Incandescent bulb luminance matching LED circuit

An incandescent bulb luminance matching LED circuit for causing the luminance of an LED to match the luminance of an incandescent bulb is disclosed. The incandescent bulb luminance matching LED circuit includes an input port (26, 56 or 82), an output port (28, 58 or 98), one or more light emitting diodes (22, 24, . . . or 88, 90 . . .), and a voltage (20 or 89) and/or current (50 or 86) compensation block. The compensation block(s) is connected in circuit with the light emitting diode(s) between the input port and the output port and compensates for voltage and/or current changes in the power applied to the input port such that the luminance of the LED is approximately the same as that of an incandescent bulb. In one embodiment, the compensation block comprises a zener diode (30) connected in series with the light emitting diode(s) (22, 24, . . .) between the input port (26) and the output port (28). In an alternate embodiment, the compensation block comprises one or more current diode(s) (60, 62, . . .) connected in parallel with the light emitting diode(s) (52, 54, . . .) between the input port (56) and the output port (58). In yet another embodiment, the compensation block comprises both a zener diode (92) connected in series with the light emitting diode(s) (88, 90, . . .) and a one or more current diode(s) (100, 102, . . .) connected in parallel with the light emitting diode(s) (88, 90, . . .).

FIELD OF THE INVENTION 
The present invention relates to luminance matching circuits, and more 
particularly, to LED circuits for causing the luminance characteristics of 
a light emitting diode (LED) to match that of an incandescent bulb. 
BACKGROUND OF THE INVENTION 
Incandescent bulbs are commonly used in a variety of applications to 
provide light. For example, incandescent bulbs may be used as a light 
source for illuminated switches, lighted panels, displays, legends, 
indicators, and in a variety of other applications. 
Although incandescent bulbs may provide a satisfactory degree of 
illumination, they also carry with them a number of disadvantages. For 
example, incandescent bulbs operate at a relatively high temperature. 
Consequently, incandescent bulbs can generate enough heat to cause burns 
when used in some applications, such as in lighted switch or panel 
applications. In addition, incandescent bulbs have a relatively short life 
span, and may require frequent replacement. Likewise, many incandescent 
bulbs are prone to failure in high vibration environments. Finally, 
incandescent bulbs operate at relatively high power levels. 
Light emitting diodes (LEDs) offer advantages over incandescent bulbs in 
each of the above areas. Thus, when compared to incandescent bulbs, LEDs 
produce less heat, operate for a longer life, are less prone to failure in 
high vibration environments, and consume less power. Because of these 
advantages, it is desirable to substitute LEDs for incandescent bulbs in 
many applications. 
Unfortunately, LEDs produce a different luminance, or brightness level, 
than incandescent bulbs given the same input current or voltage. FIG. 1 
illustrates the relative luminance of an incandescent bulb and an LED 
given a varying input voltage. The LED luminance curve is indicated by the 
reference numeral 10 while the incandescent bulb luminance curve is 
indicated by the reference numeral 12. As FIG. 1 illustrates, LEDs and 
incandescent bulbs may have quite different luminance levels over a wide 
range of input voltages. 
Similarly, FIG. 2 illustrates the relative luminance of an incandescent 
bulb and an LED over a varying input current. The LED luminance is 
indicated by the reference numeral 14, while the incandescent bulb 
luminance is indicated by the reference numeral 16. As FIG. 2 illustrates, 
LEDs and incandescent bulbs may have quite dissimilar luminance levels 
depending upon the input current level. 
While there are many uses in which it is desirable to replace an 
incandescent bulb with an LED of similar luminance, one application of 
particular importance is in aircraft cockpits. For many aircraft, display 
and indicator lights must be designed in accordance with specifications 
for brightness. In addition, under certain conditions, the aircraft pilot 
may wish to manually dim the display by adjusting a dimmer switch. If each 
of the lights has similar brightness characteristics, the display may be 
dimmed consistently. This is particularly important when the pilot is 
wearing night vision goggles. At such times, the pilot must be able to 
darken the display entirely. If any of the display lights may not be 
darkened, the night vision goggles may "bloom," rendering them practically 
useless. Accordingly, in many applications LEDs may only be substituted 
for incandescent bulbs if the brightness characteristics are the same. 
