Note: Descriptions are shown in the official language in which they were submitted.
CA 02659137 2009-03-19
,
,
LED STREET LAMP
Technical Field
[0001] This invention relates to light-emitting diode (LED) lighting devices.
Particular
embodiments provide apparatus for implementing and/or retro-fitting a street
lamp using
LEDs.
Background
[0002] Street lighting is typically provided by high-intensity discharge lamps
such as metal
halide lamps, or low pressure or high pressure sodium vapour lamps. The lamp
is typically
contained in a housing which is mounted to a pole on the road side or
suspended on a wire or
rod above the street. An outer lens cover provided on the street-facing (i.e.
downward-facing) side of the lamp may sealingly enclose the lamp in the
housing. The
cover may act as a diffuser to spread the light emitted by the lamp according
to street
illumination requirements. For example, the cover may diffuse the light to
minimize "hot
spots" (areas of concentrated light) appearing on the street.
[0003] Increasingly, LEDs are being used in street lamps to replace
conventional
high-intensity discharge lamps. LEDs have a high luminescent efficiency, which
can be
measured as a ratio of lumens output to power consumption. LEDs are also
available in a
wide range of colour temperatures (e.g. 3800 to 5000 kelvins, typically)
whereas
conventional sodium vapour lamps are limited to colour temperatures of
approximately 3000
kelvins. High-power LEDs (which typically have a power consumption of 1W or
higher)
may be used for street lighting, as they can be driven at currents on the
order of hundreds of
mAs to emit relatively more light than other LEDs. Existing LED street lamps
may be
subject to a number of drawbacks or constraints, including, for example:
= The light "spread" (i.e. cone of illumination) from prior art LED street
lamps is fairly
limited, thereby placing constraints on the height above ground at which the
lamps
can be mounted, and the lateral spacing between lamps.
= Prior art LED street lamps have a tendency to overheat. Overheating
reduces the
efficiency and reliability or operational life of the LED street lamp.
Heating, thermal
expansion and/or contraction of components in the LED street lamp may cause
the
lamp to fail.
= Many prior art LED street lamps require a custom mount and housing, and
cannot be
installed in existing lamp housings used to house conventional street lamps.
As such,
it may be difficult and costly to replace conventional street lamps with LED
street
lamps.
CA 02659137 2009-03-19
- 2 -
= Many prior art LED street lamps are unable to withstand power surges,
such as a
surge from a lightning strike.
[0004] There is a general desire to provide LED street lamps which overcome or
at least
ameliorate these and/or other drawbacks with the prior art.
Summary
[0005] Particular embodiments provide a lighting apparatus for installation in
a lamp
housing. The lighting apparatus incorporates a back plate and a front plate
which is spaced
apart from the back plate. The lighting apparatus also incorporates a
plurality of LED
assemblies arranged at spaced apart locations between the front and back
plates. At least a
portion of each LED assembly may be mounted to the back plate.
[0006] Each LED assembly comprises a high-power LED mounted to the back plate
and
oriented to emit light toward the front plate. The LED assembly further
comprises an
annular spacer having a bore therethrough, the spacer positioned such that at
least a portion
of the LED protrudes into an upper end of the bore. The LED assembly further
comprises an
optical lens positioned to receive light emitted by the LED and to direct the
light toward the
front plate. At least a portion of the lens protrudes into a lower end of the
bore and is loosely
received therein to permit limited relative movement between the lens and the
spacer. The
limited relative movement between the lens and the spacer is constrained at a
lower extreme
by the front plate and at an upper extreme by at least one of the LED and the
spacer.
[0007] The lens is moveable between a first configuration wherein an upper
portion of the
lens protrudes into the lower end of the bore of the spacer by a first
distance and the lens is
separated from the front plate by a second distance, and a second
configuration wherein a
lower end of the lens abuts against the front plate.
