Note: Descriptions are shown in the official language in which they were submitted.
CA 02475647 2004-07-23
n
, '
':Y
LED LIGHT STRING AND ARRAYS WITH IMPROVED HARMONICS
' AND OPTIMIZED POWER UTILIZATION
This application is a continuation-in-part of application serial number
10/243,835
S filed September 16, 2002, which is a continuation of copending application
serial number
09/819,736 filed March 29, 2001, which is a continuation-in-part of copending
application serial number 09/378,631 filed August 20, 1999, which is a
continuation-in-
part of copending application serial number 09/339,616 filed June 24, 1999.
This
application claims benefit of U.S. Provisional Application No. 60/119,804,
filed February
12, 1999. The disclosures of the aforementioned applications are incorporated
herein by
reference.
BACKGROUND OF THE INVENTION
1. Field of Invention
1 S The present invention relates to light emitting diode assemblies, light
strings
comprising a plurality of light emitting diode assemblies, especially light
strings
employing light emitting diodes, and related methods.
2. Description of Related Art
Light emitting diodes (LEDs) are increasingly employed as a basic lighting
source
in a variety of forms, including decorative lighting, for reasons among the
following. - : _
First, as part of an assembly, LEDs have a very long lifespan, compared with
common
incandescent and fluorescent sources. For example, a typical LED lifespan is
at least
1
CA 02475647 2004-07-23
100,000 hours. Second, LEDs have several favorable physical properties,
including
ruggedness, cool operation, and ability to operate under wide temperature
variations.
Third, LEDs are currently available in all primary and several secondary
colors, as well
as in a "white" form employing a blue sc~~:rce and phosphors. Fourth, with
newer doping
S techniques, LEDs are becoming increasingly efficient, and colored LED
sources currently
available may consume an order of magnitude less power than incandescent bulbs
of
equivalent light output. Moreover, with expanding applications and resulting
larger
volume demand, as well as with new manufacturing techniques, LEDs are
increasingly
cost effective.
Tests have established that LED light strings use only 2 to 5% of the power of
an
incandescent string, although they produce less light. However, the light
output of LEDs
has improved significantly over the past years and continues to so.
LED-containing holiday and decorative light sets, such as used for decorative
purposes such as for Christmas lighting; typically employ series-parallel
construction.
This is particularly true when LED light strings are driven using line-voltage
Alternating
Current (typically 110-220 VAC). AC driven LED light strings employing a
single series
block of LEDs, or multiple series blocks of LEDs connected in parallel and
oriented in
the same polarity exhibit a high percentage of DC and harmonic content as well
as poor
power utilization. Harmonic distortion caused by widespread use of AC driven
LED
light strings presents serious transmission problems to electric utility
providers.
When single series block, AC driven LED light strings are connected in
parallel,
DC and harmonic content becomes a random event depending on whether the
strings are
connected in an end-to-end manner using the same polarity or opposite
polarity.
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CA 02475647 2004-07-23
However, this is not a truly random event as consumers tend to orient plugs in
the same
direction. The "stacking" of plugs, or use of corrunon polarized plugs and end
connectors
in the construction of LED light strings removes the randomness factor
entirely, further
exacerbating DC and harmonic feedback problems.
AC driven LED light strings employing multiple series blocks connected in
parallel will exlubit the same unbalanced wavefonn and percent harmauc content
as
single series block LED light strings when the series blocks are arranged in
the same
polarity. LED lights strings with balanced harmonics, or minimized harmonic
distortion
has not been addressed in prior art.
SUMMARY OF THE INVENTION
This invention provides an AC driven LED light string capable of addressing
one
or more of the above-mentioned drawbacks.
This invention further provides an AC driven LED light string assembly
possessing reduced harmonic distortion when a single series block of LED lamps
is
employed.
Tlus invention further provides a method of manufacturilig AC driven LED light
strings with random polarity orientation when a single series block of LED
lamps is
employed.
This invention further provides an AC driven LED light string with balanced
harmonics and DC content when multiple series blocks of LED lamps are
employed, -
wherein harmonic distortion and DC content is self canceling.
3
CA 02475647 2004-07-23
The invention further provides an AC driven LED light string with reduced
harmonic content when multiple series blocks of LED lamps are employed and
there is an
uneven number of series blocks within the light string.
The invention further provides an AC driven LED light string with increased
electrical efficiency, optimizing the percentage of power utilized.
