GB2369256A

GB2369256A - Power supply disconnection using magnetically operated switch

Info

Publication number: GB2369256A
Application number: GB0028233A
Authority: GB
Inventors: Gerben Simon Hoeksma
Original assignee: Hubbell Lighting Ltd
Current assignee: Hubbell Lighting Ltd
Priority date: 2000-11-18
Filing date: 2000-11-18
Publication date: 2002-05-22
Anticipated expiration: 2020-11-18
Also published as: GB0028233D0; DE10156168A1; GB2369256B

Abstract

An electrical apparatus, particularly a light fitting 1, includes a holding portion 6, 7, 11 and a protective cover 2. The holding portion holds power supply circuitry. The circuitry includes a switch 14 switchable by a permanent magnet 13. A magnetic shield 16 mounted to the cover and interposed between the permanent magnet and switch is removed on opening the cover causing the switch to switch so as to break the power supply. The switch may be a reed switch. In an alternative embodiment, the permanent magnet is mounted on the cover such that the switch is operated according to the position of the cover.

Description

AN ELECTRICAL APPARATUS, A LIGHT FITTING AND METHODS OF BREAKING POWER SUPPLY THEREIN TECHNICAL FIELD The invention relates to an electrical apparatus, a light fitting and a methods of breaking power supply thereto, for example for removal or replacement of parts thereof.

BACKGROUND In light fittings particularly those used in hazardous environments it is necessary to replace lamps, i. e. light bulbs or tubes, or other items such as batteries, without inducing sparks. Sparks cause flammable materials to ignite causing explosions.

A conventional solution for the spark free replacement of lamps or batteries in hazardous areas is the use of an electromechanical driven switch which acts to deenergize the circuits to the lamps or batteries. The electromechanical driven switch has contacts which are opened by a positive mechanical drive. This both opens the contacts and also enables opening of a cover so as to enable access to the lamps or battery which are to be replaced.

This known approach requires a complex (hence expensive) mechanical construction. In the case of explosionresistant fluorescent light fittings, such a construction includes a multipole mechanical switch, that must be able to withstand the high inrush currents that are flowing if electrolytic smoothing capacitors in its electronic power

supply unit are being charged, for example when switched to an input voltage source, such as an AC mains network.

In such fittings, it is not the circuit to the lamps that is interrupted, but the low frequency AC source voltage feeding the high frequency lamp power supply unit. Since the high frequency power supply unit contains one or more electrolytic capacitors, mostly containing several Joules of energy, in this known approach it is most important to ensure that the energy stored in the capacitors is not converted into high frequency lamp current at the moment the lamps are disconnected. This is problematic.

STATEMENTS OF INVENTION In a first aspect, the invention provides a light fitting comprising a holding portion for a light bulb or tube, and a protective light transmitting cover, the holding portion including power supply circuitry, the circuitry including a switch switchable between an open state and a closed state by magnetic force from a permanent magnet, a magnetic shield being interposable between the switch and permanent magnet so as to cause the switch to switch thereby making or breaking power supply to the light bulb or tube, the magnetic shield being mounted on the cover so as to be at least partially removed from between the magnet and switch so as to cause the switch to switch upon the cover being at least partially removed so as to break the power supply to the light bulb or tube.

In preferred embodiments of the invention, the sensing device consists of a reed switch, being actuated by the field of the permanent magnet, upon removal of the shield plate fixed to the cover or similar device, the shield

plate shielding the magnetic field of the permanent magnet in the normal operating condition of the apparatus.

Preferably, the reed switch shorts the current-limited power supply to a driver circuit which generates the drive pulses for the semiconductor switches used in transmitting power to the light bulb or tube. In alternative embodiments the reed switch preferably generates a signal that blocks drive pulses generated by the driver circuit used for generating drive pulses to the semiconductor switches. In the latter case the generation of drive pulse (s) can be blocked by two independent paths, which is particularly useful if the suppression of drive pulses relies on non fail-safe parts.