[0008] The LED assemblies may be arranged so that a centre-to-centre distance
between any
two adjacent LEDs is at least 3.2 cm (1.25 inches). The LED assemblies may be
spaced
apart so that a temperature at a p-n junction of the LED of each LED assembly
does not
exceed 125 C (257 F) during operation of the lighting apparatus. The LED
assemblies may
be arranged for use with an outer lens cover having Fresnel lens patterns for
diffusing the
light emitted by the LEDs. In particular embodiments, the LED assemblies are
arranged so
that at least 80% of the total light output impinges on a central Fresnel
pattern of the cover.
CA 02659137 2009-03-19
,
,
- 3 -
[0009] In addition to the exemplary aspects and embodiments described above,
further
aspects and embodiments will become apparent by reference to the drawings and
by study of
the following detailed descriptions.
Brief Description of Drawings
[0010] In drawings which illustrate non-limiting embodiments of the invention,
Figure 1 is a bottom plan view of an LED lamp according to one embodiment
installed in a housing, shown without an outer lens cover;
Figure 2 is a cross-sectional view of the Figure 1 lamp taken along line 2-2,
shown
with an outer lens cover;
Figure 2A is a cross-sectional view of a lighting unit of the Figure 1 lamp
taken along
line 2A-2A of Figure 2;
Figure 3 is a bottom plan view of an outer lens cover that may be used with
the
Figure 1 lamp;
Figure 4 is a bottom plan view of two LEDs of a lighting unit which may be
used in
the Figure 1 lamp;
Figures 5A and 5B are side elevation views of an LED assembly according to one
embodiment which may be used in the Figure 1 lamp, showing different
configurations of
the LED assembly;
Figure 5C is an exploded view of the Figure 5A LED assembly; and
Figure 6 schematically illustrates a power supply and driving circuitry
according to
one embodiment which may be used in the Figure 1 lamp.
Description
[0011] Throughout the following description, specific details are set forth in
order to provide
a more thorough understanding to persons skilled in the art. However, well
known elements
may not have been shown or described in detail to avoid unnecessarily
obscuring the
disclosure. Accordingly, the description and drawings are to be regarded in an
illustrative,
rather than a restrictive, sense.
[0012] Figure 1 shows an LED lamp 10 according to a particular embodiment.
Lamp 10 has
opposed sides 13a, 13b, a front end 17a and a rear end 17b. Front end 17a may
be oriented
relatively more proximate to a centre of the street than rear end 17b. Lamp 10
is installed in
a housing 11. Housing 11 may be a metal casing used for conventional sodium
vapour
CA 02659137 2009-03-19
- 4 -
lamps. As seen in Figure 2, housing 11 has a generally concave interior ha and
an
opening 12 through which lamp 10 may be inserted into interior ha.
[0013] A dome or outer lens cover 14 is optionally inserted in or placed to
span opening 12
to thereby sealingly enclose lamp 10 in housing 11 (Figure 2). As seen in
Figure 3, cover 14
may have particular Fresnel lens patterns designed to diffuse light from a
sodium vapour
lamp to provide a lighting pattern according to street illumination
requirements. This is the
case, for example, where lamp 10 is retro-fit into a conventional (e.g. non-
LED) street lamp
housing. Light emitted by lamp 10 which impinges on the Fresnel lens patterns
is thereby
spread into particular lighting patterns. In the illustrated embodiment, cover
14 has a central
elongate Fresnel pattern 15a extending generally between front 21a of cover 14
(which is
positioned relatively more proximate to front end 17a of lamp 10) and rear 21b
of cover 14
(which is positioned relatively more proximate to rear end 17b of lamp 10).
Pattern 15a may
include sections, ribs, ridges, notches or the like extending in directions
substantially parallel
to the rearward and forward directions indicated respectively by arrows 44, 46
(Figure 3).