It is another feature of the invention to provide a method for manufacturing
the
above, and to provide manufacturers with test procedures to assure harmonic
content is
reduced or eliminated.
To achieve one or more of the foregoing features of the invention, and in
accordance with the purposes of the invention as embodied and broadly
described in this
document, according to a first aspect of this invention there is provided an
AC driven
LED light string employing a capacitor coupled in parallel across the light
string AC
input, or end connector terminals to greatly reduce harmonic distortion when a
single
series block of LEDs are used (FIG 6A) (unbalanced harmonics and AC waveform).
It is
well known in the art that LED polarity must be maintained within a series
block of
LEDs. Capacitor specifications and properties are also well known in the art
(Capacitors
of 0.001 uF ~0.1 uF SOOV were used depending on the AC input voltage
employed).
According to a second aspect of this invention AC driven LED light strings
employing a single series block of LED lamps should be manufactured with an
equal
number of light strings produced with all LEDs in forward bias {anode first)
and reverse
bias {cathode first). This allows true randomness when large numbers of single
series
block LED light strings are connected in an end-to-end manner. A simple
sorting and test
procedure is given later in this text to assist manufacturers and allow for an
equal number
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CA 02475647 2004-07-23
of forward and reverse bias light strings. It is possible to apply this
concept to single
LED lamps having two chips where one chip is powered in reverse bias to the
second
chip; thereby providing self canceling harmonics within the single LED lamp.
Of course,
this self canceling concept can be employ to any even number of chips in a
single LED
lamp.
According to a third aspect of the invention AC driven LED arrays or light
strings
employing an even number of multiple LED series blocks (divisible by two) has
an equal
number of series blocks parallel connected in forward bias and reverse bias.
This
arrangement of LED series blocks presents an LED light string wherein harmonic
and
DC distortion is self canceling (balanced harmonics and AC waveform) and
percentage
of power factors are optimized (FIG 2B, FIG 14, FIG 15).
A fourth aspect of the invention provides an AC driven LED array or light
string
employing an odd number of multiple LED series blocks (not divisible by two)
has an
unequal number of LED series blocks connected in parallel in either forward,
or reverse
bias. In this event the unequal number of LED series blocks connected in
forward, or
reverse bias should equal one and a capacitor should be connected in parallel
across the
AC input, or end connector terminals to nearly eliminate DC and harmonic
distortion
(FIG 6B). An alternate aspect of this invention would be to omit the
capacitor, reducing
DC and harmonic distortion somewhat, but not to the extent that it would be
reduced by
the inclusion of the capacitor (FIG 6C). It should also be noted, as the
number of series
blocks increase DC interference and harmonic distortion are automatically
reduced,
provided the maximum balance of forward to reverse bias series blocks is
maintained
(FIG 14).
5
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In accordance with the fifth aspect of this invention, power utilization
improves
when the LED light strings are manufactured using near balanced harmonics and
is
optimized when balanced harmonics is achieved (FIG 13).
In accordance with the sixth aspect of the invention, a method is provided for
making the light string of this invention, in which the forward, or reverse
bias direction of
individual series blocks is identified by visual, or mechanical means thereby
assisting the
manufacturing process and greatly reducing defects and errors (FIG 16).
In accordance with the seventh aspect of this invention a method is provided
for
manufacturers to test and sort (according to polarity, or LED orientation)
single series
block LED light strings (unbalanced harmonics), as well as LED light string
employing
multiple series blocks of LED lamps (balanced and near balanced) (FIG 17).
Many of the foregoing concepts can be used simultaneously to reduce harmonic
distortion and improve the power rating.
1 S BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are incorporated in and constitute a part of the
specification. The drawings, together with the general description given above
and the
detailed description of the certain preferred embodiments and methods given
below,
serve to explain the principles of the invention. In such drawings:
FIGS. lA and 1B show two example block diagrams of the light string in its
embodiment preferred primarily, with one diagram for a 110 VAC common
household
input electrical source (e.g., 60 Hz) and one diagram for a 220 VAC conunon
household
(e.g., 50 Hz) input electrical source.
6
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FIG. 2A shows an example schematic diagram of an embodiment of this
invention in which the diodes of the 50 LEDs (series) blocks 102 of FIG. 1 are
connected
in the same direction (unbalanced). Tlus figure is provided as an example of
the common
series block arrangement utilized by manufacturers with poor power utilization
and a
high harmonic content.
