to claim 1, wherein the smaller current is a steady current supplied to the bulb when it is not lit.

3. A method according to claim 1, wherein the smaller current comprising pulses of constant amplitude, the width of the pulses being such that the current value of the pulsed current is less than that of the full current.

4. A method according to claim 1, wherein the smaller current increases gradually from zero to full current.

5 A method according to claim 1, wherein the smaller current comprises pulses of constant amplitude with gradually increasing width from substantially zero to full current width.

6. A method according to claim 4 or 5 wherein the gradual increase is at a constant rate.

7. A method comprising two successive stages, the first stage being in accordance with claim 2 or 3 and the second stage being in accordance with claim 4 or 5 respectively.

8. A method of operating an incandescent light bulb substantially as herein described with reference to each of Figs 1 to 6 of the accompanying drawings.

9. A method of extending the life of an incondescent light bulb by operating it in accordance with the method of any of claims 1 to 8.

10. A circuit for operating an incandescent light bulb comprising current supply means and switch means, characterised in that the current supply means is responsive to activation of said switch means to supply a full current to the bulb to light it and is arranged to supply a current with a steady and smaller value or average value at other times.

11. A circuit according to claim 9, wherein a second switch means is provided for switching off the current supply means.

12. A circuit for operating an incandescent light bulb substantially as herein described.

13. A vehicle lighting system employing the method of any of claims 1 to 8 and/or the circuit of claim 10,11 or 12.

14. A vehicle lighting system according to claim 11 comprising a plurality of control switches (S1, S2,S3) connected to a first control means (10), second control means (20) responsive to the output of the first control means and having a plurality of outputs (C10, Cll, C12) for controlling respective switch means (LS1, LS2, LS3) for respective bulbs (L1, L2, L3), said switch means being connected to a voltage source (Ubatt), and said second control means (20) being arranged to control said switch means (LSl, LS2, LS3) to provide said current with a smaller value or average value to said bulbs before supplying full current.

15. A vehicle lighting system substantially as herein described with reference to Fig.7 or Fig.8 of the accompanying drawings.