Lighting or Agilent Technologies sales engineer
or from the following URL:
forward voltage can cause luminous intensity
matching variations within groups of
http://www.lumileds.com
SuperFlux emitters from the same luminous
flux and forward voltage category. For this
reason, Lumileds Lighting does not warrant
LED performance at currents less than 20 mA
(40 mA for SnapLED 150) and strongly
discourages these designs.
Note: For best matching within an array,
SuperFlux and SnapLED 70 emitters should
be operated at forward currents over 20 mA
(40 mA for SnapLED 150). At forward
currents below 20 mA (40 mA for SnapLED
150), variations in luminous efficiency and
Electrical Design Process
The electrical design is part of the overall signal
lamp design process described in AB20-1.
exceeded, portions of the electrical or thermal
design may need to be iterated in order to
reduce the forward current and/ or junction
temperature. Finally, additional prototypes of the
signal lamps should be constructed and
subjected to the appropriate reliability validation
tests.
The electrical design consists of several discrete
steps. The first step is to determine the circuit
topology and to generate an electrical
schematic of the overall circuit. First, the circuit
topology must be determined. Circuit topology
refers to the arrangement of electrical
The most important circuit topology
considerations include:
components on the electrical schematic. Next,
the circuit must be designed. Circuit design is
the process where the electrical components
are selected and component values are
• Number of LED emitters in series
• Whether LED emitters are connected in
individual series-strings, or in cross-
connected series strings
determined. Third, the operation of the circuit
must be analyzed. Circuit analysis refers to the
mathematical analysis of the variations in
voltage and current through the electrical
components due to variations in applied voltage
and component tolerances. The fourth step is
to create a breadboard of the circuit and to
measure the forward current, light output, and
thermal properties of the entire signal. If the
maximum junction temperature or the maximum
DC forward current of any of the LED emitters is
exceeded, the reliability of the LED signal lamp
may be compromised. Thus, if the circuit
validation tests indicate that these limits are
• Method of current limiting (i.e. resistors or
active circuit)
• Method of EMC protection (if any)
• Method of dimming, such as for a combined
Stop/ Tail signal
Circuit design is the solution of several
simultaneous linear equations that model the
forward current through each loop or node of the
circuit. The solution of these equations
determines the values of electrical components
that drive the LED array at the desired forward
current at the specified external supply voltage.
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