Linear lighting with selectable light output levels

Lighting circuits using LED light engines and strips of linear lighting using these circuits are described. The lighting circuits are designed to produce different levels of light output at least in part using onboard components. In some embodiments, the circuit contains several current-setting elements, such as resistors, which are coupled to their own terminals such that the light output of the lighting circuit is determined by which of the terminals are connected to power. In other embodiments, a transistor in the lighting circuit may be adapted to allow or prevent current flow in the circuit based on a control signal applied to its gate. The circuit thus has separate power and control signal lines, and an external device, such as a pulse-width modulation signal generator, may be applied to the control signal line to modulate the light output.

TECHNICAL FIELD

The invention relates to linear lighting, and in particular, to linear lighting with selectable light output levels.

BACKGROUND

Linear lighting is a class of lighting based on light-emitting diodes (LEDs) in which an elongate, narrow printed circuit board (PCB) is populated with a plurality of LED light engines, typically spaced from one another at a regular pitch or spacing. In much of the linear lighting on the market, the LED light engines are surface-mounted on the PCB, along with other components. The PCB itself may be either rigid or flexible.

Combined with an appropriate power supply, linear lighting may be considered a luminaire (i.e., a finished light fixture) in its own right. It may also be used as a raw material for the manufacture of other, more complex, luminaires.

The most popular form of linear lighting is flexible, cuttable linear lighting. In this form of linear lighting, a flexible PCB is divided into repeating blocks at defined cut points. Each repeating block is a self-contained lighting circuit that will light if connected to power. The cut points allow a manufacturer or an installer to choose the desired length of linear lighting by cutting the flexible PCB at the desired cut point and connecting the resulting length of linear lighting to power.

Linear lighting is typically a low-voltage product, operating at, e.g., 12 or 24 volts, direct current (DC), although for purposes of this description, the term “low voltage” refers to any voltage under about 50V. At a given voltage level, each repeating block of a strip of linear lighting is designed to provide a certain level of light output per repeating block, typically measured in lumens, a unit of luminous flux.

The circuitry in a typical strip of linear lighting is often simple, designed to produce a single level of light output per repeating block. If different or varied levels of light output are desired, there are two typical solutions. The first potential solution is to use an external device, such as a pulse-width modulation (PWM) dimmer, to vary light output. Yet the typical consumer-use household or commercial PWM dimmer is designed for large power loads, tends to be bulky and relatively expensive, and is not appropriate for many applications in which lower light output is desired from a strip of linear lighting.

The second potential solution is to manufacture a strip of linear lighting that is engineered to produce a lower light output. However, making a greater variety of products is not an elegant solution to the problem of varying the light output of linear lighting—it simply forces manufacturers and installers to stock a wider variety of products. Moreover, neither of these potential solutions are of much comfort to an installer who, after installing a strip of linear lighting, finds that it is simply too bright for the application and has to go to the trouble of ripping it out and installing a new one.

BRIEF SUMMARY

Aspects of the invention relate to lighting circuits for LED lighting, and to strips of linear lighting using these circuits. A lighting circuit according to one aspect of the invention includes a plurality of LED light engines, at least two current-setting elements, and a plurality of terminals. The plurality of terminals are electrically coupled to the at least two current-setting elements such that a light output of the lighting circuit is determined by which of the plurality of terminals are connected to power.

In one embodiment, the current-setting elements are resistors, several of which are arranged electrically in series with one another and with the LED light engines. The plurality of terminals includes several terminals at the cathode end of the strip of linear lighting, interposed between the resistors, such that the resistance in the circuit, and thus, the light output, are determined based on which resistor or resistors are connected within the circuit.

In another embodiment, the current-setting elements in the circuit are arranged in parallel with one another, and each of the current-setting elements is connected to its own terminal at the cathode end of the strip of linear lighting. The current in the circuit and the light output are determined based on which terminal or terminals are connected to power. The current-setting elements may be either resistors or current-setting integrated circuits.

Strips of linear lighting according to these aspects of the invention typically include an elongate, narrow printed circuit board (PCB) that is divided physically and electrically into repeating blocks, which are separated from one another at cut points. Each repeating block is a complete lighting circuit the features described above.

Another aspect of the invention relates to a lighting circuit that, in addition to a plurality of LED light engines and a current-setting element, has a transistor, and to strips of linear lighting that incorporate this lighting circuit. The lighting circuit has connections for power, and a separate control or signal input terminal that provides a voltage signal to the gate of the transistor. The transistor acts as a switch, allowing power to flow through the lighting circuit or preventing it from flowing based on the voltage signal supplied to its gate. This allows the lighting circuit to be driven by, e.g., an external pulse-width modulation (PWM) signal generator.

Other aspects, features, and advantages of the invention will be set forth in the description that follows.

DETAILED DESCRIPTION

FIG. 1is a perspective view of a strip of linear lighting, generally indicated at10, according to an embodiment of the invention. The linear lighting10includes a printed circuit board (PCB)12, on which a plurality of LED light engines14are disposed, spaced from one another at a regular spacing or pitch.

As the term is used here, “LED light engine” refers to an element in which one or more light-emitting diodes (LEDs) are packaged, along with wires and other structures, such as electrical contacts, that are needed to connect the light engine to a PCB. If the light engine is intended to emit “white” light, it may be a so-called “blue pump” light engine in which a light engine containing one or more blue-emitting LEDs (e.g., InGaN LEDs) is covered with a phosphor, a chemical compound that absorbs the emitted blue light and re-emits a broader or a different spectrum of wavelengths. In the illustrated embodiment, the light engines14are surface-mount devices (SMDs) soldered to the PCB12, although other types of light engines may be used. The particular type of light engine is not critical, and other types of light engines may be used. While multi-color RGB LED light engines that can emit a variety of colors may be used in embodiments of the invention, much of this description will assume that the LED light engines emit light of a single color.

InFIG. 1, the PCB12is a flexible PCB made, for example, of a thin MYLAR® (biaxially-oriented polyethylene terephthalate) or polyimide film, although in some embodiments, it may be a rigid PCB made of a material like FR4 or ceramic. The material of which the PCB is made is not critical, so long as it is suitable for the application in which the linear lighting10is to be used. While the LED light engines14and other devices on the PCB12are SMDs in the illustrated embodiment, other forms of mounting may also be used, including through-hole mounting.

The linear lighting10is divided into repeating blocks16. Each repeating block16is a complete lighting circuit that will light if connected to power. The repeating blocks16can be separated from one another at cut points18. In the illustration ofFIG. 1, the cut points18are marked on the upper surface20of the PCB12, e.g., by screen printing. However, in other embodiments, the cut points18may not be explicitly marked. When the cut points18are not explicitly marked, the locations of the cut points18can typically be deduced by using landmarks on the PCB12. For example, in this case, the cut points18coincide with sets of solder pads, each of which is generally indicated at22. While the term “solder pads” is used here for convenience, the sets of solder pads22may be used to make other types of electrical connections, such as connections using solderless electrical connectors.

Typically, most PCBs for linear lighting are on the order of 5-14 mm wide, although narrower and wider PCBs do exist. By joining sections of PCB12together at overlapping solder joints, a strip of linear lighting10may be made arbitrarily long. For example, 4 meter (16.4 foot) rolls of linear lighting are common in the industry, and 30 meter (100 foot) rolls of linear lighting are not unknown. Longer rolls of linear lighting10may be helpful for manufacturers and installers who use the product in great quantities; the functional maximum usable length (in industry parlance, the maximum run length) of any particular strip of linear lighting10may depend on a number of factors, and will be described in greater detail below.

In this embodiment, the devices in each repeating block16are relatively few: there are six LED light engines14and four resistors24,26,28,30. The resistors24,26,28,30are typically mounted in the interstitial space between LED light engines14or along the sides of the PCB12. Like the LED light engines14, the resistors24,26,28,30are SMDs, and may be, e.g., 0805 resistors.

The resistors24,26,28,30of the linear lighting10are but one example of a broader class of current-setting elements. As those of skill in the art will appreciate, LED light engines14require some element to set the current in the circuit. This may be done in the power supply (i.e., in the driver), or it may be done by adding components to the PCB12itself to set the current flow. Linear lighting that is designed to be used with an external driver that controls the current flow is called “constant current” linear lighting. Linear lighting that is designed to control the current flow using its own on-board circuits is often referred to as “constant voltage” linear lighting. Constant-current linear lighting is often used when the length of the linear lighting is known in advance; constant-voltage linear lighting is more versatile and more easily used in situations where the length, and resulting current draw, is unknown or is likely to vary from one installation to the next. This description assumes that the linear lighting10is constant-voltage linear lighting; thus, the presence of the resistors24,26,28,30. Some of the resistors24,26,38,30have an additional purpose that will be described in greater detail below.

FIG. 2is a schematic circuit diagram of the linear lighting10ofFIG. 1. As can be appreciated fromFIG. 2, while the repeating blocks16are physically in series with one another, they are electrically in parallel with each other between the power source and ground. Physically, the PCB12is typically constructed as a two-layer board, with the lower layer a power bus that connects to each repeating block16and the upper layer dedicated to interconnecting conductors and other structure for a circuit like that ofFIG. 2.

The circuit ofFIG. 2assumes a constant voltage input32with a variable current that increases according to the number of repeating blocks16in the strip of linear lighting10. In the embodiment ofFIGS. 1-2, there are six LED light engines14in each repeating block16. This is typical for a 24V circuit and assumes that each LED light engine14has a forward voltage in the range of about 3-3.3V. If the operating voltage or the forward voltages are different, there may be more or fewer LED light engines14.

In each repeating block16, the first two resistors24,26perform the usual function of setting the current in the circuit. These may be, e.g., 240 resistors capable of handling, e.g., 65 mW of power. Of course, the values of the resistors and their power tolerances will vary considerably from embodiment to embodiment. Since the two resistors24,26are in series with one another within the repeating block, they could be replaced by a single resistor with a resistance that is the additive sum of the resistances of the two smaller resistors. That single resistor would have an appropriate power tolerance. The advantage of dividing the total resistance and physically spacing it along the PCB12is heat dissipation: having two resistors24,26creates two less-hot spots that are spaced along the PCB12. For that reason, the necessary resistance could be divided among any number of resistors, spaced along the PCB12.

The six LED light engines14and two resistors24,26form a full, conventional constant-voltage lighting circuit. However, there are two additional resistors28,30placed electrically in series with one another and with the other components14,24,26of the repeating block16. The repeating block16is also arranged so that there are a number of terminals34,36,38at the cathode end of the strip of linear lighting10, with terminal34positioned in series before the two additional resistors28,30, terminal36placed in series between the first additional resistor28and the second additional resistor30, and terminal38placed in series after the third additional resistor30.

As shown inFIG. 1, the terminals34,36,38would typically be part of the solder pads22on the upper layer of the PCB12and would physically be placed adjacent to one another on the PCB12, although there is no requirement that this be so. Thus, the term “cathode end” of the strip of linear lighting10, and other similar terms, should be taken to refer to the circuit diagram ofFIG. 2, and not to the physical strip of linear lighting10. Additionally, as will be described below in more detail, the sense of the circuit in some embodiments may be reversed such that the terminals are on the anode end of the strip of linear lighting.

Typically, when connecting the strip of linear lighting10to power, an installer will use the set of solder pads22that is closest to one end of the strip of linear lighting10. However, that need not always be the case. Because of the arrangement of the repeating blocks16and the presence of a power bus within the strip of linear lighting10, any set of solder pads22along the strip of linear lighting10may be used. Additionally, in many strips of linear lighting, conductors are accessible from the bottom of the strip; therefore, there is no requirement that the solder pads22or other such contacts be on the upper surface of the strip of linear lighting10.

Furthermore, much of this description assumes that the strip of linear lighting10is powered from a single set of solder pads22at one end of the strip of linear lighting10. This need not be the case in all embodiments. If desired, power could be input simultaneously in multiple places along the strip of linear lighting10using any of the sets of solder pads22found along its length. The advantage of powering the strip of linear lighting10in this way is that it the light output disparity between one end of the strip of linear lighting10and the other would be reduced, thereby extending the maximum run length.

In any working circuit, only one of the three terminals34,36,38need be connected. With the arrangement shown inFIG. 2, the light output of the repeating blocks16, and thus the light output of the linear lighting10, depends on which terminal34,36,38is connected. (As those of skill in the art may appreciate fromFIG. 2, if more than one terminal34,36,38is connected, the light output is determined only by the first terminal34,36,38in the series that is connected.) When the first terminal32is connected, each repeating block16would emit at its full light output level. However, if either of the other two terminals36,38are connected instead of the first terminal, the repeating blocks16would emit light at less than full output. By Ohm's Law, the increased resistance provided by one or both of the additional resistors28,30(depending on which terminal36,38is connected) at constant voltage would cause a drop in the current, which would result in less light output from the LED light engines14. The decrease seen in any particular embodiment depends on the resistances of the two additional resistors28,30.

In the illustrated embodiment, resistor28has a resistance equal to the additive sum of the resistances of the two resistors24,26in the main portion of the repeating block16. If terminal36is connected, placing resistor28in the circuit, the light output would drop to one-half of the full light output. Resistor30has a resistance that is the additive sum of the resistances of the resistors24,26,28. If terminal38is connected, placing both resistors28,30in the circuit, the light output would drop to one-quarter of the full light output. Both of the additional resistors28,30have appropriate power capacities—for example, if the main resistors24,26are 65 mW resistors, resistor28may have a power capacity of 32 mW and resistor30may have a power capacity of 18 mW.

As a practical matter, this means that a manufacturer or an installer can choose the light output of the strip of linear lighting10from among a number of options at the time of manufacture or installation by choosing which of the terminals34,36,38to use in connecting the strip of linear lighting10to the power circuit. This may be done by soldering, by using a connector, or by any other means of making an electrical connection. As those of skill in the art will realize, while three terminals34,36,38are shown in this embodiment, there is no theoretical limit on the number of terminals, and thus, luminance options, that may be provided, but for practical reasons, it is helpful if the resulting terminals34,36,38are large enough to be used. Additionally, in this embodiment, the terminals34,36,38are cathodic terminals. That is not the only possible configuration and, as was noted briefly above, the sense of the circuit may be reversed and the terminals may be anodic terminals.

The actual resistances of the resistors24,26,28,30, and their power capacities, may vary considerably from embodiment to embodiment. In particular, for reasons of perception, it may be desirable to have light output levels other than full, one-half, and one-quarter, or to base the definitions of full, one-half, and one-quarter on perceived brightness, rather than luminous flux. In most practical embodiments, it is the perception of brightness to a human (or other animal) observer that matters, and not the luminous flux of the lighting source or the luminance of an area or object. Brightness is not necessarily proportional to luminous flux or luminance, and there are even cases in which a light source that emits a lesser luminous flux may be perceived as brighter than another light source that emits a greater luminous flux. As one example, the Helmholtz-Kohlrausch effect is a perceptual phenomenon in which intensely saturated colors are seen by the human eye as brighter than white light. For these reasons, even if full, one-half, and one-quarter are desired light output levels for a strip of linear lighting, the meanings of those terms, and the resulting resistance values of the resistors, may be chosen with respect to perceived brightness, rather than luminous flux or luminance.

In the embodiment ofFIGS. 1-2, all of the components in each repeating block16are electrically in series with one another. This need not be the case in all embodiments.FIG. 3is a circuit diagram of a strip of linear lighting, generally indicated at100, according to another embodiment of the invention. Three repeating blocks102are shown in the view ofFIG. 3, although the description above with respect to linear lighting also applies to the linear lighting100ofFIG. 3, and any number of repeating blocks102may be included in a strip of linear lighting100of arbitrary length.

