Abstract:
An electric trailer brake controller includes a deceleration sensor for generating a brake control signal. The controller also includes a device to decrease the sensitivity of the deceleration sensor to spurious inputs. Additionally, the controller includes a brake current limiting circuit that progressively reduces the current supplied to the controlled trailer brake when the brake current exceeds a predetermined level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/302,813, filed on Apr. 30, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates in general to devices for actuating trailer electric wheel brakes and in particular to enhancements for trailer electronic wheel brake controllers.  
           [0003]    Towed vehicles, such as recreational and utility trailers which are towed by automobiles and small trucks, are commonly provided with electric wheel brakes. The electric wheel brakes generally include a pair of brake shoes which, when actuated, frictionally engage a brake drum. An electromagnet is mounted on one end of a lever to actuate the brake shoes. When an electric current is applied to the electromagnet, the electromagnet is drawn against the rotating brake drum which pivots the lever to actuate the brakes. Typically, the braking force produced by the brake shoes is proportional to the electric current applied to the electromagnet. This electric current can be relatively large. For example, the electric wheel brakes on a two wheeled trailer can draw six amperes of current when actuated and the electric wheel brakes on a four wheeled trailer can draw 12 amperes of current.  
           [0004]    Automotive industry standards require that electrically-actuated vehicle wheel brakes be driven against the ground potential of the vehicle power supply. Accordingly, one end of each of the towed vehicle wheel brake electromagnets is electrically connected to the towed vehicle ground and the towed vehicle ground is electrically connected to the towing vehicle ground. The other end of each of the brake electromagnets is electrically connected through either an electric wheel brake actuator or an electric wheel brake controller to the towing vehicle power supply.  
           [0005]    Generally, electric wheel brake actuators are manually operated devices which control the magnitude of electric current supplied to the towed vehicle wheel brakes. Various electric brake controllers for towed vehicle electric brakes are known in the art. For example, a variable resistor, such as a rheostat, can be connected between the towing vehicle power supply and the brake electromagnets. Such an actuator is disclosed in U.S. Pat. No. 3,740,691. The towing vehicle operator manually adjusts the variable resistor setting to vary the amount of current supplied to the brake electromagnets and thereby control the amount of braking force developed by the towed vehicle wheel brakes.  
           [0006]    It is also known to include an integrating circuit in an electric wheel brake actuator. When the towing vehicle brakes are applied, a signal is sent to the integrating circuit. The integrating circuit generates a continually increasing voltage which is applied to the electric wheel brakes. The longer the towing vehicle brakes are applied, the more brake torque is generated by the actuator. A manually adjustable resistor typically controls the rate of integration. On such actuator is disclosed in U.S. Pat. No. 3,738,710.  
           [0007]    Also known in the art are more sophisticated electric wheel brake controllers which include electronic circuitry to automatically supply current to the towed vehicle brake electromagnets which is proportional to the towing vehicle deceleration when the towing vehicle brakes are applied. Such electronic wheel brake controllers typically include a sensing unit which generates a brake control signal corresponding to the desired braking effort. For example, the sensing unit can include a pendulum which is displaced from a rest position when the towing vehicle decelerates and an electronic circuit which generates a brake control signal which is proportional to the pendulum displacement. One such unit is disclosed in U.S. Pat. No. 4,721,344. Alternately, the hydraulic pressure in the towing vehicle&#39;s braking system or the pressure applied by the vehicle operator&#39;s foot to the towing vehicle&#39;s brake pedal can be sensed to generate the brake control signal. An example of a controller which senses the towing vehicle brake pressure to generate the brake control signal is disclosed in U.S. Pat. No. 4,398.252.  
           [0008]    Known electronic wheel brake controllers also usually include an analog pulse width modulator. The input of the pulse width modulator is electrically connected to the sensing unit and receives the brake control signal therefrom. The pulse width modulator is responsive to the brake control signal for generating an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in direct proportion to the magnitude of the brake control signal. Thus, the duty cycle of the pulse train corresponds to the amount of braking effort desired.  