Pattern 15a is configured to spread impinging light laterally (i.e. in
directions substantially
parallel to the directions indicated by arrows 42a, 42b). Pattern 15a thereby
causes light
from lamp 10 to extend laterally and downwardly (toward the street). Cover 14
also has rear
patterns 19b and side patterns 19c, 19d. Rear patterns 19b may include a V-
shaped pattern
as shown in Figure 3 which is configured to spread impinging light laterally
(i.e. in
directions substantially parallel to the directions indicated by arrows 42a,
42b) and
rearwardly (i.e. in directions substantially parallel to a direction indicated
by arrow 44). The
V-shaped pattern thereby causes light from lamp 10 to extend laterally,
rearwardly and
downwardly. Side patterns 19c, 19d are configured to spread light laterally
(i.e. in directions
substantially parallel to the directions indicated by arrows 42a, 42b).
[0014] As seen in Figure 2, lamp 10 incorporates an LED lighting unit 16
incorporating a
plurality of LED assemblies 25. LED assemblies 25 may be arranged between a
back
plate 20 and a front plate 32. At least a portion of each LED assembly 25 may
be mounted to
back plate 20. LED assemblies 25 may be arranged at spaced apart locations on
back
plate 20. Each LED assembly 25 comprises an LED 18 (e.g. high-power LED) and
components for mounting LED 18 and shaping or focusing light from LED 18,
which are
described in further detail below. Front plate 32 and back plate 20 are spaced
apart from one
another by a distance d (Figure 2). Front plate 32 may be spaced apart from
back plate 20 by
a plurality of standoffs 36 extending between front plate 32 and back plate
20. A strip 43
CA 02659137 2009-03-19
- 5 -
may extend around the periphery of front and back plates 32, 20 to sealingly
enclose the
interior components of lighting unit 16 between front and back plates 32, 20.
Strip 43 may
be made of elastic material (e.g. rubber) or semi-rigid material.
[0015] Back plate 20 may be made of aluminum. In particular embodiments, back
plate 20
is a T6061 aluminum plate having a thickness of approximately 0.64 cm (0.25
inch) to
0.953 cm (0.375 inch). An aluminum plate advantageously has good thermal
conductivity,
and absorbs heat generated during operation of lamp 10. Front plate 32 may be
made of
LexanTM plastic or the like. A LexanTM plate advantageously has a
transmissivity of at least
98%, and has a high heat tolerance and strength.
[0016] Lamp 10 incorporates a power supply 22 connected to deliver electrical
power to
LEDs 18, and driving circuitry 24 connected to control and regulate the supply
of electrical
power to LEDs 18. Lighting unit 16, power supply 22 and driving circuitry 24
may be
pre-assembled and coupled together as one module 27 for ease of installation
in housing 11.
[0017] Where lighting unit 16 is used with the Figure 3 cover 14 (e.g. where
lighting unit 16
is retro-fit into a conventional (non-LED) street lamp housing 11), LED
assemblies 25 may
be arranged between front and back plates 32, 20 as shown in Figure 1. The
Figure 1
arrangement balances two competing design constraints:
(1) For optimizing illumination through the Figure 3 cover 14, the light
emitted by
LEDs 18 should emulate the light emission from a sodium vapour lamp (or other
conventional lamp for which cover 14 may be designed). A typical sodium vapour
lamp emits light from an elongate tube which is aligned with pattern 15a so
that most
of the light from the sodium vapour lamp is directed to pattern 15a of cover
14. For
the Figure 3 cover 14, most of LED assemblies 25 should be positioned as
closely
packed together as possible in an elongate region on back plate 20 aligned
with
pattern 15a of cover 14, so as to direct most of the light toward pattern 15a
of cover
14. In some embodiments, LED assemblies 25 are arranged such that at least 80%
of
the light output from all LED assemblies 25 impinges on pattern 15a. In
particular
embodiments, LED assemblies 25 are arranged such that at least 90% of the
light
output from all LED assemblies 25 impinges on pattern 15a.
