GB2274507A

GB2274507A - A handlamp or torch with current limiting circuits

Info

Publication number: GB2274507A
Application number: GB9321567A
Authority: GB
Inventors: Peter David Weston
Original assignee: Furneaux Riddall & Co Lim
Current assignee: Furneaux Riddall & Co Lim
Priority date: 1992-10-19
Filing date: 1993-10-19
Publication date: 1994-07-27
Anticipated expiration: 2013-10-19
Also published as: GB9221887D0; GB2274507B; GB9321567D0

Abstract

A handlamp or torch having a Ni-cad rechargeable battery (BAT1), a quartz halogen bulb (LP1), and a high power setting switch (SWI) and a low power setting switch (SW2) is provided with a current limiting circuit (R2, R4, TR1, TR2) connected to the high power switch (SW1) and a current limiting circuit (R3, R6, TR3, TR4) connected to the low power switch (SW2). The two current limiting circuits are connected in series across the battery (BAT1) and the bulb (LP1). One diode (D1) connected across the two switches (SW1 and SW2) and another diode (D2) connected between the high power switch (SW1) and its current limiting circuit enable the two current limiting circuits to operate when the high power switch (SW1) is made and only the single current limiting circuit connected to the low power switch (SW2) to operate when the latter switch (SW2) is made, there being a shunt resistance (R4) connected across the collector and emitter of the transistor (TR1) of the other current limiting circuit. The switches (SW1, SW2) are in the form of reed switches which are set at an angle to the path of relative movement between them and a single actuating magnet. Shielding is provided between the two switches such that the magnetic field causing actuation of each one is shielded from the other. <IMAGE>

Description

HANDLAMPS AND TORCHES DESCRIPTION This invention relates to handlamps and torches.

Our British Patent no. 1555072 is for a torch for use by public services, such as fire, police and ambulance forces, and comprises a casing for containing a rechargeable battery which is to be used with a quartz halogen bulb. A magnet and reed switch may be used in place of the normal rocker switch to avoid providing an aperture in the casing.

An object of the present invention is to provide a handlamp or torch which is provided with a current limiting facility enabling the device to be used in a potentially explosive atmosphere and by virtue of such a facility enables NI-cad rechargeable batteries or alkaline batteries to be used in conjunction with a halogen bulb. It is also an object of this invention to provide such a handlamp or torch which can be operated selectively in either a high power setting or a low power setting.

According to one aspect of the present invention there is provided a handlamp or torch of the kind for use in potentially explosive atmospheres, having battery and bulb holding means and comprising one switch for a high power setting, another switch for a low power setting and operating means operable to actuate a selective one of the switches, wherein there is a current limiting circuit associated with each of the switches and connected in series across the battery and bulb holding means, and means are provided to enable only the current limiting means associated with said other switch to be operational when said other switch is made and to enable both the current limiting circuits to be operational when said one switch is made.

A considerable amount of space would be required for a pair of magnetically operable reed switches should such switches be used for the high and low power settings. It is a further object of this invention to provide an operating switch arrangement which enables the use of a pair of magnetically operable reed switches in a confined space.

According to another aspect of this invention there is provided an operating switch arrangement for a handlamp or torch comprising a pair of reed switches and a single actuating magnet which are arranged so that either of the reed switches is made or opened by relative movement between the magnet and the reed switches, wherein the two reeds are set at an angle with shielding means between them such that the magnetic field causing actuation of one of them is shielded from the other and vice versa. It will be understood that the reed switches are oblique to the path of relative movement between them and the magnet. Preferably the shielding means comprise a ferrous barrier.

An embodiment of the invention is illustrated in the accompanying drawings wherein: Figure 1 is a circuit diagram of the current limiting feature of the lamp or torch of the invention, and Figure 2 is a plan view of the printed circuit board incorporating the components shown in Figure 1.

The handlamp or torch of the present invention includes a halogen bulb and a cassette holding three alkaline or Ni-cad rechargeable batteries. In Figure 1 a current limiting circuit is shown within the dotted rectangle. That current limiting circuit is also shown in printed circuit form in Figure 2.

The switches SW1 and SW2 comprise a pair of magnetically operable reed switches which are incorporated in a single switching arrangement having a single common operating magnet. The switch SW1 is for the high power setting of the operating reed switch arrangement and the switch SW2 is for the low power setting.

