The low-current forward characteristics of the
same AlInGaP LED emitter are shown in Figure
3.8. This graph shows the forward voltage
versus the log of forward current. Note that a
small current flows through the emitter even at
low forward voltages below the turn-on voltage
shown in Figure 3.7. Due to the high optical
efficiency of AlInGaP material, a perceptible
amount of light is generated from the LED
emitters at forward currents as low as 10 mA.
Thus the inadvertent operation at low forward
currents can cause “ghosting” within an “off”
LED signal light.
HPWT-xx00 emitters. Operation of the LED
emitter in the reverse current region is not
recommended. Reverse currents in excess of 50
mA can cause permanent damage to the LED
junction, as discussed later in the section titled
“Electrical Transients.” The reverse breakdown
voltage is essentially constant over the –40ºC to
100ºC temperature range.
The change in luminous flux (FV) as a function of
forward current (IF) of an AlInGaP LED emitter is
shown in Figure 3.10. Note that the change in
luminous flux is roughly proportional to the
change in forward current. At forward currents
over 20 mA, the luminous flux increases at a
lower rate due to internal heating within the LED
emitter. The change in luminous flux due to a
change in forward current ( DFV / DIF) varies
somewhat from unit to unit. Figure 3.11 shows
the expected range in light output for HPWT-
xH00 emitters that were matched at 70 mA.
Note that the light output varies by a factor of 2:1
at a 20 mA forward current. Since the SnapLED
150 emitter is matched at 150 mA, then the light
output can be expected to vary by a factor of 2:1
at a forward current of 40 mA.
The forward voltage of an AlInGaP LED emitter
changes by about –2 mV per °C over
temperature. Thus, the forward voltage at a
given current is slightly lower at elevated
temperatures and slightly higher at colder
temperatures.
The reverse characteristics of an AlInGaP LED
emitter are shown in Figure 3.9. Note that a
negligible amount of reverse current (< 1 mA)
flows through the LED until the reverse
breakdown voltage is reached. The reverse
current increases quickly at voltages higher than
the reverse breakdown voltage (defined as the
voltage across the LED at which the reverse
current reaches 100 mA). The reverse
The luminous flux of an AlInGaP LED emitter
varies inversely with temperature as shown in
Figure 3.12.
breakdown voltage for AlInGaP LED emitters is
typically in the range of 20 V. However, it can
be as low as 10 V for the HPWA-xx00 and
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