21 7~8 (740-100 US) Patent Application ~or SELF-DIAGNOSTIC CIRCUIT FOR EMERGENCY LAMPHEAD
by DAVID A. VOSIKA
and ROBERT S. FELDSTEIN
Field of the Invention The present invention relates to a self-diagnostic circuit for use with an emergency lamphead. More fipecifically, the invention relates to a self-diagnostic circuit which is effective during standby operation of an emergency lamphead to indicate whether the lamphead is capable of operating in an emergency mode.
2170~8 Backqround of the Invention Emergency lighting systems are used in many types of facilities to provide DC battery-powered lighting during periods when the main AC power supply has become temporarily inoperative for some reason. Examples of such facilities include schools, hospitals, government offices, hotels and motels, industrial buildings, multi-unit dwellings, shopping malls, and airports. In many cases, these aL~cLures are very large and require that eme,gen~y la~rh~ be placed at several different locations to provide adequate co~.age. Fire safety codes require that eme~y~ lighting systems be tested periodically to ensure that they will operate ~,op~,ly ~r~n~ an em~ye~ . With a system e~ploying many separatQ lamph~A~ at scattered locations, these tests can b~ laborious and time-consuming to perform. For this reason, various types of self-diagnostic systems have been developed to facilitate the testing p,q~ .re.
A typical emergency lighting system consists of a battery for supplying power to one or more lampheA~ during an AC power 108~, a charger for charging the battery from the AC power supply during standby operation, and a relay or other type of swit~hin~ device for co~necting the la~ph~ to the battery when an AC power 1088 is detected.
When a self-diagnostic system is provided, it generally operates by briefly simulating an AC power outage and checking to be sure that the emergency lampheads illuminate properly. The test may be initiated manually, by depre~sing a pu8hbutton or operating a remote control device, or automatically in re~ponse to an internal timer.
In some cases, an intern~l control system (such as a microprocessor) automatically carries out a number of different tests in sequence, such as tests for lamp current flow, power transfer from charger to battery, and battery 217000~
voltage. If one or more of these tests fails, a light-emitting diode (LED) or other type of visual indicator may be illuminated to indicate that mainte~Ance is required.
In moro sophisticated systems employing central computer monitoring, an in~icAtion of test failure may also be produced on a computer display terminal at a central monitoring location.
In some eme-yenc~ la~rheA~ systems, the battery and charging circuitry are housed in a separate unit which is remote from somQ or all of the la~r~eA~ to which it is ~onnešted. When self-~A~qtic circuitry is provided, it will ordinarily be located in the central unit rather than in the remote lam~hoA~. This facilitates testing for proper battery and charger operation, but makes it difficult to check for ~G~e operation of the individual lam~h~A~. Problems which can render an individual lamrheA~ inoperable include a defective, hllr~e~ out or improperly connected lamp, or a wiring problem at the la~rh~A~. Most of these problems can be detected by che~i n~ for proper electrical continuity through each lamphead, but this is difficult to accomplish from a central location. The remote lamph~A~ are typically connected to each other and to the central battery and charging unit in a parallel ~daisy chain~ arrangement, and hence a self-diagnostic circuit located at the central unit cannot perform separate tefitfi on each lamphead to identify a specific la~rheA~ that requires fiervice. Typically, therefore, a central monitoring or diagnostic circuit shows that one of the lampheads is not operating for some reason, but doe~ not specify the identity or location of the inoperative lamphead. It then becomes necessary to place the entire system into emergency mode operation in order to visually identify the lamphead which is not operating.
The problem of checking for proper electrical continuity at remote lamrheA~ is more difficult to solve than might be expected. There i8 a need to minimize the number of lines or connections between the remote lamrheA~
and the central unit; therefore, the solution does not lie in r--nn- ng a large number of addit$onal wires between the remote lam~h~A~ and the central unit to support diagnostic functions. Conversely, the ~Yp~n~e and complexity of the self-~Aq~o~tic circuitry i8 ordinarily such that it is not practical to provide the circuitry at each remote lamphead location. Even if this were attempted, the~ ~daisy chain~
connection~ between remote lamph~A~ would give rise to the additio~l problem of maintA~ g proper isolation ~eL ~n the self-~Agn~t$c circu~t~ of the individual lamp~A~, 80 that the output of each diagnostic circuit will reflect the condition of its associated lamrheA~ without being affected by the condition of other lampheads.
SummarY of the Invention A primary ob~ect of the present invention i8 to provide an emergency lamphead self-diagnostic circuit which is simple and ineYr~n~ive in construction, and which can be cost-effectively integrated into each of a plurality of remote lamphe~ in a multiple-lamphead emergency lighting system.
