Key Concepts for Electrical Design of LED Signal Lamps
Presently, most current LED signal lamp
designs use resistive current limiting for the LED
array. Since most LED signal lamps are driven
from 12 to 24 V dc and require several LED
emitters, the emitters can be connected in
series and share the same supply current.
Some of the most common circuit
configurations are shown in Figure 3.1. The
series connected string circuit in Figure 3.1a
uses a separate current limiting resistor for each
string of
y
LED emitters. The paralleled string
circuit in Figure 3.1b uses a single resistor for
the entire LED array. Note that this circuit uses
x
strings with
y
LED emitters per string. The
cross connected paralleled string circuit shown
in Figure 3.1c has one or more cross
connections between the strings of LED
emitters. A mechanical analogy to this circuit is
that the circuit looks like a “ladder” with each
cross connection being a “rung” on the
“ladder.” In the diagram,
z,
refers to the number
of series connected LED emitters between each
“rung” with 1
≤
z
≤
y.
Most CHMSL designs
use either several series connected strings
(Figure 3.1a) or several cross connected series
strings (Figure 3.1c) with
z =1.
Note that in
order to obtain the same forward current for
all of LED emitters, both of the circuits
shown in Figure 3.1b and Figure 3.1c need
to have the same number of LED emitters in
each string and each “rung” for Figure 3.1c.
Note that if all LED emitters have identical
electrical forward characteristics, then the value
of the external current limiting resistor,
R,
is
equal to:
This equation is known as the linear forward
voltage model since it models the forward
voltage of the LED with a straight line. In using
this model it is important to remember that it can
only be used over a specific range of forward
currents. Outside of this range, the model will
give misleading results. Using the linear forward
voltage model, this equation can be rewritten as:
Where:
V
O
= turn on voltage of each LED emitter
R
S
= series resistance of each LED emitter
V
F
≅?
O
+ R
S
I
F
V
As shown later in the section “LED Emitter
Modeling” the forward voltage of an LED emitter
can be mathematically modeled by the following
equation:
Thus for a given
I
F
, the value of
R
depends both
on the number of LED emitters per string as well
as the number of paralleled strings.
Where:
V
IN
= input voltage applied to the circuit
V
F
= forward voltage of LED emitter at forward
current
I
F
V
D
= voltage drop across optional reverse
transient EMC protection diode
y
= number of series connected LED emitters
x
= number of paralleled strings
8
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