In a second aspect, the invention provides electrical apparatus including a holding portion for electrical circuitry and a protective cover, the electrical circuitry being in use covered by the protective cover, the holding portion and cover being relatively moveable between a closed configuration and an open configuration so as to allow access to the electrical circuitry, the circuitry including a switch which switches from a first state to a second state by magnetic force from a permanent magnet upon the cover being moved relative to the holding portion towards an open configuration so as to break supply of power to a predetermined part of the circuitry.

It will be seen that for spark free replacement of parts, drive pulses to semiconductor switches contained in the or each converter of a power supply apparatus delivering current to the parts that can be replaced, are blocked by a sensing device responsive to the position of a cover that must be moved to get access to the parts to be

replaced. The sensing device comprises the reed switch. This enables lamp or battery replacement without the risk of sparking, by fail safe cutting off of current to a lamp or to and from a battery. Such a method can be used in fluorescent lamp luminaires and fluorescent luminaires with emergency function, including those containing batteries and battery chargers.

In a third aspect, the invention provides a method of breaking power supply to a light bulb or tube of a light fitting comprising a holding portion for the light bulb or tube and a protective light transmitting cover, the holding portion including power supply circuitry, the circuitry including a switch switchable between an open state and a closed state by magnetic force from a permanent magnet, and the cover carrying a magnetic shield interposable between the switch and permanent magnet, the method comprising at least partially opening the cover so as to enable access to the circuitry, the magnetic shield being at least partially removed from between the magnet and switch so as to cause the switch to switch so as to break the power supply to the light bulb or tube.

In a fourth aspect, the invention provides a method of breaking the power supply to a predetermined part of electrical circuitry held by a holding portion of an apparatus, the apparatus also having a protective cover, the electrical circuitry being in use covered by the protective cover, the holding portion and cover being relatively moveable between a closed configuration and an open configuration so as to allow access to the predetermined part of the electrical circuitry, the circuitry including a switch which switches from a first

state to a second state by magnetic force from a permanent magnet upon the cover being moved relative to the holding portion towards an open configuration, so as to break supply of power to the predetermined part of the circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will now be described by way of example and with reference to the drawings in which: Figure 1 is a partially cut-away and diagrammatic perspective view of an explosion resistant light fitting according to the present invention; Figure 2 is a block diagram of a preferred electronic power supply for the light fitting having fluorescent lamps; Figure 3 is a block diagram of a preferred alternative electronic power supply for the light fitting having fluorescent lamps and an emergency light circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS THE LIGHT FITTING As shown in Figure 1, the light fitting 1 also called a luminaire, includes a transparent or translucent cover 2 which protects the fluorescent tube 3, the ends 5 of which each fit in a corresponding socket 6 mounted on the fixed portion 7 of the light fitting 1. The fixed portion 7 accommodates a battery 8. The cover 2 is releasably

attachable to the fixed portion 7 and includes a rim portion 9 fittable into a corresponding groove portion 10 of the fixed portion 7. Each socket 6 is mounted on a support plate 11 itself mounted on the fixed portion 11.

A shaped housing 12 is attached to one of the support plates 11. The housing 12 houses a permanent magnet 13, and a reed switch 14 of conventional type comprising a coil (not shown) wound around a glass envelope (not shown) and centrally placed reed contact in the form of a thin flat metal strip (not shown). The housing comprises a groove 15 between the permanent magnet 13 and reed switch 14 shaped to receive a shield plate 16 of soft magnetic material mounted by means of connecting portion 17 on the inside surface 18 of the cover 2 so as to be received there when the cover 2 is fitted to the fixed portion 7 of the light fitting 1.

The permanent magnet 13, reed switch 14 and shield plate 16 together constitute a magnetic sensing device responsive to the position of the cover 2 that must be removed before parts such as fluorescent tube 3 (or battery 8) can be replaced.

