the present invention provides a rotational control knob to operate a power controller which provides a duty cycle - controlled power output . fig1 is a schematic representation of an embodiment of the present invention in which a power control system 10 is provided for an electric cooktop 12 . the power control system 10 includes a power unit 14 and a plurality of user interface units 16 , 16 s . the user interface units 16 , 16 s are connected to the power unit 14 by a communication bus 18 and the power unit 14 is connected to individual heating elements 20 of the cooktop . the heating elements 20 are electrically resistive and are heated by current flowing through them . the power unit 14 includes an electronic controller for controlling power output to the heating elements 20 . further , the power unit 14 is connected to an electronic oven control unit 22 . the oven control unit 22 controls various operations of an oven ( not shown ), including the initialization of an oven cleaning cycle . the oven control unit 22 communicates bi - directionally with the power unit 14 via a two - line oven control communication bus 23 for synchronizing certain operations between the operation of the oven by the oven control unit 22 and the operation of the cooktop heating elements 20 by the power unit 14 . specifically , by way of the oven control communication bus 23 , the power unit 14 is capable of instructing the oven control unit 22 to lockout or prevent the initiation of a cleaning cycle or other operation when one or more of the heating elements 20 are in use . likewise , the oven control unit 22 is capable of instructing the power unit 14 to lockout the powering of any cooktop heating element 20 , such as when a cleaning cycle has been initiated or after a lockout button has been pressed . as used herein , the term “ lockout ” refers generally to the disabling of control or operation of some aspect of the power control system 10 . each user interface unit 16 , 16 s includes a potentiometer 24 , 24 s and a power level display 26 , 26 s . each master user interface unit 16 further includes an electronic controller 28 . a knob is attached to manually control the rotation of the potentiometer 24 , 24 s . the potentiometer 24 , 24 s acts as a rotational control input device . an angular position of the potentiometer 24 , 24 s , and thus the knob , is determined by the electronic controller 28 based upon known values representing the relationship between angular position and potentiometer resistance . the angular position is communicated to the power unit 14 via the communication bus 18 . display information is communicated by the power unit 14 back to the electronic controller 28 via the communication bus 18 . it is contemplated that other variably resistive devices , such as rheostats , or other analog input means can be substituted for the potentiometers 24 , 24 s according to the present invention . each electronic controller 28 controls its respective display 26 , 26 s based upon the display information received from the power unit 14 . each power level display 26 , 26 s is a two - digit seven - segment light - emitting diode ( led ) display for indicating a power level or setting based on a level chosen by the user using the respective potentiometer 24 , 24 s . the power level is displayed on the display 26 , 26 s as “ lo ” indicating the lowest setting , “ hi ” indicating the highest setting , or as a number from 1 . 0 to 9 . 0 in predetermined increments , indicating an intermediate setting . a larger number indicates a higher level of power . the power level display 26 , 26 s is also used for displaying other messages , as further explained herein , including warning messages and error codes . it is contemplated that other types of digital displays can be substituted for the two - digit led display 26 , 26 s , such as a liquid crystal displays ( lcds ), plasma displays , mechanical displays , cathode ray tubes ( crts ), vacuum fluorescent displays ( vfds ), discrete leds , discrete leds arranged in a clock - like fashion , led bar graphs , and the like . the display 26 , 26 s is also used in the present embodiment to display a visual indication that the respective heating element 20 has been locked out of operation by displaying “--”. the oven control unit 22 includes a buzzer or other audible warning device to emit an audible warning . further , using the oven control communication bus 23 , the power unit 14 can instruct the oven control unit 22 to emit an audible warning tone when a user attempts to operate the heating elements 20 that have been locked out . thus , the power unit 14 can cause an audible tone to be generated without requiring a separate audible warning device to be provided to the power unit 14 . in fig1 a , a simple control scheme is illustrated by way of example . the power output to a heating element 20 ′ is controlled by turning a respective potentiometer 24 ′ through its entire or full range of angular rotation . a small segment or range of the angular rotation is used to turn the heating element 20 ′ completely off . the potentiometer 24 ′ is provided with a physical detent , or other tactile indication or the like , to indicate when the “ off range ” is correctly engaged the term “ single potentiometer ” is used herein with reference to a potentiometer operating to control a single heating element over the potentiometer &# 39 ; s entire range , such as the potentiometer 24 ′ shown in fig1 a . in the embodiment of fig1 , the user interface units 16 , 16 s are provided in pairs consisting of a master unit 16 and a slave unit 16 s . the potentiometer 24 s and the display 26 s of the slave unit 16 s are connected to the controller 28 of the master unit 16 . the master unit 16 communicates with the power unit 14 for both user interface units 16 , 16 s via the communication bus 18 . the power unit 14 also delivers pulse width modulated output current to each heating element 20 . the power unit 14 controls current and / or voltage to each heating element 20 to produce the desired output power to power the heating elements 20 . the duty cycle of the output current delivered to each heating element 20 is determined by the angular position of a respective one of the potentiometers 24 , 24 s . duty cycle is expressed as a ratio of current on - time to the period ( sum of current on - time and off - time ). as explained above , the power level provided to each heating element 20 