A further ob~ect of the invention is to provide an emergency lamphead self-diagnostic circuit which can operate continuously rather than only during periodic testing cycles, ~o that component failures can be detected immediateily.
A further ob~ect of the invention is to provide an emergency lamphead self-diagnostic circuit which is compatible with existing types of emergency lighting systems, including those already incorporating other types of diagnostic or monitoring fiystems.
Still another ob~ect of the present invention is to provide an emergency la~r~eA~ self-diagnostic circuit which allows a number of remote lampheA~ to be connected to each other and to a central unit, using a minimum number of wires.
Still another ob~ect of the pre~ent invention i8 to provide an emergency lamphead self-diagnostic circuit which can be incorporated into each of a plurality of interco~n~cted remote lanr~eA~, while maintA ~ ni ng proper isolation between the diagnostic circuits of the individual lampheads.
The foregoing ob~ect~ _re substantially achieved by providing a eme~genc~ larr~A~ system which comprises at lea~t one ~mP~ye~c~ lamph~A~, a battery for supplying power to the em~yency lampheA~ during operation in an eme~ye--cy mode, a charger for charging the battery during operation in a stand~y mode, a transfer switch for switching the output of the to the battery lamphesd for operation in the emergency mode, and a self-diagnostic circuit co.~lccted to the battery and the lamphead for in~icAting during standby mode operation whether the lamphead is capable of operating in the emergency mode. The self-diagnostic circuit includes a high-imp~n~e circuit path which is conn~cted in series with the lamphead. The high-impedance circuit path includes an indicator which is energized by a battery current passing through the circuit path and the lamphead during operation in the standby mode. The battery current passing through the high-impedance circuit path i~
insufficient to illuminate the lamphead, but i~ sufficient to energize the indicator whenever proper electrical continuity exists through the lamphead.
In a preferred embodiment of the invention, the self-diagnostic circuit further comprises a second high-imre~Anc~ circuit path connected in parallel with the lamphead. The r~ron~ high-impe~nce circuit includes a second i n~ ~cAtor which is energized by a second battery current passing through the second high-imr~A~ce circuit path to ~n~cAte that the emergency lamphead is not capable of operating in the emergency mode. The flow of battery current in the second high-imr~An~e path is disabled in es~..se to the flow of ~u~.e~lL in the first high-impe~A~e circuit path, 80 that the second jn~ tor is de-energized whenever the first ~nA~cAtor is energized. The first and second indicators may, for e~ample, comprise green and red light-emitting diodes (LEDs) which are mounted on the exterior of the eme 9~ lamphead housing.
In accordance with another aspect of the present invention, a self-~A~no~tic circuit is provided for use with an eme~y~ncy lamphead system including at least one emel~e..cy la~r ~~~, a battery for supplying power to the emergency la~rh~A~ in an eme ~a~.~y mode, a charger for charging the battery in a standby mode, and a transfer switch for switching one polarity output of the battery to the lamph~ in the emergency mode. The self-diagnostic circuit comprise~ a first input terminal adapted to be connected to a first polarity output of the battery, a second input terminal to be adapted to be connected to a second polarity o~L~uL of the battery through the transfer switch, a third input terminal adapted to be connected to the second polarity o~L~uL of the battery without passing through the transfer switch, first and second output terminals adapted to be connected to the power tenminals of the emergency lamphead, with the first output terminal being coupled to the first input terminal, and a controlled switching device coupled between the second output terminal 217ű008 and the second input terminal. The controlled switching device is rendered conductive to energize the output terminals in response to battery voltage being applied between the first and second input terminals by the tranfffer ~witch in the em~l~e,~y mode, and is rendered noncon~nctive to de-energize the output terminals in respon~e to the battery voltage being removed fro~ the second input terminal by the transfer switch in the standby mode. A high-impc~-nce circuit path extends ~eL~ee~ the ~econd o~ L te~m~nal and the third input terminal, and includes an in~ator which is energized by a battery current passing through the high-imp~n~s circuit path and the emergency lamphead in the standby mode to in~icAte that the eme ~ la~rh~ is rAr~hle of operating. The i~p~Anc~ of the high-i~ro~An~e circuit path is high enough 80 that the battery current is insufficient to illuminate the emergency lamphead in the ~tandby mode, but i8 sufficient to energize the 1n~icAtor whenever proper electrical continuity exists through the emergency la~rhe~ .