           [0009]    Electronic wheel brake controllers further include an output stage which is electrically connected to the output of the pulse width modulator. The output stage typically has one or more power transistors which are connected between the towing vehicle power supply and the towed vehicle brake electromagnets. The power transistors, which are usually Field Effect Transistors (FET&#39;s), function as an electronic switch for supplying electric current to the towed vehicle brakes. The output stage may also include a drive circuit which electrically couples the output of the pulse width modulator to the gates of the FET&#39;s.  
           [0010]    The output stage is responsive to the pulse width modulator output signal to switch the power transistors between conducting, or “on”, and non-conducting, or “off”, states. As the output transistors are switched between their on and off states in response to the modulator output signal, the brake current is divided into a series of pulses. The power supplied to the towed vehicle brakes and the resulting level of brake application are directly proportional to the duty cycle of the modulator generated output signal.  
         SUMMARY OF THE INVENTION  
         [0011]    This invention relates to enhancements for trailer electronic wheel brake controllers.  
           [0012]    As explained above, electronic wheel brake controllers energize the towed vehicle brakes upon detection of the towing vehicle deceleration when the towing vehicle brakes are applied. However, other conditions, such as travel over a rough road surface may cause the pendulum in a deceleration sensor to be displaced and thereby generate a false brake control signal. Accordingly, it would be desirable to reduce the sensitivity of the sensing unit to filter out such spurious inputs.  
           [0013]    Additionally, modem towing vehicles are equipped with capacity alternators that can supply large amounts of current. Furthermore, the voltage output of such alternators tends to fluctuate with load conditions. Accordingly, the current supplied to the trailer brakes could, under certain conditions become excessive. However, use of a fuse or circuit breaker to protect the trailer brake coils is not desirable since it would have to be replaced or reset after every occurrence of excessive current. Accordingly, it would also be desirable to provide a means for limiting the current supplied from the wheel brake controller to the trailer brakes.  
           [0014]    The present invention contemplates a device for controlling the electric current supplied to at least one electric wheel brake which includes a brake control signal generator which is adapted to be connected to a vehicle stop light switch. The brake control signal generator is operative to generate a brake control signal that is a function of the towing vehicle deceleration. The device also includes a brake control signal amplifier that is connected to the brake control signal generator. A damping capacitor is coupled to the brake control signal amplifier to reduce sensitivity of the brake control signal generator. The brake control signal amplifier has an output that is connected to an output signal generator. The output signal generator has an output terminal and is responsive to an amplified brake control signal to generate an output signal at the output terminal which is a function of the brake control signal. The device further includes an electric current controller which is adapted to be connected between a vehicle power supply and the controlled electric wheel brake. The current controller also is coupled to the output terminal of the output signal generator and is responsive to the output signal to control the electric current supplied to the controlled wheel brake as a function of the output signal.  
           [0015]    The invention further contemplates a current limiting circuit coupled to the current controller and the output signal generator. The current limiting circuit is operable to modify the output signal to progressively reduce the current supplied to the controlled wheel brake upon the current exceeding a first predetermined threshold. Additionally, the current limiting circuit is operative to disable the output signal generator upon the current being supplied to the controlled wheel brake exceeding a second predetermined threshold which is greater than the first predetermined threshold. 
       
    
    
       [0016]    Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic circuit diagram for an electric brake actuator in accordance with the invention.  
         [0018]    [0018]FIG. 2 is a graph of selected voltages within the actuator shown in FIG. 1 during a brake application.  
         [0019]    [0019]FIG. 3 is a graph of selected voltages within the actuator shown in FIG. 1 during operation of the towing vehicle hazard flasher.  
         [0020]    [0020]FIG. 4 is a schematic circuit diagram for an electric brake controller in accordance with the invention.  
         [0021]    [0021]FIG. 5 is a schematic circuit diagram for an alternate embodiment of the electric brake controller shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring now to the drawings, there is illustrated in FIG. 1 a schematic circuit diagram for an enhanced electric brake actuator  10 . The actuator  10  includes an input operational amplifier U 1   c  which generates a brake control signal at its output terminal when the towing vehicle brakes are applied. The input operational amplifier U 1   c  has a positive input terminal which is connected through an input resistor R 26  to a towing vehicle stop light switch  15 . A ramp capacitor C 4 , the purpose for which will be explained below, is connected between the positive input terminal of the operational amplifier U 1   c  and ground.  
