Abstract:
A control circuit for selectively allowing a backup battery to be physically connected to a load without being electrically connected to the load includes a bistable multivibrator circuit, a power-to-load switching circuit connected to the multivibrator circuit, a delay circuit and a sleep state switching circuit. The multivibrator circuit is in a first sleep state when the backup battery is connected to the control circuit but no voltage is supplied to the control circuit from the power supply. Under these circumstances, the control circuit provides no power supply voltage or backup battery voltage to the load. The multivibrator circuit changes to a second state from the first state when a power supply voltage is provided to the control circuit after the multivibrator circuit is already in the first state. Then, the control circuit will provide either the power supply voltage or the backup battery voltage to the load. The multivibrator circuit remains in the second state unless it receives a signal from the sleep state switching circuit which places it back into the first sleep state.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to U.S. provisional patent application Ser. No. 60/636,404, filed on Dec. 15, 2004, and entitled “In an Electrically Operated Product, Circuit for Allowing a Backup Battery to be Physically Connected Without Being Electrically Connected”, the disclosure of which is incorporated herein by reference. This application claims the benefit of priority under 35 U.S.C. 119 to the aforementioned related provisional application. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an electronic circuit, designed for utilization in electrically operated devices which normally operate using standard AC line voltage as a power source, and which said devices also incorporate a backup battery as a secondary power source when AC line voltage is not available, such as during a power failure.  
         [0004]     2. Description of the Prior Art  
         [0005]     A number of circuits, such as emergency lighting circuits, are powered by AC line voltage but have a rechargeable backup battery as a secondary power source when the AC line voltage is not available. In many such circuits, relays are used to switch in the backup battery to provide power to the light or other circuit in an emergency situation when the AC power has failed. However, such a relay configuration requires a relatively large amount of current drawn from the backup battery in order to connect the battery to the light or other circuit.  
         [0006]     A problem may occur when an appliance or emergency light having a backup battery is first installed in a building, office, store or other premises under construction or renovation before AC power has been applied. If the backup battery is connected to the circuit prior to AC line voltage being supplied, obviously the backup battery will provide power to the emergency light or other circuit until the battery is entirely drained. Accordingly, installers may be required to remove the backup battery from the circuit until AC line voltage is available in the premises. This requires a return trip by the installer to the premises to reconnect the backup battery so that the backup battery will remain charged by its connection to the AC line voltage, and current will only be drawn from the backup battery when there is a power failure.  
         [0007]     There are a number of patents which describe circuits that place a backup battery in what is commonly referred to as a “sleep mode”. These include, for example, U.S. Pat. No. 6,144,186, which issued to Iilonga Thandiwe et al.; U.S. Pat. No. 6,075,742, which issued to Tom Youssef et al.; and U.S. Pat. No. 6,545,447, which issued to Gregory J. Smith. In the “sleep mode” operating condition, most if not all of the prior art circuits, for example, that disclosed in the Smith patent, draw very little current from the backup battery until the backup battery is “awakened” when a charge or load is applied to the battery that exceeds a predetermined threshold. Such a circuit does not solve the problem of having the backup battery physically connected to the load without being electrically connected, as once a load is placed on the battery without AC line voltage being connected to the circuit, the battery will be awakened from its sleep mode and provide current to the emergency light or other circuit to which it is connected.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0008]     The purposes of this circuitry are firstly, to provide a means for a backup battery to be physically connected at all times to the aforementioned device, secondly, to provide a means for the backup battery to become electrically connected to the device when AC line voltage is first supplied, thirdly, to provide a means for the backup battery to remain electrically connected to the device as a secondary power source whenever AC line voltage is subsequently terminated such as during a power failure, and lastly, to consume a minimal amount of current while performing all of these functions. Devices incorporating this circuitry will have the capability of being shipped from the manufacturer or to a customer with a charged battery pre-connected physically, but not electrically, as previously described. A typical application for this circuitry would include devices which may be installed before AC power is available, such as in new construction, and would include such devices as emergency lighting or monitoring equipment such as temperature alarms, security systems, smoke detectors and the like. A significant advantage of this circuitry is that, in scenarios such as the one previously described regarding new construction, it would eliminate the need of having personnel return to a facility to physically connect the battery once AC power is available.  
