Abstract:
A trailer light converter that is configured to operate lights of a towed vehicle from signals supplied from lighting circuits of a towing vehicle includes at least one input that is adapted to be connected with the lighting circuits of a towing vehicle and at least one output that is adapted to be connected with the lights of a towed vehicle. At least one switch is provided that is responsive to a signal applied to the at least one input to supply electrical power to the at least one output to illuminate at least one light of the towed vehicle. The at least one switch includes a switching transistor and an overload protector. The at least one switch is responsive to the signal applied to the at least one input to change to a low impedance state to supply power to the at least one output. The overload protector is responsive to an overload condition on the at least one output to turn the at least one switch to a high impedance state to remove power from the least one output.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional patent application Ser. No. 61/029,029, filed on Feb. 15, 2008, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to electronic converter circuits which are used to sense indicator signals generated by a towing vehicle and in response drive the lighting circuits of a towed trailer. 
     Devices for the interface of trailer lights to a towing vehicle are well known. Since towing vehicles started separating the turn signal lamps from the brake signal function a need has existed to perform a logical conversion to interface the combined brake/turn signals commonly used on trailers to such towing vehicles. U.S. Pat. Nos. 3,970,860, 5,241,241, and 3,849,644 illustrate lighting converter circuits suitable for switching the appropriate signal circuit to the trailer lamps. 
     Similarly, converters with buffering have been produced before as well; U.S. Pat. Nos. 5,701,116, 6,232,722, and 5,030,938 illustrate techniques by which the interface function might be accomplished without drawing the trailer light power from the towing vehicle lighting circuits. 
     SUMMARY OF THE INVENTION 
     One disadvantage found in prior systems is that the output circuitry is not protected from load faults such as would occur with a short circuited wiring harness in the trailer. If the power supply fuse did not blow to interrupt power to the lighting converter the output devices of the converter would be damaged. Also, the need to replace blown fuses is an annoyance even if they function properly. 
     A trailer light converter that is adapted to operate lights of a towed vehicle from signals supplied from lighting circuits of a towing vehicle, according to an aspect of the invention, includes at least one input that is adapted to be connected with the lighting circuits of a towing vehicle and at least one output that is adapted to be connected with the lights of a towed vehicle. At least one switch circuit is provided that is responsive to a signal applied to the at least one input to supply electrical power to the at least one output to illuminate at least one light of the towed vehicle. The at least one switch includes a switching transistor and an overload protector. The at least one switch responds to the signal applied to the at least one input to change to a low impedance state to supply power to the at least one output. The overload protector is responsive to an overload condition on the at least one output to turn the at least one switch to a high impedance state to remove power from the least one output. 
     The overload protector may be responsive to an increase in voltage across the at least one switch to turn the at least one switch to a high impedance state. The at least one switch may be a field effect transistor and the overload protector may be responsive to an increase in voltage across the source to drain junction of the field effect transistor. The trailer light converter may include a timing section that delays the overload protector from responding to a transient overload condition thereby avoiding nuisance responses. 
     The at least one switch may draw power from the battery circuit or ignition circuit of the towing vehicle or may draw power from the lighting circuits of the towing vehicle. The at least one input may include a plurality of inputs and the at least one output may include a plurality of outputs. The plurality of inputs include a right signal input, a left signal input, a stop light input and a running light input. The outputs include a right lamp output, a left lamp output and a running lamp output. The at least one switch may include a plurality of switches. One of the switches may be responsive to operation of the right signal input or the stop light input to illuminate the right light of the towed vehicle. Another of said switches may be responsive to operation of the left signal input or said stop light input to illuminate the left light of the towed vehicle. Yet another of the switches may be responsive to operation of the running light input to illuminate the running light of the towed vehicle. 
     The at least one switch may be configured to operate with the overload protector or without the overload protector. Similarly, over-temperature protection may or may not be included as required by the application. In this manner, a scaled down version of the trailer lighting converter may be provided with reduced part count. 
     These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a preferred embodiment of the protected trailer light converter that is operational from a vehicle battery circuit or ignition circuit of the towing vehicle; and 
         FIG. 2  is a schematic diagram showing another preferred embodiment of the protected trailer light converter that is operational from the lighting circuits of the towing vehicle. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now specifically to the drawings, and the illustrative embodiments depicted therein, a trailer light converter, or lighting circuit  10 , according to the various embodiments disclosed herein, include inputs adapted to be connected with the RIGHT, LEFT and STOP signals connecting to the towing vehicle ( FIG. 1 ). The RIGHT and LEFT signals are at a high potential, nominally 12V, when the appropriate signal lamp is illuminated. The STOP signal is at a high potential when the vehicles braking system is actuated. The RUNNING signal connection is at a high potential, nominally 12V, when the vehicle&#39;s marker, taillights, and clearance lights are illuminated. Finally, the BATTERY input is connected to the towing vehicle&#39;s battery circuit or ignition circuit to provide power to the towed vehicle lights. 
