Current and Voltage Regulator Circuits
This section will discuss active circuits that are
designed to drive the LED emitter array at a
constant voltage or constant current despite
input voltage or load variations. These circuits
are called voltage or current regulator circuits
because they are designed to regulate the input
voltage to generate either a fixed output voltage
or current.
and compare the load voltage with a reference
voltage. Current regulator circuits usually
measure the current through the load by
measuring the voltage drop across a “sense”
resistor in series with the load. Then the voltage
across the sense resistor is compared with a
reference voltage.
Since most LED signal lamp designs consist of
several LED emitters, they are normally arranged
in one or more series-connected strings, such as
shown previously in Figure 3.1. While it is
possible to use one voltage or current regulator
per string, due to cost considerations, most
practical designs use a single voltage or current
regulator for the entire LED array. Note that when
only a single regulator is used for the entire array
it is possible to encounter the same type of
forward current variations as described earlier in
the section “Key Concepts for the Electrical
Design of LED Signal Lamps.” Since LED
emitters are current-controlled devices,
voltage regulator circuits should use current-
limiting resistors in series with each string of
LED emitters (Figure 3.1a circuit), paralleled
string of LED emitters (Figure 3.1b circuit), or
cross-connected paralleled string of LED
emitters (Figure 3.1c circuit). For voltage
regulator circuits, Equation #3.2 can be used to
calculate the value of the external current-limiting
resistor(s) if the regulated output voltage, VOUT, is
substituted into the equation for VIN. For current
regulator circuits, external current limiting
resistors are not required but their use can
reduce forward current variations within the LED
array.
The use of voltage or current regulation
improves the operation of the LED signal lamp.
Since the drive current of the LED array remains
constant despite variations in the supply
voltage, the light output is not affected by input
voltage variations. Since the drive current
doesn’t increase due to over voltage conditions,
the LED emitters can be driven at a higher
forward current at the design voltage without
exceeding the maximum allowable forward
current at the maximum input voltage. In
addition, if the circuit is located outside of the
LED signal lamp case, the voltage or current
regulator circuit can improve the thermal
properties of the signal lamp by reducing the
power consumption within the LED signal lamp.
Block diagrams of typical voltage and current
regulator circuits are shown in Figure 3.21. The
basic elements of all of these circuits consist of
a high gain amplifier and feedback circuit, which
vary the dynamic load of a power circuit that is
either in series or parallel with the LED emitter
array. The regulator circuit modulates the
dynamic load so as to provide either a constant
voltage or current to the LED emitter array
independent of input voltage or load variations
(over some specified range). Voltage regulator
circuits measure the voltage across the load
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