Tapped variable potentiometer resistor with current sense and safety circuit

An electronic selector knob circuit (10) which uses a tapped potentiometer (POT1) to provide accurate operation without requiring factory calibration. The potentiometer (POT1) is calibrated through on-board circuitry which includes a tap voltage biasing device and which monitors operation on a continual basis through both a current monitor (16) and a voltage monitor (18). A redundant disable circuit (14) provides a redundant hardware means for shutting off all oven heating elements.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates generally to control knob circuitry and, more 
particularly, to a tapped variable potentiometer resistor circuit for a 
user actuable control knob having a current sensing capability and a 
redundancy circuit. 
Control knobs, such as those used to provide temperature selection and 
indication in a household range oven, often employ a rotary variable 
resistor which produces a voltage used to selectively energize an oven's 
heating elements in order to produce an oven temperature which corresponds 
to that selected by the user via the control knob. However, the 
potentiometer used in these control circuit devices can be slightly 
inaccurate thereby requiring factory calibration on each unit using 
microprocessor software, on-board memory, expensive factory fixturing and 
an expensive calibration process. 
The present invention provides a substantially improved control knob 
circuit which uses a tapped potentiometer to provide safe and accurate 
operation without requiring any factory calibration. The potentiometer is 
mechanically coupled to the user actuable knob and produces a voltage 
which varies in accordance with the position of the knob. The 
potentiometer tap voltage is biased enabling the knob circuit to be 
calibrated through on-board circuitry which monitors operation on a 
continual basis through interconnected current monitoring and voltage 
monitoring circuits. A disable circuit provides a redundant means for 
shutting off the oven heating elements. 
These and other features and advantages of the present invention will 
become apparent upon review of the following specification, taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the figure, the control knob circuit of the present 
invention is indicated generally at 10. While this control circuit is 
disclosed and described herein as being part of a rotary knob thermostat 
used in a household range oven, to be positioned to select a desired oven 
temperature indicated on a dial, it should become apparent that this 
circuit is equally well suited for use with other types of knobs as well 
as many other applications including control knobs for other household 
appliances such as a washer/dryer cycle setting knob. 
Circuit 10 includes four main subcircuits, the tapped potentiometer 
subcircuit 12, disable subcircuit 14, a current monitor 16 and a voltage 
monitor 18. Subcircuit 12 includes a tapped potentiometer POT1 having a 
resistor R.sub.POT and a sweeper tied to ground through resistor R1, to 
pin .mu.P2A/D of an on-board microprocessor .mu.p through a junction 20 
and a resistor R2, and to disable subcircuit 14. The microprocessor 
employed may be any suitable programmable logic device known to those 
having skill in the art and typically used in this type of application 
such as that sold under model number MN155402 by Matsushita Electric 
Industrial Company. 
Junction 20 is grounded through capacitor C1 which is also coupled through 
capacitor C2 to a 24 volt (24 V) power source. Potentiometer POT1 is 
controlled by a user through a suitable knob 21, preferably rotatably 
mounted to a control panel 23 having corresponding desired oven 
temperatures contained thereon and mechanically coupled to the sweeper of 
POT1 in a manner known to those with skill in the art. Suitable mechanical 
coupling could also be provided to operate POT1 in conjunction with a 
sliding knob or knob of any other configuration in the same fashion. 
The microprocessor, through pin .mu.P2A/D, converts a voltage produced by 
POT1 into a digital value and monitors the location of the POT1 sweeper by 
monitoring the variable voltage produced thereby. In this exemplary 
embodiment the voltage provided to the microprocessor via pin .mu.P2A/D, 
in conjunction with a sensor (not shown) which provides a signal 
indicative of the current oven temperature, is operative to control 
current flow to resistive oven heating elements or to an electrically 
actuated valve for controlling fuel flow to a gaseous burner in order to 
produce an oven temperature which corresponds with the position of the 
actuator knob as set by the user. 
The POT1 sweeper is also tied to the non-inverting input of an operational 
amplifier (op amp) U1 of subcircuit 14. The negative input of op amp U1 is 
connected to a 5 volt (5 V) source through a resistor R3 and to ground 
through resistor R4. Op amp U1 output is tied to separate disable 
circuitry (not shown) provided to turn off all relay operation, the relays 
being operative upon receipt of an electrical control signal to energize a 
heating unit in the appliance. Disable subcircuit 14 thus provides a 
hardware driven redundant shutoff circuit for turning off the broil and/or 
bake elements independent of the microprocessor. In other words, when the 
control knob is manipulated by a user to a point corresponding to an OFF 
indication, in the figure positioning the POT1 sweeper at a downwardmost 
point toward ground, the voltage at the positive input of op amp U1 
decreases to turn off the op amp and thereby cut off power to oven heating 
elements. Thus, the oven is turned off both through software via the 
microprocessor and through hardware via circuit 14. 
The current through R.sub.POT is monitored by current monitoring subcircuit 
16 to ensure POT1 does not open up. R.sub.POT is coupled to the negative 
input of op amp U2 and is powered through the output of op amp U2, through 
resistor R5. A reference voltage V.sub.REF1 is fed into the non-inverting 
input of U2 creating the drive current for POT1. The U2 output to node 26 
is also coupled through a resistor R6 and node 28, grounded through 
capacitor C3, to pin .mu.P3A/D of the microprocessor. The microprocessor 
monitors current in order to detect a breakdown due to a broken trace or 
other mechanical problem. 
The tap voltage of R.sub.POT is monitored by voltage monitor subcircuit 18 
wherein a second reference voltage V.sub.REF2 is fed to the non-inverting 
input of an op amp U3, as well as through a resistor R7 to ground. The 
output from op amp U3 is coupled to the negative input as well as through 
a resistor R8 to pin .mu.P1A/D of the microprocessor to ensure that this 
tap voltage stays within a specified range, U3 being connected as a unity 
gain follower. Pin .mu.P1A/D is also grounded through capacitor C4. U3 
biases the tap voltage, thus fixing POT1 in order to compensate for 
inaccuracies such as nonlinearities therein, as well as maintain an 
accurate mechanical vs. electrical relationship. 
Thus the control knob circuit 10 of the present invention provides a 
reliable means for selectively energizing the heating elements in a 
household range oven. A tap voltage bias eliminates the need for expensive 
factory calibration while current and voltage monitoring subcircuits 
ensure reliable operation. A redundant disable subcircuit provides an 
additional hardware means for shutting off oven heating elements in 
response to the user positioning the interconnected control knob to an OFF 
position. 
For purposes of clarity the values of the components of circuit 10 have 
been omitted from FIG. 1, but are provided in the table below. 
______________________________________ 
Resistors Capacitors Other 
______________________________________ 
R1 560 k.OMEGA. 
C1 0.1 .mu.F 
U1 LM324 
R2 10 k C2 0.1 .mu.F 
U2 LM324 
R3 39 k C3 0.1 .mu.F 
U3 LM339 
R4 1.8 k C4 0.1 .mu.F 
R5 866 
R6 10 k 
R7 560 k 
R8 10 k 
R.sub.POT 
10 k 
______________________________________ 
The foregoing discussion discloses and describes an exemplary embodiment of 
the present invention. One skilled in the art will readily recognize that 
various changes and modifications can be made thereto without departing 
from the spirit and scope of the invention as set forth in the following 
claims.