Abstract:
A circuit which limits current to a first value until a predetermined time has elapsed, then permits the current to rise to a second, acceptable value. The current limiting can be carried out during a predetermined time interval by a biased semiconductor switch. Subsequent to the current limiting time interval, the semiconductor switch can be saturated enabling the current to increase.

Description:
FIELD OF THE INVENTION 
   The invention pertains to current limiting circuits. More particularly, the invention pertains to current limiting circuits which might find applicability in electrical devices where intermittent peak current demands may greatly exceed average current draw. 
   BACKGROUND 
   It has been recognized that certain types of electrical devices can have peak current requirements which exceed average current requirements by several orders of magnitude. One form of such device is a strobe unit of the type used to indicate an emergency condition in an alarm system, such as a fire alarm. 
     FIG. 1  illustrates a known form of strobe unit  10 . The strobe unit  10  includes control circuitry  12 , which among other functions, manages charging, via circuitry  14 , a relatively large electrolytic capacitor  16 . Capacitor  16  is used to provide electrical energy to a gas discharge tube  18  which can be selectively triggered. When triggered, the electrical energy previously stored on capacitor  16  is coupled to tube  18  to provide a high intensity visual output indicative of the alarm condition. 
   When power is initially applied to such units, for example, via lines  22  as would be understood by those of skill in the art, an initial current surge which might have an amplitude as great as 10 amps can be drawn by the device  10 .  FIG. 2  is an exemplary graph illustrating an initial maximum current surge Io. The initial surge current is primarily due to a need to charge one or more electrolytic capacitors, such as capacitor  16 , in strobe unit  10 . After the initial current surge has subsided, the device  10  draws a substantially lower current, Irms, which typically depends on the candela output of the device  10 . Such currents can fall in the range of less than 50 mA to more than 500 mA. 
   By design, strobe units, such as the unit  10  flash their respective output device  18  once a second as illustrated in  FIG. 2 . Each flash produces a substantial repeat current surge, Irep, which, while less than the initial current surge Io, can still be orders of magnitude above the intermittent current demands Irms. Those of skill will understand that the peak current values illustrated in  FIG. 2  are exemplary only and could vary between the strobe units depending on their exact design. Nevertheless, in each instance, such units exhibit an initial peak inrush current followed by repetitive, though lower, repeat inrush currents. 
   It has also been recognized that there is a benefit to incorporating flexibility into strobe units, such as the strobe unit  10  by providing a candela select capability  28  (illustrated in phantom in  FIG. 1 ). This capability can be implemented with jumpers or switches located on the unit  10 . This capability enables an installer to select one of a plurality of candela outputs, at installation, and have the benefits of a common product. 
   The presence of the initial peak current draw and repetitive peak current draws, as illustrated in  FIG. 2  is undesirable. It has resulted in the use of current limiting circuitry  30 , illustrated in phantom in  FIG. 1 , in strobe units, such as the unit  10 . One such configuration has been disclosed in U.S. patent application Ser. No. 10/040,968 filed Jan. 7, 2002 and entitled “Processor Based Strobe with Feedback”, now U.S. Pat. No. 6,661,337. The U.S. Pat. No. 6,661,337 patent is assigned to the Assignee hereof and incorporated by reference herein. While the current limiting circuitry of the subject patent is effective for its intended purpose, it is primarily analog in nature and requires the inclusion of capacitors in the respective strobe units. Capacitors, of course, add both complexity and cost to such products. 