In a preferred embodiment of the invention, a second high-imp~nce circuit path is provided between the first and third input terminals, and includes a second i n~ ic~tor which is energized by a battery current passing through the second high-i~p~Ance circuit path to indicate that the emergency la~rheA~ is not capable of operating. The flow of battery current in the second high-impe~nr~ circuit path is disabled by the flow of battery current in the first high-i~r~nce circuit path, so that the second indicator~is de-energized whenever the first indicator is energized. The first and second indicators may comprise LEDs of different colors mounted to the exterior of the e~ergency lamphead housing, as described previously.
The present invention is also directed to a method for monitoring the operational status of an emergency lamphead.
The method comprises the steps of placing the lamrheA~ in series with a first ~ nA ~ cAtor circuit which produces an output in .e_po.se to a flow of current through the first inAicAtor circuit; applying a voltage across the series combination of the lamphead and the first inAicAtor circuit to produce a flow of current through the first ~ Ator circuit when electrical continuity exists through the lamphead; and limiting the current to a ~alue sufficient to produce ~n Gu~puL from the first indicator circuit but insufficient to illuminat~ the lamphead. In a preferred embodiment of the invention, the method also comprises the steps of placing a second i nAi cAtor circuit in parallel across the series combination of the l_mrh~AA and the first 1n~i~Ator circuit, with the second indicator circuit producing an ou~puL in e~onse to a flow of ~u~ a..~
through the ~cco..~ cAtor circuit; and, in the absence of electrical continuity through the lamphead, causing current to flow through the second i nA i ~Ator circuit as a result of the applied voltage to produce an output from the second in~i~Ator circuit.
srief Descri~tion of the Drawinqs Referring now to the drawings, which form a part of the original disclosure:
Figure 1 is a block diagram of an emergency lighting system employing remote la~rh~A~ that incorporate self-diagnostic circuits in accordance with the present invention;
Figure 2 is a detailed schematic diagram of a preferred self-diagnostic circuit which may be incorporated into each of the remote lampheads of Figure 1, with a 2170~08 bipolar transistor used for isolating the lampheads from each other;
Figure 3 is a detailed ~chematic diagram of a modified version of the self-diagnostic circuit of Figure 2, adapted for operation at a higher battery voltage;
Figures 4 and 5 are detailed sch~mAtic diagrams of further modifications of the self-diagnostic circuit of Figure 2, adapted for operation with emergency lighting systems that switch the opposite polarity leg of the battery circuit during the transition from standby mode operation to eme~e.~-y m~de operation; and Figure~ 6 and 7 are detailed schematic diagrams of still other mc~ f ~ ~ versions of the self-diagnostic circuit of Figure 2, employing field effect transistors (FETs) rather than bipolar transistors for lamphead isolation.
Throughout the drawings, like reference numerals will be understood to refer to like parts and components.
Detailed Description of the Preferred Embodiments An eme~gan~y lighting system 10 in accordance with a preferred embodiment of the present invention is illustrated in Figure 1. The system 10 includes a power supply transformer 12 which i~ connected to an incoming AC
power supply, 14 typically ranging from 120 to 34~ volts AC
at 50 or 60 Hz. The transformer 12 steps down the incoming AC voltage to a level that is suitable as an input to a battery charger 18. The charger 18 i8 of a conventional type and includes DC rectifying and voltage regulating circuitr~ 19 for maint~ini~g a battery 20 in a fully charged condition. The charger 18 has four output terminals which are designated B+, B-, L- and L+, respectively. The B+ and B- terminals are the battery terminals of the charger 18 and are connected to the 2170~08 positive and negative terminals of the battery 20, respectively. The L- and L+ terminals are the lamp output terminals of the charger 18 and are co~nocted in a parallel ~daisy chain~ arr_ngement, as shown, to the power terminals of a plurality of remote lamrh~A~ 22, 22' and 22~. For the purpose~ of the present invention, the B- terminals of the charger 18 and battery 20 are also r~nn~šted to each of the remote lampheA~ 22, 22' and 22~ in the same manner.
Thus, each lamphead has three input term~nals L+, L- and B-. The ter~n~ls L+ and L- are the power input terminals for operating thQ lamphead in the emQ ~n~y mode, and the terminal B- is an additional power input terminal for operating the self-diagnostic circuitry of the lamphead during standby oper~tion, as will be described shortly.
