Notification appliance circuit with energy storing notification devices

A notification appliance circuit (NAC) includes notification devices having a high capacity rechargeable energy storage device such as a supercapacitor and a strobe circuit. The supercapacitor can provide energy to produce flashes over an extended time period without fully discharging. The notification devices can also make use of the fallback power strategy in which the strobe circuit operates with reduced intensity while the supercapacitor is being recharged.

BACKGROUND

Fire alarm systems and mass notification systems typically use distributed notification devices to notify the public of the presence of fire, smoke, and other conditions. In these systems, a notification appliance circuit (NAC) is often used to connect the notification devices to a control panel.

Power for the notification device is provided over the NAC from the control panel. Primary power to control panel may be, for example, AC power derived from a utility grid. Many systems also include a battery backup power supply at the control panel in order to maintain operations when the main power supply is faulty or interrupted.

Power supplied through the NAC to notification devices may be limited by the worst case voltage to the NAC and by the voltage drop across the NAC wiring. This may result in less than optimal coverage for NAC circuits.

For example, an NAC may be designed to have 30 notification devices, each drawing 100 milliamps and having a rated spacing of 10 feet at a working voltage and current. Thus, the NAC would provide notification coverage of 300 feet. Under real world conditions, because the voltage drops from the supply through various system components, for example, a panel, circuit wiring, and the wiring of the NAC itself, the NAC may be limited to fewer devices and less coverage length because the working voltage and current for all the devices may not be provided over the entire NAC as originally designed.

One commonly used type of NAC system makes use of reverse polarity circuits that are supervised by an end of the line resistor. The notification devices themselves may be simple on/off devices with a diode that maintains the notification devices in an off state when the power on the NAC has a first polarity. The diode completes the power circuit for the notification device when the circuit polarity is reversed from the first polarity to a second polarity. Each of the notification devices has the same or similar operating characteristics in this type of system.

The NAC circuit has a supervisory state, in which the polarity of the voltage on the NAC circuit wires is such that the diodes within the notification devices are reversed biased. In the supervisory state, the NAC circuit is supervised, but the notification devices are not active.

When the polarity of the voltage on the NAC circuit wires is reversed, the NAC circuit is in an active state. The diodes within the notification devices are forward biased, allowing current to flow through the notification devices to activate the notification devices.

A notification device may provide both visual as well as audible signaling. The visual signaling can be produced by a strobe circuit that includes a light source, such as a gas filled flash tube or light emitting diodes (LEDs), as well as a driver or trigger circuit that provides the necessary voltage and current to either the light source. The strobe circuit is typically powered by a storage capacitor, which must be recharged with current from the NAC circuit after each flash produced by the strobe circuit. The current required to recharge the capacitor after every flash represents a significant portion of the total current requirement of each notification device.

SUMMARY

A notification appliance circuit with a plurality of notification devices connected by NAC wiring. Each of the notification devices includes a strobe circuit and a high capacity rechargeable energy storage device that has the capacity to store enough energy to repeatedly flash the strobe over an extended period without fully discharging.

In one embodiment of the invention, the notification devices can also make use of a fallback power strategy during a portion of the time when the notification devices are active. During a fallback power period, the strobe circuit of the notification device operates at a reduced power level. During the fallback power period, some of the current flowing through the NAC wiring is used to recharge the rechargeable energy storage device. The fallback period begins after the notification device has been active for a time period and the stored charge in the energy storage device has become partially depleted by the strobe circuit operating at a full power level.

DETAILED DESCRIPTION

FIG. 1shows notification appliance circuit (NAC) system10, which includes notification appliance circuit (NAC)12, control panel14, AC power supply16, and backup power supply18. In the embodiment shown inFIG. 1, NAC12is a two wire circuit including wires20A and20B, notification devices22, and termination resistor24. Each of the notification devices22includes high capacity energy storage device26such as a supercapacitor (SC) and strobe circuit (SCKT)28.

Control panel14is connected to one end of wires20A and20B. When a notification or alarm condition exists, control panel14activates NAC12by applying voltage of the proper polarity to wires20A and20B. The electrical power supplied over wires20A and20B activates each of notification devices22to produce an alarm or notification output, such as strobe flashes, an audible alarm, or both. Power to control panel14is normally supplied by AC power supply16. When AC power is not available, power is supplied to control panel14by backup power supply18.

When an alarm condition is not present, control panel16maintains the voltage on wires20A and20B in a reversed polarity to the polarity used during the active mode. When the reversed polarity is applied, NAC12is in the supervisory mode. Current can continue to flow through wires20A and20B and termination resistor24. This allows control panel14to monitor or supervise NAC12when the notification devices are not active. In the supervisory state, control panel14can monitor NAC12to detect open or shorted wiring strings by sensing current through termination resistor24.

During the supervisory mode, high capacity energy storage device26is charged to a fully charged state by current from NAC wires20A and20B, and then is maintained in that fully charged state until the next time NAC12is in the active state. Energy storage device26preferably is a supercapacitor (or supercapacitors) with the ability to store enough charge to operate strobe circuit28to produce flashes at a rate of, for example, 1 Hz for a period of 5 minutes or more without being recharged. Supercapacitors exhibit low leakage, so that the current required to maintain energy storage device26in a fully charged state is relatively low.