FIG. 2B shows an example schematic diagram of an embodiment of this invention
in which the diodes of the 50 LEDs (series) blocks 102 of FIG. 1 are comlected
in the
reverse direction (balanced, self canceling harmonics and DC content).
FIGS, 3A and 3B show two example block diagrams of the light string in its
embodiment prefeured alternatively, with one diagram for a 110 VAC common
household
input electrical source (e. g., 60 Hz) and one diagram for a 220 VAC common
household
(e.g., 50 Hz) input electrical source.
FIG. 4 shows an example schematic diagram of the AC-to-DC power supply
corresponding to the two block diagrams in FIG. 3 for either the 110 VAC or
the 220
VAC input electrical source.
FIGS. SA and SB show example pictorial diagrams of the manufactured light
string in either its "straight" or "curtain" form (either form may be
manufactured for 110
VAC or 220 VAC input).
FIG. 6A shows an example schematic diagram of an AC driven LED light string
employing a single series block of LEDs and including a capacitor to reduce
harmonic
noise, or interference (improved harmonics).
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FIG 6B shows an example schematic diagram of an AC driven LED light string
employing an unequal number of LED series blocks and including a capacitor to
reduce
harmonic noise, or interference (near balanced and improved harmonics).
FIG 6C shows an example schematic diagram of an AC driven LED light string
employing an unequal number of LED series blocks, but with the capacitor
eliminated to
reduce harmonic noise and interference to a lesser extent shown in figures 6A
and 6B.
FIG. 7 is a graph of current versus voltage for diodes and resistors.
FIGS. 8A and 8B are an additional schematic and block diagrams of direct drive
embodiments with near balanced harmonics.
FIG. 9 is a plot showing the alternating current time response of a diode.
FIG. 10 is a graph showing measured diode average current response for
alternating current and direct current.
FIG. 11 is a graph showing measured AllnGaP LED average and maximum AC
current responses.
FIG. 12 is a graph showing measured light output power as a function of LED
current.
FIG 13 is a chart showing power utilization (percentage of power factors) for
AC
driven LED lights strings utilizing one or more series blocks in unbalanced
(series blocks
of same bias), near balanced (unequal number of series blocks in opposed
bias), and
balanced (equal number of series blocks in opposed bias) form.
FIG 14 is a chart showing harmonic components for unbalanced, unbalanced with
improved harmonics (filtered), near balanced, near balanced with improved
harmonics
(filtered), and balanced AC driven LED light strings
8
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FIG 15 is a graph showing the AC waveform for balanced and unbalanced AC
driven LED light strings.
FIG 16 is a pictorial diagram showing a simple method of maintaining forward
and reverse bias between LED series blocks.
FIG 17 is a pictorial diagram showing a simple method of testing bias of LED
light strings employing single, or multiple seues blocks.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS AND
CERTAIN PREFERRED METHODS OF THE I1WENTION
Reference will now be made in detail to the presently preferred embodiments
and
methods of the invention as illustrated in the accompanying drawings, in which
like
reference characters designate like or corresponding parts throughout the
drawings. It
should be noted, however, that the invention in its broader aspects is not
limited to the
specific details, representative assemblies and methods, and illustrative
examples shown
and described in this section in connection with the preferred embodiments and
methods.
The invention according to its various aspects is particularly pointed out and
distinctly
claimed in the attached claims read in view of this specification, and
appropriate
equivalents.
It is to be noted that, as used in the specification and the appended claims,
the
singular forms "a," "an," and "the" include plural referents unless the
context clearly
dictates.
The term "alternating current voltage", sometimes abbreviated as "VAC", as
used
herein occasionally refers to a numerical amount of volts, for example, "220
VAC". It is
9
CA 02475647 2004-07-23
to be understood that the stated number of alternating current volts is the
nominal voltage
which cycles continuously in forward and reverse bias and that the actual
instantaneous
voltage at a given point in time can differ from the nominal voltage number. ,
In accordance with an embodiment of the present invention, an LED light string
employs a plurality of LEDs wired in series-parallel fornl, containing at
least one series
block of multiple LEDs. The series block size is determined by the ratio of
the standard
input voltage (e.g., either 110 VAC or 220 VAC) to the drive voltages) of the
LEDs to
be employed (e.g., 2 VAC). Further, multiple series blocks, if employed, 'are
each of the
same LED configuration (same number and kinds of LEDs), and are wired together
along
the string in parallel. LEDs of the light string may comprise either a single
color LED or
an LED including multiple sub-dies each of a different color. The LED lenses
may be of
any shape, and may be either clear, clear-colored, or diffuse-colored.