In order to switch between standby and emergency mode operation, the charger 18 includes an internal relay 24 whose coil 26 i~ coupled to a transistor (not shown) that senses the potential across the line (L) and neutral (N) outputs of the transformer 12. When AC power is available from the incoming power supply 14, the relay contacts are held in the unswitched (open) po~ition as shown in solid outline in Figure 1. In this condition, the lamp terminal L- is open-circuited and the lampheA~ 22, 22' and 22~ are therefore maintA i n~ in the standby or non-illuminated mode. The charging circuitry (not shown) within the charger 18 maintains the battery 20 in a fully charged condition during the standby mode. When the AS power from the incoming supply 14 is interrupted or falls below a predetermined level, the transistor energizes the relay coil 26 and causes the relay contacts to move to the switched (closed) position as shown in phantom in Figure 1.
In this position, the relay contacts connect the terminals B+ and B- of the battery 20 to the lamp output terminals L+
and L-, respectively, in order to illuminate the remote lampheads 22, 22' and 22~. Thus, the relay 24 serves as a transfer switch for automatically initiating emergency mode operation in the event of a power supply interruption, and for autom~t~cally ~e~ ing the system 10 to standby operation once power has been restored. In practice, the relay 24 switches only the negative (B-) te-minal of the battery 20 between the charging circuitry and the negative (L~ L~uL of the charger 18, as shown, and the positive (B+) termin~l of the battery 20 is permanently wired to the positive (L+) l~mp output of the charger 18. However, it is ~180 pos~ible to use the relay 24 to switch the positive (B+) termin~l of the battery 20 to the positive (L+) lamp o~l~uL of the charger 18. As another modification, it is possible to use ~ power transistor in lieu of the relay 24 to isolate the battery 20 from the lamp terminal L- or L+
during standby mode operation.
The transfor~er 12, charger 18 and battery 20 are preferably ho-~Pe1 in a single central unit 32 which is connected by means of wire runs 30, 30~ and 30~ to the remote lamp~eA~ 22, 22' and 22~. The lampheads 22, 22' and 22~ may be placed at various locations throughout a building or other structure to provide emergency lighting whe e~e. ~q~oA. Any desired number of lamph~A~ 22, 22' and 22~ may be connected to the central unit 32, sub~ect to the ~u e,.t rating of the battery 20. An example of a commercially available battery and charging assembly that may be used as the central unit 32 i8 the Model HP12100 eme~gen~y charger manufacL~,ed by ~nhhel 1 Lighting, Inc., of Christianburg, Virginia, which switches the lamp output on the positive leg of the battery, or the Hubbell Lighting M~-l PE612 emergency unit, which switches the lamp ouL~uL
on the negative leg of the battery.
As illustrated in Figure 1, all of the remote lamph~ 22, 22~ and 22~ may be essentially identical in construction. Referring to the remote l_mphead 22 for convenience, the lampheA~ will be seen to include a small housing 36 which serves the dual purpose of providing a mounting or attachment point for securing the lamphead to a shelf or wall, and enclosing a self-diagnostic circuit to be described shortly. The housing 36 carries an eme~en~y lamp 38 and lamp enclosure 40 by means of a two-axis rotatable ~oint 42, which allows the lamp 38 and enclosure 40 to be ai~ or pointed in the desired direction. On the front panel of the housing 36 ~re two light-emitting ~io~e~
(LEDs) 44 and 46 which sQrve as the ouL~L of the self-diagnostic circuit of the lamphead 22. The left-hand LED
44 is preferably gre~n in color and, when ill~minated, ~n~cAtes that proper electr~c~l continuity exists in the la~rheA~ 22. The right-hand LED 46 is preferably red in color And, when illuminated, i nA 1 cAtes that proper continuity does not exist through the lamph~A~ 22. Lack of continuity may result from several factors, including a h~r~e~ out, defective or improperly installed lamp 38 or improper or defective wiring in the l_mphead 22. As will be described below, the self-diagnostic circuit is capable of operating continuously during standby operation of the emergency lighting system 10, and hence the LEDs 44 and 46 will provide a continuous i n~ ir~tion of the status of the lamph~AA 22.
A detailed schematic diagram of a preferred self-diagnostic circuit 50 which may be incorporated into each of the emergency la~heA~s 22, 22' and 22~ of Figure 1 is illustrated in Eigure 2. The self-diagnostic circuit includes a first input terminal 52 which i8 connected to the L+ output of the charger 18 in Figure 1 (as previously noted, this terminal is permanently wired to the B+
terminal of the battery 20). The circuit 50 also includes a second input terminal 54 which i8 connected to the L-output of the charger 18. During standby operation of the emergency lighting sy~tem 10, the relay 24 of Figure 1 maintains the input terminal 54 in a open-circuit condition; however, during emergency mode operation, the relay 24 connects the input terminal 54 to the B- terminal of the battery 20. A third input terminal of the ~elf-diagnostic circuit 50, in~ic~ted at 56 in Figure 2, i8 co~,octed directly to the B- term;nal of the battery 20 without passing through the contacts of the relay 24.