Each repeating block102has six LED light engines14, a configuration which, like the one above, assumes 24V DC input power with about a 3-3.3V forward voltage for each LED light engine14. The difference in the strip of linear lighting100relative to the strip of linear lighting10described above lies in the configuration of the resistors104,106,108. As can be seen inFIG. 3, there are three resistors104,106,108in each repeating block100, each resistor104,106,108in parallel with the others.

The repeating block102has three terminals110,112,114at the cathode end of the strip of linear lighting100, each one connected to one of the resistors104,106,108. As described above, these terminals110,112,114may be part of a set of solder pads on the upper surface of a PCB12, although other arrangements are possible. The fourth terminal116is the anode, which may also be a solder pad. As with the embodiment described above, the sense of the circuit may be reversed such that in some cases, the terminals110,112,114are anodic and the fourth terminal116is the cathode.

In the arrangement ofFIG. 3, one or more of the resistors104,106,108are connected to the circuit simultaneously based on which of the three terminals110,112,114are connected to the circuit—and unlike in the linear lighting10described above, more than one terminal110,112,114may be connected at once. The resistor or resistors104,106,108that are connected determine the total resistance and, therefore, the luminous flux. Each resistor104,106,108may have a different resistance. Depending on the desired light output, one, two, or all three resistors104,106,108may be connected to the circuit. If soldering multiple wires to solder pads is undesirable, other solutions, like jumpers or DIP switches, may be used to connect multiple resistors104,106,108at once. As those of skill in the art will understand, if all three resistors104,106,108have different resistances, then there are seven different possible combinations, and thus, seven different potential light output levels. As one example, resistor104may have a resistance of 3.36 kΩ and a power capacity of 19 mW, resistor106may have a resistance of 1.68 kΩ and a power capacity of 37 mW, and resistor108may have a resistance of 840Ω and a power capacity of 74 mW. The resistances of the resistors104,106,108could be chosen to create the greatest range of differences in perceived brightness among the various light output options, or the resistances could be chosen to provide fine gradations in perceived brightness around a general level of light output.

As was noted above, resistors are but one example of current-setting devices that may be used in linear lighting circuits, and embodiments of the invention may include other types of current-setting devices. For example,FIG. 4is a circuit diagram of a strip of linear lighting, generally indicated at200, according to another embodiment of the invention. Like the previous embodiments, in the view ofFIG. 4, three repeating blocks202are shown, arranged electrically in parallel with one another along the strip of linear lighting200, although any number of repeating blocks202may be present in a strip of linear lighting200. Each repeating block202assumes a 24 VDC input and has six LED light engines14, like the other embodiments.

The difference in the repeating blocks202, as compared with those of other embodiments, lies in their current-setting components. Instead of resistors104,106,108, each repeating block202has three current source/driver integrated circuits204,206,208. As in the strip of linear lighting100ofFIG. 3, these are arranged in parallel with one another. Also similar toFIG. 3, each integrated circuit204,206,208has its own terminal210,212,214at the cathode end of the strip of linear lighting200.

The integrated circuits204,206,208perform the current-setting function in the strip of linear lighting200, serving as constant current drivers for the LED light engines14. Typically, each integrated circuit204,206,208is designed to supply a different current level. The current levels may be, e.g., 2 mA, 4 mA, and 8 mA, for example, although as explained above, the current levels may be chosen in accordance with the perception of brightness, rather than the luminous flux they allow. As with the linear lighting100ofFIG. 3, the light output of the linear lighting200would depend on which of the terminals210,212,214are connected to power. Thus, as the linear lighting200ofFIG. 4demonstrates, embodiments of the invention may include any element or group of elements that can control or regulate the current in a circuit, and are not limited to resistors.

While current-setting ICs204,206,208may be slightly more expensive than resistors, it is possible that their use may result in a lower total resistance and allow for longer maximum run lengths.

There is a commonality in all of these embodiments: each repeating block includes at least one additional terminal, coupled to at least one additional current-setting element, be it a resistor or a current-control IC, and the terminal or terminals that are actually connected to the circuit determine the ultimate light output of the repeating block and of the strip of linear lighting as a whole.