Moreover, each
LED may have internal circuitry to prov3e for intermittent on-off blinking
and/or
intermittent LED sub-die color changes. Individual LEDs of the light string
may be
arranged continuously (using the same color), or periodically (using multiple,
alternating
CIP colors), or pseudo-randomly (any order of multiple colors). The LED light
string
may provide an electrical interface to couple multiple light strings together
in parallel,
and physically from end to end. Fiber optic bundles or strands may also be
coupled to
individual LEDs to diffuse LED light output in a predetermined manner.
An LED light string of embodiments of the present invention may have the
following advantages. The LED light string may last far longer and require
less power
consumption than light strings of incandescent lamps, and the light string may
be safer to
operate since less heat is generated. The LED light string may have reduced
cost of
CA 02475647 2004-07-23
manufacture by employing series-parallel blocks to allow operation directly
from a
standard household 110 VAC or 220 VAC source, either without any additional
circuitry
(AC drive), or with only minimal circuitry (DC drive). In addition, the LED
light string
may allow multiple strings to be conveniently connected together, using
standard 110
VAC or 220 VAC plugs and sockets, desirably from end-to-end.
Direct AC drive of LED light string avoids any power conversion circuitry and
additional wires; both of these items add cost to the light string. The
additional wires
impose additional mechanical constraint and they may also detract
aesthetically from the
decorative string. However, direct AC drive results in pulsed lighting.
Although this
pulsed lighting cannot be seen at typical AC drive frequencies (e.g. 50 or 60
Hz), the
pulsing apparently may not be the most efficient use of each LED device
because less
overall light is produced than if the LEDs were continuously driven using DC.
However,
this effect may be compensated for by using higher LED current duuing each
pulse,
depending on the pulse duty factor. During "off' times, the LED has time to
cool. It is
shown that this method can actually result in a higher efficiency than DC
drive,
depending on the choice of AC current.
FIGS. lA and 1B show the embodiment of an LED light string in accordance with
the present invention, and as preferred primarily through AC drive. In FIG.
lA, the two
block diagrams correspond to an exemplary string employing 100 LEDs, for
either 110
VAC (top diagram) or 220 VAC (bottom diagram) standard household current input
(e.g.,
50 or 60 Hz). In the top block diagram of FIG. IA, the input electrical
interface consists
merely of a standard 110 VAC household plug 101 attached to a pair of drive
wires.
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With the average LED drive voltage assumed to be approximately 2.2 VAC in FIG.
lA, the basic series block size for the top block diagram, corresponding to
110 VAC
input, is approximately SO LEDs. Thus, for the 110 VAC version, two series
blocks of S0
2.2 VAC LEDs 102 are coupled in parallel to the drive wires along the light
string. The
S two drive wires for the 110 VAC light string terminate in a standard 110 VAC
household
socket 103 to enable multiple strings to be connected in parallel electrically
froW end-to-
end. The series block quantity of 50 LED lamps illustrated in Fig. lA is used
for
example purposes only and could be larger, or smaller depending on the AC
input voltage
applied as well as the type of LED lamps used and the presence of current or
voltage
limiting circuitry (specifically not shown) as known in the art.
In the bottom block diagram of FIG. 1B, the input electrical interface
likewise
consists of a standard 220 VAC household plug 104 attached to a pair of drive
wires.
With again the average LED drive voltage assumed to be approximately 2.2 VAC
in FIG:
1 B, the basic series block size for the bottom diagram, corresponding to 220
VAC input;
1 S is 100 LEDs. Thus, for the 220 VAC version, only one series block of 100
LEDs 105 is
coupled to the drive wires along the light string. The two drive wires for the
220 VAC
light string terminate in a standard 220 VAC household socket 106 to enable
multiple
strings to be connected in parallel from end-to-end. Note that for either the
110 VAC or
the 220 VAC light string, the standard plug and socket employed in the string
varies in
accordance to the country in which the light string is intended to be used.
The series
block quantity of 100 LED lamps illustrated in Fig. 1B is used for example
purposes only
and could be larger, or smaller depending on the AC input voltage applied as
well as the
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CA 02475647 2004-07-23
type of LED lamps used and the presence of current or voltage limiting
circuitry
(specifically not shown) as known in the art.
