GB2288890A - A lamp economising circuit for hazard warning devices - Google Patents

Description

2288890 1 Title: HAZARD WARNING DEVICES

Description of Invention

This invention is concerned with hazard warning devices, particularly of the kind utilising an intermittently flashing warning lamp.

A conventional hazard warning device comprises a light source (bulb), an energy source (battery) and a control circuit to cause the bulb to be powered in a desired mode. ln one mode, the bulb is powered intermittently for a brief time interval (referred to as an operative time period) the length and frequency of which may vary. For example, In a "flashina'mode, the operative period (pulse duration) may be between 10 and 300 ms, and the pulse frequency may be between 50 and750 pulses per minute (ppm).

Three major problems have been identified in relation to hazard warning lamps of this kind.

First, the conventionally used energy source is a dry cell battery (nonrechargeable) which provides an output voltage during its working life which falls roughly into three zones, namely:- i) an initial high voltage level; a normal discharae voltaye; a terminal low discharae voltaae.

The initial level is characterised bv a voltaee which falls rapidly from 1 tz an initially high level to the normal working level over a short period of time, possibly less than 10% of the working life of the battery.

A hazard warning lamp uses a bulb which is designed to provide the required light output at the normal discharge level of the battery, and thus when the lamp is run with the new battery, more light is produced than is desired, which is enercy wasteful, and the bUlb is rLin at a hiuher volta-e level than is gy c 11) necessarv, resulting in a reduction in the life of the bulb.

1 1 Because of the requirement to provide a specified light output, the lamp cannot be run during the third period. despite the fact that the battery still contains useable electrical energy.

Tlis problem is reduced in accordance with this invention, by the characteristic, that the control circuit is operative to power the bulb during each operative period by the use of high frequency pulses, selected in accordance with the available voltage whereby to power the bulb during an operative period is at or close to the desired level.

If desired high frequency pulses of standard width may be utilised, the 0 1 desired power level being achieved b frequency modulation, so that when the c y 1 battery is exhibiting an initially high output voltage, the frequency at which pulses of standard width are applied to the lamp is reduced, whilst conversely when the battery exhibits a low power level preceding the end of its useful life, pulses are applied at a higher frequency.

Alternatively high frequency pulses may be applied to the lamp at a constant rate, determined by a time pulse produced by the control circuit of constant frequency, the width of each pulse (i.e. the time of its duration) being varied to ensure the application of the desired energy level during each operative time period.

In this manner more efficient use is made of the initial life of the batterv bv ensurine, that the bulb is not over powered, and further, more use may be made of the end of the life of the battery, to the point where the pulses may be substantially continuous over each operative period.

If desired the control circuit may comprise means to detect the available volta!e and to control the pulse veneration in accordance with the available voltage to ensure optimum light output.

Alternatively or in addition the control circuit may comprise a sensing means to sense the level of light produced by the bulb and to control the pulse generation to produce optimum light output.

1 v 1 i 3 Thus on detection of more light than is required, as may occur on the installation of a new batten, into the hazard warning lamp, the control circuit may respond to reduce the frequency of the pulses where frequency modulation is utilised, or the width of the pulses may be reduced, where pulse modulation is utilised, to reduce the energy applied to the bulb during each operative period, to the point where the desired quantity of light is being produced.

Under such circumstances feedback control is utilised to achieve optimum results.

Conversely on the detection of a falling light level, as may occur when the battery is towards the end of its normal working life, the control circuit may operate to increase the frequency or width of the pulses, to increase the energy applied to the bulb.

Whilst this aspect of the invention has been described above, either in relation to pulse frequency modulation or pulse width modulation, if desired of course a combination of pulse width modulation and pulse frequency modulation may be utilised if desired.

A second problem which exists in the operation of hazard warning devices is that, particularly during the intervals between each operative period, the filament of the bulb cools, and it has been determined that this repeated cooling and reheating is a major factor determining the life of a bulb.

According to this invention there is also provided a hazard warning. lamp where the Ocontrol circuit is operative prior to each operative period to apply power to the bulb at a level below that which produces any effective light but which is sufficient to cause an increase in the temperature of the bulb filament.

In this manner, i.e. by the use of a "prewarm", thermal shock to the bulb filament may be reduced to a significant extent.

The power applied to the bulb prior to each operative period may be a generally constant voltage at a level of between 10% and ")0% of the normal operatina voltage, or mav be a rising voltage.

C c 1 c -- 4 Preferably, however, the power applied to the bulb prior to each operative period is at the normal operating voltage, but in the form of high frequency pulses spaced to provide insufficient energy to cause the bulb to develop any significant light output. For example, during the "prewarm" period pulses of normal operating voltage may be applied having a pulse width of between 5% and 30% of the width required to power the bulb at the normal operating level.

Alternatively, pulses of standard width may be utilised, but may be applied during the "prewarm" period at a frequency insufficient to cause the bulb to provide any significant light output.