(2) For optimizing the operational reliability and life span of lamp 10 and
its
components, LED assemblies 25 should be spaced sufficiently far apart from one
another so that the heat generated by LED 18 at each LED assembly 25
contributes
CA 02659137 2009-03-19
- 6 -
only minimally to the heating of other LED assemblies 25 (such as neighbouring
or
adjacent LED assemblies 25). It is desirable to minimize the likelihood of
overheating of any LEDs 18 and/or other components of lamp 10.
100181 In particular embodiments, to minimize the likelihood of failure of
LEDs 18 caused
by overheating, LED assemblies 25 should be spaced sufficiently far apart from
one another
such that a temperature at the p-n junction of each LED 18 does not exceed 125
C (257 F)
during operation of lamp 10 within the nominal power rating of LEDs 18. It has
been found
that for a lamp 10 incorporating 5W LuxeonTM LEDs 18 which may be driven at a
maximum
current of 350 mA, a centre-to-centre distance r (see Figure 4) between any
two adjacent
LEDs 18 of at least 3.2 cm (1.25 inches) is desirable to minimize the
likelihood of failure of
LEDs 18 caused by overheating. For a lamp 10 incorporating 1W LuxeonTM LEDs 18
which
may be driven at a maximum current of 350 mA, a centre-to-centre distance r
between any
two adjacent LEDs 18 of at least 1.905 cm (0.75 inches) is desirable to
minimize the
likelihood of failure of LEDs 18 caused by overheating.
100191 In other embodiments, LED assemblies 25 may be positioned in other
arrangements
than as shown in Figure 1. The arrangement may be optimized for use of
lighting unit 16 in
a lamp 10 with a cover 14 incorporating different Fresnel lens patterns than
as shown in
Figure 3 for diffusing the light. LED assemblies 25 may be arranged so as to
direct light to
impinge on specific areas of the cover, to produce output light patterns
according to street
illumination requirements.
100201 In still other embodiments, lighting unit 16 may be used in a lamp 10
without a cover
for diffusing light. For example, some conventional (non-LED) street lamps do
not
incorporate a cover 14 (i.e. opening 12 in housing 11 is left uncovered).
Other conventional
(non-LED) street lamps may include a cover 14, but cover 14 does not
incorporate Fresnel
lens patterns and does not act as a diffuser. Where lighting unit 16 is used
in a lamp 10
without a cover for diffusing light, LED assemblies 25 may be arranged in rows
between
front and back plates 32, 30. The LED assemblies 25 in each row may be spaced
evenly
apart. In particular embodiments, the centre-to-centre distance between
adjacent LEDs 18 in
a given row is at least 3.2 cm (1.25 inches).
[0021] A lighting unit 16, having LED assemblies 25 positioned in the Figure 1
arrangement, may also be retro-fit in a lamp 10 which: (a) does not have a
cover 14; (b) has a
CA 02659137 2009-03-19
- 7 -
cover 14 without Fresnel lens patterns; or (c) has a cover 14 with different
Fresnel lens
patterns than as shown in Figure 3.
[0022] Each LED assembly 25 may incorporate an optical lens 30 for spreading
or
dispersing the light from LED 18. The amount of dispersion of the output light
beam 52 is
related to the particular lens angle of lens 30 (see Figure 5B). Typically,
the lens angle is in
the range of 50 to 120 . A lens 30 having a relatively higher lens angle
shapes the light to
produce an output beam 52 with greater dispersion than a lens 30 having a
relatively lower
lens angle. In some embodiments, all lenses 30 in lighting unit 16 have a lens
angle of 40 .
In other embodiments, at least 80% of the LED assemblies 25 in lighting unit
16 incorporate
a lens 30 having a lens angle of 40 or higher. In particular embodiments, at
least 5% of the
LED assemblies 25 incorporate a lens 30 having a lens angle of 10 or less
(e.g. 5 lens
angle). Lenses 30 having a relatively lower lens angle (e.g. lens angle of 10
or less) can be
used for shaping light that is to travel a relatively further distance. For
example, it may be
desirable for light that impinges on cover 14 near the front 21a of cover 14
to travel a
relatively further distance in the forward direction 46 to illuminate an area
across the street.
