Device for activating trailer electric wheel brakes

An electric trailer brake control device includes a delay circuit to prevent actuation of the trailer brakes when the towing vehicle hazard flasher is actuated.

BACKGROUND OF THE INVENTION
 This invention relates in general to devices for actuating trailer electric
 wheel brakes and in particular to enhancements for trailer electric wheel
 brake actuators and controllers.
 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.
 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.
 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.
 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.
 Also known in the art are more sophisticated electric 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 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's braking system or the pressure applied by the vehicle operator's
 foot to the towing vehicle'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.
 Known electronic 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.
 Electronic 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'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's.
 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
 This invention relates to enhancements for trailer electric wheel brake
 actuators and controllers.
 As explained above, electric wheel brake actuators energize the towed
 vehicle brakes upon actuation of the towed vehicle brakes. Typically, the
 stop light switch in the towed vehicle is electrically coupled to the
 actuator. Upon closure, the stop light switch supplies a signal to the
 actuator to cause the actuator to energize the towed vehicle brakes.
 Towing vehicles also include a hazard flasher which causes the vehicle
 brake lights to flash as a warning of a hazardous condition to other
 vehicle operators. While hazard flashers are typically used when the
 towing vehicle is stopped, towing vehicle operators will also actuate the
 hazard flasher while the towing vehicle is in motion. An example is during
 operation at a reduced rate of speed, such as during a hill climb.
 However, because of common wiring, the hazard flasher can supply a signal
 to the brake actuator which causes the actuator to energize the trailer
 wheel brakes as the brake lights flash. Accordingly, it would be desirable
 to prevent operation of the brake actuator when the hazard flasher is
 operating.
 Electric wheel brake actuators also typically include a manual brake
 control which can be used by the towing vehicle operator to apply the
 trailer brakes independently of the towing vehicle brakes. If the towing
 vehicle brakes are then applied, the actuator will respond to the greater
 of actuator generated brake signal or the manual brake signal. If the
 actuator gain is set at too high a value, the actuator generated brake
 signal may overpower the manual brake signal, causing an over application
 of the trailer wheel brakes. Accordingly, it also would be desirable to
 disable the portion of the actuator which generates the brake signal when
 the manual brake control is being used.
 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. The device also includes an output
 signal generator which is connected to the brake control signal generator.
 The output signal generator has an output terminal and is responsive to
 the 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. A delay circuit is connected to the output
 signal generator and is operative to delay generation of the brake control
 signal until a predetermined time period elapses following closing of the
 stop light switch. The predetermined time period is selected to be greater
 than the on-time of the vehicle hazard flasher.
 In the preferred embodiment, the delay circuit includes a resistor and a
 delay capacitor adapted to be connected between the stop light switch and
 ground with the output signal generator coupled to the delay capacitor.
 Accordingly, the first predetermined time period is a function of a delay
 time constant defined by the resistor and the delay capacitor.
 Additionally, the delay circuit includes a device for discharging the
 delay capacitor when the stop light switch is opened.
 The invention also contemplates a manual brake control signal generator and
 an isolation circuit connected to the manual brake control signal
 generator and the brake control signal generator. The isolation circuit is
 operative to disable the brake control signal generator when the manual
 brake control signal generator is actuated by the vehicle operator.
 In the preferred embodiment, the isolation circuit includes a switching
 device adapted to be connected between the stop light switch and the brake
 control signal generator. The switching device also is connected to the
 manual brake control signal generator. The switching device is in a
 non-conducting mode when the manual brake signal generator is actuated and
 in a conducting mode when the manual brake control generator is not
 actuated.
 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.
 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 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 U1c which generates a brake
 control signal at its output terminal when the towing vehicle brakes are
 applied. The input operational amplifier U1c has a positive input terminal
 which is connected through an input resistor R26 to a towing vehicle stop
 light switch 15. A ramp capacitor C4, the purpose for which will be
 explained below, is connected between the positive input terminal of the
 operational amplifier U1c and ground.
 The actuator 10 further includes first and second operational amplifiers,
 which are identified by the designators U1a and U1b, respectively. The
 output terminal of the input operational amplifier U1c is connected to a
 positive input terminal of the first operational amplifier U1a. Thus, the
 brake control signal is applied to the positive input terminal of the
 first operational amplifier U1a. The first operational amplifier U1a also
 has a negative input terminal which is connected to an output terminal of
 the second operational amplifier U1b. The first and second operational
 amplifiers U1a and U1b 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 U1a. In the preferred
 embodiment, the threshold voltage is approximately two volts. The PWM
 output signal has a duty cycle which is proportional to the magnitude of
 the brake control signal.
