Abstract:
An LED driver circuit that includes first and second switching mechanisms. The first mechanism is in a first switching condition when a source voltage is at or below a first threshold and moves to a second switching condition when the source exceeds the first threshold. The second mechanism is in a third switching condition when the source voltage is applied thereto and is at or below a second threshold less than the first threshold and moves to a fourth switching condition when the source voltage is applied thereto and is between the first and second thresholds. The third and fourth switching conditions each cause a safe current to flow through the LED. The second switching condition causes a safe current, preferably none, to flow through the LED, and the first switching condition causes the source voltage to be applied to the second mechanism. Also, a switching assembly including the circuit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an LED driver circuit, and in particular to a circuit for protecting an LED or similar lighting device from transient voltages in, for example, a vehicle electrical system. 
   2. Background Information 
   The instrument panel, center console and/or dashboard of a vehicle, such as an automobile or a truck, typically contains a number of switches for activating various components of the vehicle, such as, without limitation, the headlights, hazard lights and front and rear fog lights. A light emitting diode (LED) or other suitable light source is often provided in connection with each such switch in order to illuminate the area surrounding the switch assembly. 
   In addition, during operation of a vehicle, relatively large transient voltages may occur. These transient voltages are of relatively short duration and may result from a loose battery connection or other causes. If electrical devices on the vehicle, such as the LEDs described above, are exposed to the relatively large transient voltages, the devices could be damaged (all devices and components are on the same electrical system). LEDs used in conjunction with switch assemblies can typically have a maximum current rating on the order of 25 mADC, meaning they can only safely operate, without being damaged, at currents at or below such values. Thus, LED protection circuits have been developed for protecting LEDs from potentially damaging transient voltage spikes. 
   As is known, vehicles typically employ either a 12 volt or a 24 volt (larger vehicles) electrical system, and the prior art LED protection circuits that have been developed have been designed to operate only in the particular electrical system (12 volt or 24 volt) in question.  FIG. 1  is a circuit diagram of LED protection circuit  5 , which is one example of a prior art LED protection circuit for use in connection with a 24 volt vehicle electrical system. As seen in  FIG. 1 , the 24 volt electrical signal (actually, the signal may typically range from between 18 volts and 32 volts) is applied, when appropriate, across terminals  10 A and  10 B. LED protection circuit  5  includes resistors  15  and  20  connected in parallel, each of which is preferably a 3K Ω resistor. Zener diode  25  is provided between resistor  15  and terminal  10 B, and is chosen such that it will not begin conducting current until an upper threshold voltage is reached, such as about 36 volts. Zener diode  25  may be a 39 VDC zener diode, which will typically begin to conduct at about 35–36 volts. LED  30 , which is the LED to be protected by the LED protection circuit  5 , is provided between resistor  20  and terminal  10 B. Also, resistor  35 , such as a 51 Ω resistor, is provided in series with LED  30 . In operation, if an appropriate 24 volt (actually 18 volt to 32 volt) signal is applied across terminals  10 A and  10 B (such as when the associated switch is in an “on” position), zener diode  25  will not conduct current, and the voltage will be applied across parallel resistors  15  and  20  and LED  30 . As a result, a current of about 16 mADC will flow through LED  30 , which is within the operating range of LED  30 . If, however, a voltage of about 36 volts or greater, as a result of a transient condition, is applied across terminals  10 A and  10 B, then zener diode  25  will conduct and will clamp the excess voltage down and shunt harmful current away from LED  30 , thereby protecting LED  30 . Resistor  35  and diode  40  are provided to protect LED  30  in the event circuit  5  is improperly connected to the voltage source (i.e., backwards). 
     FIG. 2  is a circuit diagram of LED protection circuit  5 ′, which is one example of a prior art LED protection circuit for use in connection with a 12 volt vehicle electrical system. LED protection circuit  5 ′ is identical to LED protection circuit  5 , except that the values of resistor  15 ′, resistor  20 ′, and zener diode  25 ′ are chosen to provide an appropriate current to LED  30  when a 12 volt (actually typically ranging from between 9 volts and 16–17 volts) signal is applied across terminals  10 A and  10 B, and to protect LED  30  from voltages that exceed about 18 volts. As will be appreciated, two different circuits are required, depending on the type of electrical system (12 or 24 volt) being employed. Suppliers and maintenance personnel must thus stock both types of protection circuits. 
