GB1558473A - Power supplies for alarm systems - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO POWER SUPPLIES FOR ALARM SYSTEMS (71) We, NITTAN COMPANY LIMITED, a Japanese Company of 1-11-6 Hatagaya, Shibuya-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to rectifier circuits for alarm systems, such as fire alarms or burglar alarms.

The invention is applicable to detectors of a wide variety of types such as mechanical detectors and, more particularly, to the many forms of fire detectors that have recently been developed, namely optical smoke detectors, ionization smoke detectors and flame detectors. Such detectors generally have an open or closed contact, that is to say a contact that only becomes effective when the detector senses conditions due to an incipient fire.

One such "contact" that is frequently used compares a silicon controlled rectifier (SCR) or an arrangement of semi-conductor devices having a similar function, which is caused to conduct when it receives a gating signal and sends an alarm signal to a relay device in a control panel which maintains an associated fire alarm or extinguishing device in operation so long as the detector holds its operating condition.

A D.C. supply is necessary for such an alarm system and this has to be supplied by way of a rectifier from the usual A.C. mains.

For a detector using semiconductor devices, including, for example, an SCR as described above, hitherto a smoothing or stabilizing circuit has been used in association with the rectifier, However, a smoothing circuit has an undersirably wide voltage variation with load and a stabilizing circuit is costly.

It is a primary object of the invention to provide a circuit having the function of providing a holding voltage for a detector in an alarm system in its operating condition which otherwise would be reset if the supply was allowed to become too low.

It is a further object of the invention to provide comparatively simple and cheap means for providing a control panel in an alarm system with D.C. power that is sufficiently smooth and stable for the conditions of the system.

According to the invention, an alarm circuit comprises a rectifier circuit arranged to receive a single-phase a.c. power supply, a detector, a capacitor circuit comprising capacitor means, electronic unidirectional conductor means and electronic regulator means, the capacitor circuit being connected between the output of said rectifier circuit and supply terminals for the detector responsive to events necessitating the actuation of an alarm device, the capacitor circuit being formed and arranged so that when in use the capacitor means is charged for part only of the cycles of the a.c. power supply until the voltage regulator means reaches a critical voltage whereby the capacitor means discharge during other parts of the half cycles, the charge being first by way of the voltage regulator means and the unidirectional conductor means or only the unidirectional conductor means and then, when the voltage applied to the regulator means falls below the critical voltage, by way of the unidirectional conductor means, whereby the diminution of the output voltage applied between said terminals that occurs between, the peaks of the half cycles is limited, and means responsive to the detector for actuating an alarm device supplied by energy derived from said power supply, the limitation of the diminution of the output voltage being sufficient to enable the detector to hold its responsive condition so long as power is being supplied thereto.

In order that the invention may be clearly understood and readily carried into effect, circuits in accordance therewith will now be described by way of example, with reference to the accompanying drawings, in which: Figures 1 (a) and I (b) are circuit diagrams showing two prior circuits for the supply of power to alarm systems; Figures 2 (a) and 2 (b) are graphs connecting voltage and time, substantially as these parameters apply respectively to the prior circuits of Figures 1 (a) and 1 (b); Figures 3 (a) and 3 (b) are circuit diagrams sho-.ving the fundamental elements in two examples of the present invention; Figure 4 is a graph connecting voltage and time, substantially as these parameters apply to the circuits of Figures 3 (a) and 3 (b); and Figures 5 (a) and 5 (b) show two alarm circuits respectively incorporating the features cf Fibres 3 (a) and 3 (b).

Referring to the known circuit of Figure 1(a), terminals 1, 2 for an alarm system are connected to a single phase A. C. supply by way of a transformer 3 and rectifier bridge 4.

When in use, the terminals will be connected to relays or devices functioning as relays allocated to the respective zones to be protected and each controlled by a detector in its associated zone. The voltage between terminals 1, 2 under load is as shown by a full line A in Figure 2 (a) and under no load by a broken line B in Figure 2 (a). In each case the effective voltage is indicated diagrammatically by a line C for load and D for no load. However, the periodic return of the voltage to zero under load results in semi-conductor switching devices in the alarm circuits being reset and during reset the detector sometimes shows an unstable operation. This is particularly the case with modern sensitive detectors. In any event the detector cannot be relied on to hold an operating condition with this power supply.The voltage can be prevented from returning to zero by the introduction of a capacitor 7 as shown in the prior proposals of Figure I (b). This results, however, in load and no load voltage curves as shown in Figure 2 (b) but the effective voltage variation between lines C1 and D1 is very high in relation to the load current variation. The rectifier bridge, shown more diagrammatically in Figure I (b) and also in Figures 3 and 5, is similar to that shown in Figure 1 (a).