 The output terminal of the first operational amplifier U1a is connected to
 the base of a driver transistor Q4. The collector of the driver transistor
 Q4 is connected to the gate of an output power Field Effect Transistor
 (FET) Q1. The power FET Q1 is connected between the towing vehicle power
 supply 16 and the towed vehicle electric brake coils 18 (one shown). An
 actuation transistor Q5 is connected between the emitter of the driver
 transistor Q4 and ground. The actuation transistor Q5 has a base terminal
 connected through an actuation Zener diode D3 and a series connected pair
 of resistors, R19 and R35, 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 Q5 to be in a conducting state and
 thus enables the control of the output FET Q1 by the driver transistor Q4.
 When the stop light switch 15 is closed, the ramp capacitor C4 charges
 through the input resistor R26 with a time constant which is a function of
 the product of the ramp capacitor C4 and the input resistor R26.
 Accordingly, as the stop light switch 15 remains closed, an increasing
 voltage is applied to the positive input terminal of the input operational
 amplifier U1c. In response to the increasing voltage, the operational
 amplifier U1c generates an increasing ramped brake control signal which is
 applied to the positive input terminal of the first operational amplifier
 U1a. The first and second operational amplifiers U1a and U1b 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 Q4.
 The driver transistor Q4 is responsive to the output signal to switch the
 power FET Q1 between its non-conducting and conducting states with the
 duration of the conducting states increasing as the ramp capacitor C4
 charges. As the power FET Q1 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 R26 and ramp capacitor C4.
 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'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 C3 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 C3 also is connected through a coupling diode D6 to
 the positive input terminal of the first operational amplifier U1a. The
 first voltage divider 21, which includes a pair of resistors, R27 and R28,
 is connected between the collector of an isolation transistor Q7 and
 ground. The isolation transistor Q7 has an emitter connected through a
 plurality of diodes, D13, D14 and D15, to the stop light switch 15. As
 will be explained below, during normal operation of the actuator 10, the
 isolation transistor Q7 is in its conducting state.
 The hazard delay circuit 20 also includes a second voltage divider 22,
 which includes a pair of resistors, R29 and R30, connected between the
 collector of the isolation transistor Q7 and ground. The center tap of the
 second voltage divider 22 is connected to the base of a discharge
 transistor Q6. Thus, the second voltage divider 22 functions to bias the
 discharge transistor Q6. The emitter of the discharge transistor Q6 is
 connected through a first discharge diode D11 to the center tap of first
 voltage divider 21 and thereby to the non-grounded first end of the delay
 capacitor C3. The emitter of the discharge transistor Q6 also is connected
 through a second discharge diode D7 to the positive input terminal of the
 input operational amplifier U1c and thereby to the non-grounded side of
 the ramp capacitor C4.
 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 C3 and the ramp capacitor C4 are discharged. Also, the base of
 the discharge transistor Q6 is at ground potential, which causes the
 discharge transistor Q6 to be in its conducting state. Accordingly, when
 the stop light switch 15 is closed, as shown at t.sub.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 Q6 which causes the transistor Q6 to switch to
 its non-conducting state, blocking current flow through the first and
 second discharge diodes D11 and D7. The delay capacitor C3 proceeds to
 charge through the resistor R27 to a voltage level determined by the ratio
 of the resistors in the first voltage divider 21, as shown in the curve
 labeled "C3" in FIG. 2. Simultaneously with the charging of the delay
 capacitor C3, the ramp capacitor C4 charges through the input resistor R26
 causing the input operational amplifier U1c to generate a ramped brake
 control signal, as shown by the curve labeled "RAMP" in FIG. 2. Both the
 voltage across the delay capacitor C3 and the ramped brake control signal,
 RAMP, generated by the input operational amplifier U1c are applied to the
 positive input terminal of the first operational amplifier U1a. As can be
 seen in FIG. 2, initially, the voltage across C3 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 C3. When the voltage across the delay capacitor C3
 increases to the predetermined threshold level, which occurs at t.sub.2 in
 FIG. 2, the first operational amplifier U1a begins to generate the PWM
 output signal which causes actuation of the towed vehicle brakes.