         [0023]    The actuator  10  further includes first and second operational amplifiers, which are identified by the designators U 1   a  and U 1   b , respectively. The output terminal of the input operational amplifier U 1   c  is connected to a positive input terminal of the first operational amplifier U 1   a . Thus, the brake control signal is applied to the positive input terminal of the first operational amplifier U 1   a . The first operational amplifier U 1   a  also has a negative input terminal which is connected to an output terminal of the second operational amplifier U 1   b . The first and second operational amplifiers U 1   a  and U 1   b  are responsive to the brake control signal exceeding a threshold voltage to generate a PWM output signal at an output terminal of the first operational amplifier U 1   a . In the preferred embodiment, the threshold voltage is approximately two volts.  
         [0024]    The PWM output signal has a duty cycle which is proportional to the magnitude of the brake control signal.  
         [0025]    The output terminal of the first operational amplifier U 1   a  is connected to the base of a driver transistor Q 4 . The collector of the driver transistor Q 4  is connected to the gate of an output power Field Effect Transistor (FET) Q 1 . The power FET Q 1  is connected between the towing vehicle power supply  16  and the towed vehicle electric brake coils  18  (one shown). An actuation transistor Q 5  is connected between the emitter of the driver transistor Q 4  and ground. The actuation transistor Q 5  has a base terminal connected through an actuation Zener diode D 3  and a series connected pair of resistors, R 19  and R 35 , to the towing vehicle stop light switch  15 . Closure of the stop light switch  15  upon application of the towing vehicle brakes causes the actuation transistor Q 5  to be in a conducting state and thus enables the control of the output FET Q 1  by the driver transistor Q 4 .  
         [0026]    When the stop light switch  15  is closed, the ramp capacitor C 4  charges through the input resistor R 26  with a time constant which is a function of the product of the ramp capacitor C 4  and the input resistor R 26 . Accordingly, as the stop light switch  15  remains closed, an increasing voltage is applied to the positive input terminal of the input operational amplifier U 1   c . In response to the increasing voltage, the operational amplifier U 1   c  generates an increasing ramped brake control signal which is applied to the positive input terminal of the first operational amplifier U 1   a . The first and second operational amplifiers U 1   a  and U 1   b  co-operate to generate a PWM output signal having a constant frequency and a ramped duty cycle which is proportional to the magnitude of the brake control signal. The PWM output signal is applied to the base of the driver transistor Q 4 .  
         [0027]    The driver transistor Q 4  is responsive to the output signal to switch the power FET Q 1  between its non-conducting and conducting states with the duration of the conducting states increasing as the ramp capacitor C 4  charges. As the power FET Q 1  remains in its conducting state for a longer portion of each switching cycle, the average current supplied to the brake coils  18  increases. Thus, the magnitude of the current supplied to the brake coils  18  increases as a function of the time constant determined by the product of the input resistor R 26  and ramp capacitor C 4 .  
         [0028]    The towing vehicle also includes a hazard flasher switch  19 , which is connected in parallel across the stop light switch  15 . As explained above, the operation of the towing vehicle&#39;s hazard flasher switch  19  can cause false actuation of the towed vehicle brakes. Accordingly, the present invention contemplates including a hazard delay and automatic reset circuit, which is shown in FIG. 1 within the dashed lines labeled  20 , in the actuator circuit  10 . The hazard delay circuit  20  includes a delay capacitor C 3  which has a first end connected to a center tap of a first voltage divider  21  and a second end connected to ground. The first end of the delay capacitor C 3  also is connected through a coupling diode D 6  to the positive input terminal of the first operational amplifier U 1   a . The first voltage divider  21 , which includes a pair of resistors, R 27  and R 28 , is connected between the collector of an isolation transistor Q 7  and ground. The isolation transistor Q 7  has an emitter connected through a plurality of diodes, D 13 , D 14  and D 15 , to the stop light switch  15 . As will be explained below, during normal operation of the actuator  10 , the isolation transistor Q 7  is in its conducting state.  