Whenever AC drive is used and two or more series are incorporated in the light
string, the first series blocks would be driven by either the positive or
negative half of the
AC voltage cycle. The only requirement of this embodiment is that; in each
series block,
the LEDs within the series block are wired with the same polarity and
subsequent series
blocks are wired in reverse polarity relative to the first series block, or
alternating series
blocks. For this arrangement, the series block of LED lamps should be
manufactured
with a substantially equal number of light strings produced with all LEDs in
forward bias
(anode first) and reverse bias (cathode first). This allows true randomness
when large
numbers of single series block LED light strings are connected in an end-to-
end manner.
A simple sorting and test procedure is given later in this text to assist
manufacturers and
allow for an equal number of forward and reverse bias light strings. It is
possible to
apply this concept to single LED lamps having two chips where one chip is
powered in
reverse bias to the second chip; thereby providing a self canceling hamonics
within the
single LED lamp. Of course, this self canceling concept can be employ to any
even
number of chips in a single LED lamp.
In AC driven LED light strings employing a single series block of LED lamps,
whether or not there is a substantially equal number of LEDs in forward bias
(anode first)
and reverse bias (cathode first), harmonic distortion is greatly reduced when
a capacitor is
connected in parallel across the AC input, or end connector terminals (FIG 6A)
of the -
light string. In the one tested embodiment, a 0.01 uF, SOOV capacitor at 120
VAC, 60 Hz
13
CA 02475647 2004-07-23
was used. Tllcorporating this harmonic filter dramatically reduces harmonic
distortion
and DC feedback entering into the power grid (see FIG 14).
Harmonic distortion is greatly reduced when the number of series blocks
oriented
in forward bias and reverse bias are nearly equal, differing by a single
series block as
S shown in FIG 6C and FIG 8A (near balanced). As the number of series blocks
contained
in the LED light set increases power utilization improves (FIG 13) and
relative harmonic
distortion decreases (FIG 14).
Harmonic distortion can be further reduced when the number of forward and
reverse bias series blocks are nearly equal and a capacitor is connected in
parallel across
the AC input terminals, or end connection terminals (FIG 6A and FIG 6B). It
appears as
if the harmonic filter (capacitor) is no longer required when 5 or more near
balanced
series blocks are employed as relative harmonic distortion is decreased to a
harmless
level.
Harmonic balance is achieved (FIG 15) and power utilization is maximized (FIG
13) when the number of series blocks oriented in forward bias and reverse bias
are equal
(Fig 2B).
It should be noted that the arrangement of forward bias to reverse bias series
blocks is not order dependant. The important factor is to maintain an equal,
or near equal
number of series blocks oriented in forward and reverse polarity (bias).
Figures 2A and 2B show two schematic diagram implementations of the top
diagram of FIG. lA, where the simplest example of AC drive is shown that uses
two -
example series blocks of 50 LEDs, connected in parallel and powered by 110
VAC. In
the top schematic diagram of FIG. 2A both of these LED series blocks are wired
in
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CA 02475647 2004-07-23
parallel with the polarity of both blocks in the same direction (or,
equivalently, if both
blocks were reversed). With this block alignment, both series blocks flash on
simultaneously, using electrical power from the positive (or negative; if both
blocks were
reversed) portion of the symmetric AC power cycle only. This results in poor
power
utilization and harmonics identical to single series block (unfiltered) LED
light strings.
The disadvantage of this configuration is that, since both blocks flash on
simultaneously, they both draw power at the same time, and the maximum current
draw
during this time is as large as possible. A further disadvantage of this
arrangement is
power utilization is minimized and a high percentage of DC and harmonic
distortion
enters the utility grid. An example of the AC waveform exhibited by LED light
sets
using this configuration is shown in FIG 15 as a single string. The flash
rate, at 50-60
Hz, cannot be seen directly by human eye and is instead integrated into a
continuous light
stream.
FIG. 2B shows the alternative implementation for FIG. 1 A, where again, two
exemplary series blocks of 50 LEDS are connected in parallel and powered by
110 VAC.
In this alignment, the two series blocks are reversed, relative to each other,
in
polarity with respect to the input AC power. Thus, the two blocks flash
alternatively,
with one block flashing on during the negative portion of each AC cycle. The
symmetry,
or "sine-wave" nature of AC allows this possibility and is shown in FIG 15 as
balanced
strings. In accordance with the present invention, the advantage is that,
since each block
flashes alternatively, drawing power during opposite phases of the AC power,
the power
utilization of the device is maximized (FIG 13) and DC interference and
harmonic
distortion become self cancelling (FIG 14), resulting in a harmonically
balanced light set.