 In the preferred embodiment, the curve labeled C3 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.sub.3, the ramp brake control signal
 generated by the input operational amplifier U1c exceeds the voltage
 across the delay capacitor C3 and causes the duty cycle of the PWM output
 signal to ramp up to a maximum of 100 percent, which is reached at
 t.sub.4. The duty cycle remains at 100 percent until the stop light switch
 15 is released at t.sub.5. The slope of the ramp brake control signal,
 RAMP, generated by the input operational amplifier U1c is adjustable with
 the Automatic Gain Control (AGC), R8. Thus, under normal operating
 conditions, the delay capacitor C3 and ramp capacitor C4 function to
 slightly delay the application of and provide a soft turn-on to the towed
 vehicle brakes.
 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.sub.6
 and opens at t.sub.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 R27 and C3 is selected such that the difference
 between t.sub.1 and t.sub.2 is slightly greater than the on-time of the
 towing vehicle hazard flasher switch 19, which is the difference between
 t.sub.6 and t.sub.7. In the preferred embodiment, the time constant
 provides a difference between t.sub.1 and t.sub.2 which is approximately a
 half second. The ramp RC circuit comprising R26 and C4 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 C3
 charges sufficiently to initiate generation of a PWM output signal, as
 shown in the middle and lower curves in FIG. 3.
 When the actuator input voltage returns to zero, the base of the discharge
 transistor Q6 is pulled to ground, causing the discharge transistor Q6 to
 switch to its conducting state. When the discharge transistor Q6 begins to
 conduct, the delay capacitor C3 begins discharging through the first
 discharge diode D11 and the ramp capacitor C4 begins discharging through
 the second discharge diode D7 to prepare the circuit 20 for the next
 on-cycle of the hazard flasher. It will be appreciated that the discharge
 transistor Q6 and discharge diodes D11 and D7 also begin to conduct to
 discharge the delay and ramp capacitors C3 and C4 upon the stop light
 switch 15 opening at the end of a normal braking cycle.
 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 R7 which is connected between the towing vehicle
 power supply 16 and ground. The slider tap of the potentiometer R7 is
 connected to the positive input terminal of the first operational
 amplifier U1a. Movement of the potentiometer R7 from its "OFF" position
 generates a manual brake control signal which is applied to the first
 operational amplifier U1a. However, if the automatic gain control of the
 input operational amplifier U1c is set too high, an application of the
 towing vehicle brakes could cause the input operational amplifier U1c 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.
 As shown in FIG. 1, the manual brake control signal potentiometer R7 is
 ganged to a manual control potentiometer switch S1. In the preferred
 embodiment, the potentiometer R7 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 S1 is closed.
 One side of the switch S1 is connected to the vehicle power supply 16. The
 normally open contact of the switch S1 is connected through the coil of a
 relay RE1 to ground. The relay RE1 includes a set of normally open
 contacts connected between the power supply 16 and the stoplight lamp. The
 normally open contact of the switch S1 is connected to the base of the
 isolation transistor Q7, the second operational amplifier U1b and the
 vehicle stop lights (one shown).
 The operation of the manual stop light and automatic isolation circuit 30
 will now be described. During normal operation, the switch S1 is open,
 causing the base of the isolation transistor Q7 to be at ground potential.
 Accordingly, the isolation transistor Q7 is normally in its conducting
 state which allows power to flow from the stop light switch 15 to the
 delay and ramp capacitors, C3 and C4. However, upon movement of the slider
 of the manual brake control signal potentiometer R7 to generate a manual
 brake control signal, the switch S1 is closed. When the switch S1 closes,
 a voltage is applied to the base of the isolation transistor Q7 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 Q7 in a non-conducting state, the delay and ramp capacitors, C3
 and C4, 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 U1c to generate a brake control
 signal. As described above, closure of switch S1 supplies power to the
 second operational amplifier U1b which enables the generation of a PWM
 output signal from the first operational amplifier U1a 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).
 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 R11, R12 and R13 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 Q1. As described above, the power output FET Q1 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 Q1 is connected thorough a resistor R16 to a base terminal of a
 first sensor transistor Q2. The first sensor transistor Q2 has an emitter
 terminal connected to the power supply 16 and a collector terminal
 connected through a sensor capacitor C2 to ground.
 The collector terminal of the first sensor transistor Q2 also is connected
 to a bias circuit 42 comprising a pair of resistors, labeled R17 and R33,
 connected in series. The center tap of the bias circuit 42 is connected to
 the base of a second sensor transistor Q3. The emitter of the second
 sensor transistor Q3 is connected to ground while the collector of the
 second sensor transistor Q3 is connected through a blocking diode D8 to
 the positive input terminal of a first operational amplifier U1a. The
 blocking diode D8 blocks current from flowing back to the first
 operational amplifier input terminal from the current limiting circuit 40.
