Current regulating circuit and light emitting diode device having the same

A current regulating circuit is for connection in series between a light emitting diode (LED) and a power source, and includes: a first resistive unit having a first resistance that is proportional to an operation temperature of the LED when the operation temperature is above a predetermined threshold temperature; and a second resistive unit connected in series with said first resistive unit, and having a second resistance that is inversely proportional to the operation temperature of the LED when the operation temperature is above the predetermined threshold temperature. When the operation temperature of the LED is above the predetermined threshold temperature, an effective resistance of said current regulating circuit attributed to said first and second resistive units is proportional to the operation temperature of the LED, and absolute value of a rate of change of the first resistance is larger than that of the second resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Application No. 201010182030.0, filed on May 19, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current regulating circuit and a light emitting diode device having the same, more particularly to a current regulating circuit for regulating current flowing to a light emitting diode and a light emitting diode device having the same.

2. Description of the Related Art

Referring toFIG. 1, a conventional light emitting diode device900is powered by an alternating current (AC) power source, and includes a light emitting diode (LED)910and a resistor920, which has a temperature-insensitive internal resistance (seeFIG. 2).

After a long duration of operation at a working temperature (Tw) higher than room temperature (i.e., 25 degrees Celsius), ageing of the LED910will cause an internal resistance thereof to vary, causing a voltage difference across the light emitting diode910to vary, which in turn causes a current flowing across the light emitting diode910to fluctuate. As a result, brightness of light emitted by the LED910becomes unstable, and durability of the LED910is reduced.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a current regulating circuit capable of regulating current flowing to a light emitting diode (LED), thereby stabilizing brightness of light emitted by the LED.

Accordingly, a current regulating circuit of the present invention is for connection in series between a light emitting diode (LED) and a power source, and comprises: a first resistive unit having a first resistance that is proportional to an operation temperature of the LED when the operation temperature of the LED is above a predetermined threshold temperature; and a second resistive unit connected in series with the first resistive unit, and having a second resistance that is inversely proportional to the operation temperature of the LED when the operation temperature of the LED is above the predetermined threshold temperature.

When the operation temperature of the LED is above the predetermined threshold temperature, an effective resistance of the current regulating circuit attributed to the first and second resistive units is proportional to the operation temperature of the LED, and absolute value of a rate of change of the first resistance is larger than that of the second resistance.

Another object of the present invention is to provide an LED device capable of emitting light with stable brightness even after long-term use.

Accordingly, a light emitting diode (LED) device of the present invention is adapted to be connected to a power source. The LED device includes an LED, and a current regulating circuit connected to the LED, and adapted to be connected to the power source such that the LED, the current regulating circuit, and the power source are connected in series. The current regulating circuit includes: a first resistive unit having a first resistance that is proportional to an operation temperature of the LED when the operation temperature of the LED is above a predetermined threshold temperature; and a second resistive unit connected in series with the first resistive unit, and having a second resistance that is inversely proportional to the operation temperature of the LED when the operation temperature of the LED is above the predetermined threshold temperature.

When the operation temperature of the LED is above the predetermined threshold temperature, an effective resistance of the current regulating circuit attributed to the first and second resistive units is proportional to the operation temperature of the LED, and absolute value of a rate of change of the first resistance is larger than that of the second resistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 3, the first preferred embodiment of a light emitting diode (LED) device100, according to the present invention, includes an LED1, and a current regulating circuit2connected in series between the LED1and an alternating current (AC) power source3such that current flowing to the LED1is regulated when an operation temperature of the LED1is in a temperature range above a predetermined threshold temperature (Tb), and when an internal resistance of the LED1starts to vary in the temperature range due to ageing, thereby enabling the LED1to emit light with stable brightness.

The current regulating circuit2includes a series connection of first and second resistive units4,5. The AC power source3, the first resistive unit4, the LED1, and the second resistive unit5are connected in series in the order given and form a loop. Referring toFIG. 4, Curve (L) defines a relationship between an effective resistance of the current regulating circuit2, which is a contribution of a first resistance of the first resistive unit4and a second resistance of the second resistive unit5, and the operation temperature of the LED1. Curve (L) is divided at the predetermined threshold temperature (Tt) into first and second curve sections (L1, L2).