Thuo~ a voltage is ~-~nt between the third input termin~l 56 and the first input termlnal 52 during st~ndby operation of the eme ye~y lighting syste~, and this provides power for the operation of the self-~lAr -eLic circuit 50.
The Oelf-~AgnoOtic circuit 50 also includes first and second output ter~n~lR 58 ~nd 60, .FOp~cLively. The firOt ou~u~ terminal 58 is co~ ed directly to the firOt input terminal 52, a8 shown. The o~L~L terminals 58 and 60 are connected to the lamp leads 62 and 64, r~O~ecLively, of the eme~yen~y lamphead circuit. For the purposes of illustration, the emergency la~pheA~ circuit is illuOtrated in Figure 2 as including only the lamp 38. In reality, however, the lamphead circuit will also include the lamp socket and it~ associated wiring. By connecting the output te-minals 58 and 60 of the self-diagnostic circuit 50 across the entire lampheA~ circuit, lack of electrical continuity at any point in the lamphead circuit can be detected.
The self-diagnostic circuit 50 includes two high-impedance circuit paths 66 and 68, with the first high-impedance;circuit path being connected in Oeries with the lamphead circuit and the second high-impedance circuit path 68 being connected in parallel with the lamphead circuit.
The first high-impedance circuit path 66 includes a silicon junction diode 70 and a green LED 72 connected in series (and in the same polarity orientation) between the output terminal 60 and a common node 74. A resistor 76 is connected between the common node 74 and the third input terminal 56 to provide the circuit path 66 with the desired imr~nce. Preferably, the resistor 76 has an imr~n~e with is much higher (e.g., by two orders of magnitude or more) than the impe~nce of the lamp 38 and associated lamphead circuitry. Thus, for example, a lamrh~A~ circuit ut~ inq a 6-volt, 25-watt lamp 38 will have a cold DC
resistance or i~rs~n~ value of ~ o 1mAtely 0.5 ohms and a hot DC res~st~nc~ or imp~ s value of approximately 1.5 ohms. In this example, a resistor 76 having a value of 390 ohms m~y be u~ . The resistance value is chor3n 80 that current flow and power dissipation in the high-imp~nce circuit path 66 will be _inimized, with the current held to a value insufficient to illllminate the lamp 38. At the same time, however, the voltage and cul~nt applied to the LED 72 are sufficient to illtlminate the LED
when continuity exists through the la~r~ circuit.
The second high-imre~nce circuit path 68 i~ connected in parallel across the lamr~A~ circuit and includes two silicon ~unction diodes 78 and 80 and a red LED 82, all connected in series (and in the ~ame polarity orientation) between the first input terminal 52 and the common node 74.
A bypass resistor 84 is connected in parallel across the LED 82. The re~istor 76 connected between the common node 74 and the third input terminal 56 i8 ~hared with the first high-impedance circuit path 66 and provides the second high-impe~Ance circuit path 68 with an equivalent resi~tance. As in the case of the fir~t high-impe~Ance circuit path 66, the resistor 76 limits current flow and power di~sipation in the second high-imre~nce circuit path 68 under conditions when the red LED 82 is illuminated.
_ 15 -The self-diagnostic circuit 50 also includes a bipolar NPN transistor 86 which has its collector connected to the second output terminal 60 and its emitter connected to the second input terminal 54. A leakage bypass resistor 88 is connected between the ba~e of the transistor and the second input terminal 54. A biasing resistor 90 and two ~ ?~ 92 and 94 of the same polarity are ~o----ecLed in series between the first input terminal 52 and the node 96 between the resistor 88 and the basQ of thQ transistor 86. In this way, base drive is provided to the transistor 86 when a sufficient voltage appears between the first and ~econd input terminals 52 and 54. When the transistor is conducting, current is allowed to pass between the collector and emltter of the tr~nsistor 86, thereby illuminating the emer~en~ lamp 38. The transistor 86 serve~ as a controlled swi~chi nq device for providing isolation between the self-diagno~tic circuit 50 and the ~elf-~iA~noatic circuits of other ro~nected lampheads, as will be expl~ine~ in more detail shortly. The current conduction capability of the transistor 86 is sufficient to handle the current drawn by the lamp 38 when the latter is in its energized or illllm;nated condition.
The operation of the self-diagnostic circuit 50 of Figure 2 will be evident from the foregoing description.