is displayed on the respective power level display 26 , 26 s . in the embodiment of fig1 , the output power provided to the heating elements 20 is fixed as 240 vac , which would typically be provided from two - phase utility power . it should be appreciated that maximum output power is equal to the maximum output voltage multiplied by the unmodulated output current . thus , it is contemplated that the voltage of the output power could also be modulated , in addition to the duty cycle of the current , by the power unit 14 to control the output power . for example switching from 240 vac to 120 vac , by utilizing a single phase of the two - phase utility power , could be used to provide additional control , especially for achieving lower power outputs . for a single potentiometer , such as in the example of fig1 a , the relationships between angular position , display information and output power are determined according to table 1 , below . the output power is expressed as a percentage of maximum output power , or the duty cycle times 100 percent . since the power level is controlled electronically , the relationship between the potentiometer angular position and the power output can be non - linear and even non - uniform such that the relationship cannot be expressed as an equation . for example , the power level is incremented in steps of 0 . 2 from 1 . 0 to 3 . 0 and in larger steps of 0 . 5 from 3 . 0 to 9 . 0 . this allows more control in the lower heating ranges , which is useful for cooking and keeping food warm . turning the potentiometer to above 330 degrees and below 30 degrees , in the off range , turns the power completely off . as referred to herein , zero degrees is at a 12 o &# 39 ; clock position on the potentiometer and succeeding degrees are measured in a clockwise fashion . alternatively , as embodied in the various alternative control schemes of fig3 - 8 , one potentiometer can be used to control two or more power outputs , and thus two or more heating elements . a potentiometer being used in this way is referred to herein as a “ dual potentiometer .” according to this alternative embodiment of the present invention , one portion of the total angular rotation of a dual potentiometer controls power to a first element and the other portion of the angular rotation controls power to both the first element and a second element . table 2 , below , illustrates the operation of a dual potentiometer according to this alternative control scheme . the specific numbers or values shown in tables 1 and 2 are given by way of example and can be modified as appropriate to meet the needs of a particular application . fig2 is a plot of potentiometer position versus duty cycle ( in percent of maximum power ) as embodied by the control schemes of tables 1 and 2 above . as set forth in tables 1 and 2 , each “ potentiometer position ” relates to an angular range of potentiometer rotation . thus , although the potentiometer rotates smoothly throughout its range , the duty cycle is controlled in discrete steps corresponding to the specific ranges of potentiometer rotation set forth in tables 1 and 2 . the minimum duty cycle of the present embodiment is 1 %, as shown in fig2 . fig3 shows another embodiment in which a dual potentiometer 124 is arranged to control a dual heating element 120 , having concentrically arranged inner heating element 120 b and outer heating element 120 a . the left portion 124 l of the angular rotation of the dual potentiometer 124 , from 190 to 330 degrees , controls power to the inner heating element 120 b only , and the right portion 124 r of the angular rotation of the dual potentiometer 124 , from 170 to 30 degrees , controls both heating elements 120 a , 120 b simultaneously . fig4 shows another embodiment using a dual potentiometer 224 a to control a single heating element 220 a and a separate bridge heating element 220 b . the bridge heating element 220 b provides heating between the single heating element 220 a and a second heating element 220 c spaced apart from the single element 220 a . the dual potentiometer 224 a operates similarly to the dual potentiometer 124 a of the embodiment of fig3 . specifically , the left portion 224 a l of the angular rotation of the dual potentiometer 224 a controls power to the single heating element 220 a only , and the right portion 224 a r of the angular rotation of the dual potentiometer 224 a , controls both the single heating element 220 a and the bridge element 220 b simultaneously . power to the second single heating element 220 c is controlled by a single potentiometer 224 b . fig5 shows an embodiment using two potentiometers 324 a , 324 b to control three heating elements : two single heating elements 320 a , 320 c and a bridge heating element 320 b . the first potentiometer 324 a controls the first single heating element 320 a around its entire angular rotation 324 a 1 . the second potentiometer 324 b is a “ modified single potentiometer ,” wherein 324 b controls the second single heating element 320 c over most of its angular rotation 324 b m , except that a small range 324 b b of the angular rotation is used to enable bridge control . a physical detent , or the like , indicates that the second potentiometer 324 b is set on the bridge control range 324 b b . when bridge control is enabled by the second potentiometer 324 b , the first potentiometer 324 a simultaneously controls all three heating elements 320 a - c over its entire angular rotation 324 a 2 . this allows all three heating elements 320 a - c to be easily and accurately set to the same power level . fig6 shows an embodiment which uses principles from both the embodiment of fig4 and the embodiment of fig5 . like the embodiment of fig5 , a second potentiometer 424 b , being a modified single potentiometer , controls only a second single heating element 420 c over most of its angular rotation 424 b m and places the first potentiometer 424 a in bridge control mode at a bridge control range 424 b b . the first potentiometer 424 a of fig6 is a dual potentiometer and operates much like the first potentiometer 224 a of fig4 , controlling the first heating element 420 a over the left portion of rotation 424 a l 1 and controlling both the first heating element 420 a and the bridge heating element 420 b over the right portion 424 a r 1 of angular rotation . when the first potentiometer 424 a of fig6 is placed in bridge mode by the second potentiometer 424 b , the first