         [0029]    The hazard delay circuit  20  also includes a second voltage divider  22 , which includes a pair of resistors, R 29  and R 30 , connected between the collector of the isolation transistor Q 7  and ground. The center tap of the second voltage divider  22  is connected to the base of a discharge transistor Q 6 . Thus, the second voltage divider  22  functions to bias the discharge transistor Q 6 . The emitter of the discharge transistor Q 6  is connected through a first discharge diode D 11  to the center tap of first voltage divider  21  and thereby to the non-grounded first end of the delay capacitor C 3 . The emitter of the discharge transistor Q 6  also is connected through a second discharge diode D 7  to the positive input terminal of the input operational amplifier U 1   c  and thereby to the non-grounded side of the ramp capacitor C 4 .  
         [0030]    The operation of the hazard delay and automatic reset circuit  20  will now be explained. Selected voltages within the actuator  10  during a normal brake actuation, without the hazard flasher in operation, are illustrated in FIG. 2. Before actuation of the stop light switch  15 , both the delay capacitor C 3  and the ramp capacitor C 4  are discharged. Also, the base of the discharge transistor Q 6  is at ground potential, which causes the discharge transistor Q 6  to be in its conducting state. Accordingly, when the stop light switch  15  is closed, as shown at t 1  in the top curve in FIG. 1, the power supply voltage is applied to the second voltage divider  22 . A portion of the power supply voltage appears on the base of the discharge transistor Q 6  which causes the transistor Q 6  to switch to its non-conducting state, blocking current flow through the first and second discharge diodes D 11  and D 7 . The delay capacitor C 3  proceeds to charge through the resistor R 27  to a voltage level determined by the ratio of the resistors in the first voltage divider  21 , as shown in the curve labeled “C 3 ” in FIG. 2. Simultaneously with the charging of the delay capacitor C 3 , the ramp capacitor C 4  charges through the input resistor R 26  causing the input operational amplifier U 1   c  to generate a ramped brake control signal, as shown by the curve labeled “RAMP” in FIG. 2. Both the voltage across the delay capacitor C 3  and the ramped brake control signal, RAMP, generated by the input operational amplifier U 1   c  are applied to the positive input terminal of the first operational amplifier U 1   a . As can be seen in FIG. 2, initially, the voltage across C 3  increases at a faster rate that the brake control signal, RAMP. Accordingly, the first operational amplifier is initially responsive to the voltage across the delay capacitor C 3 . When the voltage across the delay capacitor C 3  increases to the predetermined threshold level, which occurs at t 2  in FIG. 2, the first operational amplifier U 1   a  begins to generate the PWM output signal which causes actuation of the towed vehicle brakes.  
         [0031]    In the preferred embodiment, the curve labeled C 3  initiates a PWM output signal having a duty cycle of 8 to 12 percent, as illustrated in the bottom curve in FIG. 2. The reduced duty cycle provides a “soft turn-on” for the towed vehicle brakes. At t 3 , the ramp brake control signal generated by the input operational amplifier U 1   c  exceeds the voltage across the delay capacitor C 3  and causes the duty cycle of the PWM output signal to ramp up to a maximum of 100 percent, which is reached at t 4 . The duty cycle remains at 100 percent until the stop light switch  15  is released at t 5 . The slope of the ramp brake control signal, RAMP, generated by the input operational amplifier U 1   c  is adjustable with the Automatic Gain Control (AGC), R 8 . Thus, under normal operating conditions, the delay capacitor C 3  and ramp capacitor C 4  function to slightly delay the application of and provide a soft turn-on to the towed vehicle brakes  
         [0032]    Selected voltages within the actuator  10  with the hazard flasher actuated are shown in FIG. 3. When the hazard flasher of the towing vehicle is actuated, the hazard flasher switch  19  is periodically moved between open and closed positions. Thus, the hazard flasher switch  19  closes at t 6  and opens at t 7  in FIG. 3. Accordingly, the input voltage to the actuator  10  consists of a pulse train, as illustrated by the stop light voltage curve shown at the top of FIG. 3. The time constant for the delay RC circuit comprising R 27  and C 3  is selected such that the difference between t 1  and t 2  is slightly greater than the on-time of the towing vehicle hazard flasher switch  19 , which is the difference between t 6  and t 7 . In the preferred embodiment, the time constant provides a difference between t 1  and t 2  which is approximately a half second. The ramp RC circuit comprising R 26  and C 4  has a time constant which is longer than the delay RC time constant. Accordingly, if the input voltage to the actuator  10  is generated by the hazard flasher, the input voltage to the actuator  10  will go to zero before the delay capacitor C 3  charges sufficiently to initiate generation of a PWM output signal, as shown in the middle and lower curves in FIG. 3.  