Whilst it is contemplated specifically that the "prewarm" period may commence shortly before commencement of an operative period (i.e. pulse "on"), e.g. at a point of ms prior to such commencement, if desired the bulb may be powered continuously between successive operative periods (between pulses) at a low transient voltage level, sufficient to reduce significantly the temperature to which the bulb filament falls on termination of the application of energy, and hence significantly to reduce the level of thermal shock to the filament on commencement of each operative period.

Similarly, such transient voltage may be continuous, or may be in the form of continuous high frequency pulses, the frequency and/or pulse width of which beine, insufficient to cause the bulb to cenerate any significant quantity of light.

A third problem is that most control circuitry is operative to produce a large overshoot current on initial application of power to the bulb, which is wasteful of energy and reduces the life of the bulb.

For example, on commencement- of a normal operative period, the current may rise briefly to a level of 10007)f the normal operating level.

This problem is reduced in accordance with this invention, by the characteristic, that the bulb is powered by voltage pulses. the voltage level, frequency and/or duration of which is selected to reduce current overshoot.

i 2k Thus, for each operative period the voltage pulses may rise in level over a relatively short period of time so as to smooth out current overshoot.

Alternatively voltage pulses of uniform voltage level may be utilised, the level of power being applied to the bulb being increased over a short period of time by increasing the frequency of application of the pulses, and/or increasing the width of the pulses.

There will now be given a detailed description, to be read with reference to the accompanying drawings, of a hazard warning lamp which is a preferred embodiment of this invention, having been selected for the purposes of illustrating the invention by way of example.

In the accompanying drawings- Figure 1 is a schematic view of the circuitry of the embodiment; Figure 2 is a graph indicating the typical fall off in the output of a dry cell battery; Figure 3 is a graph illustrating the application of voltage pulses at a standard frequency to a bulb of the warning lamp; Figures 4 and 5 illustrate an increase in pulse width to compensate for a reduced output voltage, utilising an unchanged frequency; Figure 6 illustrates an alternative form of powering, the lamp, utilising pulses of standard width, being applied at varying frequency; Figure 7a is a graph showing a typical current overshoot on powering up the bulb; Figure 8 illustrates the application of a "prewarm" period utilising c c) pulses applied at a fixed frequencv, but of varying pulse width: and Figure 9 illustrates schematically an effective form of hazard warning lamp.

6 The hazard warning lamp which is the preferred embodiment of this invention comprises a power source in the form of a dry cell battery 6 an output of which is applied to a band gap reference and comparator chain controller 7. An output is then applied to a main pulse generator 10, from which an output is applied to and/or decoder 12, for application through a driver FET 13 to the lamp bulb 14.

The output from a photodiode 16, measuring the intensity of light falling on the lamp, is fed to a comparator, at which it is compared to a desired light level, and an output is fed to a selector 18, operative to select pulse width.

The hazard warning lamp also comprises an encoder 20, by which the mode of operation of the lamp may be set.

In its simplest form the output from the battery 6 causes the generator 10 to generate pulses at a standard frequency, set by an oscillator 22 operating at 40 Khz, which in each operative period ("pulse on") of the bulb are applied by the aecoder 12 to the bulb. In the event that the output provided by such energisation of the bulb produces an output intensity which is too low, a signal fed backwardly from comparator 16 causes the width of the pulses produced by generator 10 to increase, until the required light output is produced.

Ficure 2 illustrates the ty 0, pical dry cell discharge curve, section A of which being the initial high output, falling rapidly at section B to the normal operative level, prior to commencement at section C of the more quickly falling voltage which marks the end of the useful life of the battery.

When the output of the battery corresponds to section A, each operative period may consist of pulses having pulse width p as shown in Figure 3, whilst durina section B of the battery output, pulse width c 1 -q as shown in Figure 4 may be utilised. During the terminal stage of the life of the battery, during section C, a pulse width r as shown in Figure 5 may he utilised, in which the pulses are almost constant in duration.

Notwithstanding. the use of the invention as described so far represents a significant increase in the life of the battery, firstly by utilising the energy 0 1 W 1 7 available during section A more efficiently, and secondly by allowing the energy which is none the less available during section C to he utilised, whilst maintaining the capability of meeting the light output parameters.

Alternative to the use of pulse modulations, in which pulses are generated at a standard frequency under the control of an oscillator, if desired frequency modulation may be utilised, and Figure 6 illustrates the way in which frequency modulation may be utilised during the generation of two pulses operative for a time period t the first X being produced by a high output voltage, corresponding to the use of a new battery, the second Y corresponding to a low output voltage, corresponding to the battery being at or close to the end of its useful life.

