GB2349519A - Bipolar LED indicator; fire alarm systems - Google Patents

Abstract

A bipolar indicator has LED's 2 and 3 connected in inverse parallel and mounted in the same envelope (14, Fig.5) for emitting light of the same colour. The indicator is useful in a fire alarm system to provide a remote indication that a fire detector 3 has responded and changed state, for example in situations (Fig.3) where a local detector status indicator LED 1 cannot be observed. The remote indicator can be connected to the detector 3 without the need to observe polarity and thus the risk of otherwise damaging a single LED. The LED's 2, 3 may be connected in series with a current limiting resistor R2. Alternatively, the resistor R2 may be omitted if the LED's 2, 3 are connected across a zener diode (ZD1, Fig.2) which is in series with the local status indicating LED 1. The resistor R2 may be incorporated in the same envelope as LED's 2, 3, the envelope preferably having three lead-outs T1, T2, T3 thus giving the options of including or not including the resistor R2 in the circuit.

Description

BIPOLAR LED INDICATOR This invention relates to a bipolar LED indicator which can be used to indicate an active state in a device which is powered by either AC, or DC. The advantage of the bipolar indicator is that it not sensitive to polarity. It can be connected without observing polarity and thus without risking damage caused by reverse polarity. It also inherently resistant to damage to high voltage transients because each LED limits the reverse voltage across the other LED to the forward bias voltage, a voltage, which is usually, much less than the rated maximum reverse voltage. The invention can be applied, for example, to a fire detecting system where it is used as a remote indicator.

Most fire alarm systems operate on a DC supply, for example 18 volts DC, because a 24-volt battery is used to power the system in the event of mains failure. Where the fire alarm system is installed in a large building, each room is equipped with at least one fire detector, e. g. a smoke detector, which is connected by a line pair to a central control unit (CCU). The fire detector usually includes a base which is fixed to the ceiling of a room (and is connected to the line pair), and a detecting head which is removably fitted to the base. Several fire detectors are usually connected to the same line pair and several line pairs may be connected to the CCU. Either one end of each line pair is connected to the CCU or both ends are connected to form a loop. The line pair supplies current for operating the detectors and it also enables signalling or communication to take place between each fire detector and the CCU.

If a fire breaks out in a room, the fire detector in the room responds by changing from a passive (or standby) state to an active (or alarm) state. The active state is usually latched until reset by removing the supply by manual intervention. To give an example of one type of system, the detector normally presents a high impedance across the line pair, but when it responds to fire, it changes to an active state where it places a low impedance across the line pair. The voltage drop or increase in current, due to the low impedance on the line, is sensed by the CCU, which then causes an alarm to be given, e. g. by activating alarm devices, such as bells or sirens. It is naturally of primary importance to give a fire alarm, but it is also important to provide guidance as to the site of the fire. In order to indicate which detector has responded to a fire, the head of the detector can be fitted with a visible indicator or LED. This is illuminated when the detector changes from its passive to its active state. The LED is mounted onboard the detecting head and projects through an opening in the head moulding so that it can be seen by personnel, such as a fire officer or security person, who can then tell which detecting head has responded to a fire and hence determine the site of the fire.

This works well where all of the detecting heads are located so that their onboard LEDs are visible. However, this is not always the case, because some fire detectors may be installed in ceiling or floor voids, or in locked rooms, or other inaccessible places, where the onboard LED cannot be readily seen.

In order to deal with this problem, it is known to connect a remote LED to a part of the circuitry of the smoke detector which supplies a suitable operating voltage to the remote LED (i. e. when the detecting head is in the active or alarm state). A resistor is usually, but not always, connected in series with the remote LED to limit the current to a level below the LED's rated maximum current or to limit the alarm signalling impedance or both. The resistor may be in the detector or in the housing of the remote LED or split between the two.

An LED is likely to be damaged by even the momentary application of reversed polarity in excess of its rated reverse voltage, typically 5 volts or less. This can occur where wiring, which is connected to the head and then carried through a roof or ceiling void to the remote LED, is reversed at either end. This can easily occur when using fire resistant mineral insulated cable with unmarked conductors. The problem is even more acute where the detectors can be wire without regard to the polarity of the supply because, when a remote LED is connected, the polarity of the supply wiring has to be traced back to the CCU.

In a conventional remote LED, it is known to connect a rectifying diode in series with the LED, in the same polarity as the LED, so that the LED turns on when the rectifying diode conducts. When connected in reverse polarity, virtually all the reverse voltage is developed across the rectifying diode and the LED is protected. In another configuration, a rectifying diode is connected in parallel with the LED in opposite polarity so that when the supply polarity is reversed, the rectifying diode conducts and limits the reverse voltage across the LED to a safe level. However, a problem of both configurations is that the remote LED is operable with one polarity only. In the other polarity it would not provide any illumination after the detector has changed from the passive to the active state. This fault could remain unnoticed.