         [0033]    When the actuator input voltage returns to zero, the base of the discharge transistor Q 6  is pulled to ground, causing the discharge transistor Q 6  to switch to its conducting state. When the discharge transistor Q 6  begins to conduct, the delay capacitor C 3  begins discharging through the first discharge diode D 11  and the ramp capacitor C 4  begins discharging through the second discharge diode D 7  to prepare the circuit  20  for the next on-cycle of the hazard flasher. It will be appreciated that the discharge transistor Q 6  and discharge diodes D 11  and D 7  also begin to conduct to discharge the delay and ramp capacitors C 3  and C 4  upon the stop light switch  15  opening at the end of a normal braking cycle.  
         [0034]    As explained above, the actuator  10  includes a manual brake control which can be used by the towing vehicle operator to apply the trailer brakes independently of the towing vehicle brakes. The manual brake control includes a potentiometer R 7  which is connected between the towing vehicle power supply  16  and ground. The slider tap of the potentiometer R 7  is connected to the positive input terminal of the first operational amplifier U 1   a . Movement of the potentiometer R 7  from its “OFF” position generates a manual brake control signal which is applied to the first operational amplifier U 1   a . However, if the automatic gain control of the input operational amplifier U 1   c  is set too high, an application of the towing vehicle brakes could cause the input operational amplifier U 1   c  to generate a greater than needed brake control signal. Accordingly, the present invention further contemplates that the actuator  10  includes a manual stop light and automatic isolation circuit, which is labeled  30  in FIG. 1.  
         [0035]    As shown in FIG. 1, the manual brake control signal potentiometer R 7  is ganged to a manual control potentiometer switch S 1 . In the preferred embodiment, the potentiometer R 7  includes a return spring which urges the potentiometer slider to the OFF position. When the towing vehicle operator manually moves the slider from the OFF position, the switch S 1  is closed. One side of the switch S 1  is connected to the vehicle power supply  16 . The normally open contact of the switch S 1  is connected through the coil of a relay RE 1  to ground. The relay RE 1  includes a set of normally open contacts connected between the power supply  16  and the stop light lamp. The normally open contact of the switch S 1  is connected to the base of the isolation transistor Q 7 , the second operational amplifier U 1   b  and the vehicle stop lights (one shown).  
         [0036]    The operation of the manual stop light and automatic isolation circuit  30  will now be described. During normal operation, the switch S 1  is open, causing the base of the isolation transistor Q 7  to be at ground potential. Accordingly, the isolation transistor Q 7  is normally in its conducting state which allows power to flow from the stop light switch  15  to the delay and ramp capacitors, C 3  and C 4 . However, upon movement of the slider of the manual brake control signal potentiometer R 7  to generate a manual brake control signal, the switch S 1  is closed. When the switch S 1  closes, a voltage is applied to the base of the isolation transistor Q 7  which causes the transistor to switch to its non-conducting state. Also, the relay contacts close to illuminate the stop light lamp. With the isolation transistor Q 7  in a non-conducting state, the delay and ramp capacitors, C 3  and C 4 , are isolated from the stop light switch  15 . Accordingly, actuation of the stop light switch  15  when the manual control is in use will not cause the input operational amplifier U 1   c  to generate a brake control signal. As described above, closure of switch S 1  supplies power to the second operational amplifier U 1   b  which enables the generation of a PWM output signal from the first operational amplifier U 1   a  in response to the manual brake control signal. As described above, power also is supplied to illuminate the towing and towed vehicle stop lights (one shown).  