As such, lenses 30 having a relatively lower lens angle may be used for LED
assemblies 25
that are positioned relatively closer to front end 17a of lamp 10.
[0023] In the illustrated embodiment of Figure 1, each LED assembly 25
incorporates a
lens 30 having a 40 lens angle, except for two LED assemblies 25a which are
positioned
closest to front end 17a of lamp 10. LED assemblies 25a may incorporate lenses
30 having a
5 lens angle. A lens 30 having a 5 lens angle shapes light, which may then
be directed,
through cover 14, to a further distance in a direction indicated by arrow 46
(Figure 1) than a
lens 30 having a 40 lens angle.
[0024] As seen in Figures 5A, 5B and 5C, each LED assembly 25 is arranged
between back
plate 20 and front plate 32. In the illustrated embodiment, each LED assembly
25
incorporates a high-power LED 18, which may have a power consumption in the
range of
1W to 5W. For example, in particular embodiments, LEDs 18 comprise 5W LEDs
(e.g.
LuxeonTM 5W LEDs). In other embodiments, LEDs 18 comprise 1W LEDs (e.g.
LuxeonTM
1W LEDs). Each LED 18 typically has a body 18a (which contains the LED
semiconductor
diode), a cap or lens 18b extending from body 18a, and a base 18c to which
body 18a is
mounted. Base 18c of LED 18 is mounted to back plate 20. Base 18c may be
attached to
back plate 20 using a curable, heat-resistant adhesive 33 such as J-B WeIdTM,
ceramic or the
CA 02659137 2009-03-19
- 8 -
like. LED assembly 25 also incorporates an optical lens 30 and an annular lens
spacer 28
positioned between optical lens 30 and LED 18. Spacer 28 may be a
frustroconically annular
spacer, having a bore 29 therethrough for receiving LED 18 in its upper end
29a, and for
receiving optical lens 30 in its lower end 29b. Spacer 28 is oriented so that
the diameter of
bore 29 increases from its upper end 29a to its lower end 29b (i.e. the
diameter of bore 29 is
larger at locations closer to front plate 32). The shape of bore 29 may permit
maximum
divergence of light from LED 18. Surfaces of bore 29 of spacer 28 may be at
least 80%
reflective at the central wavelength of LED 18. In other embodiments, spacer
28 need not be
frustoconical in shape. For example, spacer 28 may comprise a generally
cylindrical spacer
having a bore 29 for receiving LED 18 and optical lens 30.
[0025] Lens 30 is loosely received in bore 29 of spacer 28. LED 18, spacer 28
and lens 30
may be aligned to share a common axis R. In the illustrated embodiment, axis R
coincides
with the axis 51 of lens 30 (Figure 5B). Lens 30 may be rotatable about axis R
relative to
LED 18 and/or spacer 28. As described in more detail below, the loose fit
between lens 30
and spacer 28 permits limited relative movement between lens 30 and spacer 28,
which is
constrained at a lower extreme by front plate 32 and at an upper extreme by
LED 18 and/or
spacer 28.
[0026] In the illustrated embodiment, cap 18b of LED 18 protrudes into upper
end 29a of
bore 29, and surfaces of bore 29 at its upper end 29a engage with body 18a of
LED 18. In
particular embodiments, spacer 28 is bonded to body 18a of LED 18 using a high-
temperature silicone. In other embodiments, spacer 28 is not bonded to body
18a of
LED 18, and LED 18 is loosely received in upper end 29a of bore 29 to permit
spacer 28 to
be rotatable about axis R relative to LED 18 and/or lens 30, and/or moveable
to a limited
extent relative to LED 18 and/or lens 30.
[0027] Upper portion 30a of lens 30 includes a concave recess 31 for
accommodating
cap 18b of LED 18. Upper portion 30a of lens 30 protrudes into lower end 29b
of bore 29.