In this embodiment, the first resistive unit4includes a series connection of first and second thermistors (R1, R2). Referring toFIG. 4, Curve (L+) defines a relationship between the first resistance of the first resistive unit4, which is attributed to respective resistances of the first and second thermistors (R1, R2), and the operation temperature of the LED1. Curve (L+) is also divided at the predetermined threshold temperature (Tt) into first and second curve segments (La, Lb). The first thermistor (R1) has a positive temperature coefficient, and the second thermistor (R2) has a negative temperature coefficient. Absolute value of the temperature coefficient of the first thermistor (R1) is not smaller than that of the second thermistor (R2) such that the first resistance of the first resistive unit4is proportional to or is in a positive relation with the operation temperature of the LED1when the operation temperature of the LED1is above the predetermined threshold temperature (Tt). In the present embodiment, the temperature coefficients of the first and second thermistors R1, R2are 1300 mΩ/° C. and −1000 mΩ/° C., respectively.

In this embodiment, the second resistive unit5includes a series connection of third and fourth thermistors (R3, R4). Referring toFIG. 4, Curve (L−) defines a relationship between the second resistance of the second resistive unit5, which is attributed to respective resistances of the third and fourth thermistors (R3, R4), and operation temperature of the LED1. Curve (L−) is also divided at the predetermined threshold temperature (Tt) into third and fourth curve segments (Lc, Ld). The third thermistor (R3) has a positive temperature coefficient, and the fourth thermistor (R4) has a negative temperature coefficient. Absolute value of the temperature coefficient of the third thermistor (R3) is not greater than that of the fourth thermistor (R4) such that the second resistance of the second resistive unit5is inversely proportional to or is in a negative relation with the operation temperature of the LED1when the operation temperature of the LED1is above the predetermined threshold temperature (Tt). In the present embodiment, the temperature coefficients of the third and fourth thermistors (R3, R4) are 500 mΩ/° C. and −2000 mΩ/° C., respectively.

It is to be noted that actual values of the temperature coefficients of the first, second, third, and fourth thermistors (R1, R2, R3, R4) are not limited to such. For example, in other embodiments, the current regulating circuit2may be configured such that the first and second thermistors (R1, R2) of the first resistive unit4have respective positive temperature coefficients, and that the third and fourth thermistors (R3, R4) of the second resistive unit5have respective negative temperature coefficients.

It is worth noting that Curve (L) is the summation of Curves (L+) and (L−). That is, the first curve section (L1) is the summation of the first and third curve segments (La, Lc), and the second curve section (L2) is the summation of the second and fourth curve segments (Lb, Ld). Furthermore, in the present embodiment, the effective resistance of the second resistive unit5at an initial temperature (e.g., room temperature) is not smaller than one-half of that of the first resistive unit4at the initial temperature.

Moreover, in the present embodiment, when the operation temperature of the LED1is equal to or lower than the predetermined threshold temperature (Tt), the effective resistance of the current regulating circuit2is substantially non-varying. Specifically, when the operation temperature of the LED1is equal to or lower than the predetermined threshold temperature (Tt), absolute value of a rate of change of the first resistance is approximate to that of the second resistance, and absolute value of a rate of change of the effective resistance is below a predetermined value. In this present embodiment, the predetermined value is 5%. In other embodiments, the predetermined value can be configured according to design need. Since the internal resistance of the LED1is substantially non-varying when the operation temperature of LED1is equal to or below the predetermined threshold temperature (Tt), an overall resistance of the LED1and the current regulating circuit2is substantially non-varying when the operation temperature is equal to or below the predetermined threshold temperature (Tt), such that the LED1emits light with stable brightness.

Since the effective resistance of the current regulating circuit2is proportional to the operation temperature of the LED1when the operation temperature is above the threshold temperature (Tt) (i.e., the second curve section L2of Curve L), when current flowing from the AC power source3to the LED1is reduced as a result of an increase in the internal resistance of the LED1caused by ageing and long duration of operation in the temperature range above the predetermined threshold temperature (Tt), the reduction in current flowing to the LED1causes the effective resistance of the current regulating circuit2to decrease due to a reduction in the operation temperature of the LED1, which in turn cancels out the increase in the internal resistance of the LED1, thereby regulating current flowing to the LED1.