     The following modes of operation will be described with the trailer light converter module: 
     OFF 
     Running Lights On 
     Turn-on into overload 
     Clearing of fault condition. 
     Right Turn 
     Left Turn (same as right) 
     Stoplight 
     Right Turn and Stoplight 
     Left Turn and Stoplight 
     Hazard flasher modes. 
     The following text discusses converter function in each of these modes. As the output stage operation and protection mechanism is common to all channels (RIGHT, LEFT, and RUNNING) it will be discussed using the RUNNING light circuit first. 
     OFF Condition: 
     In the OFF condition the signal levels on the RIGHT, STOP, LEFT, and RUNNING inputs are all at a low potential. In this condition, input transistors Q 3 : 1 , Q 3 : 2 , and Q 6  are all biased “OFF”, allowing R 3 , R 7 , and R 16  to turn the output switching transistors U 1 , U 2 , and U 3  “OFF” as well. No output current is sourced to the output terminals and the towed vehicle lights remain “OFF” as well. 
     RUNNING Lights ON: 
     When the towing vehicle running lights are turned “ON” the RUNNING signal input changes to a high potential, nominally 12V. This turns input transistor Q 6  “ON” and turns output transistor U 5  “ON” by pulling its gate towards GROUND though R 15  and C 3 . As output transistor U 5  turns ON the voltage drop between the drain and source terminals of this field effect transistor (FET) approaches zero volts, allowing Q 2 : 2  to turn OFF. As output transistor U 5  is now ON, it allows current to flow from the BATTERY terminal to the RUN_OUT terminal which then energizes the running lamps on the towed vehicle. 
     RUNNING Lights with Faulted Load: 
     If the RUN_OUT circuit (connected to the towed vehicle running lights) were faulted, such as with a short circuit to ground, the circuit will protect itself as follows: 
     If the fault occurs while the RUNNING light channel is actuated Input transistor Q 6  is ON and output transistor U 5  is ON. At the moment of the overload the MOSFET U 5  will attempt to pass an excessively high amount of current to the faulted load. This will result in a high voltage drop between the Drain and Source leads of U 5  added to the voltage drop over resistor R 112 , which will turn overload protector Q 2 : 2  ON. When Q 2 : 2  is turned ON a timing circuit made up of a capacitor C 3  will charge through a resistor R 14 , turning output transistor U 5  OFF after a time determined by resistor R 14  and capacitor C 3 . In this way a time delay is accomplished to prevent nuisance over-current trips and yet protect the circuitry in the case of a faulted load condition. 
     If the RUNNING light channel is activated into a shorted channel a similar turn-off mechanism will occur: When the RUNNING light input is activated Q 6  will turn ON, turning U 5  ON through the action of R 15  and C 3 . Since the load is faulted, the voltage drop over transistor U 5  and resistor R 112  will be excessive and Q 2 : 2  will be ON, causing C 3  to charge through R 14  and after a time delay set by R 14  and C 3  will turn U 5  OFF. In this way a time delay is accomplished to allow for the inrush over-current of typical incandescent lamps and yet protect the circuitry in the case of a faulted load condition. Note that the voltage drop over U 5  is now Vbat; Q 2 : 2  will remain ON, latching the circuit OFF. 
     Once the over-current protection is engaged, the channel must be turned OFF for a time sufficient for resistor R 15  to discharge C 3 , then back ON again to reactivate the load. A temperature sensitive resistor RT 3  provides a pre-bias to the protector mechanism. If the temperature of the assembly is excessive, the over-current shutdown will occur at a lower current. 
     RIGHT Turn Indication: 
     Right Turn indication is performed by the towing vehicle by pulsing the RIGHT input ON, then OFF at a nominally 1 Hz rate. During the ON portion of the cycle the RIGHT input to the module is at a high potential (nominally 12V) while the LEFT and STOP inputs are nominally low. This applies a forward bias to Q 3 : 1  which turns on, turning U 1  ON in similar manner as previously described for the RUNNING light section. Note that a faulted load will result in the channel turning off by the same mechanism as described in the RUNNING light circuit description as well. During the OFF portion of the blinker cycle the RIGHT input is low, Q 3 : 1  and U 1  are turned OFF, and the R_OUT terminal is a low potential (same as the OFF condition described above). 
     LEFT Turn Indication: 
     LEFT turn indication is essentially the same as right turn indication with Q 3 : 2 , Q 1 : 2 , D 3  and U 2  corresponding to Q 3 : 1 , Q 1 : 1 , D 2  and U 1  of the right channel. 
     STOP Light Indication: 
     STOP light indication is performed by the towing vehicle by applying a high potential to the STOP input with the RIGHT and LEFT inputs held low. This applies a forward bias to Q 3 : 1  and Q 3 : 2  which in turn biases U 1  and U 2  ON to switch power to both the RIGHT and LEFT trailer lamps simultaneously. 
