1. TITLE:
The Title of Invention is "Wide voltage range LED assembly with variable intensity for Aircraft panel backlight illumination".
2. FIELD OF INVENTION:
The present Invention generally relates to Aircraft instrument panel lightning System and in particular relates to the design and development of wide voltage range led assembly for backlight illumination of cockpit instruments.
3. BACKGROUND OF INVENTION:
Backlight illumination is provided, in aircraft's cockpit instruments for clear visibility of inscriptions and failure indications to the pilot. As aircraft power supply is in range of 24-27V DC, the most suited component for backlight illumination is high voltage incandescent bulbs. Some instruments have integrated ambient light sensors which provide voltage variation across these bulbs according to measured ambient light in the cockpit. This voltage •variation, in the range of 16-27 VDC controls the luminous intensity of incandescent bulbs, providing enhanced vision and relief to the pilot's eyes according to the light conditions of the cockpit. The filament based technology of incandescent bulbs is very old and inefficient. The filament's metallurgical properties and heat produced during its operation are responsible for its limited life, high power consumption and reduced intensity over the total technical life span of parent instrument. Moreover, high cost is involved in manufacturing of incandescent bulbs of aircraft grade.
The prior art US8004216B2 discloses a variable LED intensity system which is configured to provide a change in luminance versus input voltage that corresponds to a desired transfer function, such as the dimming characteristics of an incandescent lamp, which closely resembles the response of human eye. Although this patent can achieve desired voltage vs luminance function, the size and mounting configuration makes its scope of application very limited and unsuitable for aircraft instruments backlight illumination.
The proposed configuration of LED assembly has very few components, simple design and can be calibrated or configured to be used as a direct substitute to axial incandescent . bulbs for aircraft instruments backlight illumination.
Challenges:
1. The Voltage available at the input of incandescent Lamp in aircrafts is in range of 16-27V DC, which is suitable for filament based lamps only. The LED operating voltages are in rage of 1.8 - 2.2V DC. First, a simple potentio-divider approach was used to achieve the required voltage drop. This design approach failed to provide desired levels during different modes of working of instruments.

Depending upon the ambient light sensor in instrument, when measured by a photometer, brightness of light signal indicators should be in range of:
- In DAY mode (Maximum ambient light) -600 -2600 Cd/m2.
- In NIGHT mode(Minimum ambient light, dark) -25 -100 Cd/m2
Consequently, an approach was devised to control the current flowing through LED to achieve desired luminous intensity at both day and night modes of operation.
4. BRIEF SUMMARY OF INVENTION:
Technology up-gradation is need of the hour. Backlight illumination is provided in aircraft's cockpit instruments for clear visibility of inscriptions and failure indications to the pilot. As aircraft power supply is in range of 24-27V' DC, the most suited component for backlight illumination is high voltage incandescent bulbs. However due to high manufacturing costs, limited lifespan, unwanted heat radiation, power inefficient and failure prone nature, an attempt is made to design LED based backlight illumination technique which can directly replace old incandescent bulbs. The proposed LED assembly used two resistors for precise control of in-circuit current for achieving desired luminous intensity in wide range of aircraft DC voltage inputs.
5. BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is the graphical relationship representation between High brightness LED forward
current vs. forward voltage.
Fig. 2 is the graphical relationship representation between High brightness LED Luminous
intensity vs. Forward current
Fig. 3 is the Schematic view of two LED assemblies connected in series for current simulation
on MICROCAP software.
Fig 4 is the Schematic view of single LED assembly of one embodiment of the present
invention.
Fig. 5 is the view of components placement on FR-4 PCB to mimic the fit and form of T1
incandescent bulb mounted on support.
Fig. 6 is the comparison of actual images of T1 incandescent bulb mounted on support and
LED assembly being referred as invention.
Fig. 7 is the flowchart of design approval process for accepting the LED assembly as a
substitute of T1 incandescent bulb mounted on support.

6. DETAILED DESCRIPTION OF INVENTION:
With" several iterations on design and simulation software (Micro CAP), a new type of technique was evolved in which two resistors were used. One as current limiting resistor, to provide stable current levels to LED and some voltage drop and second resistor was used in parallel to LED for bypassing extra current being fed at the input of LED. Current bypass resistor allowed us to precisely control the LED current and thus the light intensity.
The relationships between Forward Voltage, current and luminous intensity of the LED used, are shown in Fig.1 and Fig.2. The proposed circuit can be adapted to wide range of LEDs with different Forward Voltage, current and luminous intensity relationships.
Final circuit with simulated parameters is shown in Fig.3.
In this arrangement, two LED assemblies are conncted in series. One LED assembly consists of a small signal general purpose'diode, one high brightness LED, and two reistors.
Diode D5/D4 are used to provide reverse voltage protection to LED. Resistor R1 is used to provide the finely tuned voltage drop, so that the LED can operate within 1.8V to 2.2V according to required luminous intensity. Resistor R2 is used as current bypass resistor for bypassing extra current being fed at the input of LED. By selecting appropriate combination of R1 and R2, it is possible to operate LED within its absolute ranges and controlcurrent passing through LED to achieve desired luminous intensity.
In this particular requirement, the value of two resistors (series and bypass) was so adjusted that LED current is limited to 20mA in day mode (full voltage) and to 6mA in Night Mode (lowest Voltage).This provided the required luminous intensity during both testing modes.
Though the proposed circuit is designed to work within 16-27V DC, it can be modified to work for even a wider range and for different luminous intensities, based on the required applications.
Schematic diagram for single LED assembly is shown in Fig.4.
PCB Design and mechanical dimensions:
The LED assembly was developed as a substitute to filament based incandescent bulb of T1 size, which is to be press-fitted inside a indicator switch. As the mechanical dimensions of bulb mounted on support were limited to 14mm*5.4mm, small size SMD components were selected for implementing the design. Ref Fig.5.

Fig. 6 is the side by side comparision of real images of T1 incandescent bulb mounted on support and LED assembly being referred as invention.
Exposed copper contact points were provided on PCB for proper connections with spring type contacts. This eliminated the need of a separate support and also the contact was more firm than original part.