forward currents. For SuperFlux and SnapLED
emitters, the light output varies by a factor of
2:1 at a forward current of 20 mA (40 mA for
the SnapLED 150). Thus, even if all of the LED
emitters are driven at the same forward current,
there would likely be unacceptable light output
matching if the Tail signal is driven at a low DC
forward current.
reduced luminous intensity. The Stop signal
might operate the LED array at a high DC
forward current. Then for the Tail signal, the
array would be operated at the same peak
forward current with a low duty cycle (ratio of
“on” time to “on” plus “off” time). This
approach provides light output matching under
both levels of luminous intensity. A
recommended PWM circuit is shown in the
section “Special Considerations for Dual
Luminous Intensity Operation.”
For best matching, it is recommended that a
pulse width modulation (PWM) circuit be
designed to operate the signal lamp at
Theory
Overview of Electrical, Optical, and
Thermal Characteristics of an LED Emitter
In order to properly design an LED signal light,
it is important to have a basic understanding
of the electrical, optical, and thermal
the incremental forward voltage. Although this
graph has been used traditionally to describe the
forward characteristics of a diode, in reality LED
emitters are best thought of as current controlled
devices, not voltage controlled devices. For an
LED emitter, the optical properties are best
described as a function of current, not a function
of voltage. In addition, operation of the LED
emitter at a constant current gives the best
control of light output. In contrast, operation of
the LED emitter at a constant voltage allows a
larger variation in forward current and light output
from device to device.
characteristics of an LED emitter.
The typical forward current (IF) versus forward
voltage (VF) characteristic for an AlInGaP LED
emitter under positive (forward) bias is shown in
Figure 3.7. On a linear scale of forward current
versus forward voltage, negligible current flows
until a threshold voltage, also known as the
turn-on voltage (VO), is exceeded. Above this
voltage, the current increases proportionally to
Figure 3.7 Typical Forward Current versus
Forward Voltage for HPWA-xHOO LET Emitter
(Linear Scale).
Figure 3.8 Typical Forward Current versus Forward
Voltage for HPWA-xHOO LED Emitter (Semi-Long
Scale.)
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