         [0009]     It is possible to perform the functionality for which this circuitry is intended by using a latching relay, but there are a number of significant advantages that this circuitry offers over a relay based configuration: firstly, the circuitry, which will be subsequently disclosed, is less expensive than utilizing a relay, secondly, this circuitry can be configured in less space than a relay would require, making it possible to easily have the circuitry added to the respective device or even built into the battery itself, and thirdly, this circuitry will consume an extremely small fraction of the current required by a relay configuration. This last advantage is of particular importance when a device utilizing this circuitry is actually operating off of the battery, such as during a power failure. At such times, the life of the battery is dependent on the amount of current that is being drawn by the circuitry that it is powering. The minimal amount of current that is drawn by this circuitry will help to prolong battery life when such power failures occur.  
         [0010]     In accordance with one form of the present invention, a circuit which allows a backup battery to be physically connected without being electrically connected includes a control circuit having a bistable multivibrator which senses whether the backup battery is connected to the circuit and the load prior to AC line voltage being supplied to the circuit. The circuit of the present invention allows the backup battery to be physically connected to the load, but not electrically connected, during the condition when AC line voltage has not yet been supplied to the circuit. The circuit of the present invention also senses the presence of AC line voltage and, accordingly, changes state to allow power to be supplied to the load either by the backup battery or the AC line voltage.  
         [0011]     These and other objects, features and advantages of the present invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic of the aforementioned circuitry of the present invention as utilized in a typical application.  
         [0013]      FIG. 2  is a schematic of a variation of the circuitry shown in  FIG. 1  and formed in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     With reference to  FIGS. 1 and 2 , a description of the control circuit of the present invention will now follow.  
         [0015]     PS 1  is a block representation of a typical power supply, which is designed to take AC line voltage, normally 120VAC, change the line voltage to a lower voltage and then convert the lower voltage to DC voltage. This resultant lowered DC voltage is typically what is required to operate many electrical or electronic devices, and PS 1  is the primary power source for device operation. BAT  1  is a battery, such as a 6 volt rechargeable battery, which serves as a secondary source of device operation when PS 1  is not activated, such as during a power failure. D 1  and D 2  are diodes such as a 1N4001, and they are configured so that, while voltage supplied by PS 1  and BAT  1  are joined as one power source for device operation, power from PS 1  cannot feed back into BAT  1 , and power from BAT 1  cannot feed back into PS 1 . The joined cathode connection of D 1  and D 2  is then connected to the emitter of Q 2 . Q 1  and Q 2  are PNP transistors such as a PN2907, connected in a Darlington configuration, and together, Q 1  and Q 2  serve as a switch (i.e., a power-to-load switching circuit) for turning power, received from the cathode junction of D 1  and D 2 , on and off to DC 1 , DC 1  being the device circuitry operated by PS- 1  or BAT 1 . If the base of Q 1  is HI, (logic 1), then power to DC 1  will be off. If the base of Q 1  is LO, (Logic 0), then power to DC 1  will be on.  