The present invention is directed to providing a compensation circuit for 
matching the luminance of an LED to that of an incandescent bulb over a 
wide range of input currents or input voltages. 
SUMMARY OF THE INVENTION 
In accordance with this invention, an incandescent bulb luminance matching 
LED circuit that compensates at least one parameter of an input power 
source to cause the luminance of an LED to match that of an incandescent 
bulb in response to changes to the at least one parameter is provided. The 
circuit includes an input terminal, an output terminal, an LED, and a 
diode. The diode is connected in circuit with the LED between the input 
and output terminals. In a first embodiment of this invention, the diode 
is a zener diode connected in series with the LED between the input and 
output terminals. In this manner, the LED luminance is matched to an 
incandescent bulb luminance at at least one input voltage level. 
In accordance with other aspects of this invention, resistors are provided 
in series and in parallel with the zener diode. The resistors enable the 
LED luminance to more closely approximate the incandescent bulb luminance, 
and to match the incandescent bulb luminance at at least two input voltage 
levels. 
In accordance with further aspects of this invention, in a second 
embodiment of this invention the diode is a current diode connected in 
parallel with the LED between the input and output terminals. In this 
manner, LED luminance is matched to incandescent bulb luminance as a 
function of input current. 
In accordance with still other aspects of this invention, multiple current 
diodes may be used to enable the LED luminance to be matched to an 
incandescent bulb luminance at any desired input current level. 
In accordance with still further aspects of this invention, the luminance 
compensation circuit may drive a plurality of LEDs. 
In accordance with yet other aspects of this invention, the circuits of the 
first and second embodiments, described above, may be used together so 
that LED luminance is matched to the luminance of an incandescent bulb 
over a wide range of input voltages and currents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 3 illustrates a luminance compensation circuit formed in accordance 
with this invention. As will be better understood from the following 
description, a luminance compensation circuit formed in accordance with 
this invention includes a circuit for causing the luminance of an LED to 
more closely approximate the luminance of an incandescent bulb. 
The circuit illustrated in FIG. 3 includes an input port 26, a compensation 
block 20, one or more LEDs 22, 24, . . . connected in parallel, and an 
output port 28. The compensation block 20 provides input voltage 
compensation and includes a zener diode 30 connected in parallel with a 
first resistor 32. The zener diode 30 and the first resistor 32 are 
connected in series with a second resistor 34. The input voltage, 
V.sub.IN, is applied to the input port 26, i.e., the junction between the 
cathode of the zener diode 30 and the first resistor 32, and the output of 
the compensation block 20 is applied to the anodes of the LEDs 22, 24, . . 
. . The voltage return, V.sub.OUT, is at the outport port 28. 
When a relatively low input voltage (i.e., a voltage below the zener 
breakdown, or threshold, level) is applied to the input port 26, current 
will initially flow through the first resistor 32 and the second resistor 
34, producing a corresponding voltage drop across the first resistor 32 
and the second resistor 34. As a result, the voltage present at the LEDs 
22, 24, . . . is much lower than the voltage at the input port 26. 
As the voltage at the input port 26 is increased to levels at or above the 
zener threshold level, current will flow from the input port 26 through 
the zener diode 30, largely bypassing the first resistor 32. Thus, at 
input voltage levels approximately above the zener breakdown level, the 
input voltage is dropped across the second resistor 34 and the zener 
voltage is dropped across the first resistor 32. Consequently, there is a 
smaller relative reduction in the voltage level present at the anodes of 
the LEDs 22, 24, . . . for input voltages greater than the zener breakdown 
level than for input voltages less than the zener breakdown level. 
FIG. 5 is an illustration of the relative luminance of an incandescent bulb 
and an LED driven by a compensation circuit of the type shown in FIG. 3. 
The reference numeral 40 refers to the luminance level of the LED, while 
the reference numeral 42 refers to the luminance of the incandescent bulb. 
In this illustration, the zener diode 30, the first resistor 32, and the 
second resistor 34 were selected so that the LED luminance matches the 
incandescent luminance at input voltage levels of 12 and 26.5 volts. In 
between, the compensation block 20 compensates the input power source 
applied to the input port 26 such that the luminance of the LED 22 closely 
approximates that of an incandescent bulb. 