   There thus is a continuing need for current limiting circuitry which could be incorporated into strobe units, such as the exemplary strobe unit  10 , to limit not only the initial peak in rush current but subsequent repetitive peak current values. Preferably, such circuitry could be implemented so as to minimize any additional costs without unduly requiring additional capacitors for the purpose of smoothing, and or limiting the initial peak current as well as repetitive peak current draws. Such circuitry might be useful in connection with other types of devices which draw substantial initial in-rush currents and/or subsequent repetitive peak currents. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an exemplary prior art strobe unit; 
       FIG. 2  is a graph of initial peak and repetitive peak currents for strobe units, such as that of  FIG. 1 ; 
       FIG. 3  is one form of a current limiting circuit; 
       FIG. 4  is a block diagram of a current limiter in accordance with the invention; 
       FIG. 5  is a schematic diagram of an exemplary current limiting circuit in accordance with the invention; 
       FIG. 6  is a schematic diagram of an alternate embodiment of the current limiter of  FIG. 5 ; and 
       FIG. 7  is yet another alternate embodiment of the current limiter of  FIG. 5 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
   A circuit which embodies the invention includes a current sensor which senses and controls current flow through a current limiting device. In a disclosed embodiment, a current sensing threshold is established which is sufficiently low that it could meet the requirements of the lowest candela setting in a multi-candela strobe unit. 
   The current limiting device operates to limit current flow for a predetermined time interval before being disabled. The predetermined time interval is selected such that the capacitors of the device are charged to a sufficiently high voltage. 
   The difference between the voltage across the capacitor(s) and the applied source voltage, which are coupled together through the current limiting device, makes it possible to limit the resulting current surge to an acceptable value. Subsequently, the capacitors of the device can continue to be charged through a low impedance, fully saturated, current limiter. The described embodiment permits higher currents and results in less losses throughout the balance of the charging cycle. 
   The above-described embodiment is particularly advantageous in that it can be incorporated into a multi-candela output device. Thus, the benefits of multi-candela devices continue to be available to customers who install them. Their visual output levels can continue to be set in the field either manually by the installer, or if desired, downloaded from a remote source. One model, thus can be used to meet a variety of output requirements thereby improving manufacturing efficiency and reducing inventory costs. 
     FIG. 3  illustrates a basic form of a current limiting circuit  40 . The circuit  40  includes a load capacitor  16 - 1  which is to be charged to a predetermined voltage. Capacitor  16 - 1  might, for example, correspond to the capacitor  16  of a strobe unit such as the unit  10  of  FIG. 1 . Merely coupling Vsource to capacitor  16 - 1 , as would be understood by those of skill in the art, results in undesirable peak current values as illustrated in  FIG. 2 . 
   When the applied voltage across R 1 , from Vsource, approaches 600 mV, transistor Q 1  turns on forcing Q 2  to partly shut off to start limiting current. Surge current is limited by choosing an appropriate value for resistor R 1 . As an example, consider a strobe, such as strobe  10  of  FIG. 1 , that has IRMS=50 mA. R 1  could be selected such that the peak current would be limited to less than 250 mA but greater than 50 mA if 50 mA is required to deliver the required candela. 
   Diode D 1  protects the circuit from damage in the event of accidental reverse polarity connection to the power source, Vsource. D 1  is not necessary in order for the circuit to function properly if the source polarity is connected correctly. 
   The circuit  40  of  FIG. 3  is not suitable for use in field selectable candela products. In those products, the end user selects a candela setting, and IRMS can vary from 50 to 500 mA, based on the candela selection. There is no single value of R 1  that can provide enough current at the highest candela setting without exceeding a maximum surge current at the lowest candela setting. 
     FIG. 4  illustrates a current limiting circuit  42  in accordance with the invention. The circuit  42  is advantageous in that it can be used with variable candela strobe units. 
   The circuit  42  includes a single threshold current sensor  42 - 1  which is coupled to and which provides control signals to a current limiting switch  42 - 2 . When the source voltage Vsource couples electrical energy to the circuit  42  to charge a load capacitor Cload, current sensor  42 - 1  responds to the incoming current required to charge the load capacitor. 
   If the incoming current exceeds a predetermined threshold, the current sensor  42 - 1  causes the current limiting switch to go from a highly conductive, low impedance, state to a less conductive, higher impedance, state. The peak value of the incoming current Iin can thus be limited as required. 
   Circuit  42  also includes timing circuitry  42 - 3  coupled to a control input of the current limiting switch  42 - 2 . After a predetermined interval, the timing circuitry  42 - 3  disables the current limiting function of the switch  42 - 2 . The current at that point in time increases based on the difference between the value of the source voltage, Vsource, and the voltage across the capacitor Cload and also based on the impedance of the current limiting switch  42 - 2 . 