The cover 2 which protects the fluorescent tube 3 in the light fitting 1 for use in hazardous areas with explosion risk, is such that the soft magnetic shield plate 16 shields the magnetic field the permanent magnet 3 from the reed switch 4. The cover must be opened for lamp 6 or battery 7 replacement, and on opening the cover 2 the reed switch 14 closes.

THE ELECTRONIC POWER SUPPLY

The light fitting 1 includes an electronic power supply circuit 50 as shown in Figure 2, in which the AC input voltage 20 is rectified by a bridge rectifier 21, then passed to a Power Factor Correction circuit (PFC, 25) which operates as a boost converter to increase the voltage level. The output of the boost converter is connected to a smoothing capacitor 39. The voltages mentioned below are with respect to common rail 24. The DC voltage on rail 26 is converted by a DC/AC converter 27 into high frequency AC starting voltage and lamp current on rail 38 for lamp 3. Part of the AC sine wave shaped output voltage of the converter 27 is supplied via capacitor 29 to parallel stabilising zener diode 31, rectified by diode 33, and smoothed by capacitor 34.

The resulting DC voltage on rail 35 (typical 12 VDC) is used to power both the drive circuit of Power Factor Correction circuit 25 and DC/AC converter 27. To start the whole circuit, a bootstrap start resistor 36 initially supplies a small current from the rectified DC voltage on rail 23 (current through the resistor typically being between 0.1 and 1 mA). The drive circuits supplied from rail 35 are dimensioned so that they initially draw low supply current until a start-up voltage threshold on rail 35 is exceeded (typically 10 V). After start-up on exceeding the start-up threshold, the supply of the rectified current from capacitor 29 is sufficient for the larger supply current to flow during normal operation (typically 5-30 mA total).

On opening the cover 7, the reed switch 14 is closed, shorting the current limited supply to the DC/AC converter 27 and Power Factor Correction Circuit 25. As a result,

high frequency current can no longer flow through lamp 3. The DC blocking capacitor 37 connected to the return lamp lead 51 in DC/AC converter 27 prevents DC current from flowing through the lamp 3.

It will be seen that the drive pulses for the semiconductor switches within the converters are suppressed as a result of reed switch closure.

The whole circuit of the power supply circuit 50 remains connected to the AC input voltage 20. For removal, the lamp 3 is rotated which results in the contacts (not shown) in lampholder socket 6 being shielded so as to prevent unintended touching of the contacts inside the lampholders.

Another light fitting 1'will now be described which is shown in Figure 3. It has essentially the same structure as the light fitting described and shown in Figures 1 and 2 but with a modified power supply circuit 50'. This light fitting 1'which is shown in Figure 3, is a luminaire with an emergency light function.

As shown in Figure 3, the rectifier 21', power factor correction circuit 25'and DC/AC converter 27'function as described above in relation to the circuitry shown in Figure 2. The high rail 26', low rail 24', capacitor 38' and lamp 3'are also as in the light fitting shown in Figures 1 and 2. To the positive rail 23', a further DC/DC converter 40 is connected charging battery 8, on command of signal 47, generated by a microcontroller 44.

During emergency operation, in absence of mains voltage a back-up DC/DC converter 42 converts the battery voltage

into a higher voltage supplied via diode 43 to the DC input 26 of DC/AC converter 27.

On closing the reed switch 14', a signal is sent to the microcontroller 44, the controller blocking the further charging DC/DC converter 40 to signal 47, blocking the back-up DC/DC converter 42 to signal 46 and blocking the DC/AC lamp voltage converter 27'to signal 45. As a result the lamp or the battery can be removed or placed without the risk of sparking.

For additional safety, an extra independent circuit (prot, 49) provides an extra signal 48 to the charging DC/DC converter 40, the back-up DC/DC converter 42 and the DC/AC lamp converter 27', blocking the three converters in an independent way. A preferred way to block the converters 40,42, 27'is to short the timing capacitors of each of the RC oscillators (not shown) of the respective converters 40,42, 27' which generate the time bases of the drive circuits (parts of converters 40,42, 27' respectively) used for the generation of drive pulses for the semiconductor switches in the converters. This shorting can be accomplished by a signal transistor (not shown) that is driven by the blocking signal.