The reeds of the reed switches SW1 and SW2 are set at an angle of about 45 degrees to the path of movement of a slider operating member (not shown) which is mounted on the casing of the handlamp or torch for rectilinear sliding movement between a high power operating position, an off position and a low power operating position. The slider carries the single magnet (not shown). There is a ferrous barrier (not shown) located between the two reed switches, conveniently substantially parallel to them, such that the magnetic field causing actuation of one of the reeds is shielded from the other reed, and vice versa.

The current limiting circuit is in effect two current limiting circuits in series, a first being operatively associated with the low setting switch SW2 only, whilst they are both operatively associated with the high setting switch SW1. The first limiting circuit which is operatively associated with switch SW2 comprises resistor R6 connected between one terminal of the battery BAT1 and the emitter of transistor TR3, the resistor R6 also being connected to the base of transistor TR4 whose collector is connected to one terminal of switch SW2 (the low position) via resistor R3. The limit current is defined by the voltage drop across resistor R6 necessary to bring transistor TR4 into conduction.This voltage drop is preferably of the order of 0.7V giving a nominal current of 2.12A. The second duplicate current limiting circuit consisting of transistors TR1 and TR2 and resistors R2 and R5 is connected in series with the first limiting circuit and to one terminal of switch SW1 and with the addition of a resistor R4 shunted across the emitter and collector of transistor TR1. Diodes D1 and D2 steer bias current from the operating reed switch to enable the current limiting circuits as required.On the HIGH setting of lamp LP1, with the contacts of switch SW1 closed, both limiting circuits conduct giving maximum voltage to the lamp LP1, whilst on the low setting with the contacts of switch SW2 closed, only limiter transistors TR3 and TR4 conduct causing resistor R4 to reduce the available lamp voltage.

Should any failure occur in the first limiting circuit, viz resistors R3, R6 and transistors TR3, TR4, the second limiting circuit i.e. resistors R2, R5 and transistors ml, TR2 will continue to limit the current to the predetermined design value with, in the worst case, i.e. with a fully charged battery and short-circuited lamp, up to 2A passing through shunt resistor R4 and the remainder through transistor TR1 of the second limiting circuit. In the event of any failure in the second limiting circuit, the first limiting circuit will continue to limit the current to the design value, with all current passing through transistor TR3 of the first limiting circuit 1.Resistor R1 is connected across lamp LP1 to ensure that all bias current is limited in the event of failure of resistors R3 or R2 of the first and second limiting circuits respectively.

The fuse F1 connected across resistor R1 prevents the maximum fault current (nominally 2.12A) from flowing for longer than approximately 30 m5, thereby avoiding excessive discharge of the battery BAT1 under prolonged fault conditions.

For the purposes of analysing potential faults, the worst operating conditions would involve the torch being set to high brightness (switch SW1) with a fully charged battery BAT1. These conditions would cause both transistors TR3 and TR1 of the first and second current limiting circuits respectively being biassed on.

In the event of any failure involving the second limiting circuit including even a short circuit, the current is limited by the combination of components of the first limiting circuit. If the current rises through resistor R6 of the first limiting circuit, to a nominal 2.12A, thereby developing a nominal potential of 0.7V, transistor TR4 of the first limiting circuit will begin to conduct resulting in the bias current through resistor R3 being diverted away from transistor TR3 of the first limiting circuit, thereby limiting the maximum current flow to nominally 2.12A. Worst case current flow occurs if resistor R6 is at the lower end of its tolerance spread and the Vbe of transistor TR4 is at the upper end of its tolerance spread (approximately 0.8V).

Under this worst possible case situation, the fault current would still be limited to a maximum of 2.69A.

Similarly in the event of any failure involving transistors TR3 or TR4 or resistors R3, R6 of the first current limiting circuit, the current is limited by the combination of components of the second current limiting circuit including shunt resistor R4. If the current rises through resistor R5 to the nominal 2.12A, transistor TR2 of the second current limiting circuit begins to conduct resulting in the bias current supplied through resistor R2 being diverted away from transistor TR1 thereby limiting the maximum current to the nominal 2.12A.

