luminous flux and forward voltage at 70 mA
(150 mA for the SnapLED 150). Thus, their
matching is best when driven at high forward
currents. As discussed in the section “Electrical,
Optical, and Thermal Properties of an LED
Emitter,” Figure 3.11 shows the expected range
in light output for HPWT-xH00 emitters that are
matched at 70 mA. Note that the light output
varies by approximately 2:1 at a 20 mA forward
current. Note that since the SnapLED 150 is
matched at 150 mA, then the light output would
vary by about 2:1 at 40 mA. Thus, even if a
well-controlled constant current circuit drives
the LED emitters, the light output matching
might be unacceptable for the Tail function if
driven by a low DC forward current.
Figure 3.20 shows a PWM Tail circuit. When the
input voltage is applied to the Stop pin, the
cross-connected paralleled string circuit is
operated at a dc forward current determined
external resistor, RSTOP. The value for RSTOP can be
determined using Equation #3.2. When the input
voltage is applied to the Tail pin, the NE555
oscillator is energized. Resistors, R1 and R2, and
capacitor, C1, determine the frequency and duty
cycle of the oscillator. The values shown
generate a 2K Hz frequency and a 10% duty
cycle (output low). When the output of the
NE555 (pin 3) is low, a high-current switching
transistor is turned-on, which supplies current to
the LED array. External resistor, R TAIL, determines
the peak current of the Tail circuit. The value for
R TAIL can be determined using a similar equation
as used previously for RSTOP and including the
extra voltage drop across the high-current
switching transistor. Thus for the same value of
peak forward current, R TAIL < RSTOP. Diodes D1
and D2 protect the circuit from negative EMC
transients. Zener diode D 3 protects the NE555
from positive EMC transients.
A PWM circuit is recommended for best
luminous intensity matching for the Tail function.
This circuit could drive the LED array at a high
forward current for the Stop function and drive
the LED array at the same peak forward current
but at a low duty cycle for the Tail function. With
this approach, the matching of the LED array
will be the same regardless of whether the
signal is operating in Stop or Tail mode.
Figure 3.20 Stop/Tail LED
Signal Lamp Circuit that Uses
a PWM Scheme to Generate
the Reduced Light Output of
the Tail Signal.
30
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