   In addition, LED protection circuits such as those shown in  FIGS. 1 and 2  are currently mounted to the switch assembly as shown in  FIG. 3 . Specifically, the LED protection circuit is implemented by attaching discrete electrical components  45  (e.g., the components shown in  FIGS. 1 and 2 ) to a circuit board substrate material  50 , such as FR-4, G-10 or the like. LED  30  is then attached to the circuit board substrate material  50  at appropriate electrical contacts. The circuit board substrate material  50  containing the LED protection circuit is then attached to a connector base  60  which forms a part of the switch assembly of the vehicle. Connector base  60  may be made of, for example, glass filled valox, and includes electrical connectors  65  for connecting the switch assembly to the vehicle electrical assembly. The problem with such a configuration is that the circuit board substrate material  50  is responsible for about a third of the price of the finished switch assembly as a whole (e.g., the circuit board substrate material  50  typically costs on the order of $0.50 with the cost of the whole switch assembly being approximately $1.50). 
   SUMMARY OF THE INVENTION 
   These needs, and others, are addressed by the present invention which provides a driver circuit for protecting an LED or similar lighting device from transient voltage conditions, wherein the LED has a maximum operating current and driver circuit has a source voltage applied thereto. The driver circuit includes a first switching mechanism having a first switching condition and a second switching condition. The first switching mechanism is in the first switching condition when the source voltage is at or below a first threshold value and moves to the second switching condition when the source voltage exceeds the first threshold value. The driver circuit also includes a second switching mechanism having a third switching condition and a fourth switching condition. The second switching mechanism is in the third switching condition when the source voltage is applied thereto and is at or below a second threshold value that is less than the first threshold value and moves to the fourth switching condition when the source voltage is applied thereto and exceeds the second threshold value and is at or below the first threshold value. The third switching condition causes a first current that is less than the maximum operating current to flow through the LED, and the fourth switching condition causes a second current that is less than the maximum operating current to flow through the LED. In addition, the second switching condition of the first switching mechanism causes a safe level of current, preferably substantially none, to flow through the LED and preferably prevents the source voltage from being applied to the second switching mechanism (thereby protecting the LED from transients), and the first switching condition of the first switching mechanism causes the source voltage to be applied to the second switching mechanism in order to power the LED. In one embodiment, the first and second currents are substantially equal to one another such that the LED generates substantially the same amount of light in each condition. 
   Preferably, the first switching mechanism and the second switching mechanism are provided on a single integrated circuit. In addition, the second switching mechanism preferably includes a parallel resistor combination including a first resistor in parallel with a second resistor, wherein when the second switching mechanism is in the third switching condition the source voltage is applied to a first series connection including the LED connected in series with the parallel resistor combination, and wherein when the second switching mechanism is in the fourth switching condition the source voltage is applied to a second series connection including the LED connected in series with the first resistor and not the second resistor. This configuration ensures that a safe current is supplied to the LED. In order to operate with both 12 volt and 24 volt vehicle electrical systems, the first threshold value may about 36 volts and the second threshold value may be about 17 volts. 