Figure 5B shows LED assembly 25 in a fully protruding configuration, in which
lens 30 is
fully protruding into bore 29 by a distance h (i.e. lens 30 can extend no
further into bore 29,
as upper portion 30a of lens 30 is abutting against body 18a of LED 18, the
surface of recess
31 of lens 30 is abutting against cap 18b of LED 18, and/or the conical outer
surface of lens
30 is abutting against the corresponding surface of bore 29). In this fully
protruding
configuration (Figure 5B), a gap or separation k is provided between lens 30
and front plate
CA 02659137 2009-03-19
-9-
32. LED assembly 25 may assume a plate-contacting configuration during
operation (Figure
5A). When LED assembly 25 is directed to emit light generally downwardly (in
generally
the same direction as gravity g), gravitational forces pull loose fit lens 30
downwardly
toward front plate 32, thereby causing lens 30 to assume the plate-contacting
configuration
shown in Figure 5A. In the plate-contacting configuration, lens 30 is
partially protruding
into bore 29 by a distance h-k, and a lower end of lens 30 abuts against front
plate 32 (i.e. no
gap is provided between lens 30 and front plate 32). In the plate-contacting
configuration,
spaces 41 are formed between recess 31 of lens 30 and cap 18b of LED 18, and
spaces 40 are
formed between spacer 28 and body 18a of LED 18. Spaces 40, 41 permit air
circulation,
which assists in conveying heat away from LED 18 and other components such as
lens 30
and spacer 28 (e.g. by way of convection). The heat dissipation provided by
air flow through
spaces 40, 41 may also prevent further damage to lighting unit 16 (and other
components of
lamp 10) in the event of a failure of any one LED 18.
[0028] To prevent lens 30 from separating completely from spacer 28 as lens 30
moves from
a fully protruding configuration (Figure 5B) to a plate-contacting
configuration (Figure 5A),
front plate 32 and back plate 20 are spaced apart such that when lens 30 is
fully protruding
into bore 29 by a distance h, the resulting separation k between lens 30 and
front plate 32 is
less than distance h. If k is less than h, a portion of lens 30 remains
protruding into bore 29
(and lens 30 may remain generally aligned along axis R), even as gravitational
forces cause
lens 30 to fall downwardly to contact front plate 32. In some embodiments, the
ratio of h to
k is in the range of 1.2 to 10. In particular embodiments, the ratio of h to k
is in the range of
2 to 6. In some embodiments, k is less than 0.25 cm (0.1 inch). In one
particular
embodiment, k is 0.16 cm (0.0625 inch).
[0029] In some embodiments, spacer 28 is not bonded to body 18a of LED 18.
Spacer 28
may be moveable relative to LED 18 and/or lens 30. In operation, when LED
assembly 25 is
directed to emit light generally downwardly (in generally the same direction
as gravity g),
gravitational forces pull spacer 28 downwardly toward front plate 32, until it
reaches a
lowered configuration where it can move no lower as a result of surfaces of
bore 29 of spacer
28 engaging with the upper portion 30a of lens 30. In this lowered
configuration, spaces
may be provided between spacer 28 and body 18a of LED 18, and between spacer
28 and
cap 18b of LED 18. The spaces may assist in conveying heat away from LED 18
(and from
other components of lamp 10).
CA 02659137 2009-03-19
- 10 -
[0030] Standoffs 36 may be used to maintain a separation distance d between
back plate 20
and front plate 32 (Figure 2). Standoffs 36 may comprise bolts, screws, or the
like. In the
illustrated embodiment, standoffs 36 are screws which are screwed through
front plate 32
and into back plate 20.
[0031] In some embodiments, back plate 20 incorporates one or more heat sink
structures 48
for carrying heat away from lamp 10 (Figure 2A). Heat sink structures 48 may
project from
an upper surface of back plate 20 (i.e. on a side opposite LED assemblies 25).