In an actual embodiment, corresponding to the relative luminance curve 
shown in FIG. 5, the chosen zener diode 30 had a rating of 6.8 volts, the 
chosen first resistor had a value of 1500 ohms and the chosen second 
resistor had a value of 150 ohms. Thus, at input voltages below 
approximately 6.8 volts, little or no current flow passed through the 
zener diode 30. At such voltage levels, the resistance of the first 
resistor 32 determined the brightness of the LED 22 and the point at which 
the luminance of the LED 22 matched that of an incandescent bulb because, 
relatively, the second resistance had little effect. The value of the 
second resistor 34 determines the luminance of the LED 22 for input 
voltage levels greater than the zener threshold level. The value of the 
second resistor 34 also determines the point at which the luminance of the 
LED 22 will be equal to that of an incandescent bulb at high input levels. 
Those of skill in the art will recognize that the LED luminance can be 
matched to an incandescent bulb luminance at virtually any voltage level 
by selecting the proper zener diode 30, first resistor 32, and second 
resistor 34. 
As illustrated in FIG. 3, and noted above, a single compensation circuit 
may be formed to drive a single or multiple LEDs. An array of LEDs may 
also be driven by the compensation circuit, where the array of LEDs is 
comprised of one or more LEDs connected in series and one or more series 
strings of LEDs connected in parallel. The relative luminance of all of 
the LEDs 24 will be matched to the luminance of an incandescent bulb. In 
one actual embodiment of the invention, a single compensation circuit is 
used to drive a total of five LEDs. 
Those skilled in the art will further appreciate that the compensation 
block 20 can be implemented using resistors arranged other than as 
illustrated in FIG. 3. One alternative is illustrated in FIG. 4. In FIG. 
4, the zener diode 30 of the compensation block 20 is connected in series 
with a first resistor 38. The zener diode 30 and first resistor 38 are 
connected in parallel with a second resistor 36. 
The operation of the alternate embodiment illustrated in FIG. 4 is similar 
to the operation of the embodiment illustrated in FIG. 3. When a low input 
voltage (that is, a voltage less than the zener breakdown voltage level) 
is applied to the input port 26, except for a slight leakage current 
through the zener diode 30, all of the current through the compensation 
block 20 passes through the second resistor 36. The portion of the input 
voltage present at the input port 26 dropped across the second resistor 36 
reduces the voltage at the anodes of the LEDs 22, 24, . . . . When the 
voltage at the input port 26 increases to a level above the zener 
breakdown voltage level, the majority of the current through the 
compensation block passes through the zener diode 30 and the first 
resistor 38. Very little current passes through the second resistor 36. 
Those of skill in the art will appreciate that the values of the zener 
diode 30, the first resistor 38, and the second resistor 36 can be 
selected to allow the luminance of the LED 22 to match that of an 
incandescent bulb at virtually any voltage level. 
Unfortunately, the luminance of the LEDs 22, 24, . . . varies with 
increasing temperatures. In turn, the temperature of the LEDs 22, 24, . . 
. increases with increasing current through the LEDs 22, 24 . . . . 
Moreover, the actual resistances of the first and second resistors and the 
luminance of the LEDs 22, 24, . . . may vary substantially from their 
nominal or advertised values. Accordingly, the luminance of the LEDs 22, 
24, . . . may be best matched to that of an incandescent bulb by trimming 
or tuning the resistor values while monitoring the luminance of the LEDs 
22, 24, . . . . 
In another alternate embodiment of this invention, the compensation block 
can be formed to cause the luminance of an LED to match that of an 
incandescent bulb over a variety of input currents. As illustrated in FIG. 
6, the circuit of this alternate embodiment includes an input port 56, a 
compensation block 57, one or more LEDs 52, 54, . . . , and an output port 
58. Because the compensation block 57 of this alternate embodiment is 
intended to adjust the LED luminance as a function of input current, the 
compensation block 57 is connected in parallel with the LEDs 52, 54, . . . 