   In addition, a further aspect of the invention relates to switch assembly for a vehicle electrical system including a connector base having a plurality of electrical connectors for connecting the switch assembly to the vehicle electrical system, an LED having a maximum operating current, and a driver circuit in the various embodiments described above that is electrically connected to the LED for protecting the LED from transient voltage conditions present in the vehicle electrical system. The driver circuit is electrically connected to one or more of the electrical connectors and has a source voltage from the vehicle electrical system applied thereto. Preferably, all or part of the driver circuit is provided on a single integrated circuit mounted on the connector base. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
       FIGS. 1 and 2  are circuit diagrams of prior art LED protection circuits; and 
       FIG. 3  is a front schematic view of a portion of prior art switch assembly that includes an LED and an LED protection circuit provided on a printed circuit board; 
       FIG. 4  is an LED protection circuit (driver circuit) according to an embodiment of the present invention; 
       FIGS. 5 and 6  are top and front schematic views of an LED protection circuit (driver circuit) formed at least partially on an integrated circuit according to one embodiment of the present invention; and 
       FIG. 7  is a front schematic view of a portion of switch assembly according to an aspect of the present invention that includes an LED and an LED protection circuit formed at least partially on an integrated circuit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4  is a circuit diagram of LED protection circuit  70  according to one embodiment of the present invention that may be used interchangeably in connection with either a 12 volt or a 24 volt vehicle electrical system. Preferably, LED protection circuit  70  is implemented in the form of an integrated circuit (IC) (as opposed to discrete electrical components) according to one of several methods known in the art. LED protection circuit  70  includes pins  75 A and  75 B across which the electrical system voltage (12 or 24 volts) is applied. As seen in  FIG. 4 , LED protection circuit  70  includes four bipolar junction transistors  80 ,  85 ,  90 , and  95 . Resistor  100 , preferably a 20K Ω resistor, is provided between the collector and the base of transistor  80 . In addition, resistor  100  and the collector of transistor  80  are connected to pin  75 A as shown. The emitter of transistor  80  is connected to the base of transistor  85 . The collector of transistor  85  is connected to pin  75 A and the emitter of transistor  85  is connected to resistors  105  and  110  provided in parallel as shown. Preferably, resistor  105  is a 1K Ω, 1 Watt resistor and resistor  110  is a 1.2K Ω, 1 Watt resistor. Diode  115  is provided between the opposite ends of resistors  105  and  110 . LED  120  is provided between the junction of resistor  110  and diode  115  on one end and pin  75 B on the other end. Zener diode  125 , resistor  130  and resistor  135  are provided in series between pins  75 A and  75 B as shown in  FIG. 4 . Preferably, zener diode  125  is a 39 VDC zener diode and resistors  125  and  130  are 100K Ω resistors. The base of transistor  90  is connected at a location between resistor  130  and resistor  135 , the collector of transistor  90  is connected to the base of transistor  80 , and the emitter of transistor  90  is connected to pin  75 B. As also shown in  FIG. 4 , zener diode  140 , resistor  145  and resistor  150  are provided in series between the emitter of transistor  85  and pin  75 B. Preferably, zener diode  140  is a 20 VDC zener diode and resistors  145  and  150  are 20K Ω resistors. Finally, the base of transistor  95  is connected to a location between resistor  145  and resistor  150 , the collector of transistor  95  is connected to the junction of resistor  105  and diode  115 , and the emitter of transistor  95  is connected to pin  75 B. 
   In operation, when 12 volts from a vehicle electrical system (the voltage will actually range from between about 9 and 16–17 volts) is applied across pins  75 A and  75 B, zener diode  125  will not conduct (the voltage is not high enough), and, as a result, transistor  90  will be in a switched off (non-conducting) condition. The 12 volt signal will be applied to resistor  100 , thereby causing approximately 11.3 VDC volts to be applied to the base of transistor  80 . This voltage will cause transistor  80  to be switched on, which in turn will result in about 10.7 VDC to be applied to base of transistor  85  and thereby cause transistor  85  to be switched on. With transistor  85  turned on (conducting current from collector to emitter), the 12 volt signal is applied to the junction of the emitter of transistor  85  and zener diode  140 . The 12 volts is not, however, sufficient to cause zener diode  140  to conduct, and, as a result, transistor  95  is in a switched off condition (no voltage is applied to its base). The 12 volt signal is thus applied to the parallel combination of resistors  105  and  110 , thereby causing approximately 20 mADC (a safe level) to flow through LED  120 . When 24 volts from a vehicle electrical system (the voltage will actually range from between about 18 and 32 volts) is applied across pins  75 A and  75  B, zener diode  125  will still not conduct (the voltage is not high enough), and transistor  90  will be in a switched off condition. As described above, the 24 volt signal will be applied to resistor  100 , thereby causing transistors  80  and  85  to be switched on. With transistor  85  turned on and conducting current from collector to emitter, the 24 volt signal is applied to the junction of the emitter of transistor  85  and zener diode  140 . The 24 volts, unlike the 12 volts, is sufficient to cause zener diode  140  to conduct, and, as a result, approximately 0.7 volts is applied to the base of transistor  95  causing it to be switched on and conduct from collector to emitter. With transistor  95  turned on, current flow is shunted away from LED  120 , with the resulting current flowing through LED  120  through resistor  110  being about 22 mADC (a safe level). Because substantially the same amount of current is passed through LED  120  in either situation just described (20 mADC and 22 mADC), the LED  120  will generate substantially the same amount of light in each case. If, however, a transient voltage spike exceeding, for example, 36 volts is applied across pins  75 A and  75 , zener diode  125  will conduct, and a voltage of approximately 0.7 volts will be applied to the base of transistor  90 . As a result, transistor  90  will be turned on and will conduct current form its collector to its emitter. This condition will result in no voltage being applied to the base of transistor  80 , thereby causing it and transistor  85  to be in an off, non-conducting condition. LED  120  is thus protected from the harmful transient voltage, as substantially no current will flow through it in this condition. Alternatively, the circuit may be arranged to provide only a safe level of current to flow through LED  120  under transient voltage conditions. As will be appreciated, LED protection circuit  70  may be used to provide protection to LEDs from transients in both 12 volt and 24 volt electrical system vehicles. 