During standby operation of the emergency lighting system 10, battery voltage is provided between the first and third input terminals 52 and 56, respectively, but the second input terminal 54 is open-circuited. In this condition, no current flows through the circuit path consisting of the resistors~88 and 90 and diodes 92 and 94, and hence no base drive is provided to the transistor 86. The transistor 86 is thus maint~ine~ in a nonconducting (cutoff) state. At the same time, however, the output voltage of the battery 20 in Figure 1 is applied across the first and third input terminals 52 and 56, and (assuming proper lamphead continuity) this re~ults in voltages being applied across both the first and second high-impq~n~ circuit paths 66 and 68. The resulting current in the first high-impe~nce circuit path 66 illuminates the green LED 72, i~icAting that proper continuity exists through the lamp 38 and associated lamphead circuitry. Afi i8 known, the voltage drop across a silic~n ~unction diode in the conducting state is approximately 0.7 volt, while the voltage drop across an LED in the con~ ting state is a~.o~imately 2 volts. Thus, assuming for the ~u~GOe of example that the battery 20 of Figure 1 ~ ~1..~ an ou~uL of 6.8 volts at full charge, the ag~ te voltage drop across the series-connected diode 70 and green LED 72 in the first-high imre~r9 circuit path 66 will ~e a~ imately 2.7 volts.
This leaves s~ ~imately 4 volts to be divided between the lamp 38 (and associated lamphead circuitry) and the resistor 76. R~CAt~ the imF~n~s of the resistor 76 is much greater than that of the lamphead, virtually all of this voltage will A~reAr across the resistor 76. It follows that, in the case of the second high-impe~nce circuit path 68, there is only approximately 2.7 volts to be divided among the diodes 78 and 80 and red LED 82. This potential is insufficient to place all three devices into conduction. The resulting non-illuminated condition of the red LED 82 provides an additional indication that proper continuity exists through the lamphead 22. The bypass resistor 84 prevents any illumination of the LED 82 from the very small current passing through the second high-impedanceicircuit path 68.
Let is now be assumed that the emergency lighting system is still operating in a standby condition, but that proper electrical continuity does not exist through the lamphead 22 due to a burned-out bulb 38 or one of the other conditions mentioned earlier. In this situation, no current can flow through the first high-impedance circuit path 66, and hence the green LED 72 is no longer illuminated. This provides an indication that a problem exi~ts at the la~rh~A~ 22 requiring service. With the first high-imFe~-n~e circuit path 66 no longer conducting, the voltage across the resistor 76 is no longer held at 4 volts and can transition to a lower value. With a batterv voltage of 6.8 volts applied across the first and third input termln~ls 52 and 56, the ~ 78 and 80 and red LED
82 of the -~c~n~ high-imFs~-n~s circuit path will pnD~u_e an a~ g~te voltage drop of approxlmately 3.4 volts, leaving approx~mately 3.4 volts across the resistor 76.
The diodes 78 and 80 and red LED 82 are now in ~-o~ ion, and the ill~minated condition of the red LED 82 (together with the non-illuminated condition of the green LED 72) in~ te~ that ~L~e~ electrical continuity does not exist in the lamphead 22. This provides a warning to mainten~n~
personnel that the lamphead 22 is not capable of operating in the emergency mode, and that bulb replacement or other service is required.
As noted previously, eme~ye-~y mode operation is initiated at the charger 18 of Figure 1 by csnn~cting the B- battery terminal to the L- lamp ou~u~ terminal. This has the effect, in the self-diagnostic circuit 50 of Figure 2, of electrically co~lrl i ng the second and third input terminals 54 and 56 to each other and thereby placing the transistor 86 into saturation. With the transistor 86 conducting, the first high-impedance circuit path 66 is bypassed and the green LED 72, if previously illuminated, is now exting~ he~. Thus, during emergency mode operation, the bulb 38 of a functioning lamphead will be illuminated but the green LED 72 will not. However, whether or not the red LED 82 was illuminated prior to the initiation of emergency mode operation (indicating a burned-out bulb 38 or other problem in the lamphead), it will be illuminated for the duration of the emergency.
This i8 a result of the fact that the second high-imp~nre circuit path 68 is connected across the battery terminal6, and hence receives battery voltage even when an open-circuit condition exists within the lamphead. The illumination of the red LED 82 i n~ i cAtes that emergency mode operation is in effect and provides a positive ~ n~ ~ cAtion that battery voltage is available at the lamrh~. Thus, the u~er i~ alerted that ~ny failure of the lamp 38 to illuminate is due to a bulb failure or other problem at the lamphead itself, rather than to a defect in the wiring leA~in~ to the lamphe_d.