 The operation of the current limiting circuit 40 will now be described.
 When the output FET Q1 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 Q2 begins to conduct which
 causes the sensor capacitor C2 to begin to charge. It will be appreciated
 that the load current flowing through the output FET Q1 fluctuates as the
 PWM output voltage switches the FET Q1 between its conducting and
 non-conducting states. Accordingly, the current flowing to the sensor
 capacitor C2 also fluctuates. The sensor capacitor C2 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 C2 is applied to the base of the second sensor transistor Q3,
 which turns on and thereby reduces the brake control signal applied to the
 positive input terminal of the first operational amplifier U1a. 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 Q1 and, thereby, reduces the load current supplied
 to the electric trailer brake coils 18.
 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 Q2 and Q3 and thereby
 progressively reducing the duty cycle of the PWM output voltage. Upon the
 load current reaching a second predetermined threshold, the second
 transistor Q3 becomes fully conducting, providing a direct connection
 between the positive input terminal of the first operational amplifier U1a
 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 Q1 and providing short circuit protection for
 the actuator 10. In the preferred embodiment, the sensor transistors Q2
 and Q3 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 Q1
 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 C2 and/or the resistors R17 and R33.
 Upon shut off of the output FET Q1, the first sensor transistor Q2 also is
 shut off as the current flow though the current sensor 41 stops. The
 sensor capacitor C2 then begins to discharge through the bias resistors
 R17 and R33. As the sensor capacitor C2 discharges, the conduction of the
 second sensor transistor Q3 is progressively reduced, allowing the voltage
 at the positive input terminal to the first operational amplifier U1a to
 increase. In the preferred embodiment, the time constant for the
 combination of the sensor capacitor C2 and the resistors R17 and R33 is
 selected such that, for brake currents in excess of 20 amps, the sensor
 capacitor C2 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 Q1 is greatly reduced. The invention
 also contemplates using power FET's having a lower internal resistance
 than in prior art controllers to further reduce heating and associated
 power losses.
 The invention further contemplates that the brake actuator 10 includes a
 plurality of the voltage regulation diodes labeled D13, D14 and D15 which
 are connected between the stop light switch 15 and the positive input
 terminal of the input operational amplifier U1c. The regulation diodes D13
 thorough D15 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.
 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 R27 and R28 in the first voltage
 divider 21 to assure that the actuator 10 has a consistent turn on duty
 cycle for the PWM output signal.
 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 U1a. As described above,
 the first operational amplifier U1a cooperates with a second operational
 amplifier U1b to generate a PWM output signal for controlling the output
 power FET Q1. The PWM output signal has a duty cycle which is a function
 of the brake control signal.
 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 Q1 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 Q1 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.
 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 D4, is connected between the voltage input terminal of
 the second operational amplifier U1b and ground. The first zener diode D4
 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
 D7, is connected between the voltage input terminal of the HED 56 and
 ground. The second zener diode D7 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 D10, is connected between the voltage input
 terminal of the manual brake control signal potentiometer P2 and ground.
 The third zener diode D10 functions to regulate the voltage supplied to
 the potentiometer P2 and thus prevent overloading the potentiometer P2
 while assuring generation of consistent manual brake control signals. A
 fourth zener diode D3 is connected between the stop light switch 15 and
 the base of the actuation transistor Q5. The fourth zener diode D3
 provides a threshold voltage which must be exceed before the output power
 FET Q1 can be turned on. It will be noted that the fourth zener diode D3
 also is included in the improved actuator circuit 10 shown in FIG. 1.
 Additionally, the zener diodes, D3, D4, D7 and D10 are selected to have a
 positive temperature coefficient to prevent a temperature increase from
 decreasing the duty cycle of the PWM output signal.
 The invention also contemplates the inclusion of a damping capacitor C13
 which is connected between the output terminal and the negative input
 terminal of the input operational amplifier U1c. The damping capacitor C13
 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 C13 is a 1.0
 micro-farad capacitor. Damping can be further increased by connecting an
 optional second damping capacitor C14 in parallel to the damping capacitor
 C13, as shown in FIG. 4.
 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 U1a. The voltage divider 57 includes a pair of
 resistors R27 and R28 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
 U1a 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 R27 and
 R28, 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 R27 and R28 in the voltage
 divider 57 to assure that the controller 50 has a consistent turn on duty
 cycle for the PWM output signal.
 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.