     STOP Light and RIGHT Turn Indication: 
     When a RIGHT turn is being indicated simultaneously to a STOP light indication it is desired that the LEFT trailer lamp be turned on steadily while the RIGHT trailer lamp flashes ON/OFF in response to the towing vehicle signal light. This is accomplished by the action of diodes D 2   a  and D 2   b . During the portion of the blink cycle when the module RIGHT input is high it can be seen that both the STOP input and the RIGHT input are at a high level. In this condition the diodes D 2   a  and D 2   b  are both reverse biased and no current can flow through Q 3 : 1 , causing U 1  to be turned OFF and the RIGHT output to be low. During the portion of the blink cycle when the RIGHT input is low (the STOP input is high) Q 3 : 1  is biased ON through R 10 . Current flows through Q 3 : 1  through D 2 :a and R 2  to ground (or the towing vehicle lamps through the RIGHT input) to turn U 1  ON and operate the trailer&#39;s RIGHT side lamp(s). 
     STOP and LEFT Turn Indication: 
     This mode of operation can be seen to be essentially the same as STOP and RIGHT turn indication with Q 3 : 2 , Q 1 : 2 , D 3  and U 2  corresponding to Q 3 : 1 , Q 1 : 1 , D 2  and U 1  of the right channel. 
     HAZARD Flasher Operation: 
     In HAZARD flasher operation the RIGHT and LEFT signals both flash on/off together. Operation of the controller module can be seen to be the same as simultaneous operation in both RIGHT turn and LEFT turn indication modes, both with and without STOP light indication as described above. 
     Operation of the Non-Battery Powered Version: 
     Operation of a non-battery powered trailer light converter  110  includes all operating modes described above but power is derived from the vehicle lighting circuits instead of from a battery connection as used above ( FIG. 2 ). 
     OFF State: 
     In the OFF state all inputs are at a low potential; no power is passed to any of the trailer lights. 
     Running Lights on: 
     The trailer running lights are connected to the vehicle running lights through a positive temperature coefficient (PTC) circuit protection device. When the vehicle running lights are turned on the trailer running lights will be powered through the device. If a fault occurs, which draws excessive current through the PTC device, it will switch to a high impedance state, effectively disconnecting the load. 
     Right Turn: 
     During the ON portion of the blink cycle power from the RIGHT input will be passed through diode D 1  to the MOSFET Q 2 . Since the STOP signal is at a low potential the gate terminal of Q 2  will be pulled low through D 2  and Q 2  will be turned ON allowing current to flow from the RIGHT input to the RIGHT output to the trailer lights. During the OFF portion of the cycle there is no power to the circuit and the trailer lamps are OFF. 
     Right Turn into Shorted Load: 
     If the trailer load is faulted such as with a short circuit Q 2  will attempt to power the load normally. When the current through Q 2  reaches an excessive value the voltage drop over Q 2  and R 110  will reach a value such that Q 1 : 1  is turned on. The resulting current through Q 1 : 1  will charge C 1  in a time determined by C 1  and R 1 ; this will turn Q 2  OFF and will protect the module and vehicle circuitry. This delay is selected to allow the turn-on time of incandescent lamps with their characteristic inrush current. 
     Temperature sensitive resistor RT 1  provides a pre-bias to the protector mechanism. If the temperature of the assembly is excessive, the over-current shutdown will occur at a lower current. 
     STOPLIGHT Indication: 
     When the RIGHT turn signal is off and the STOP light input is high power from the RIGHT input will be passed through diode D 1  to the MOSFET Q 2 . Since the RIGHT input is at a low potential the gate terminal of Q 2  will be pulled low through D 2  and Q 2  will be turned ON allowing current to flow from the STOP input to the RIGHT output to the trailer lights. 
     Right Turn and STOP Indication: 
     During the ON portion of the blink cycle both the RIGHT input and the STOP input will be high. Since both sections of D 2  are high the gate terminal of Q 2  will remain high and Q 2  will be OFF. During the OFF portion of the blink cycle D 2  will conduct, pulling the gate of Q 2  low and allowing it to turn-on. This allows current to flow from the STOP input to the R_OUT terminal and lights the RIGHT trailer lamp. 
     LEFT Channel Operation: 
     Operation of the LEFT channel can be seen to be the same as the RIGHT channel with corresponding components and signals. 
     The described embodiments provide a cost-effective approach to a buffered lighting converter. The device will perform the required interface and logical conversion necessary to connect the combined brake and turn signals commonly used by trailers to vehicles with separate STOPLIGHT and TURN signals. In addition, the circuit as built above will provide a protected output stage which drives the output lamps (trailer lights) in response to the towed vehicle lights. Power to the trailer lights may be drawn from a dedicated battery wire or ignition wire; whereby only a signal current of approximately 10 milliamperes is drawn from the towing vehicle lighting circuits to operate the converter logic. 
     The various trailer light converter embodiments disclosed herein provide build options for construction of non-protected units for more cost-sensitive applications. By deleting the overload protection components, a converter may be provided with the same nominal function, but without the overload protection. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.