   In accordance with the invention, circuit  44 , illustrated in  FIG. 5 , operates as a 50 mA current limiter. The circuit of  FIG. 4  is more flexible than is the circuit in  FIG. 3 . Circuit  44  could be used to charge capacitor  16 - 1  to energize gas tube  18 - 1  when triggered. The circuit of  FIG. 4  has the capability of disabling the current limiting function when the strobe capacitance  16 - 1  has been sufficiently charged to prevent an unacceptable inrush current surge. 
   Initially when the voltage across resistor R 1  approaches 600 mV, transistor Q 1  turns on forcing FET Q 2  to operate to start limiting current. R 1  is selected such that the inrush current is below a pre-set threshold. For example, resistor R 1  can be selected to limit in-rush current to a level acceptable for the lowest candela setting of a field selectable multi-candela strobe. 
   A timer or microprocessor IC 1 , controls the amount of time the current limiting function is enabled and then disables current limiting function by turning on Q 4 . This in turn will cause FET Q 2  to conduct heavily. The amount of time the current limiting is enabled ensures that the difference between load capacitance voltage and source voltage, coupled with the impedance between the source and load capacitance  16 - 1 , will not result in an unacceptable current surge, once the current limiting function has been disabled. The strobe capacitor  16 - 1  can then continue to be charged through a fully saturated current limiting device such as transistor Q 2 . A single value can now be used for R 1  in a multi-candela strobe to provide enough current at the highest candela setting without exceeding a pre-set maximum surge current at the lowest candela setting. 
   Capacitor C 2  has been selected to limit the initial inrush current surge during power up. During power up, IC 1  will go through a power up and initialization process before it can take control. As those of skill in the art will recognize, capacitor C 2  may not be needed depending upon the characteristics of D 2 , D 3 , IC 1 , Q 2 , Q 3  and Q 4 . It also may not be needed depending on the values of R 2 , R 3  and R 4 , the allowable range of input voltages, or the allowable range of candela settings. 
   Control circuit IC 1  can be implemented as a programmed microprocessor. It can also be implemented by various timing devices or similar circuits. 
   Transistor Q 2  is illustrated as a P Channel MOSFET. It can be replaced by other devices. Alternately, a current limiter  44 ′ can be placed in the negative leg of a circuit, see  FIG. 6 . For example, transistor Q 2  can be an N Channel MOSFET located in the negative leg of a corresponding circuit as in  FIG. 3 . Other circuit implementations come within the spirit and scope of the invention. 
   Zener D 2  protects the transistor Q 2  from damage due to excessive voltage across its gate and source. Zener diode D 2  may not be necessary in order for the circuit to function properly in cases such as (but not limited to): if the source voltage is sufficiently low or transistor Q 2  is not a MOSFET. 
   Diode D 3  protects transistor Q 3  from damage due to reverse over voltage across its base and emitter. Diode D 3  may not be necessary in cases such as (but not limited to): if the source voltage is sufficiently low or transistor Q 3  is not a NPN transistor. 
   Resistor R 4  is a pull down for the base-emitter of transistor Q 2 . It is not needed if diode D 3  is not used. 
   Resistor R 2  is a pull-up resistor for transistor Q 3 . It feeds current to the base of transistor Q 3  when current limiting function is disabled. Q 4  is on to disable current limiting. 
   Resistor R 3  is a pull down resistor for transistor Q 2 . 
   Capacitor  16 - 1  is illustrated as a single capacitor. Those of skill will understand that it represents all of the capacitances of the device or strobe. 
   It should be noted that while the above describes a fire alarm strobe application, the same type of circuitry could also be used for a fire alarm sounder application or any other application where in-rush current control is needed, inside or outside the fire protection industry. The particular type of application is not a limitation of the invention. 
     FIG. 7  illustrates another embodiment of a current limiting circuit  50  in accordance with the invention. In the circuit  50 , transistor Q 3  of  FIG. 5  has been replaced with fixed resistor R. 
   From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.