. More specifically, the compensation block 57, which provides input 
current compensation, includes one or more current diodes 60, 62, . . . 
connected between the input port 56 and the output port 58. 
At relatively low input current levels, specifically current levels below 
the rated level of the current diodes 60, 62, . . . , all of the current 
through the circuit flows through the current diodes 60, 62, . . . , 
bypassing the LEDs 52, 54, . . . . When the current at the input port 56 
exceeds the rated current level of the current diodes 60, 62, . . . , the 
current in excess of the rated current level passes through the LEDs 52, 
54, . . . . Appropriate choice of the current diodes enables the 
compensation block 57 to cause the luminance of the LED 52 to generally 
match that of an incandescent bulb. 
As noted above, a single current diode 60, or a plurality of current diodes 
60, 62, . . . (illustrated by dashed lines in FIG. 6) may be connected in 
parallel. Appropriately selecting the type and number of current diodes 
60, 62, . . . allows the relative luminance of the LED 52 to be matched to 
the luminance of an incandescent bulb at virtually any input current 
level. 
As also noted above, the current compensation block shown in FIG. 6 may be 
used to drive a plurality of LEDs. That is, one or more LEDs 52, 54, . . . 
(shown in dashed lines in FIG. 6) may be connected to the compensation 
block 57. Those having skill in the art will further recognize that 
additional, optional circuitry such as circuitry for current or voltage 
regulation or circuit protection (indicated by the reference numeral 64), 
consistent with the present invention, may be included, if desired. 
FIG. 7 is an illustration of the relative luminance of an incandescent bulb 
and an LED driven by the compensation circuit formed in accordance with 
this invention and illustrated in FIG. 6. The reference numeral 70 refers 
to the luminance of the LED 52, while the reference numeral 72 refers to 
the luminance of an incandescent bulb. As is illustrated in FIG. 7, the 
compensation circuit of this invention enables the luminance of the LED to 
relatively more closely approximate the luminance of the incandescent 
bulb. 
In many applications, it may be preferable to match LED luminance to that 
of an incandescent bulb across a wide range of both input voltages and 
input currents. In such cases, a voltage compensation block (such as 
depicted in FIGS. 3 and 4) may be used in conjunction with a current 
compensation block (depicted in FIG. 6) in the same circuit. This 
alternate embodiment is depicted in FIG. 8. The circuit of this alternate 
embodiment includes an input port 82, a voltage compensation block 89, a 
current compensation block 86, one or more LEDs 88, 90, . . . , and an 
output port 98. The voltage compensation block 89 is similar to the 
compensation block 20 depicted in FIG. 3, and includes a zener diode 92 in 
parallel with a first resistor 94. The zener diode 92 and first resistor 
94 are connected in series with a second resistor 96. The voltage 
compensation block 89 is connected between the input port 82 and the 
anodes of the LEDs 88, 90, . . . . The current compensation block 86 is 
connected between the input port 82 and the output port 90, in parallel 
with the voltage compensation block 89 and the LEDs 88, 90, . . . . As 
with the compensation block 57 depicted in FIG. 6, the current 
compensation block 86 includes one or more current diodes 100, 102, . . . 
. The inclusion of both the current compensation block 86 and the voltage 
compensation block 89 enables the luminance of the LEDs 88, 90, . . . to 
approximate the luminance of incandescent bulbs over a wide range of input 
currents and voltages. 
An incandescent luminance matching circuit formed in accordance with the 
present invention offers many advantages over the prior art. Most 
importantly, LEDs may be substituted for incandescent bulbs in 
applications that require LED luminance to be matched to incandescent 
luminance over a wide range of input voltages or currents. Additionally, 
an incandescent luminance matching circuit formed in accordance with the 
present invention provides for an LED light source that produces less 
heat, operates for a longer life, is less prone to failure in high 
vibration environments, and consumes less power when compared to an 
incandescent bulb. Because of these many advantages, LEDs may be readily 
substituted for incandescent bulbs in many applications. 
Those skilled in the art will further appreciate that the present invention 
can be implemented using devices arranged other than as described in the 
preferred embodiment. Consequently, within the scope of the claims, it is 
to be understood that the invention can be practiced otherwise than as 
specifically described herein.