US3041458A - Fire detection system - Google Patents

Description

INVENTOR. RICHARD C. ROX BERRY BY s R. C. ROXBERRY FIRE DETECTION SYSTEM Filed Aug. 11, 1959 June 26, 1962 R iwmu Pm v ilniteci rates senate Patented June 2 6, 1962 ice 3,941,458 DETECTHON SYSTEM Richard C. Roxherry, Morristcwn, NJL, assignor to McGraw Edison Company, Elgin, EL, a corporation of Delaware Filed Aug. 11, 1%9, Ser. No. 832,940 5 Claims. (ill. 25t)-83.3)

This invention relates to novel fire detection systems using ultraviolet detection tubes adapted for A.C. operation such as are described and claimed in the pending application Serial No. 801,625 of Dennis H. Howling, filed March 24, 1959. More particularly, the invention relates to a simple and reliable operating circuit for this new type of detector tube.

A feature of the detector tube disclosed in the above Howling application is that when excited by ultraviolet light it will conduct alternating current responsive to an applied alternating voltage and can be worked in a low impedance circuit for maximum power efficiency since the tube is quenched at each half cycle by the reversal of the applied voltage. The present invention resides especially in a circuit for utilizing this detector tube in an eflicient, practical and dependable manner.

An object of the invention is to provide an AC. fire detection system of the ultraviolet detector tube type using very simple and economical circuitry.

Another object is to provide such novel fire detection system wherein proper integration and dropout timing is accomplished in a very simple manner to provide a system which will respond reliably to ultraviolet radiation from flame and give a suitable alarm without producing any alarm responsive to random ultraviolet excitations.

These and other objects and features of my invention will be apparent from the following description and the appended claims.

In the description of my invention reference is had to the accompanying circuit diagram of a fire detection sys tem incorporating the invention.

The fire detection system shown in the accompanying drawing uses an ultraviolet sensing tube of the character above-mentioned. This tube has two electrodes which have both mechanical and electrical symmetry permitting each electrode to operate equally well as cathode and anode. The tube can therefore be operated from an AC. voltage source with a first advantage that quenching is obtained at the end of each half cycle by the reversal of the applied voltage, and with a second advantage that since quenching is no longer a matter of circuit design the load impedance can be chosen to match the internal resistance of the tube for maximum power efficiency. Power outputs of as much as 5 watts are therefore feasible to enable direct operation of the power type control apparatus without need for any amplification.

One or more of the detection tubes, there being three shown by way of example referred to as a, 10b, and 130, are connected in parallel through cable sections 11a, 11b, and 1 10, the cable sections being for example in series to constitute in effect a single cable with taps leading olf to the individual tubes. At one end this cable is connected to a high voltage energizing circuit 12 coupled through a power transformer 13 to an AC. voltage source 14-. The source may be a 115 volt line of from 60 to 400 c.p.s. The transformer 13 has a stepped up ratio adapted to provide typically about 500 volts R.M.S. in the energizing circuit across the detector tubeswhich is a voltage the peak value of which is about midway between the striking voltage and the self counting voltage for the tubes. Also included serially in the energizing circuit 12 is the primary winding of an output transformer 15. This is a step-down transformer having typically a 10 to 1 turns ratio. The secondary winding of the transformer 15 is center tapped and grounded at 16 and the end terminals of the secondary winding are connected through respective half-wave rectifiers 17 and through a common resistor 18 to an RC. integrating network comprising a resistor 19 and condenser 20 in parallel. Connected across this network are the base and emitter of a common emitter transistor circuit 21. Between the emitter and ground is connected a load device comprising the coil 22 of a relay 23 having switch contacts 24. The collector is provided with power from the secondary Winding 25 of the power transformer 13. This winding is center tapped to ground and connected at its ends through rectifiers 26 to the collector electrode of transistor 21. The relay contacts 24 are connected by a lead wire 27 through an alarm device 28 and secondary winding 29 of the power transformer 13.

By way of example the resistor 18 may have 200 ohms, the resistor 19, 39,000 ohms, the condenser 20, microfarads and the relay 22, 1000 ohms. The RC. charge time of the condenser 20 is therefore very short, measuring of the order of 20 microseconds, but the RC discharge time of the condenser 2% through the resistor 19 and transistor 21 is relatively long, being from 2 to 3 seconds. The fact that the response of the circuit is very fast does not necessarily mean that the operate time of the system is equally as fast since the operate time depends also upon the intensity of the source of light being detected. To avoid any possibility of false alarms from background noise and random excitations the system is to require a given number of pulses within a given integration time to operate the relay, the number of pulses so required depending upon the sensitivity of the detector tube and of the relay, and on the integration time of the detector circuit. For example, a fire having an intensity at the detector tube of one foot-candle radiates only enough photons to excite the tube about ten times per second. Such low count rate will require about one second to build up sufficient charge in the condenser 20 to operate the relay. On the other hand, if the source being detected is an intense one providing a saturated excitation triggering the detector tube at the start of each half cycle of the applied voltage, the delay in the oper ation of the system is determined by the timing characteristics of the electrical circuits and the make time of the relay. The release or integration time-i.e., the time for the relay to drop out after a complete cessation of any excitation of the tubeis determined wholly by the electrical circuit constants. The use of a relatively long release time of 2 to 3 seconds is advantageous in eliminating possible erratic on-oif operation of the relay.