         [0037]    The actuator  10  also includes an output current limiting and short circuit protection circuit  40 . The circuit  40  includes a current sensor  41  comprising a plurality of low valued resistors which are connected in parallel. In the preferred embodiment, three 0.10 ohm resistors, which are labeled R 11 , R 12  and R 13  in FIG. 1, are connected in parallel; however, more or less resistors can be utilized. The current sensor  41  is connected between the power supply  16  and the source terminal of the output power FET Q 1 . As described above, the power output FET Q 1  has a drain terminal connected through the coils  18  (one shown) of the electric wheel brakes to ground. The end of the current sensor  41  connected to the source terminal of the FET Q 1  is connected thorough a resistor R 16  to a base terminal of a first sensor transistor Q 2 . The first sensor transistor Q 2  has an emitter terminal connected to the power supply  16  and a collector terminal connected through a sensor capacitor C 2  to ground.  
         [0038]    The collector terminal of the first sensor transistor Q 2  also is connected to a bias circuit  42  comprising a pair of resistors, labeled R 17  and R 33 , connected in series. The center tap of the bias circuit  42  is connected to the base of a second sensor transistor Q 3 . The emitter of the second sensor transistor Q 3  is connected to ground while the collector of the second sensor transistor Q 3  is connected through a blocking diode D 8  to the positive input terminal of a first operational amplifier U 1   a . The blocking diode D 8  blocks current from flowing back to the first operational amplifier input terminal from the current limiting circuit  40 .  
         [0039]    The operation of the current limiting circuit  40  will now be described. When the output FET Q 1  conducts, a load current flows through the current sensor  41 . The load current causes a voltage to appear across the current sensor  41  which is directly proportional to the magnitude of the load current. When the voltage across the current sensor  41  exceeds a first predetermined threshold, the first transistor Q 2  begins to conduct which causes the sensor capacitor C 2  to begin to charge. It will be appreciated that the load current flowing through the output FET Q 1  fluctuates as the PWM output voltage switches the FET Q 1  between its conducting and non-conducting states. Accordingly, the current flowing to the sensor capacitor C 2  also fluctuates. The sensor capacitor C 2  smoothes the fluctuations and charges to a voltage which is proportional to the average load current supplied to the brake coils  18 . The voltage across the sensor capacitor C 2  is applied to the base of the second sensor transistor Q 3 . which turns on and thereby reduces the brake control signal applied to the positive input terminal of the first operational amplifier U 1   a . The reduced brake control signal causes, in turn, a reduction in the duty cycle of the PWM output voltage. The reduced duty cycle reduces the on time of the output FET Q 1  and, thereby, reduces the load current supplied to the electric trailer brake coils  18 .  
         [0040]    If the current supplied to the trailer brake coils  18  further increases, the voltage across the current sensor  41  also increases, progressively turning on the first and second sensor transistors Q 2  and Q 3  and thereby progressively reducing the duty cycle of the PWM output voltage. Upon the load current reaching a second predetermined threshold, the second transistor Q 3  becomes fully conducting, providing a direct connection between the positive input terminal of the first operational amplifier U 1   a  and ground. When this occurs, the brake control signal is shunted to ground and the operational amplifier PWM output signal goes to zero, turning off the output FET Q 1  and providing short circuit protection for the actuator  10 . In the preferred embodiment, the sensor transistors Q 2  and Q 3  in the current limiting circuit  40  begin conducting when the brake current reaches 13.5 to 18 amps and complete shut off of the output FET Q 1  occurs when the output current reaches approximately 20 to 24 amps. The current values can be adjusted by selecting other values for the sensor capacitor C 2  and/or the resistors R 17  and R 33 .  