potentiometer 424 a controls all three heating elements 420 a - c over either portion 424 a l 2 , 424 a r 2 of its angular rotation . fig7 is a variation on the embodiment of fig6 . the first potentiometer 524 a normally acts as a dual potentiometer , independently controlling the first heating element 520 a over its left portion 524 a l and controlling both the bridge element 520 b and the first heating element 520 a over its right portion 524 a r . when bridge control is enabled , the first potentiometer 524 a acts as a single potentiometer . that is , when the second potentiometer 524 b , being a modified single potentiometer , is placed in its bridge range 524 b b , the first potentiometer 524 a controls all three heating elements 520 a - c over its entire range 524 a e of angular rotation . this provides more precise control of power than the scheme of fig6 . fig8 is an additional embodiment for controlling two single heating elements 620 a , 620 c and a bridge heating element 620 b . first and second potentiometers 624 a , 624 b are both dual potentiometers . the first potentiometer 624 a controls the first single heating element 620 a over the left portion 624 a l of its angular rotation and controls both the first single heating element 620 a and the bridge heating element 620 b simultaneously over the right portion 624 a r of its angular rotation . the second potentiometer 624 b controls the second single heating element 620 c over the right portion 624 b r of its angular rotation and controls all three heating elements 620 a - c simultaneously over the left portion 624 b l of its angular rotation . when the second potentiometer 624 b is controlling all three heating elements 620 a - c , the first potentiometer 624 a is disabled from controlling any of the heating elements 620 a - c . referring again to fig1 , thermal limiters 30 are provided to prevent the heating elements 20 from overheating and potentially causing damage , such as when the heating elements 20 are covered by a flat glass cooking surface . each limiter 30 comprises two bimetallic thermostatic switches or limiter elements : a high temperature switch and a low temperature switch . the high temperature switch in each limiter 30 is connected directly to a corresponding heating element 20 . the high temperature switch opens at temperatures above t hi , such as 500 degrees celsius , thus disconnecting power from the heating element 20 . once the heating element 20 cools below t hi , the high temperature switch closes , reconnecting power to the heating element 20 . it is contemplated that the high temperature switch could be connected in a different manner , for example by being connected via the controller of the power unit 14 rather than directly to the heating element 20 . the low temperature switch in each limiter 30 is connected to the power unit 14 . the low temperature switch opens when the temperature falls below t lo , such as 50 or 70 degrees celsius . when the low temperature switch is closed , the power unit 14 causes a heat warning to be displayed on the seven - segment power level display 26 , 26 s , such as “ he ” for element , “ hs ” for hot surface , “ hc ” for hot cooktop , or other appropriate display , indicating that the cooking surface at the respective heating element 20 is too hot to touch . alternatively , a warning lamp or indicator could be used to display the heat warning . as a further alternative , the low temperature switch or limiter element can be replaced by a timing mechanism which causes the heat warning to be displayed for a predetermined period of time , after which the respective heating element 20 should have predictably fallen below t lo . the timing mechanism can be implemented by the electronic controller of the power unit 14 , or by some other known means . nonvolatile memory , such as an eeprom , can be provided to the power unit 14 to retain timing information in the event of a power failure . fig9 illustrates a communication and power connection arrangement according to an embodiment of the present invention including a power board 714 and two master user interface units 716 l , 716 r . communication between the master user interface units 716 l , 716 r and the power board 714 is accomplished by a one wire serial communication bus or wire 718 provided in a wiring harness 730 . in addition to the communication wire 718 , the 5 - wire harness 730 also includes + 12 vdc , ground , + 5 vdc , and an identification wire . with the exception of the identification wire , each of the 5 wires is connected from the power unit 714 to each of the master user interface units 716 l , 716 r . the identification wire 732 carries a + 5v identification signal from the power unit 714 to the right master user interface unit 716 r , telling the unit 716 r that its position is “ right .” since there is no connection between the identification wire 732 and the left master user interface unit 716 l , the unit 716 l will not receive the identification signal , causing the unit 716 l to identify its position as “ left .” it should be appreciated that the “ right ” and “ left ” positions can be transposed without departing from the present invention . potentiometer angle information from a master interface unit 716 l , 716 r or a slave user interface unit 716 ls , 716 rs is digitally encoded by the microprocessor in the respective master user interface unit 716 r , 716 s and sent to the power unit 714 via the communication bus 718 , similarly to that described above with reference to fig1 . likewise , digital display information is sent from the power unit 714 to the user interface units 716 l , 716 r via the communication bus 718 . an identification code is included in each communication to identify the sender or recipient user interface unit as the left master unit 716 l , the left slave unit 716 ls , the right master unit 716 r , the right slave unit 716 rs . the identification code also indicates whether the corresponding potentiometer is being used as a single or dual potentiometer , whereby the power board 714 controls the user interface unit 716 and its corresponding heating element according to the appropriate set of data , as exemplified in tables 1 and 2 . it should be evident that this disclosure is by way of example and that various changes may be made by adding , modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure . the invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited .