FIELD OF THE INVENTION

[0001] The present invention relates to LED light bulbs, which remain lit for some period of time after power is removed, and more particularly, to the use of a capacitor in parallel with at least one LED of one or more LEDs in an LED light bulb to produce a gradually diminishing light.

BACKGROUND OF THE INVENTION

[0002] A light bulb typically turns off immediately when the power to it is removed. This is natural since there is no energy storage inside a light bulb.

However, in some cases it might be desirable for the bulb to remain on for some time after power is removed. For example, a bedside reading lamp might conveniently remain on for some time (e.g., seconds) after turning off, in order to permit the user to get in to bed in a not totally dark environment. For this purpose, it might be acceptable for the bulb to not be as bright as when powered on, and for the brightness to gradually decay.

[0003] Three common technologies for light bulbs are presently available, including incandescent, fluorescent and LED light bulbs. Fluorescent bulbs which are designed to screw in to conventional sockets are generically referred to as compact fluorescent lamps or CFLs.

[0004] Incandescent bulbs have no electronics, and draw very large power. It thus appears to be difficult to create an incandescent bulb that delayed its turn off on loss of power. CFLs, while possessing electronics and drawing far less power than incandescents, require special circuitry to maintain filament temperature when operating at less than full brightness, and thus it may be expensive to create a CFL that delayed its turn off on loss of power.

[0005] However, LED bulbs also possess electronics, and draw even less power than CFLs. Furthermore, LED bulbs require no special circuitry to operate at less than full brightness, and have the further advantage that they may have multiple LEDs, permitting only some subset of the total number of LEDs to be powered on during loss of power to the bulb. Accordingly, it would be desirable to provide an LED light bulb, which remains lit for some period of time after power is removed, and more particularly, to power some or all of the total number of LEDs at reduced power to produce a gradually diminishing light.

SUMMARY OF THE INVENTION

[0006] This invention has the object of developing an LED light bulb with delayed turn-off on power loss such that the above-described primary problem is effectively solved. As set forth above, it would be desirable to provide a light bulb whose light output gradually dims and eventually turns off when power is removed, and wherein the time to complete off (or dark state) is controllable with an electronic circuit element. In accordance with an exemplary embodiment, the apparatus includes an LED light bulb and a capacitor (which is preferably a large-value capacitor) in parallel with one or more of the LEDs. In accordance with an exemplary embodiment, the capacitor provides energy to the paralleled LEDs after power is removed from the bulb. In use, the capacitor gradually discharges through the LEDs, providing gradually decreasing light for a time related to the characteristics of the LEDs and the capacitance of the capacitor. [0007] hi accordance with an embodiment, a delayed turn-off LED light bulb comprises: a bulb; a plurality of LEDs inside said bulb; and a capacitor in parallel with one or more LEDs in the plurality of LEDs.

[0008] hi accordance with another embodiment, a circuit for a delayed turn-off LED light bulb comprises: a plurality of LEDs; and a capacitor in parallel with at least one LED within the plurality of LEDs, and wherein the capacitor produces a light output from the at least one LED upon removal of current from the circuit. [0009] In accordance with a further embodiment, a circuit for a delayed turn-off LED light bulb comprises: one or more LEDs; and a capacitor in parallel with at least one LED of the one or more LEDs, and wherein the capacitor produces a light output from the at least one LED upon removal of current from the circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

[0011] FIG. 1 shows a cross-sectional view of an LED light bulb according to one embodiment.

[0012] FIG. 2 is a circuit schematic of a string (or plurality) of LEDs in an LED light bulb with a capacitor in parallel with at least one of the LEDs.

[0013] FIG. 3 is a representation of typical light output as a function of time in such a circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [0014] Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. [0015] According to the design characteristics, a detailed description of the current practice and preferred embodiments is given below.