In systems comprising sensitive detectors, there are sometimes false alarms caused by special enviromental conditions or by a faulty condition of the detector. It is, therefore, highly desirable for the detector to hold the operating condition to enable the operating detector to be located by an indication lamp on the detector or by removing the detector from its socket. Then the cause of the false alarm can be ascertained and remedied.

A simple and inexpensive power supply which is constructed in accordance with the invention and which will enable a detector connected between terminals 1, 2 to hold its operating condition, is shown in Figure 3 (a).

This provides load and no load voltage curves as shown in Figure 4. When the rectifier bridge 4 is conducting to supply power to the detector, a capacitor 5 is charged up to the Zener voltage of a Zener diode 6 through a resistor 8. When the zener voltage is reached, the capacitor 5 discharges through Zener diode 6 and diode 9. When the voltage is reduced below the Zener voltage the capacitor 5 discharges to the detector through diode 9.

Figure 3 (b) shows the introduction of an npn transistor 10 into the circuit of Figure 3 (a). This enables the use of a comparatively large capacitor 5 without the necessity of using a corresponding high powered Zener diode 6 which would involve a resistor 8 having a comparatively large wattage rating, that would limit the rate of charging the capacitor 5. The capacitor 5 is charged through the transistor 10 until the Zener voltage is reached. Then the capacitor 5 first discharges through the Zener diode 6 to switch the transistor and then discharges through diode 9.

Figure 5 (a) shows a complete alarm circuit incorporating the power supply circuit of Figure 3 (a), wherein the treminals 1, 2 are connected to one or more detectors 14 connected in parallel. The detector, or each detector 14, comprises an SCR 17 which, when an alarm becomes necessary, is triggered by a signal from a sensor 15, such for example as an ionization smoke detector, through a triggering circuit 16. The triggering of the SCR 17 causes a warning light 18, individually associated with the detector, to light up so that, where there are more than one detector, the detector that has operated can be located. The triggering of the SCR 17 also causes a relay 12 to be actuated so as to close contacts 21, thereby switching alarm indicating devices into circuit with the transformer 3. The alarm indicating devices include visual indicators such as lamps 19 and a sound generator 20.A diode 11 isolates this load from the circuit comprising the elements 5, 6, 8 and 9, so that when SCR 17 is triggered the current through these elements flows only through the relay 12 and detector or detectors 14. A diode 13 bypasses the relay 12 so as to prevent any reverse current from operating the relay 12.

Figure 5 (b) shows a multi zone alarm system incorporating a modification of the power supply circuit of Figure 3 (b). As shown, ten zones are connected in parallel to the terminals 1, 2. The zones respectively comprise a detector, or group of parallel detectors, 14-1 14-10. The detectors, or groups thereof, respectively control relays 12-1 .....12-10 which in turn control contacts 21-1 .....21-10 and these respectively control lamps or other alarm indicators 19-1 19-10. Thus, by observing which of these lamps or other indicators have been actuated, the affected zone can be identified. A single sound generator 20 is common to all indicators 19.

Diodes 22-1 22-10 are provided for the alarm indicator circuits 19-1 19-10 to allow the sound generator to be operated by any of the zones but permits the separate indication from the respective indicator to indicate the active zone.

A diode 24 has been incorporated to pr- vent reverse voltage breakdown between the emitter and base of the transistor 10. An additional resistor 23 limits the collector current of the transistor 10 to within its maximum rating.

Typical values in volts, ohms and microfarads are shown in Figures 5 (a) and 5 (b) applied to various elements therein.

WHAT WE CLAIM IS: 1. An alarm circuit comprising a rectifier circuit arranged to receive a single phase a.c.

power supply, a detector, a capacitor circuit comprising capacitor means, electronic unidirectional conductor means and electronic regulator means, the capacitor circuit being connected between the output of said rectifier circuit and supply terminals for the detector responsive to events necessitating the actuation of an alarm device, the capacitor circuit being formed and arranged so that when in use the capacitor means is charged for part only of the half cycles of the a.c.

power supply until the voltage regulator means reaches a critical voltage whereby the capacitor means discharge during other parts of the half cycles, the charge being first by way of the voltage regulator means and the unidirectional conductor means or only the unidirectional conductor means and then, when the voltage applied to the regulator means falls below the critical voltage, by way of the unidirectional conductor means, whereby the diminution of the output voltage applied between said terminals that occurs between the peaks of the half cycles is limited, and means responsive to the detector for actuating an alarm device supplied by energy derived from said power supply, the limitation of the diminution of the output voltage being sufficient to enable the detector to hold its responsive condition so long as power is being supplied thereto.