Some fire detecting systems employ low voltage AC instead of DC. In this case, a rectifying diode is again connected in series or parallel with the LED. However, in both configurations, the LED is only powered during each alternate half cycle of AC, and hence provides only half the illumination.

In another arrangement the LED is connected to the output of a bridge rectifier, but this is expensive because it uses four rectifier diodes instead of one to protect the LED and usually requires a printed circuit board on which to mount the diodes, resistor and LED.

In any event, attempts to provide a reliable, low cost solution to the problem have not been successful.

The invention provides a bipolar indicator for indicating an active state in a device which is operatively connected to a line pair that is powered by AC or DC; the bipolar indicator comprising terminals for connection to the device; and at least two oppositely poled LEDs connected in parallel to said terminals, the LEDs being mounted in the same envelope for emitting light of the same colour.

Optionally, the parallel connected LEDs are connected in series with a resistor to provide protection against transient voltages that could damage either of the LEDs. In this case, said terminals provide the option of including the series connected resistor or not with the parallel connected LEDs.

A bipolar indicator according to embodiments of the invention has the following advantages: (a) It is not sensitive to polarity. It can therefore be connected without observing polarity and thus without damaging either LED due to reverse polarity. For example, when the device is powered by DC and the device is in its active state, DC operating current is supplied to said terminals. The LED which then conducts (with the correct polarity) limits the voltage across the other LED, so that one LED always protects the other from damage against reverse polarity.

(b) It is versatile because it can be used with an AC instead of a DC system without any modification. In this case, one of the LEDs will be illuminated on every half cycle of AC, thereby protecting the other LED, and the indicator will also be brighter than low cost prior art indicators which are illuminated on alternate half cycles. Moreover, no bridge rectifier is required.

(c) It is not necessary to provide any rectifier diodes to prevent destruction of the LED due to reverse polarity connection.

(d) As both LEDs emit light of the same colour, it makes no difference which way around the indicator is connected to the line pair. One or other of the LEDs will be illuminated and hence one will emit light of the accepted colour to show that the fire detector has transitioned from the passive to the active state.

(e) By placing both LEDs in the same envelope, they can be fitted to the usual holder and the envelope will appear like a single bulb. This makes installation and maintenance easier.

The invention also provides a fire detecting system comprising: a CCU connected to at least one line pair; a plurality of fire detectors connected across the line pair, each detector having a passive state and an active state, the detector changing from the passive state to the active state when responding to a fire, each detector having an onboard indicator LED which is illuminated when the detector has changed to its active state; and a remote indicator located remotely and connected to at least one fire detector, characterised in that the remote indicator comprises at least two LEDs connected in parallel, but with opposite polarity, both LEDs emitting light of the same colour; the arrangement being such that when one LED in the parallel connection conducts, it is illuminated and it also limits the reverse bias voltage across the other LED to a level which prevents the other LED from being damaged by reverse line polarity.

The detectors can be of various types, for example, where each detector applies a high impedance cross the line pair in the passive state and a low impedance in the active state. Alternatively, the detector may have some form of switch which changes state when the detector changes from the passive state to the active state when responding to a fire. In the case of analogue addressable detectors which receive signals from the CCU, the switch and therefore the remote LED may be under the control of the CCU.

An embodiment of the invention will now be described with reference to the accompanying schematic drawings. Fig. 1 illustrates a remote bipolar indicator, incorporating a series resistor, connected to a fire detector in a fire detecting system; Fig. 2 illustrates the same remote bipolar indicator connected, without including the series resistor, to a fire detector in a fire detecting system; Fig. 3 shows an installation with a fire detector connected to a remote indicator; Fig. 4 is a schematic circuit diagram of the bipolar indicator; and Fig. 5 shows LEDs in the same envelope.

Referring to the drawings, a fire detecting system will now be described by way of example only. It will be understood that the principles of the invention can be applied more broadly to devices which require some form of bipolar indicator.

The fire detecting system includes a central control unit (CCU), not shown but of known construction, to which is connected at least one line pair to which a plurality of fire detectors is connected. Fig. 1 shows part of the circuitry of such a fire detector 3 having terminals 4 and 5. Terminal 4 is connected to one line of the line pair 1,2 between the CCU and an end-of-line device (EOL), not shown, but of standard construction. Terminal 5 is connected to the other line of the line pair between the CCU and EOL. The bridge rectifier 9 in the detector 3 permits the connection of the line pair 1,2 to terminals 4 and 5 to be independent of polarity.