The bipolar transistor 86 in the self-diagnostic circuit 50 of Figure 2 provides isolation between different lampheads when a plurality of lampheads 22, 22' and 22~ are connected together in a parallel ~daisy chain~ arrangement as illustrated in Figure 1. In the absence of the transistor 86, a common path would exist through the second input terminals 54 of the lamph~A~ and would allow the green LED 72 of a given lamphead to be illl~minated even when proper continuity does not exist through that particular lamphead due to a hl~r~e~-out bulb 38 or other problem. When the transistor 86 is in saturation, the voltage drop between its collector and emitter is negligible (about 0.1 volt), and hence the light output of the lamphead 22 in the emergency mode is not significantly affected. It will also be appreciated that the operation of the ~self-diagnostic circuit S0 of Figure 2 is essentially transparent from the standpoint of the first and second input terminals 52 and 54; that is, the lamphead circuit behaves in essentially the same manner (in terms of voltage and current characteristics) whether or not the self-diagnostic circuit is connected. The only differences are a slight increase in emergency mode current attributable to the base circuit of the transistor 86, and an added voltage drop attributable to the collector-to-emitter voltage across the transistor. Both of these factors can be minimized by appropriate choice of the transistor 86. It will be appreciated that the ~transparency~ of the self-diagnostic circuit 50 is advantageous in that it allows a lamphead incorporating the self-diagnostic circuit to be used with existing types of chargers 18 or ce~LLal units 32 (including those incorporating other types of ~Aqnostic and self-testing circuits) without requiring any special modifications.
As will be evident from the foregoing description of the self-diagnostic circuit 50, the alternative operation of the green and red LEDs 72 and 82 arises from the fact that the agy~e~ate diode voltage drop in the second high-impe~ circuit path 68 i~ greater than that in the first high-imr~n~e c~rcuit path 66. In the illustrated embodiment, this results from the use of two series-connected diodes 78 and 80 in the second high-i~re~Ance circuit path 68 and one diode 70 in the first high-impedance circuit path 66, as shown. However, the same result may be obtA i n~ by increasing the number of ~io~e~
in each circuit path while maintAining the total number of diodes in the circuit path 68 at least one greater than the total number of diodes in the circuit path 66. It is also possible to reduce the number of diodes in each of the circuit paths 66 and 68 by one, but this would subject the LED 72 to~reverse bias potentials that may be damaging over time. The connection of the bypass resistor 84 in parallel across the red LED 82 prevents the red LED from glowing when the green LED 72 is illuminated, by bypassing any current that may occur through the diodes 78 and 80.
Figure 3 illustrates a modified version S0-1 of the ~elf-diagnostic circuit 50 of Figure 2 which is adapted for 12-volt rather than 6-volt operation. Nost of the circuit com~n~ts are identical and have been designated by corresponding reference numerals. However, in order to reduce power dissipation at the higher voltage level, the resistor 76 is replaced by a resistor 98 having a higher resistance value (preferably 1 kilohm). In addition, the biasing resistor 90 of Figure 2 is replaced by two higher-value resistors 100 and 102 connected in parallel. In this way, the current is split between the two resi~tors ~o that resistors having lower ~e~ ratings c~n be used. Finally, the 6-volt lamp of Figure 2 is replaced by a 12-volt lamp 104 preferably having the sAme 25-watt power rating.
Figure 4 illustrates a further mQAifi~Ation 50-2 of the self-diagnostic circuit 50 of Figure 2, adapted for use with an emergency lighting system in which the positive leg (B+) of the battery 20 of Figure 1 is switched during the transitlon between standby and emergency operation. The circuit is essentially equivalent to that shown in Figure 2, except that the polarities of the diodes 70, 78, 80, 92 and 94 and LEDs 72 and 82 are reversed. In addition, the bipolar NPN tran~istor 86 of Figure 2 is replaced by a bipolar PNP transistor 106. The operation of the two circuits is substantially the same, except for the directions of voltage drops and current flow Figure 5 illustrates a modification 50-3 of the self-diagnostic circuit 50-2 of Figure 4, which is adapted for 12-volt rather than 6-volt operation. The circuit i8 equivalent in most respects to that of Figure 4, except for the substitution of resistors 98, 100 and 102 having values equivalent to those of Figure 3. In addition, as in the circuit of Figure 3, a 12-volt lamp 104 is substituted for the 6-volt lamp 38.