CA 02475647 2004-07-23
For AC drive with non-standard input (e.g., three-phase AC) the series blocks
may similarly be arranged in polarity to divide power among the individual
cycles of the
multiple phase AC. Tlus may result in multiple polarities employed for the LED
series
blocks, say three polarities for each of the three positive or negative
cycles.
As an alternative preference to AC drive, FIGS. 3A and 3B shows two block
diagrams that correspond to an exemplary string employing 100 LEDs and DC
drive, for
either 110 VAC (top diagram) or 220 VAC (bottom diagram) standard household
current
input (e.g., 50 or 60 Hz). In the top block diagram of FIG. 3A, the input
electrical
interface consists of a standard 110 VAC household plug 301 attached to a pair
of drive
wires, followed by an AC-to-DC converter circuit 302. As in FIG. 1, with the
average
LED drive voltage assumed to be approximately 2.2 VAC iii FIG. 3A, the basic
series
block size for the top block diagram, corresponding to 110 VAC input, is
approximately
SO LEDs. Thus, for the 110 VAC version, two series blocks of 50 LEDs 303 are
coupled
in parallel to the output of the AC-to-DC converter 302 using additional feed
wires along
1 S the light string. The two drive wires for the 110 VAC light string
terminate in a standard
110 VAC household socket 304 to enable multiple strings to be connected in
parallel
electrically from end-to-end.
In the bottom block diagram of FIG. 3B, the input electrical interface
likewise
consists of a standard 220 VAC household plug 305 attached to a pair of drive
wires,
followed by an AC-to-DC converter circuit 306. With again the average LED
drive
voltage assumed to be approximately 2.2 VAC in FIG. 3B, the basic series block
size for -
the bottom diagram, corresponding to 220 VAC input, is 100 LEDs. Thus, for the
220
VAC version, only one series block of 100 LEDs 307 is coupled to the output of
the AC-
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CA 02475647 2004-07-23
to-DC converter 306 using additional feed wires along the light striilg. The
two drive
wires for the 220 VAC light string terminate in a standard 220 VAC household
socket
308 to enable multiple strings to be connected in parallel from end-to-end.
Note that for
either the 110 VAC or the 220 VAC light string, the standard plug and socket
employed
in the string varies in accordance to the country in which the light string is
intended to be
used.
FIG. 4 shows an example schematic electrical diagram for the AC-to-DC
converter employed in both diagrams of FIG. 3. The AC input to the circuit in
FIG. 1 is
indicated by the symbol for an AC source 401. A varistor 402 or similar fusing
device
may optionally be used to ensure that voltage is limited during large power
surges. The
actual AC to DC rectification is performed by use of a full-wave bridge
rectifier 403.
This bridge rectifier 403 results in a rippled DC current and therefore serves
as an
example circuit only. A different rectification scheme may be employed,
depending on
cost considerations. For example, one or more capacitors or inductors may be
added to
reduce ripple at only minor cost increase. Because of the many possibilities,
and because
of their insignificance, these and similar additional circuit features have
been purposely
omitted from FIG. 4.
The same, or substantially similar principals of balanced harmonics and power
utilization are applicable to Figures 3 and 4 as previously explained and
illustrated in this
text. Further explanation has been purposefully omitted as being redundant and
easily
understood by those skilled in the art.
For either the 110 VAC or the 220 VAC version of the LED light string, and
whether or not an AC to-DC power converter is used, the final manufacturing
may be a
17
CA 02475647 2004-07-23
variation of either the basic "straight" string form or the basic "curtain"
string form, as
shown in the top and bottom pictorial diagrams in FIGS. SA and 5B. In the
basic
"straight" form of the light string, the standard (110 VAC or 220 VAC) plug
501 is
attached to the drive wires which provide power to the LEDs 502 via the series-
parallel
feeding described previously. The two drive and other feed wires 503 are
twisted
together along the length of the light string for compactness and the LEDs 502
in the
"straight" form are aligned with these twisted wires 503, with the LEDs 502
spaced
uniformly along the string length (note drawing is not to scale). The two
drive wires in
the "straight" form of the light string terminate in the standard
(correspondingly, 110
VAC or 220 VAC) socket 504. Typically, the LEDs are spaced uniformly every
four
inches.