         [0016]     Central to the circuit operation is IC 1 A, IC 1 B, IC 1 C and IC 1 D, which together, comprise a four input NAND gate integrated circuit, wholly identified as IC 1 . Many of the other electronic devices heretofore and hereinafter described, such as Q 1 , (PN2907), or D 1 , (1N4001), are typical devices which may be interchanged with other similar devices. This however, is not so of IC 1 . In the preferred embodiment, IC 1  is a CMOS device, identified as part #4011. While there are a number of other similar devices to the 4011 in terms of logic function, the two important advantages of the 4011 are that firstly, the 4011 has a wide operating voltage range (3-18 VDC, typical), making it suitable for application in power supply/battery configurations of different voltages, and secondly, the 4011 consumes very little current (6 μuA.), thereby causing a minimal current drain while performing its intended tasks. IC 1 B and IC 1 C are configured in the circuit so as to create a bistable multivibrator circuit or “flip flop”, a circuit which functions as a one bit memory. Pin  8  of IC 1 B and Pin  12  of IC 1 C are inputs to the flip flop, and Pin  10  of IC 1 B (also including Pin  13  of IC 1 C by means of connection in the circuitry), is the output of the flip flop. The truth table of any one of the four NAND gates comprising IC 1  is as follows: If either or both inputs=0, the output=1. If both inputs=1, the output is 0. Therefore, with IC 1 B and IC 1 C configured as shown, the output of the flip flop will be at either 0 or 1, depending on the last 0 that is present at either of the inputs. If the last 0 is at Pin  8  of IC 1 B, then the output will be 1. If the last 0 is at Pin  12  of IC 1 C, then the output will be 0. The remaining NAND gates comprising IC 1 , IC 1 A and IC 1 D, are configured to operate as inverters: A 0 on the input produces a 1 on the output and a 1 on the input produces a 0 on the output. R 1  and R 2  provide a voltage divider network at the input of IC 1 A. The resistance value of R 1  and R 2  will vary, depending on the operating voltage of PS 1  and BAT 1 . If PS 1  is on, a voltage (logic 1) will be supplied at the input of IC 1 A. If PS 1  is off, this will supply a 0 at the input of IC 1 A. In the absence of voltage from PS 1 , the input of IC 1  will be pulled to ground (logic 0) by R 2 . R 3  and C 1  provide an RC network defining an integrator circuit at the input of IC 1 C, which integrator circuit functions as a delay circuit for the signal received at the Pin  12  input of IC 1 C, which is either the backup battery voltage or the power supply voltage. If voltage is applied across R 3  and C 1 , the junction of R 3  and C 1  (also connected to the input of IC 1 C) will momentarily be logic 0 and then will rise to logic 1 as C 1  charges through R 3 . PB 1  is a momentary pushbutton switch, providing a means for manually making the input of IC 1 C a logic 0 when so desired to put the control circuit back into sleep mode.  
         [0017]     Based upon the aforementioned logic description, circuit operation is as follows: If BAT 1  is connected prior to PS 1  providing power, the input of IC 1 A will be 0 and the output of IC 1 A will be 1. Therefore, the input of IC 1 B (Pin  8 ) will be 1. Simultaneously, the input of IC 1 C (Pin  12 ) will be 0 temporarily and then will rise to 1 as C 1  charges. Therefore, the last 0 will be at Pin  12  of IC 1 C and the output of the flip flop (Pin  10 ) will be 0. This 0 is then inverted by IC 1 D so that a logic 1 (through R 4 , which is a current limiting resistor) appears at the base of Q 1 . As a result, the switch formed by Q 1  and Q 2  is off and device circuitry is not activated. At this time, BAT 1  is physically connected to DC 1 , but it is not electrically connected, and both inputs of the flip flop are at logic 1. This is considered to be a “sleep” condition for BAT 1 , whereby the only circuitry being activated is the described circuit itself. At this time, total current draw from BAT 1  is 6 μA., which is the current required to operate the IC 1 . When PS 1  is turned on, the presence of voltage at R 1  makes the input of IC 1 A a logic 1, which then gets inverted to a 0 by IC 1 A so that Pin  8  becomes a logic 0, where it will remain for as long as PS 1  is on. The result is that the output of the flip flop (Pin  10 ) changes to logic 1, which then gets inverted by IC 1 D, so that the base of Q 1 , through R 4 , is now 0, and as a result, the switch formed by Q 1  and Q 2  is on and DC 1  is now activated. Throughout this sequence, Pin  12  of IC 1 C remains at Logic 1. This, then, is the circuit status for as long as PS 1  remains activated. In the event that PS 1  becomes de-activated, such as in a power failure, the resultant absence of voltage at R 1  makes the input of IC 1 A a logic 0, which then gets inverted to a 1 by IC 1 A so that Pin  8  becomes a logic 1, where it will remain for as long as PS 1  is off. At this time, Pin  8  of IC 1 B and Pin  12  of IC 1 C are both at logic 1. However, since the last 0 was at Pin  8  when PS 1  was activated, then the output at Pin  10  remains at 1, which, as previously described, will maintain DC 1  in an activated state. At this time, DC 1  will run off of power provided by BAT 1 . Once power is restored, DC 1  will continue to operate, only the system will have switched back to power as supplied by PS 1  as previously described. After such a power failure occurs, and if BAT 1  is rechargeable, R 5  provides a means for current to flow back into BAT 1  from PS 1  once power has been restored so as to recharge the battery. Should it be desired to restore the “sleep” condition, such as prior to when product is to be shipped, this is accomplished by removing power from PS 1  and then momentarily pressing PB 1 . This will reverse the status of the flip flop and restore the sleep condition as previously described. The sleep condition for the battery can also be restored when power from PS 1  is removed by temporarily disconnecting and then reconnecting the battery.  