Figure 6 illustrates a still further modification 50-4 of the self-diagnoRtic circuit 50 of Figure 2. In this modification, a field effect tran~i~tor (FET) 108 is substituted for the bipolar (~unction) transistor 86 of Figure 2, and the resistors 88 and 90 and diodes 92 and 94 are deleted. The FET embodiment is advantageous in that the gate of the FET provides a higher input i~sn-~ than the base of a ~unction transistor, thereby reducing power dissipation and parasitic current losses. Also, since the high input imp~ r~ of the FET me~ns that essentially zero CUrrQnt i8 required to control co~ tion of the FET, no additional components are required to provide biasing current. Thi~ results in a lower component count, lower cost and t~ circuit board area. An N-ch~nn~l metal-o~cide-sem~ on~lllctor field effect transistor (MOSFET) is illustrated in Figure 6, but other types of field-effect devices, such as P-c~-n~ OS~ E or ~unction field effect transistors (JFETs), may be used in other embod~ent~.
Figure 7 illustrates a modification 50-5 of the self-diagnostic circuit shown in Figure 6, which is adapted for 12-volt rather than 6-volt operation. This embodiment is similar to that of E'igure 6, except that the higher value resistor 98 of Figures 3 and S is Rubstituted for the resistor 76 of Figure 6, and the 12-volt lamp 104 of Figures 3 and 5 is substituted for the 6-volt lamp 38 of Figure 6. Also, an FET 110 having a gate resistance suited for 12-volt operation is sub~tituted for the 6-volt FET 108 of Figure 6. It will be appreciated that the self-diagno~tic circuits of Figures 6 and 7 can be further modified along the lines of Figures 4 and 5, for use with emergency lighting system~ which switch on the positive side of the battery 20 in Figure 1. This can be accomplished by substituting a P-channel MOSFET for the N-channel MOSFET 108 of Figure 7.
Table 1 below is a truth table which summarizes the states of the green and red LEDs 72 and 82 during standby and emergency mode operation. A~ ~n~irAted in Table 1, the green LED i~ on only during ~tandby operation when proper electrical continuity exists at the lamphead. The red LED
is on during standby operation when a lack of electrical continuity i8 detected in the la~rh~A~, and i8 always on during emergency operation.
Table 1 Mode Lamphead Green LED Red LED
Standby Good On Off Standby Bad ,Off On Emergency Good Off On Emergency Bad Off On Preferred values for the electrical components used in the self-diagnostic circuits of Figures 2 - 7 are provided in Table 2 below. Resistor values are expressed in ohms (D) or kilohms (R). All resistors are ~-watt unless otherwise noted.
Table 2 Component Value or Type Lamp 38 6 volts, 25 watts mas.
Diodes 70, 78, 80, 92, 94 lN4001 LEDs 72, 82 20 milliamp~, 2.1 volts Resistor 76 390 ohms Resistor 84, 98 lR
Tr~n~istor 86 MJE3055T with heat ~ink Resi~tor 88 4.7K
Resistor 90 22 ohms (2 watts) Resistor~ 100, 102 100 ohms (2 watts) Lamp 104 12 volts, 25 watts Transistor 106 MJE2995T with heat sink FET 108 Phillips BUR 553-SOB or Motorola MTP-3055EL
FET 110 Phillips BUR 453-SOB or Motorola MTP-3055E
The self-diagnostic circuits illustrated in Figures 2 - 7 are advantageous in that they can be used to provide a continuous indication of the operating status of an emergency lamphead during standby mode operation. The self-diÓgnostic circuits are simple in design and employ only small number of relatively inexpensive components, thereby making it practical to incorporate the circuits into individual remote lampheads in a multiple-lamphead system. The self-diagnostic circuits are compatible with 217000~
existing types of emergency lamphead systems, including those already incorporating centralized self-testing circuits, and provide ~uitable isolation when multiple remote lampheA~ are connected together. The self-~iAgno~tic circuit in each lamphead requires only one addition~l conductor (co e~~ i ng to the third input terminal 56 in Figure~ 2 - 7) to prov$de power to the circuit during standby mode operation, and this co~ ctor may have a very small diameter since the current drawn by the self-diagno~tic circuit is quite low.
While only a limited number of exemplary embodiments have been chosen to illustrate the present invention, it will b~ understood by tho~e skilled in the art that various modification~ can be made therein. For example, it will be apparent that while the self-diagnostic circuits of the present invention are well-suited to continuous operation, they can be adapted for use in a periodic or inte~m1ttent testing mode if desired. Mo eG~e~, although the self-diagnostic circuits include bipolar transistors or FETs for isolation purposes, these components (together with the resistors and ~i~AeS used for biasing the bipolar transistors) can be deleted if the self-diagnostic circuit is used for only a single lamphead. These and other modifications are in~en~e~ to fall within the spirit and scope of the invention as defined in the appended claims.