As those of skill in the art will note, all of the above-described embodiments assume a DC voltage input. However, there are situations in which a time-varying voltage is used with linear lighting, e.g., the use of a pulse-width modulation (PWM) signal in order to change the duty cycle and effective light output of a strip of linear lighting.

Certain embodiments of the invention may include a component or components to make the strip of linear lighting more compatible with PWM signals, as well as other types of time-varying signals.

FIG. 5is a circuit diagram of a strip of linear lighting, generally indicated at300, according to another embodiment of the invention. Like the embodiments described above, the strip of linear lighting300is divided into repeating blocks302by cut points304. The repeating blocks302, three of which are shown inFIG. 5, are electrically in parallel with one another between power306and ground308. In the illustrated embodiment, much like the other embodiments, power is assumed to be a 24 VDC supply, and each repeating block302has six light-emitting diodes14and two resistors24,26.

Each repeating block302has one additional component that, like the others, may be, e.g., surface mounted on the PCB12of the strip of linear lighting300: a transistor, specifically an N-channel CMOS field-effect transistor (FET)310. The drain312of the transistor310is connected to the cathode end of the strip of linear lighting300and the source314is connected to ground308. The gate316of the transistor302is connected to a signal line318that runs the length of the strip of linear lighting300in parallel with power306and ground308. The signal line318would typically have a terminal in the sets of solder pads22on the top surface of the PCB12, although as was described above, other locations are possible.

Arranged this way in each repeating block302, the transistor310acts as a switch. If the signal318applied to the gate316of the transistor310is equal to or exceeds the threshold voltage of the transistor310, current flows through the circuit. The threshold gate-to-source voltage of a typical transistor310is typically much lower than the voltage used to drive the repeating blocks302, e.g., about 1.5 volts. Thus, the strip of linear lighting300ofFIG. 5takes DC power at a first, higher voltage (e.g., 12 or 24V) and has a separate control signal, which may be of much lower voltage (e.g., 1.5-3V). That control signal may be modulated, e.g., with a PWM scheme, to change the duty cycle of the lighting and create an effective light output that is proportional to the duty cycle of the control signal.

In order to create an appropriate control signal, the strip of linear lighting300may be connected to a signal generator320, such as a PWM signal generator. This may take the form of a small module connected to one end of the strip of linear lighting300. Of course, any other type of signal generator may be used, and in some embodiments, signals other than PWM may be used, e.g., to create a strobing effect or some other desired lighting effect.

With any oscillating signal, there is always a risk of generating electromagnetic interference. The sharp corners of a square-wave PWM signal may increase this risk. If necessary, each repeating block302may include a capacitor, or another such filtering element, to smooth the signal somewhat and reduce the possibility of electromagnetic interference. The control signal may also be programmed with slower rise and fall times to reduce electromagnetic interference.

There are several potential advantages to the arrangement shown inFIG. 5. For one, because PWM is performed on a lower-voltage control signal, the hardware used as the PWM generator320can be lower-voltage, lower-power, and potentially smaller than traditional PWM dimmers used to modulate, e.g., a 120V AC power signal or a 24V, 4 A DC power signal. The PWM generator320itself may have switches or a knob to allow the light output to be adjusted at the time of installation. The arrangement of the linear lighting300may also allow for greater compatibility with a variety of other types of modulation schemes and electronics, because only a very low voltage must be modulated to achieve the same effect that previously required the modulation of a 12V or 24V signal.

FIG. 6is a circuit diagram of a strip of linear lighting, generally indicated at400, that is a variation on the strip of linear lighting300ofFIG. 5. The strip of linear lighting400has repeating blocks402that are virtually identical to the repeating blocks302ofFIG. 5. In particular, they include the transistor310described above, as well as the signal line318to which a PWM signal generator320is attached. The difference between the repeating blocks302,402lies in the current-setting elements: the repeating blocks402ofFIG. 6are devoid of resistors24,26and instead use current-setting ICs404in their place. As explained above, this may allow for a longer functional maximum run length.

While the invention has been described with respect to certain embodiments, the description is intended to be exemplary, rather than limiting. Modifications and changes may be made within the scope of the invention, which is defined by the appended claims.