In the basic "curtain" form of the light string, as shown pictorially in the
bottom
diagram of FIGS. SA and SB, the standard (110 VAC or 220 VAC) plug 501 again
is
attached to the drive wires which provide power to the LEDs 502 via the series-
parallel
feeding described previously. The two drive and other feed wires 503 are again
twisted
together along the length of the light string for compactness. However, the
feed wires to
the LEDs are now twisted and arranged such that the LEDs are offset from the
light string
axis in small groups (groups of 3 to 5 are shown as an example). The length of
these
groups of offset LEDs may remain the same along the string or they may vary in
either a
periodic or pseudo-random fashion.
Within each group of offset LEDs, the LEDs 502 may be spaced uniformly as -
shown or they may be spaced nonuniformly, in either a periodic or pseudo-
random
fashion (note drawing is not to scale). The two drive wires in the "curtain"
form of the
18
CA 02475647 2004-07-23
light string also terminate in a standard (correspondingly 110 VAC or 220 VAC)
socket
504. Typically, the LED offset groups are spaced uniformly every six inches
along the
string axis and, within each group, the LEDs are spaced uniformly every four
inches.
In any above version of the preferred embodiment to the LED light string,
blinking may be obtained using a number of techniques requiring additional
circuitry, or
by simply replacing one of the LEDs in each series block with a blinking LED.
Blinking
LEDs are already available on the market at comparable prices with their-
continuous
counterparts, and thus the light string may be sold with the necessary (e.g.,
one or two)
additional blinkers included in the few extra LEDs.
In accordance with the present embodiment FIG. 6A illustrates a single series
block of LEDs wherein a capacitor is connected in parallel in order to
minimize harmonic
distortion when a single series block of LEDs is employed. Additional,
optional
components to this circuit have been intentionally eliminated as they are
known in the art
and are not critical to the desired result.
In accordance with the present embodiment FIG 6B illustrates an unequal number
of LED series blocks wherein a capacitor is connected in parallel in order to
minmize
harmonic distortion.
In accordance with the present embodiment FIG 6C illustrates a lower cost,
near-
balanced alternative wherein harmonic distortion is reduced, however, not to
the extent
afforded by FIG 6B. As noted earlier in this text, as the total number of
series blocks
increase, this near balanced alterative approaches the self cancelling and hal-
monically
balanced design of FIG 2B. This is further illustrated by FIG $A and
predicated on the
total number of forward bias to reverse bias series blocks being unequal by
one.
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CA 02475647 2004-07-23
Many LED light string designs use one or more impedance elements in series
between the LED network and the power supply while other designs are free of
impedance circuitry. Current-saturated transistors are a less common method of
current
limiting. A resistor is often used for the impedance element due to low cost,
high
S reliability and ease of manufacture from semiconductors. For pulsed-DC or AC
power,
however, a capacitor or inductor may instead be used as a series block
impedance
element. With AC power, even though the waveform shape may be changed somewhat
by capacitors or inductors, the overall effect of these reactive elements is
basically the
same as a resistor, in adding constant impedance to the series circuit due to
the single AC
frequency involved (e.g., 60 Hz). In any case, the fundamental effect of
current-limiting
circuitry is to partially linearize or limit the highly nonlinear current
versus voltage
characteristic response curve of the diode, as shown in Figure 7 and has
little or no effect
on the harmonics of the circuit.
Fig 8A shows the preferred embodiment of the invention, wherein a network of
diodes, consisting of LEDs, is directly driven by the AC source. Near harmonic
balance
is achieved without the inclusion of the harmonic filter shown in FIG 6A and
6B and as
stated previously this near balanced alterative approaches the self cancelling
and
harmonically balanced design of FIG 2B as the number of series blocs employed
increases. Once again, the difference between positively and negatively
oriented series
blocks should always be one. Additional elements of this circuit, such as
impedance
devices have been purposely eliminated from these figures as they are known in
the art
and have little, or no effect on circuit hanx~oucs. Figure 8B is a block
diagram of the
above schematic, where a combination plug/socket is drawn explicitly to show
how
CA 02475647 2004-07-23
multiple devices can be directly connected either on the same end or in an end-
to-end
fashion, without additional power supply wires in between. This end-to-end
connection
feature is particularly convenient for decorative LED light strings.
The invention in FIGS. 8A and 8R may have additional circuitry, not explicitly
drawn, to perform functions other than current limiting. For example, logic
circuits may
be added to provide various types of decorative on-off blinking. A full-wave
rectifier
may also be used to obtain lugher duty factor for the diodes which, without
the rectifier,
would turn on and off during each AC cycle at an invisibly high rate (e.g., 50
or GO Hz).