         [0041]    Upon shut off of the output FET Q 1 , the first sensor transistor Q 2  also is shut off as the current flow though the current sensor  41  stops. The sensor capacitor C 2  then begins to discharge through the bias resistors R 17  and R 33 . As the sensor capacitor C 2  discharges, the conduction of the second sensor transistor Q 3  is progressively reduced, allowing the voltage at the positive input terminal to the first operational amplifier U 1   a  to increase. In the preferred embodiment, the time constant for the combination of the sensor capacitor C 2  and the resistors R 17  and R 33  is selected such that, for brake currents in excess of 20 amps, the sensor capacitor C 2  will maintain a sufficiently high charge to keep the brake current at zero for three cycles of the PWM signal. Thus, the actuator off-time is increased to approximately 11 milliseconds from a typical off-time of approximately 3 milliseconds in prior art actuators. As a result, the heating of the power FET Q 1  is greatly reduced. The invention also contemplates using power FET&#39;s having a lower internal resistance than in prior art controllers to further reduce heating and associated power losses.  
         [0042]    The invention further contemplates that the brake actuator  10  includes a plurality of the voltage regulation diodes labeled D 13 , D 14  and D 15  which are connected between the stop light switch  15  and the positive input terminal of the input operational amplifier U 1   c . The regulation diodes D 13  thorough D 15  reduce the input voltage supplied to the actuator  10  from the stop light switch and compensate for variation of the towing vehicle alternator voltage. When conducting, the voltage across each of the regulation diodes is fixed by the diode forward emf and does not vary with the supplied voltage as the voltage across a resistive voltage divider would. While three regulation diodes are shown in FIG. 1, it will be appreciated that the invention also can be practiced with more or less diodes.  
         [0043]    The invention further contemplates stabilizing the voltages within the actuator and controller circuit  10  with selected use of one percent tolerance resistors. Such resistors do not vary with temperature changes or the age of components. In the preferred embodiment, one percent resistors are utilized for the resistors R 27  and R 28  in the first voltage divider  21  to assure that the actuator  10  has a consistent turn on duty cycle for the PWM output signal.  
         [0044]    The invention also contemplates utilizing the output limiter and short circuit protection circuit  40  in an enhanced electric brake controller  50 , as illustrated in FIG. 4. Components shown in FIG. 4 which are similar to components shown in FIG. 1 have the same numerical designators. The electric brake controller  50  is similar to the actuator  10 , but includes a brake control signal generator  55 . In the preferred embodiment shown in FIG. 4, the brake control signal generator  55  includes a pendulum device (not shown) which co-operates with a Hall Effect Device (HED)  56  to generate a brake control signal which is proportional to the deceleration of the towing vehicle. The brake control signal is applied to the positive input terminal of the first operational amplifier U 1   a . As described above, the first operational amplifier U 1   a  cooperates with a second operational amplifier U 1   b  to generate a PWM output signal for controlling the output power FET Q 1 . The PWM output signal has a duty cycle which is a function of the brake control signal.  
         [0045]    As shown in FIG. 4, the enhanced controller  50  includes the output limiter and short circuit protection circuit  40  described above. The protection circuit  40  monitors the current flowing through the output FET Q 1  and is operable to reduce the duty cycle of the PWM output signal as the current increases above a predetermined first threshold. The protection circuit  40  is further operable to turn off the output FET Q 1  if the current exceeds a second predetermined threshold. Similar to the actuator  10  described above, the controller off-time is increased to approximately 11 milliseconds from a typical off-time of approximately 3 milliseconds in prior art cintrollers.  