In the
illustrated embodiment, two longitudinally-extending heat sink structures 48
are arranged so
as to extend between a front end 17a and rear end 17b of lamp 10. Each heat
sink
structure 48 comprises a plurality of longitudinally-extending fins 48a
radiating or fanning
outwardly from a central base portion 49 of heat sink structure 48. Central
base portion 49
of heat sink structure 48 may be mounted to back plate 20. Fins 48a may extend
generally
upwardly (i.e. away from back plate 20). Air channels 50 are provided between
adjacent fins
48a and between fins 48a and back plate 20 to facilitate the transfer of heat
from heat sink
structures 48 into the surrounding air. Heat sink structures 48 may be made of
aluminum.
[0032] In particular embodiments, power supply 22 is configured to limit the
output current
so that the LEDs 18 may be operated at approximately 80% to 85% of their
maximum rated
current in continuous wave (CW) operation. LEDs 18 may have a maximum rated
current of
350 mA. In the illustrated embodiment of Figure 6, power supply 22
incorporates a
transformer 38 (e.g. 120V to 18V transformer). The AC output voltage of the
transformer is
rectified by a full wave bridge rectifier 37 which may have a rating of at
least 4 A. The DC
output voltage of rectifier 37 may be smoothed by stabilizing capacitors 35
and regulated to
15 V by voltage regulators 39.
[0033] LEDs 18 of lighting unit 16 may be electrically connected in a
plurality of parallel
circuits 26, each having a plurality of LEDs 18 electrically connected in
series. In the
illustrated embodiment of Figure 6, driving circuitry 24 is configured to
drive three parallel
circuits 26. Each circuit 26 may have eight LEDs 18 connected in series (not
shown on
Figure 6) (i.e. so that there are a total of twenty-four LEDs 18 in lighting
unit 16). If an
LED 18 of a circuit 26 should fail, lamp 10 can still operate with the sixteen
LEDs 18 in the
remaining two circuits 26. In other embodiments, driving circuitry 24 is
configured to drive
more than three parallel circuits 26, each having a plurality of LEDs 18
connected in series.
In some embodiments, no more than 35% of the total number of LEDs 18 of
lighting unit 16
CA 02659137 2015-10-22
- 11 -
are connected in series in any particular parallel circuit 26. In other
embodiments, no more
than 25% of the total number of LEDs 18 are connected in series in any
particular parallel
circuit 26.
[0034] Power supply 22 may include a surge protection mechanism 34 to shunt
excess
current away from LEDs 18 in the event of a surge (such as may be caused by a
lightning
strike). In the illustrated embodiment of Figure 6, surge protection mechanism
34
incorporates three metal oxide varistors (MOVs) (labelled as MOV1, MOV2 and
MOV3)
connected as shown in Figure 6 for shunting excess current from hot to
neutral, from hot to
ground and/or from neutral to ground. MOV1 is connected between hot terminal
53 and
ground 57. MOV2 is connected between neutral terminal 55 and ground 57. MOV3
is
connected between hot terminal 53 and neutral terminal 55. In particular
embodiments, each
of the MOVs has a varistor voltage of 240 V, a maximum continuous DC voltage
of 200
VDC, a maximum continuous AC voltage of 150 VAC, a surge capability of up to
6.5 kA,
and an energy rating of 95 J. Using three MOVs in surge protection mechanism
34 as seen
in Figure 6 advantageously provides alternate shunt paths for excess current
at both the hot
and neutral terminals 53, 55. In the event that one MOV should fail, the
remaining MOVs
may still operate to shunt excess current away. In other embodiments, surge
protection
mechanism 34 may include only one MOV connected between the hot and neutral
terminals
53, 55.
[0035] While a number of exemplary aspects and embodiments have been discussed
above,
those of skill in the art will recognize certain modifications, permutations,
additions and
sub-combinations thereof The scope of the claims should not be limited by the
preferred
embodiments set forth in the examples, but should be given the broadest
interpretation
consistent with the description as a whole.