[0016] FIG. 1 shows a cross-sectional view of an LED light bulb 10 showing the shell (or bulb) 20 enclosing an LED (light-emitting diode) 30 according to one embodiment. The light bulb 10 includes a screw-in base 40, which includes a series of screw threads 42 and a base pin 44. The screw-in base 40 is configured to fit within and make electrical contact with a standard electrical socket (not shown). The electrical socket is preferably dimensioned to receive an incandescent or other standard light bulb as known in the art. However, it can be appreciated that the screw-in base 40 can be modified to fit within any electrical socket, which is configured to receive an incandescent bulb, such as a bayonet style base. In use, the screw-in base 40 makes electrical contact with the AC power in a socket through its screw threads 42 and its base pin 44. [0017] FIG. 2 is a circuit schematic 100 of a string (or plurality) of LEDs 110 in an LED light bulb, with a capacitor 120 in parallel with at least one of the LEDs 130 within the string or plurality of LEDs 110. It can be appreciated that the capacitor 120 can be in parallel with one or more of the LEDs 130, or potentially in parallel with all of the LEDs 110 together. Alternatively, rather than a plurality or string of LEDs 110, in accordance with another embodiment, the light bulb 10 can include a single LED 130 (i.e., one LED) and a capacitor 120 in parallel with the single LED 130. [0018] In a preferred embodiment, the capacitor 120 is sized such that the discharge time into the LED 130 is a substantial number (i.e., at least 2 to 10 seconds) of seconds, hi accordance with an exemplary embodiment the capacitor 120 is across (or in parallel with) only one of the one or more LEDs 130 in the string of LEDs 110. In accordance with another embodiment, the capacitor includes a resistor 122 in series with the capacitor 120. It can be appreciated that the capacitor 120 and the one or more LEDs 130 in parallel with the capacitor 120 are preferably ground-referenced 124.

[0019] In accordance with a preferred embodiment, the LED light bulb 10 contains a string or plurality of LEDs 110, all of which are energized in series during normal operation. In accordance with an exemplary embodiment, at least one of the LEDs 130 has a large- value capacitor 120 in parallel with the at least one of the LEDs 130. The capacitor 120 preferably has a value of at least 470 millifarads (or 0.47 farads). At "turn on" (i.e., when a source of power is supplied to the circuit 100), the capacitor 120 is in a discharged state, and then is gradually charged by the current being supplied to the string or plurality of LEDs 110. During the charging time (i.e., while a source power is supplied to the circuit 100), the at least one LED 130 with which the capacitor 120 is in parallel gradually ramps up to the same brightness as the other LEDs 110 in the string. In steady-state (i.e., once an equilibrium has been reached), the capacitor 120 is charged to the voltage across the LED 130, and the string current flows entirely through the LED 130, so that the at least one LED 130 is at the same brightness as the other LEDs 110 in the string.

[0020] At "turn off (i.e., when the source of power is removed or disconnected from the circuit 100), the LED string 110 current is immediately cut off, and all of the non-paralleled LEDs 110 immediately turn off. However, the capacitor 120 across the at least one LED 130 begins to discharge through the at least one LED 130, which maintains the light output from the at least one LED 130 for a period of time related to the I- V (current- voltage) characteristics of the at least one LED 130 and the value of the capacitance associated with the at least one LED 130. In addition, it can be appreciated that the speed (or rate) in which the light from the at least one LED 130 decays can be dependent on the I-V (current- voltage) characteristics of the at least one LED 130. [0021] In accordance with an exemplary embodiment, the performance of the bulb 10 at "turn off thus consists of two aspects. First, the light output immediately jumps down to 1/N of the "on-state" light, where N is the total number of LEDs 110 in the string, including the paralleled LED or LEDs 130. Secondly, the light gradually decays as the capacitor 120 discharges through the at least one LED 130, and at some point being so low as to be effectively off (i.e., no longer emitting visible light).

[0022] FIG. 3 is representation of typical light output as a function of time in a circuit such as that shown in FIG. 2. In this representation or graph, power to the bulb, and hence the LED string, is cut off at time 140. At this point, the light from the light bulb drops from its steady-state value of 100% to a fraction of this value, represented here as 20%, corresponding to one out of a total of five LEDs in the bulb. After this, the light output of the light bulb drops, until a time 150 at which there is essentially no further light coming out of the bulb.

[0023] It will be apparent to those skilled in the art that various modifications and variation can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.