The presence of shunt resistor R4 causes the limit current to be shared between transistor TR1 and resistor R4. Under severe fault conditions, for example when the lamp Lop1, transistor TR3 and resistor R6 of the first current limiting circuit are short circuited, at the highest possible input voltage, i.e. 4.5V with a fully charged battery, nominally 3.8V drop will occur across resistor R4 with the remaining 0.7V being dropped across resistor R5. Under these conditions, a nominal 1.73A will flow through shunt resistor R4 resulting in 0.39A current flow through transistor TR1 of the second current limiting circuit to give a nominal total of 2.12A.

Currents in the second current limiting circuit are the same as for the first current limiting circuit resulting again in a worst case fault current of 2.69A.

Thus is will be seen that the use of a pair of reeds for the high and low positions of the operating reed switch, with the current limiting circuits associated with the reeds, provides current limitation under the worst possible fault conditions allowing the lamp or torch to be used with intrinsic safety in potentially explosive atmospheres.

Furthermore the current limiting feature allows the device to be used with rechargeable Ni-cad batteries or with alkaline batteries together with a halogen bulb.

The points P1 to P7 in the circuit diagram correspond to the points P1 to P7 of the printed circuit board of Figure 2 which also shows the resistors R1 to R6, the transistors TR1, TR2, TR3, TR4, fuse F1 and diodes D1 and D2 connected between the switch terminals of SW1 and SW2.

It will be appreciated that the invention is capable of considerable modification and is not to be deemed limited to the values of the components shown in the drawings and referred to by way of example only.

Claims

1. A handlamp or torch for use in potentially explosive atmospheres, having battery and bulb holding means and comprising one switch for a high power setting, another switch for a low power setting and operating means operable to actuate a selected one of the switches, wherein there is a current limiting circuit associated with each of the switches and connected in series across the battery and bulb holding means, and means are provided to enable only the current limiting circuit associated with said other switch to be operational when said other switch is made and to enable both of the current limiting circuits to be operational when said one switch is made.

2. A handlamp or torch according to Claim 1 wherein the switches are magnetically operable reed switches.

3. A handlamp or torch according to Claim 2 wherein shielding means are located between the reed switches.

4. A handlamp or torch according to Claim 3 wherein the shielding means comprise a ferrous barrier.

5. A handlamp or torch according to any one of Claims 2 to 4 wherein the switches are oblique to the path of relative movement between them and a single common operating magnet.

6. A handlamp or torch according to any one of Claims 1 to 5, wherein each of the current limiting circuits comprises the emitter of one transistor and the base of another transistor being connected in parallel to a battery terminal through a resistor, the base of said one transistor and the collector of said other transistor being connected in parallel via another resistor to one terminal of the respective switch, the current limiting circuit associated with said one switch being connected to said battery terminal via the other current limiting circuit by being connected to the collector of said one transistor of that other current limiting circuit.

7. A handlamp or torch according to Claim 6 including a diode connected across said one terminal of said switches between those switches and the respective other resistor of the respective current limiting circuit and allowing current flow from said one switch to the other switch, there being another diode between said one terminal of said one switch and said other resistor of the respective current limiting circuit permitting current flow from said one switch to the respective current limiting circuit.

8. An operating switch arrangement for a handlamp or torch comprising a pair of reed switches and a single magnet which are arranged so that either of the reed switches is made or opened by relative movement between the magnet and the reed switches, wherein the two reeds are set at an angle with shielding means between them such that the magnetic field causing actuation of one of them is shielded from the other and vice versa.

9. An operating switch arrangement for a handlamp or torch according to Claim 8, wherein the shielding means comprise a ferrous barrier.

10. A handlamp or torch of the kind for use in potentially explosive atmospheres, having battery and bulb holding means and comprising an operating switch arrangement according to Claim 8 or Claim 9, wherein there is a current limiting circuit associated with each of the reed switches and connected in series across the battery and bulb holding means so as to limit current through a lamp fitted into the bulb holding means, one of the reed switches being for a high power setting and the other being for a low power setting, and means being provided to enable only the current limiting means associated with said other reed switch to be operational when said other reed switch is made and to enable both the current limiting circuits to be operational when said one reed switch is made.

11. A handlamp or torch of the kind for use in potentially explosive atmospheres substantially as described hereinbefore with reference to and as illustrated in the accompanying drawings.

12. An operating switch arrangement substantially as described hereinbefore with reference to and as illustrated in the accompanying drawings.