In another embodiment (not shown) another possible way to block the converters is to use a logic function block such as a 2 input AND port. One input of the AND port receives periodic pulses and the other input receives the active low blocking signal. The output of the AND port generates the drive pulses that activate the semiconductor switches of the converters.

As in the case of the lamps 3 and lampholder socket 6 contacts, mechanical shielding measures are taken in the electrical battery connection so as to prevent open electrical contacts from being inadvertently touched.

This is because these contacts remain galvanically connected to the AC input voltage. Both embodiments described above, use only one lamp. In some alternative embodiments more lamps are used although there is usually only one DC/AC lamp converter.

Generally more than one lamp converter is used, the current limited power supply to the drive circuits of all lamp converters must be shorted, or a fail safe or double protected system must shut down all lamp converters upon the protective cover being opened.

As a further alternative (not shown) an inverted system is possible where the reed contact is closed in the normal operating position of the cover, for example by the magnetic field of a permanent magnet fixed to the protective cover. In that case the reed switch can be switched in series with the power supply to the drive circuit of the lamps, or a double protected logical signal can be derived from the reed relay to be input to the microcontroller 44 for suppression of drive pulses from the drive circuits (not shown) of converters 40,42, 27'.

This thus blocks all converter circuits delivering power to serviceable parts.

Generally the use of a reed switch with its hermetic glass seal enables a reliable contact independent of environmental influences like deposition of contaminants,

and the contact if mechanically protected can reasonably be assumed not to fail.

The invention in its preferred embodiments enables advantageously spark free disconnection of parts connected to an output of an apparatus using a high frequency converter without the need for an expensive mechanical construction and without the risk of the energy stored in electrolytic capacitors being converted into output current at the moment the parts are disconnected.

The preferred spark free removal or placement of parts according to the invention exploits the fact that electronic devices powering fluorescent lamps or charging batteries using a high frequency converter, contain semiconductor switches, that must be activated by drive pulses to become conductive. By suppressing the drive pulses to the semiconductor switches the current to the lamps or batteries connected to the electronic apparatus is stopped in a fail safe manner, that is, such that even if one of the semiconductor switches forms a short or open as a result of a defect, or if the drive circuit, generating the drive pules, has a defect, no current can flow through, say, the lamps or to or from the battery.

In the preferred embodiments conversion is advantageously prevented of energy stored in an electrolytic capacitor of the power supply apparatus into an output current at the moment the connections to the parts are broken, thus preventing sparking at the moment of disconnection.

Claims

CLAIMS :

1. A light fitting comprising a holding portion for a light bulb or tube, and a protective light transmitting cover, the holding portion including power supply circuitry, the circuitry including a switch switchable between an open state and a closed state by magnetic force from a permanent magnet, a magnetic shield being interposable between the switch and permanent magnet so as to cause the switch to switch thereby making or breaking power supply to the light bulb or tube, the magnetic shield being mounted on the cover so as to be at least partially removed from between the magnet and switch so as to cause the switch to switch upon the cover being at least partially removed so as to break the power supply to the light bulb or tube.

2. A light fitting according to claim 1, in which the switch is a reed switch.

3. A light fitting according to claim 1 or claim 2, in which the magnetic shield is of soft magnetic material.

4. A light fitting according to any preceding claim, in which the circuit includes at least one electrical voltage converter comprising at least one semiconductor switch activated in use by drive pulse (s), the switch upon opening preventing drive pulse (s) being transmitted to said at least one semiconductor switch.

5. A light fitting according to claim 4, in which the switch when closed acts to short-circuit power away from drive circuitry which otherwise acts to generate the drive

pulse (s) for said at least one semiconductor switches.