Thus, the overall resistance attributed to the LED1and the current regulating circuit2is substantially non-varying, such that brightness of light emitted by the LED1is stabilized, and durability of the LED1is relatively improved. In addition, when current flowing from the AC power source3to the LED1varies due to variation in voltage of the AC power source3, the current regulating circuit2is also able to reduce the variation in current attributed to the variation in the supply voltage.

In other words, when the operation temperature of the LED1is above the predetermined threshold temperature (Tt), the absolute value of the rate of change of the first resistance (i.e., the second curve segment Lb) is larger than that of the second resistance of the second resistive unit5(i.e., the fourth curve segment Ld), such that the effective resistance of the current regulating circuit2is proportional to the operation temperature when the operation temperature is above the predetermined threshold temperature (Tt). Specifically, in the present embodiment, when the operation temperature is above the predetermined threshold temperature (Tt), the absolute value of the rate of change of the first resistance within a temperature increment of 10 degree Celsius is at least twice larger than that of the second resistance.

In the present embodiment, when the operation temperature of the LED1is above the threshold temperature (Tt), a rate of change of the effective resistance of the series connection of the first resistive unit4and the second resistive unit5(i.e., a rate of change of the effective resistance of the current regulating circuit2) within the temperature increment of 10 degrees Celsius is at least 5% (preferably, 5% to 8%). It is to be noted that the rate of change of the effective resistance of the series connection may be changed to meet design needs by implementing the first and second resistive units4, independently using different combinations of thermistors. For example, in an application where the threshold temperature (Tt) is 70 degree Celsius and the operation temperature of the LED1increases from 70 degree Celsius to 80 degree Celsius, the rate of change of the effective resistance of the current regulating circuit2within the temperature increment of 10 degrees Celsius is at least 5%. Furthermore, in other embodiments, the temperature increment is not limited to such, and may be adjusted according to electrical characteristics of the LED1.

Referring toFIG. 5, in the second preferred embodiment, the first resistive unit4includes a first thermistor (R5) with a positive temperature coefficient, and the second resistive unit5includes a second thermistor (R6) with a negative temperature coefficient. Absolute value of the temperature coefficient of the first thermistor (R5) is not smaller than that of the second thermistor (R6).

Referring toFIG. 6, the temperature coefficient of the first thermistor (R5) is 1500 mΩ/° C. and is represented by Curve (L61), the temperature coefficient of the second thermistor R6is −4000 mΩ/° C. and is represented by Curve (L62), and summation of which is represented by Curve (L63). Apparently, the current regulating circuit2of the second preferred embodiment also has a substantially flat effective resistance when operation temperature of the LED1is between 30 degree Celsius and 70 degree Celsius, and an increasing effective resistance when the operation temperature of the LED1is above 70 degree Celsius.

Moreover, in other embodiments with a configuration similar to that of the second preferred embodiment, the first and second resistive units4,5may be configured such that first and second thermistors (R5, R6) have temperature coefficients of 400 and −500, respectively, so that a sum of the first and second resistances of the first and second resistive units4,5(i.e., the effective resistance of the current regulating circuit2) is substantially non-varying when the operation temperature is in the temperature range from 25 to 50 degrees Celsius. However, configuration of the current regulating circuit2is not limited to such, as long as the effective resistance of the current regulating circuit2is substantially non-varying within a predetermined temperature range below the threshold temperature (Tt), and increases with the operation temperature of the LED1when the operation temperature is above the threshold temperature (Tt) (i.e., a tangential line of the curve (L) at a temperature above the threshold temperature (Tt) has a positive rate of slope).

In summary, the current regulating circuit2of the present invention is able to regulate current flowing to the LED1when the operation temperature of the LED1is in the temperature range above the predetermined threshold temperature (Tt), and when the internal resistance of the LED1starts to vary in the temperature range due to ageing, such that brightness of light emitted by the LED1is stabilized and that durability of the LED1is relatively improved.