Abstract:
A flashlight is provided with a special circuit for limiting the surge current through the flashlight bulb. The circuit includes a secondary incandescent filament used to energize a transistor gradually upon the activation of flashlight. The transistor in turn gradually energizes the main bulb.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains to a flashlight with soft turn on control, and more particularly to a flashlight with circuit for limiting the surge or inrush current through an incandescent bulb thereof. 
     2. Description of the Prior Art 
     Typically, a flashlight has to be relatively small so that it can be hand-held and to minimize its storage space. Therefore the controls for the flashlight are made as simple as possible. Thus, a standard flashlight with a single incandescent bulb is normally provided with only a mechanical on/off switch for feeding current to the bulb from a power source such as one or more batteries. The bulb, switch and batteries are normally enclosed in a common housing. One problem which this arrangement is that the initial or surge current to the bulb is substantial. This large surge current (sometimes termed inrush current) seriously degrades the bulb because it imposes a large thermal and mechanical strain on its incandescent filament and switch contacts. This strain reduces the useful life of the bulb and often leads to its premature failure. For example, a bulb used in a flashlight which is activated frequently has a much shorter life than a bulb used in a flashlight which is activated only rarely even if the second flashlight is on for a longer time than the first flashlight. This problem is aggravated even further if the flashlight is provided with an activating switch with bounce, i.e. a switch which closes and opens several times in quick succession every time the switch is turned on. Thus a single activation of the switch causes several sequential current surges through the fillament in a relatively short time period. 
     SUMMARY OF THE INVENTION 
     In order to solve the above described problem, it is an objective of the present invention to provide a control circuit for a flashlight for limiting the surge current to its incandescent lamp. 
     A further objective is to provide a circuit which has a small number of components thereby minimizing the size and cost of the flashlight. 
     Other objectives and advantages of the invention shall become apparent from the following description of the invention. A flashlight in accordance with this invention comprises a housing which includes a power source, light generating means, activating means for selectively coupling the light generating means to the power source and a control circuit for limiting the surge current to the light generating means during initial activation. The control means includes, a switch element connected in series with said light generating means and a biasing network for activating the switch element gradually in response to said activating means. Preferably the biasing network includes a temperature-dependent variable resistance element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a circuit for a typical prior art flashlight; 
     FIG. 1B shows the resistance of a typical incandescent bulb filament as a function of temperature; 
     FIG. 1C shows the surge current through the fillament of the flashlight of FIG. 1; 
     FIG. 2A shows a circuit for a flashlight constructed in accordance with this invention; 
     FIG. 2B shows the base voltage of the switching element for the circuit of FIG. 2A; 
     FIG. 2C shows the current through the primary fillament of FIG. 2A; 
     FIG. 3 shows an alternate embodiment of the invention; 
     FIG. 4 shows two filaments in a common envelope; and 
     FIG. 5 shows a flashlight with the circuits of FIG. 2A or 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A-1C illustrate the surge current through a prior art flashlight filament. As previously explained, in a typical flashlight, batteries B generating a constant voltage V are merely connected in series with a switch S and a light bulb L having an incandescent fillament with a resistance R1. As shown in FIG. 1B, the initial resistance of the fillament, Rcold is relatively low. As a result when switch S is closed (at t=0), a very high surge current flows through the fillament. The value of the current, Io is given by: 
     