The LEDs themselves may be a mixture of any type, including any size, shape,
material,
color or lens. One vital feature of the diode network is that all diodes are
configured to
minimize, or eliminate harmonic interference.
Figure 9 shows the peak voltage drown, Vpk, is less than or equal to the diode
maximum voltage, V",aX. For AC voltages below the diode voltage threshold,
Vt,,, the
current is zero. As the voltage increases above V~, to its peak value, Vpk,
and then falls
back down again, the diode current rises sharply in a nonlinear fashion, in
accordance to
its current versus voltage characteristic response curve, to a peak value,
Ipk, and then the
diode current falls back down again to zero current in a syn nnetric fashion.
Since the
voltage was chosen such that Vpk <_ V~, then the peak diode current satisfies
Ipk <_ h"a,;.
The average diode current, I$,,g, is obtained by integrating the area under
the current spike
over one full period. This is in keeping with the balanced and unbalanced AC
wavefonn
of LED light strings shown in FIG. 15.
Figure 10 shows that if one used DC voltages for tile diode in an AC circuit,
the
resulting average AC diode current would be much higher than the DC current
expected.
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CA 02475647 2004-07-23
This is also in keeping with the balanced and unbalanced AC waveform of LED
light
strings shown in FIG 15.
FIG. 12 illustrates the relationship of input current to relative light
intensity for
LEDs.
FIG. 13 illustrates the power utilization of incandescent, unbalanced, near
balanced, and balanced LED light strings in keeping with the present
embodiment of this
invention.
FIG. 14 illustrates relative DC and harmonic distortion components of
unbalanced, unbalanced with a harmonic filter, near balanced, near balanced
with a
harmonic filter; and harmonically balanced LED light strings. This is in
keeping with the
present embodiment of this invention and gives graphic representation of the
improved
harmonics and reduced DC distortion. It should be added that various LED light
string
designs were tested, including various impedance devices. The results did not
deviate
from those shown in FIG. 14.
1 S FIG. 15 illustrates the AC waveform for balanced and v.W balanced LED
light
strings in keeping with the present embodiment of this invention.
FIG. 16 illustrates a simple, visu~.l method whereby manufacturers can
maintain
balanced series block polarity within an LED light string. In addition,
mechanical
methods such as circuit connectors of varying sizes and shapes can also be
employed in
order to properly orient LED series blocks. An equally simple, yet effective
method
would be use of an audible tone or visual signal (such as lights of different
colour) to _
identify series block polarity and to assist manufacturing. These details are
purposefully
omitted due to the broad array of methods that could be employed.
22
CA 02475647 2004-07-23
FIG. 17 shows an example test station whereby manufacturers cm check series
block polarity within LED light strings containing an infinite number of
series blocks.
The test station employs variable rate, rectified AC power (DC power can be
used as an
alternative), so a single test station is suitable for testing products being
shipped to
various countries worldwide. The operator only needs orient the AC plug in the
same
direction when unpolarized plugs are used (polarized plugs will be self
orienting).
LED light sets containing a singly series block of LED lights will only
illuminate
in the positive (forward) bias, making it easy for manufacturers to produce
one half of the
light sets with positive orientation and one half of the light sets with
negative orientation.
When LED light sets containing multiple series blocks are tested, only the
series
blocks in forward (positive) orientation will illuminate. One half of the
series blocks will
illuminate in "balanced" LED light strings (as illustrated). The difference
between
illuminated and non-illuminated series blocks in near balanced LED lights
strings will
always be one series block.
1 S It will be understood that various changes in the details, materials and
arrangements
of the parts which have been described and illustrated in order to explain the
nature of
this invention may be made by those skilled in the art without departing from
the
principle and scope of the invention as expressed in the following claims.
The foregoing detailed description of the preferred embodiments of the
invention
has been provided for the purpose of explaining the principles of the
invention and its
practical application, thereby enabling others skilled in the art to
understand the invention
for various embodiments and with various modifications as are suited to the
particular
use contemplated. This description is not intended to be exhaustive or to
limit the
23
CA 02475647 2004-07-23
invention to the precise embodiments disclosed. Modifications and equivalents
will be
apparent to practitioners skilled in this art and are encompassed within the
spirit and
scope of the appended claims.
24