2. An alarm circuit according to Claim 1, in which the capacitor circuit comprises a Zener diode so connected in parallel with a capacitor that when the capacitor voltage reaches a predetermined level, the Zener voltage is exceeded so that the Zener diode conducts thereby preventing the capacitor voltage from exceeding a predetermined voltage and limiting said diminution of the said output voltage.

3. An alarm circuit according to Claim 2, in which the capacitor is charged by way of a resistor and discharged through the Zener diode and through a further diode in parallel with the resistor.

4. An alarm circuit according to Claim 2, in which the capacitor is charged by way of a resistor and transistor and discharged by way of the Zener diode and transistor.

5. An alarm circuit according to any one of the preceding claims, in which the detector comprises a sensor responsiveto events needing the actuation of the alarm device, a triggering circuit responsive to the sensor and an SCR arranged to be triggered by the triggering circuit.

6. An alarm circuit according to any one of the preceding claims, in which a relay is connected in circuit with the detector to be actuated when the detector detects an event necessitating an alarm, the alarm device being controlled by the relay and having the rectifier as its source of power.

7. An alarm circuit substantially as described with reference to Figure 3 (a) of the accompanying drawings.

8. An alarm circuit substantially as described with reference to Figure 3 (b) of the accompanying drawings.

9. An alarm circuit substantially as described with reference to Figure 5 (a) of the accompanying drawings.

10. An alarm circuit substantially as described with reference to Figure 5 (b) of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. any of the zones but permits the separate indication from the respective indicator to indicate the active zone. A diode 24 has been incorporated to pr- vent reverse voltage breakdown between the emitter and base of the transistor 10. An additional resistor 23 limits the collector current of the transistor 10 to within its maximum rating. Typical values in volts, ohms and microfarads are shown in Figures 5 (a) and 5 (b) applied to various elements therein. WHAT WE CLAIM IS:

1. An alarm circuit comprising a rectifier circuit arranged to receive a single phase a.c.

power supply, a detector, a capacitor circuit comprising capacitor means, electronic unidirectional conductor means and electronic regulator means, the capacitor circuit being connected between the output of said rectifier circuit and supply terminals for the detector responsive to events necessitating the actuation of an alarm device, the capacitor circuit being formed and arranged so that when in use the capacitor means is charged for part only of the half cycles of the a.c.

power supply until the voltage regulator means reaches a critical voltage whereby the capacitor means discharge during other parts of the half cycles, the charge being first by way of the voltage regulator means and the unidirectional conductor means or only the unidirectional conductor means and then, when the voltage applied to the regulator means falls below the critical voltage, by way of the unidirectional conductor means, whereby the diminution of the output voltage applied between said terminals that occurs between the peaks of the half cycles is limited, and means responsive to the detector for actuating an alarm device supplied by energy derived from said power supply, the limitation of the diminution of the output voltage being sufficient to enable the detector to hold its responsive condition so long as power is being supplied thereto.

2. An alarm circuit according to Claim 1, in which the capacitor circuit comprises a Zener diode so connected in parallel with a capacitor that when the capacitor voltage reaches a predetermined level, the Zener voltage is exceeded so that the Zener diode conducts thereby preventing the capacitor voltage from exceeding a predetermined voltage and limiting said diminution of the said output voltage.

3. An alarm circuit according to Claim 2, in which the capacitor is charged by way of a resistor and discharged through the Zener diode and through a further diode in parallel with the resistor.

4. An alarm circuit according to Claim 2, in which the capacitor is charged by way of a resistor and transistor and discharged by way of the Zener diode and transistor.

5. An alarm circuit according to any one of the preceding claims, in which the detector comprises a sensor responsiveto events needing the actuation of the alarm device, a triggering circuit responsive to the sensor and an SCR arranged to be triggered by the triggering circuit.

6. An alarm circuit according to any one of the preceding claims, in which a relay is connected in circuit with the detector to be actuated when the detector detects an event necessitating an alarm, the alarm device being controlled by the relay and having the rectifier as its source of power.

7. An alarm circuit substantially as described with reference to Figure 3 (a) of the accompanying drawings.

8. An alarm circuit substantially as described with reference to Figure 3 (b) of the accompanying drawings.

9. An alarm circuit substantially as described with reference to Figure 5 (a) of the accompanying drawings.

10. An alarm circuit substantially as described with reference to Figure 5 (b) of the accompanying drawings.