The wires of the cable sections 11a, 11b and have individual shields 30 to 31 shown grounded at 30a and 31a respectively. The distributed capacity of the one cable wire to its associated shield 30 forms effectively a capacity shunt 30c across the secondary winding of the power transformer 13 as dottedly shown. This shunt capacity provides no signal power to the output transformer 15. The distributed capacity of the other cable lead to its shield 31 provides in effect a shunt capacity 310 across the primary winding of the output transformer. This capacity also provides no signal power to the output transformer. Therefore, this mode of cable connection and shielding prevents the cable capacity from shunting the detector tubes with the result that the only signal fed to the output transformer is that obtained via the detector tubes.

The capacity 31c will however resonate with the leakage inductance of the output transformer. When this resonant frequency is set at the frequency of the power source, the capacity 310 is helpful in increasing the average output power per pulse derived from the detector tubes.

The only limit on the cable capacity-and hence cable lengthis the power dissipation limit of the detector tubes. A typical limit on distributed capacity of each side of the cable to ground is 5,000 micromicrofarads. Since commercial cables are available having a nominal shunt capacity of 25 micromicrofarads per foot the de tectortubes may be at a distance as great as 200 feet from the control equipment generally referred to as 32 in the drawing. V

The embodiment of my invention herein particularly shown and described is intended to be illustrative and not necessarily limitative of my invention since the same is subject to changes and modifications without departure from the scope of my invention, which I endeavor to express according to the following claims.

I claim:

1. A fire detection system comprising a detector tube responsive to ultraviolet light and adapted for A.C. operation, an AC. source of potential, a high voltage energizing circuit for said tube coupled to said source, a low impedance load device, step-down transformer coupling between said energizing circuit and load device, said energizing circuit having a' ground connection at one'side of the primary winding of said output transformer and said detector tube being connected between the other side of said primary winding and said source, a pair of individually shielded leads connecting said detector tube to said energizing circuit, means connecting the shields of said leads to said ground connection whereby the distributed capacity of one lead to ground is connected across said source and the distributed capacity of the other lead to ground is connected across said primary winding, and said output transformer having a leakage inductance resonant with the distributed capacity of said other lead at the frequency of said source.

2. A fire detection system comprising a detector tube responsive to ultraviolet light and adapted for AC. operation, a high voltage drive circuit for said tube adapted to be operated from an AC. source of potential, a stepdown output transformer having its primary winding serially connected with said tube in said drive circuit, a full wave rectifier connected to the secondary winding of said coupling transformer, an R.C'. integrating circuit connected to the output of said rectifier, an amplifier having an input circuit connected across said integrating circuit, a relay connected to the output of said amplifier, said R.C. integrating circuit including a condenser having a charge-up period via said rectifier, coupling transformer and drive circuit of the order of 20 milliseconds, and said condenser having a discharge period via the resistance of said R.C. integrating circuit and said amplifier of the order of 2 seconds.

3. A fire detection system comprising a detector tube responsive to ultraviolet light and having a symmetrical electrode arrangement adapting the tube for A.C. operation, a high voltage drive circuit including said tube, a step-up transformer having a primary winding adapted to be connected to a relatively low voltage source of alternating current and a secondary winding connected in said drive circuit, a step-down coupling transformer having a primary winding serially connected in said drive circuit and a tapped secondary winding, a pair of rectifiers oppositely series connected across said secondary winding of said coupling transformer, a filter network connected between the'junction between said rectifiers and the center tapof said tapped secondary winding, a transistor amplifi'er having an input circuit connected to the output of said filter network, and a control relay connected to the output of said transistor amplifier.

4. The fire detection system set forth in claim 3 wherein said network has a predetermined integrating time constant, and the sensitivity of said tube and relay and the gain of said amplifier are set to cause said relay to operate within one second when said tube is exposed to a fire having a radiation intensity at the tube of one foot-candle.

5. A fire detector system comprising an ultraviolet detector tube adapted to pass alternating current when subjected to an applied alternating current potential and excited by ultraviolet light, an AC. source of potential, a load device, an alternating current circuit serially including said source, detector tube and load device and corn prising individually shielded lead wires respectively connecting one terminal of said tube to one terminal of said source and the other terminal of said tube to one terminal of said load device, and a common ground connection for said shields and for the remaining other terminals of said source and load device whereby the distributed capacities of said lead wires are connectedacross said source and load device respectively.

References Cited in the file of this patent UNITED STATES PATENTS 1,822,061 Roberts Sept. 8, 1931 2,125,073 Knowles July 26, 1938 2,411,531 Engelhart g Nov. 26, 1946 2,448,503 Wilson Aug. 31, 1948 2,577,973 MacDougall Dec. 11, 1951 2,678,400 McKay May 11, 1954 2,678,401 Jaeger May 11, 1954 2,737,643 Marsden Mar. 6, 1956

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