   In the LED protection circuit  70 , zener diode  125  and transistor  80 ,  85 , and  90  form part of a first switching mechanism that allows safe level source voltages to be applied to the remainder of the circuit to provide a current for LED  120  in a first switching condition, shunts away harmful level source voltages (and prevents them from being applied to the remainder of the circuit) in a second switching condition. In addition, when the safe level voltages are applied to the remainder of the circuit, zener diode  140  and transistor  95  act as a second switching mechanism that causes a safe (below the maximum operating current) current to flow through the LED  120  regardless of the actual voltage level of the source voltage (e.g., 12 volts or 24 volts). 
   As noted above, all of the circuit elements shown in  FIG. 4  except for LED  120  are, in one embodiment, provided on a single integrated circuit by known methods of IC fabrication. In another embodiment, resistors  105  and  110  are too large to be integrated into the integrated circuit and therefore are not provided on the integrated circuit, but instead are connected to the integrated circuit externally, for example as shown in  FIGS. 5 and 6 . In particular,  FIGS. 5 and 6  show integrated circuit package  155  having leads  160 ,  165 ,  170  and  175 .  FIGS. 5 and 6  also show resistors  105  and  110 , each in the form of a resistor package, that are external to integrated circuit package  155 . Lead  160  is connected at one end to resistor  110  and at the other end to diode  115  within integrated circuit package  155  (See  FIG. 4 ). Lead  165  is connected at one end to resistor  110  and at the other end to resistor  105  and the emitter of transistor  85  within integrated circuit package  155  (See  FIG. 4 ). Lead  170  is connected at one end to resistor  105  and at the other end to diode  115  within integrated circuit package  155  (See  FIG. 4 ). Lead  175  is connected at one end to resistor  105  and at the other end to resistor  110  and the emitter of transistor  85  within integrated circuit package  155  (See  FIG. 4 ). In addition, leads  180  and  185  are provided for connecting LED  120  to integrated circuit package  155  in the manner shown in  FIG. 4  (the opposite end of lead  180  is connected to the junction of diode  115  and resistor  110 , and the opposite end of lead  185  is connected to pin  75 B). 
   According to an aspect of the invention, as shown in  FIG. 7 , one or more integrated circuit packages  155  (with internal or external resistors  105  and  110 ) containing LED protection circuits  70  are mounted directly to a connector base  60  which forms a part of a switch assembly of the vehicle in question, thereby eliminating the circuit board substrate material  55  (and the associated cost) that was required in the prior art as shown in  FIG. 3 . Appropriate electrical connections are made to the vehicle electrical system via pins  75 A and  75 B, which are connected to the appropriate electrical connectors  65 . One or more LEDs  120  may then be connected to the integrated circuit packages  155 , with the LEDs  120  being protected by the LED protection circuits  70  provided therein. 
   While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art of various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, while the present invention has been described in connection with the supply of current to an LED, other types of similar devices for providing an indicator, a display or a light may be used, referred to herein as lighting devices. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.