6. A light fitting according to claim 4 in which the switch when open acts to prevent power being supplied to one or more drive circuits which generate the drive pulse (s) for said at least on semiconductor switch.

7. A light fitting according to any of claims 4 to 6 in which the switching of the switch causes a logical signal to be generated which is input to a microcontroller for suppression of the drive pulse (s).

8. A light fitting according to any preceding claim, in which the magnetic shield lies between the permanent magnet and the switch when the cover is in its closed configuration relative to the holding portion.

9. A light fitting according to any preceding claim in which the permanent magnet and the switch are held in a housing having a groove between the permanent magnet and reed relay shaped to accommodate the shield plate.

10. Electrical apparatus including a holding portion for electrical circuitry and a protective cover, the electrical circuitry being in use covered by the protective cover, the holding portion and cover being relatively moveable between a closed configuration and an open configuration so as to allow access to the electrical circuitry, the circuitry including a switch which switches from a first state to a second state by magnetic force from a permanent magnet upon the cover being moved relative to the holding portion towards an open configuration so as to break supply of power to a

predetermined part of the circuitry.

11. Apparatus according to claim 10 in which the predetermined part is a light bulb or light tube.

12. Apparatus according to claim 10 or claim 11, in which the predetermined part is a battery.

13. A method of breaking power supply to a light bulb or tube of a light fitting substantially as hereinbefore described with reference to the Figures.

13. A method of breaking power supply to a light bulb or tube of a light fitting comprising a holding portion for the light bulb or tube, and a protective light transmitting cover, the holding portion including power supply circuitry, the circuitry including a switch switchable between an open state and a closed state by magnetic force from a permanent magnet, and the cover carrying a magnetic shield interposable between the switch and permanent magnet, the method comprising at least partially opening the cover so as to enable access to the circuitry, the magnetic shield being thereby at least partially removed from between the magnet and switch so as to cause the switch to switch so as to break the power supply to the light bulb or tube.

14. A method of breaking the power supply to a predetermined part of electrical circuitry held by a holding portion of an apparatus, the apparatus also having a protective cover, the electrical circuitry being in use covered by the protective cover, the holding portion and cover being relatively moveable between a closed configuration and an open configuration so as to allow access to the predetermined part of the electrical circuitry, the circuitry including a switch which switches from a first state to a second state by magnetic force

from a permanent magnet upon the cover being moved relative to the holding portion towards an open configuration, so as to break supply of power to the predetermined part of the circuitry.

15. A light fitting substantially as hereinbefore described with reference to the Figures.

16. A method of breaking power supply to a light bulb or tube of a light fitting substantially as hereinbefore described with reference to the Figures.

Amendments to the claims have been filed as follows

CLAIMS :

2. A light fitting according to claim 1, in which the switch is a reed switch.

5. A light fitting according to claim 4, in which the switch when closed acts to short-circuit power away from drive circuitry which otherwise acts to generate the drive pulse (s) for said at least one semiconductor switch.

6. A light fitting according to claim 4, in which the switch when open acts to prevent power being supplied to one or more drive circuits which generate the drive pulse (s) for said at least one semiconductor switch.

9. A light fitting according to any preceding claim in which the permanent magnet and the switch are held in a housing having a groove between the permanent magnet and the switch shaped to accommodate the magnetic shield.

10. Apparatus according to any preceding claim, in which the predetermined part is a battery is provided to supply emergency back-up lighting.

11. A method of breaking power supply to a light bulb or tube of a light fitting comprising a holding portion for the light bulb or tube, and a protective light transmitting cover, the holding portion including power supply circuitry, the circuitry including a switch switchable between an open state and a closed state by magnetic force from a permanent magnet, and the cover carrying a magnetic shield interposable between the switch and permanent magnet, the method comprising at least partially opening the cover so as to enable access to the circuitry, the magnetic shield being thereby at least partially removed from between the magnet and switch so as to cause the switch to switch so as to break the power supply to the light bulb or tube.

12. A light fitting substantially as hereinbefore described with reference to the Figures.