         [0018]     This, then, describes the operation of the circuit. If desired, other capabilities can be added as well. For example, another momentary pushbutton switch could be connected to the junction of R 1 , R 2  and the input of IC 1 A. The other connection on this pushbutton switch would be made to “+” (positive) side of the battery. This pushbutton switch would provide a means of re-connecting the battery electrically without the presence of power to PS 1 . Another capability would be to provide the control circuit with a sleep state switching circuit having the ability to automatically put BAT 1  into sleep mode if the battery is becoming too discharged while operating DC 1  during a prolonged power failure. This could easily be accomplished by adding a comparator, a resistor network, connected across BAT 1  to function as a voltage divider, and a fixed voltage reference to the circuit. The voltage reference would be connected to the “−” (inverting) input of the comparator and the junction of the voltage divider resistors would be connected to the “+” (non-inverting) input of the comparator. The output of the comparator would be connected to Pin  12  of IC 1 C. Resistor values and voltage reference would be selected in accordance with requirements so that operation would be as follows: A fully charged battery would generate a voltage at the “+” (non-inverting) input of the comparator that is higher than the voltage generated at the “−” (inverting) input by the voltage reference. As a result, the output of the comparator would be a 1. As the BAT 1  discharges from driving DC 1 , the voltage at the “+” (non-inverting) input would gradually drop. When this voltage dropped below the voltage reference voltage on the “−” (inverting) input, the output of the comparator would change to a 0, and this would have the same effect as pressing PB 1 , which would put BAT 1  into sleep mode. This added capability would prevent BAT 1  from becoming completely discharged in a prolonged power failure. A preferred configuration of the aforementioned capability is as shown in  FIG. 2  wherein the aforementioned components are contained within the device circuitry, DC 1 . When so configured, these circuit components will only be active when power is being supplied by PS 1  or by BAT 1  as in during a power failure. Therefore, these components will not cause any additional current drain on BAT 1  when the circuitry is in the sleep condition. Referring to  FIG. 2 , IC 2  is a comparator such as an LM339, R 6  and R 7  are resistors comprising the voltage divider connected to the “−” inverting input (Pin  6 ), and a Zener diode or other voltage reference component VR 1  (such as an LM285Z), combined with resistor R 8 , provides a fixed voltage reference connected to the “+” non-inverting input (Pin  7 ). Note that Pin  12 , which is the operating voltage connection for IC 2 , as well as R 6  and R 8 , are all connected to the collector of Q 2  and therefore, as previously mentioned, are only powered when PS 1  is activated or when BAT 1  is operating the system in a power failure. With reference to IC 2 , other comparators may be utilized as well, but the LM339 is suitable because it has a wide operating voltage range ( 2-36  VDC, typical), making it suitable for application in power supply/battery configurations of different voltages. The selection of values of R 6  and R 7  and the value of the reference voltage of VR 1  will vary depending upon the operating voltage of PS 1  and BAT 1  and the desired minimal voltage at which BAT 1  would be returned to the sleep condition as it discharges during a prolonged power failure. The value of R 8 , while not critical, would be in the range of 10K ohms. It is also preferred that resistor R 9  is added between the output (Pin  1 ) of IC 2  and the input (Pin  12 ) of IC 1 , as this will help to limit the current that will flow when the output of IC 2  changes state as a discharging battery reaches the threshold point where it is returned to the sleep condition. The value of R 9 , while not critical, must be very low (such as 100 ohms) compared to the value R 3  (also not critical, but may be a value such as 100K ohms) so that a voltage divider is not created between R 9  and R 3  at the input of IC 1 C (Pin  12 ) which would affect the logic 0 condition generated by IC 2  in a “return to battery sleep” condition caused by a suitably discharged battery. This logic 0 condition thus generated at the input of IC 1 C (Pin  12 ) is essential for automatically returning the battery to the sleep condition in a prolonged power failure.  
         [0019]     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.