Detector 3 has not been shown in full detail, since fire sensor circuit 6, shown in broken line, represents much more detailed circuitry which can sense a fire. On sensing a fire, the fire sensor circuit 6 triggers the thyristor 7 so as to cause conduction in the low value resistor R1 and LED1 which is thereby illuminated to indicate that detector 3 has changed from its normal passive (high impedance) state to an active (low impedance) state.

In a conventional fire detecting system, a remote unipolar indicator comprises a single LED 2 and resistor R2 connected in series to terminals 4 and 8 in the correct polarity which must be determined and observed.

The remote indicator is usually some distance away from the detector 3. As shown in Fig. 3, detector 3 may be located on the ceiling of a locked room 10 having no inspection window, whereby LED 1 cannot be seen from an adjoining room 11.

However, remote LED 2 is connected to detector 3 by a length of wiring 12 so that LED 2 can be seen by an occupant of room 11. The remote indictor is mounted in a housing 13 provided with wiring terminals and fixing points (not shown).

A characteristic of the detector and remote indicator arrangement shown in Fig. l is that when a detector is activated the load placed across the line pair is higher that for a detector without a remote indicator. However, if the wiring 12 to the remote indicator are subject to a short circuit fault, the impedance across the line pair 1,2 will also be close to a short circuit. Therefore, if local codes require a CCU to indicate a fault in the event of a short circuit in a line pair, the activation of a detector with a short circuit across the wiring to its remote indicator would be misinterpreted as a fault in the line pair.

Fig. 2 shows an alternative detector and remote indicator arrangement in which the supply to the remote indicator is derived from a zener diode ZD1 having a zener voltage just slightly greater than the forward voltage of LED2 and LED3, the LEDs not being connected in series with a resistor. In this arrangement, when the detector is activated, the load placed across the line pair is substantially the same whether or not the remote indicator is connected to the detector. Furthermore, the load is only marginally increase if the lines to the remote indicator become short-circuited.

The three terminal (T1, T2, T3) bipolar remote indicator incorporating a series resistor as shown in Fig. 4 is suitable for use with either of the detector types and circuit arrangements shown in Figs. 1 and 2.

In accordance with the invention, a bipolar indicator includes LED 3 connected in parallel with LED 2, but with the opposite polarity, both LED 2 and LED 3 being mounted in the same envelope 14 (like a bulb) and emitting light of the same colour (e. g. red). In this case polarity does not need to be observed when connecting the bipolar indicator (LED 2 and LED 3, R5) to detector terminals 4 and 8. The reason for this is that the LED with the same polarity as the line pair will conduct and be illuminated and it will also limit the reverse bias voltage across the other LED to a safe level thereby preventing the other LED from being damaged. Preferably the parallel connected LEDs are connected in series with a resistor in a housing 13 with terminals T1, T2, T3 for optionally including or not a resistor R2 in series with the parallel connected LEDs.

The resistor R2 (not seen in Fig. 5) can be incorporated within the envelope 14 with lead-outs 15 to reduce the electronic component count to one. Preferably the envelope is fitted with a third lead-out to give the option of including or not the series resistor.

The bipolar indicator according to the invention can also be used with a device connected to a line pair providing low voltage AC (instead of DC). In this case, no bridge rectifier is required to avoid problems with reverse polarity connections. Each LED will conduct on respective half cycles, hence providing full brightness.

Claims (5)

1. CLAIMS 1. A bipolar indicator for indicating an active state in a device which is operatively connected to a line pair that is powered by AC or DC; the bipolar indicator comprising terminals for connection to the device ; and at least two oppositely poled LEDs connected in parallel to said terminals, the LEDs being mounted in the same envelope for emitting light of the same colour.
2. 2. A bipolar indicator according to claim 1, wherein the parallel connected LEDs are connected in series with a resistor to provide protection against transient voltages, the LEDs and resistor being connected to terminals which provide the option of including the resistor or not.
3. 3. A bipolar indicator substantially as herein described with reference to the accompanying drawings.
4. 4. A fire detecting system comprising: a CCU connected to at least one line pair ; a plurality of fire detectors connected across the line pair, each detector having a passive state and an active state, the detector changing from the passive state to the active state when responding to a fire, each detector having an onboard indicator LED which is illuminated when the detector has changed to its active state; and a remote indicator located remotely from the fire detector, characterised in that the remote indicator comprises at least two LEDs connected in parallel, but with opposite polarity, both LEDs emitting light of the same colour; the arrangement being such that when one LED in the parallel connection conducts, it is illuminated and it also limits the reverse bias voltage across the other LED to a level which prevents the other LED from being damaged by reverse line polarity.
5. 5. A fire detecting system substantially as herein described with reference to the accompanying drawings.