In addition to providing a means for utilising the output of a battery more efficiently, the use of the pulse control of the bulb also effects a reduction in current overshoot, as is shown in Figures 7a Figure 7a is a graph showing current in mA against time, showing the overshoot in current on commencement of an operative period to 340 niA, prior to the current reducing to the steady state - -.. Conversely by the use of powering the bulb by width modulated pulses, the overshoot is reduced significantly prior to the reduction to the steady state It will of course be appreciated that such overshoot reduction further increases the life of the bulb filament.

The hazard warning lamp which is preferred embodiment of this invention also comprises a "prewarm generator" 30, operative to apply power to the bulb filament for a short period of time prior to commencement of each operative period, to reduce thermal shock to the bulb filament.

Thus -prior to commencement to an operative period a series of voltage pulses are constant voltage but increasing pulse widw are applied to the bulb, insufficient to increase the temperature of the bulb filament to a level at which ariv sianificant quantity of illumination is produced, but sufficient to reduce the thermal shock applied to the filament on 8 commencement of the operative period. As will be seen, overshoot is filter reduced.

Figure 8 is that showing current with time, and illustrates that, in addition to the "prewarm" effect, the application of the lower level voltage prior to commencement of the period, "overshooC of current is further significantly reduced.

Alternative to the prewarm and/or the overshoot reducing voltage being applied to the lamp in the form of a voltage which is commenced a short time prior to commencement of the operative period, if desired the prewarm and/or overshoot reducing voltage may be applied to the bulb on a constant basis (i.e. operating throughout the cycle of flashing).

The current flowine, throuah the bulb is indicated by 1 in Figure 8, illustrating how current overshoot can be reduced to an effectively negligible level by appropriate pulse width modulation.

Additionally the hazard warning lamp which is the preferred embodiment of this invention comprises a warning LED, to indicate that the battery is close to the end of its operative life, and is in need of replacement, and in the event that the lamp fails.

Fiaure 9 illustrates an alternative form of hazard warning lamp, comprising a photosensitive switch A, an output of which is applied to a logic control and memory unit B, having mode select inputs.

A battery unit C comprises a voltage comparator, to which a reference input voltage is applied, and which produces an output signal comparing the output voltage of the batten, with the reference voltage. A signal is also applied in the event of a low batterv voltaee sisinal to an LED driver/contraflash, to provide visual indication of batten, status.

A master oscillator and sub-divider E generates a frequency signal 1 C7 which is applied to pulse width modulator F, to which an output from the lamp current sensina and control system is applied, to determine the optimum width of pulses required to feed the lamp through the lamp driver array.

c 1 A f 11 9 Durina the switched off time of the lamp the circuit will apportion enough power to keep the filament in a quiescent state between pulses in readiness for the next fully switched on interval, this facility reducing the transient current required by the filament as distinct from when it is switched on from cold, which in turn conserves the energy source and reduces fatigue of the lamp mechanically through the reduction of expnnsion and contraction of the filament.

All of the modes of operation and lamp electrical currents required of the hazard warning lamp will be integrated into an ASIC (application specific itkF-GXA',eD-circuit) and selectable by way of user programmable options, which can be achieved in several ways.

The ASIC will be constructed to operate from a wide range of power supply voltages, typically 3.4 volts to 14.0 volts, and when switched on the ASIC will automatically detect the supply voltage and store the voltage value, and the power supply source will be constantly monitored to detect a spent energy source. The detection of a spent energy source will trio er a drive circuit to flash an LED, tlg operable if the lamp is in flashing mode during the inoperative period. The lamp current feedback information will also be used to detect the broken light, and this will also flash the LED at a different rate. As the electrical current to the lamp is under high speed voltage it will be possible to use the ASIC to power a low voltage lamp:)r very high brightness LED from a higher voltage source, thus extending the time from fully charged energy source to spent condition.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination Of Such features, be utilised for realising the invention in diverse forms thereof.

J1 c Cl a irr's 1) A application specific int.-----rated circuit for the incandescent f41ament- larn.ps tnal-- w41' reduce the wasted electrical current at- the switching on 4.

C0r, t rol of 4enrush of 2) A application specific integrated circuit- as i- 1) allow custom external ad,ius4-.,-nen4..s for various m.cdes of in the control of light sources.

J.!n;z -!- - 1? 1 1. - - W! i 1 operation 3) A application specific integrated circuit as in l) that will automatically detect and signal a spent energy source.

4) A application specific integrated circuit as in 1) that will detect automatically, a open circuit lamp or light emitting diode and signal this status.

5) A application specific integrated circuit as in l.) that will operate on a wide voltage range and control lamps of differing operational voltage at their optimum operating conditions.

6) A application specific integrated circuit'. as in 1) that will conserve energy by way of high speed digital proportional switching of an energy source.

7) As in 1) a circuit that will conserve lamp life 8) A application specific integrated circuit, as in 1)that wil.11 conserve lamp and energy source life by -na,nta4n4!nc a quiescent current to the lamp during shcrt]a,-r-.p switched off periods.

9) A application specific integrated circutt as in 1) that will be more cost effective to produce than conventional circuitry.