         Io=V/Rcold 
    
     As the filament heats up, its resistance R1 slowly rises toward a maximum value Rhot as shown in FIG. 1B, and the current I through the filament similarly decreases toward a constant value Idc. 
     According to the present invention, a flashlight is provided which includes a circuit for limiting the surge current through the filament. The components of the flashlight are shown, in a diagramatic form, in FIG. 2A. The circuit shown in this Figure includes batteries B which generate a constant voltage V, and which are in series with a primary bulb Lp with an incandescent filament having a resistance Rp, and a transistor T. There is also provided a biasing network comprising a resistor Rc in series with a manual switch S and a secondary bulb Ls having an incandescent fillament with a resistance Rs. The resistor Rc forms a voltage divider with the secondary bulb for controlling the transistor T as shown. The secondary bulb Ls is a high resistance low light output bulb such as a T3/4 or another similar bulb used for instrument panels. Such bulbs typically have a very long useful life. As shall be explained in more detail below, the present application increases the life of this bulb even farther. 
     The circuit operates as follows. When switch S is closed (at t=0), the cold resistance of bulb Ls is relatively low (as indicated in FIG. 1B). The value of Rc is selected so that initial voltage at the base of the transistor T, Vb is below the transistor turn-on threshold level and therefore no current flows through primary bulb Lp. For example if the cold resistance of bulb Rs is about 10 ohms and resistor Rc has a value of 47 ohms then for 1.5 volt batteries the initial voltage at the transistor base, Vo is: 
     
         Vo=1.5(10/57)=0.26 volts 
    
     As the filament of the secondary bulb heats up, its resistance and, correspondingly, the voltage Vb, increases until Vb reaches a level Von (at a time ton) so that transistor T starts conducting. As the voltage Vb increases gradually after ton the transistor passes through its active region and the collector current I through the transistor and primary bulb rises gradually as shown in FIG. 2C. After transistor T reaches saturation, the current I may have a brief excursion above the steady state of final level Idc while the fillament of bulb Lp heat up, however it will never reach the surge levels indicated in FIG. 1C. Furthermore, if switch S bounces as it is closed, the effects of the bounce are eliminated by the transistor and the secondary light bulb by preventing the instantaneous high output current therethrough. 
     As shown in FIG. 2B, after t=ton, voltage continues to rise gradually until it reaches its final value Vdc which is dependent on the corresponding hot resistance of secondary bulb fillament Rs. For example, of the hot resistance of Rs is about 100 ohms, Vdc is: 
     
         Vdc=1.5(100/147)=1 volt. 
    
     Preferably, secondary bulb Ls is selected so that its rating operating voltage is above Vdc. For Vdc=1 volt as described above, the operating voltage of Ls may be 1.5 volts. As a result, bulb Ls is operated below its rated voltage so that it will be barely on, and its light output will be at most minimal. As a result, bulb Ls will operate for a very long period of time without replacement. It is expected that the bulb may be operating up to 10,000 hours or more. Instead of a bulb, a capacitor may be used theoretically to achieve the results described above. From a practical view point this would be undesirable because the capacitor would have to have a value in excess of 10,000 microfarads. Such capacitors are very bulky, expensive and unreliable. However, the operation of the circuit may be slowed down by the use of an additional small capacitor C (having a value in order of one microfarad) connected in parallel with bulb Ls as shown in FIG. 2A. This capacitor would also assist in eliminating the effect of bouncing by switch S as described above. 
     One disadvantage of the circuit in FIG. 2A may be that a small leakage current may flow though the primary bulb Lp and transistor T even when the switch S is open, thereby draining the batteries. Therefore, it may be advantageous to put switch S in series with both the biasing network and the primary bulb Lp. Such a configuration is shown in FIG. 3. The two embodiments of FIGS. 2A and 3 both reduce switch contact wear. 
     A flashlight with a circuit as shown in FIGS. 2A or 3 is shown in FIG. 5. The flashlight includes a housing H with a switch S, batteries B, a primary bulb Lp, and a control package P containing resistor Rc and transistor T. Capacitor C may also be included in the package, if necessary. 
     Physically the secondary bulb Ls may be provided as separate element within the control package P. In this configuration, the primary bulb Lp may be changed as required without affecting the rest of the control package. 
     Alternatively, the bulbs may be provided in a common envelope as shown in FIG. 4. In this configuration, the two bulbs are changed simultaneously. 
     Obviously, numerous changes may be made to this invention without departing from its scope as defined in the following claims.