         [0046]    The present invention contemplates use of zener diodes to regulate voltages in the brake controller circuit  50  shown in FIG. 4. A first zener diode, which is labeled D 4 , is connected between the voltage input terminal of the second operational amplifier U 1   b  and ground. The first zener diode D 4  functions to regulate the voltage supplied to the operational amplifier and thus prevent overloading the operational amplifier while assuring consistent operation of thereof. A second zener diode, which is labeled D 7 , is connected between the voltage input terminal of the HED  56  and ground. The second zener diode D 7  functions to regulate the voltage supplied to the HED  56  and thus prevent overloading the HED  56  while assuring generation of consistent automatic brake control signals. A third zener diode, which is labeled D 10 , is connected between the voltage input terminal of the manual brake control signal potentiometer P 2  and ground. The third zener diode D 10  functions to regulate the voltage supplied to the potentiometer P 2  and thus prevent overloading the potentiometer P 2  while assuring generation of consistent manual brake control signals. A fourth zener diode D 3  is connected between the stop light switch  15  and the base of the actuation transistor Q 5 . The fourth zener diode D 3  provides a threshold voltage which must be exceed before the output power FET Q 1  can be turned on. It will be noted that the fourth zener diode D 3  also is included in the improved actuator circuit  10  shown in FIG. 1. Additionally, the zener diodes, D 3 , D 4 , D 7  and D 10  are selected to have a positive temperature coefficient to prevent a temperature increase from decreasing the duty cycle of the PWM output signal.  
         [0047]    The invention also contemplates the inclusion of a damping capacitor C 13  which is connected between the output terminal and the negative input terminal of the input operational amplifier U 1   c . The damping capacitor C 13  slows changes in the automatic brake control signal to prevent false brake applications caused by road surface irregularities displacing the pendulum device. In the preferred embodiment, the damping capacitor C 13  is a 1.0 micro-farad capacitor. Damping can be further increased by connecting an optional second damping capacitor C 14  in parallel to the damping capacitor C 13 , as shown in FIG. 4.  
         [0048]    The controller  50  further includes a voltage divider  57  which supplies a minimum brake control signal to the positive input terminal of the first operational amplifier U 1   a . The voltage divider  57  includes a pair of resistors R 27  and R 28  which are connected between the stop light switch  15  and ground. When the stop light switch  15  is closed, a small voltage is applied to the positive input terminal of the first operational amplifier U 1   a  to actuate the trailer wheel brakes before the towing vehicle has decelerated sufficiently for the pendulum device  55  and HED  56  to generate an automatic brake control signal. In the preferred embodiment, the minimum brake control signal is equivalent to a ten percent brake application; however, by adjusting the values of the resistors R 27  and R 28 , other amounts of brake application can be provided, such as a six percent initial application. Also in the preferred embodiment, one percent resistors are utilized for the resistors R 27  and R 28  in the voltage divider  57  to assure that the controller  50  has a consistent turn on duty cycle for the PWM output signal.  
         [0049]    An alternate embodiment  60  of the circuit  50  is shown in FIG. 5. Components in FIG. 5 that are similar to components shown in FIG. 4 have the same numerical designators. The circuit  60  includes a voltage stabilizing circuit  62  that replaces three of the voltage regulating circuits included in the circuit  50  shown in FIG. 4. The voltage stabilizing circuit  62  includes a series connection of a resistor R 31  with a cathode of a Zener diode D 7 . The anode of the Zener diode D 7  is connected to ground while the end of the resistor R 31  that is opposite from the Zener diode D 7  is connected to the power supply  16  through either the stop light switch  15  or the relay RE 1 . A regulated voltage supply appears at the cathode of the Zener diode D 7 . The cathode of the Zener diode D 7  is connected to the voltage input terminal of the second operational amplifier U 1   b , the voltage input terminal of the HED  56  and the voltage input terminal of the manual brake control signal potentiometer P 2 . Accordingly, two of the Zener Diodes, D 4  and D 10 , that are included in the circuit  50  shown in FIG. 4 are eliminated. This not only reduces the cost of the circuit  60 , but also eliminates variation in the regulated voltage supplied to the components due to the tolerance differences from use of three Zeners.  
         [0050]    Additionally, in the circuit  60 , a minimum turn on potentiometer P 4  is connected between the manual control signal potentiometer P 2  and ground. The minimum turn on potentiometer P 4  provides an initial input signal to the positive input terminal of the first operational amplifier U 1   a . Thus, the potentiometer P 4  replaces the voltage divider  57  shown in FIG. 4 and provides an adjustable initial voltage.  
         [0051]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. For example, the isolation circuit  30  included in the actuator  10  shown in FIG. 1 also can be included in the brake controller  50  illustrated in FIG. 4.