When control panel14switches NAC12to an active state, each of the notification devices22must be powered so that it can provide a visual or audible notification, or both. In some jurisdictions, it is required that an nominal 24 volt NAC excitation voltage with a steady current limit appropriate for powering a specified number of notification devices that are wired in parallel. The number of notification devices22that can be connected in NAC12, and therefore how far wires20A and20B can run, is dependent upon the maximum current draw of each notification device22. NAC system10reduces the maximum current supplied by control panel14when notification devices22are active by the use of high capacity energy storage devices26. Because energy storage device26can operate strobe circuit28over an extended time period without the need to be recharged after each flash, the current draw of notification devices22in the active state can be the current required to operate all of the circuitry other than the strobe circuit plus some charging current for partially recharging energy storage device26. As a result, a reduction in the overall current draw of NAC12during the active state can be achieved. Lower current draw offers the opportunity to increase the number of notification devices22and extend the coverage of NAC12.

FIG. 2is a block diagram of notification device22. In this embodiment, notification device22includes high capacity energy storage device26, strobe circuit28(which includes trigger circuit30and flashtube (FT)32), microcontroller (μC)34, clock36, output setting storage38, and power conditioning circuitry40.

Microcontroller34monitors the status of voltage on NAC lines20A and20B to determine when NAC12is in a supervisory state, and when it is in an active state. Microcontroller34provides control signals to power conditioning circuit40to control changing of energy storage device26and control signals to trigger circuit30to control the timing and intensity of flashes produced by flashtube32. Microcontroller34receives clock signals from clock36and instructions for output settings from output settings storage38.

Power conditioning circuitry40controls the charging of energy storage device26. In one embodiment, energy storage device26is a single supercapacitor which, when fully charged, has a voltage of between 300 to 400 volts. Power conditioning circuitry40charges energy storage device26during the supervisory mode until energy storage device26is fully charged. Once a full charge has been achieved, power conditioning circuitry40monitors the state-of-charge, and supplies additional charging current as needed to keep energy storage device26in a fully charged state. Once energy storage device26is fully charged, the amount of current draw required to maintain a full charge is very low.

When microcontroller34senses a change in polarity on wires20A and20B indicating an active mode, microcontroller34begins providing trigger pulses to trigger circuit30. The trigger pulses cause trigger circuit30to supply current from energy storage device26to flashtube32, which is a gas-filled flash tube, such as a xenon flash tube. The flashes produced by trigger circuit30and flashtube32will continue as long as notification device22and NAC12are in an active state. For an initial period of 5 minutes or more, the flashes produced by flashtube32have a duration of about 300 microseconds to 500 microseconds at a rate of 1 Hz. This results in a duty cycle of about 0.005 percent. With an average current of 100 milliamperes, the instantaneous current drawn from energy storage device26during one of the pulses may be on the order of 2000 amperes.

Energy storage device26has a storage capacity large enough to operate flashtube32for an extended period of time, such as 5 minutes or more, at a 1 Hz strobe rate without requiring a full recharge while in the active state. As a result, it is not necessary to deliver charging current sufficient to fully recharge energy storage device26after each flash, as has been the case with prior art notification devices that use an ordinary capacitor to store charge that is delivered to a flashtube. Power conditioning circuitry40may provide some recharging of energy storage device26throughout the period in which notification device22is in the active mode and strobe flashes are being generated. This charging current, however, does not need to be enough to replace the current drawn in generating a flash, because the storage capacity of energy storage device26is large enough to produce current to operate the flashtube for an extended period of time without fully recharging. For example, the charging current provided to energy storage device26while notification device22is active may be in the order of 2 milliamps.

If notification device22remains active for a long period of time, the net charge stored by energy storage device26will decrease. Based on the amount of time elapsed during the active mode, or based upon a sensed level of charge (or voltage) on energy storage device26, microcontroller34may initiate a fallback power operation in which intensity of the strobe flash is reduced so that less power is consumed, and the charging of energy storage device26between flashes is increased to build up the net charge stored in energy storage device26. Microcontroller34can control the intensity of the flashes by changing the voltage of the pulses supplied by trigger circuit30to flashtube32. Changing the voltage to flashtube32changes the brightness or intensity of the strobe flashes.

Restricting the full intensity to about 5 minutes has an additional benefit: it will greatly reduce the wear and tear on the notification devices over their operational life. The vast majority of the activations are for the non-emergency purposes of system maintenance testing and occupant training drills, rather than for actual emergency events. Thus, the reduced intensity is particularly appropriate in view of the non-emergency usage.

When the active state ends, flashtube32is no longer being flashed and no longer consuming power from energy storage device26. During the supervisory state, power conditioning circuitry40provides charging current to energy storage device26to recharge device26to its fully charged state, so that it is ready for the next time an active state occurs. Once a full charge is achieved, power conditioning circuitry40reduces the amount of charging current to only that which is needed to offset the loss of charge caused by leakage.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. For example, although the invention has been described in the context of a NAC system in which switching between supervisory and active states is achieved by reversing polarity of the NAC wires, the invention is applicable to other NAC configurations that do not rely upon polarity reversal to initiate an active or alarm state. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.