Electronic thermostat

A silicon junction diode is used as one element of a resistance bridge to rm the temperature sensing element of an electronic thermostat. Current through the diode is maintained at approximately constant value so that the negative temperature coefficient characteristic of the diode can be utilized. The values of the fixed resistors in the bridge are selected so as to provide a null at a preselected temperature. Used in conjunction with a voltage comparator circuit having positive feedback, a digital control signal is provided whenever the temperature of the sensing element rises above the preselected null value. The addition of a single NOR gate allows the combination of this digital control signal with an external control signal.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of electronic 
temperature sensing and control and more particularly to temperature 
sensing by the use of electrical bridge networks. More particularly, the 
present invention relates to the combination of temperature sensing 
electrical bridge networks with circuitry providing digital control 
signals used to actuate machinery or equipment. 
Thermostats used to control the operation of machinery and equipment have 
generally been designed around a mechanical means for temperature sensing 
This mechanical means usually takes the form of a bimetallic strip 
configured in such a manner that expansion or contraction of the strip 
would trigger a mechanical switch. With repeated industrial use, the 
contacts of such switches tend to deteriorate and malfunction. Applicant's 
electronic thermostat however contains no mechanical parts to deteriorate 
or malfunction and provides increased stability dispite repeated shock, 
vibration, and aging. Because the controls for most machinery and 
equipment now use standard digital logic levels, applicant's electronic 
thermostat has been designed to provide a standard logical "on" signal 
when the temperature of the sensor rises above a preselected value. 
Although resistor bridge networks and even those including one of more 
diodes have been used for the linear measurement of temperature, such 
diode bridge networks have not been combined with the necessary circuitry 
to produce the digital control signals necessary to produce an electronic 
thermostat. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides an electronic thermostat 
capable of generating a digital control signal. This control signal is 
provided by combining a temperature sensing resistive bridge utilizing the 
negative temperature coefficient property of a silicon junction diode with 
a voltage comparator circuit and NOR gate in a positive feedback 
arrangement. This arrangement provides for a digital control signal to be 
generated and latch "on" in the presence of a temperature exceeding some 
predetermined threshold value. The amount of hysteresis in the On-Off 
switching points may be controlled by choice of a feedback resistor value. 
Furthermore, the use of a NOR gate and positive feedback allows the 
control signal generated by this electronic thermostat to be combined with 
a control signal from some external source, such as a time delay circuit. 
This combination allows applicant's invention to be used in situations 
requiring the coincidence of two or more events before switching is to 
occur. Normally one of these events will be a temperature rising above 
some preselected value and the other will be a period of time having 
elapsed. 
The temperature sensing element is a resistive bridge having four legs, one 
of which is a silicon junction diode. The diode is forward biased with an 
approximately constant current source so that the voltage drop across it 
is a function of temperature. The values of the constant resistors in the 
other three legs are chosen to produce a bridge null at a predetermined 
temperature. The opposite nodes bridge outputs are coupled to the positive 
and negative inputs of a voltage comparator so that the output of the 
comparator will become positive when the voltage at the positive input of 
the comparator exceeds the voltage at its negative input. A resistor is 
connected between the output of the voltage comparator and its positive 
input port to produce positive feedback and a latching effect. 
To combine the control signal generated by this electronic thermostat with 
an external control signal, a NOR gate is interposed between the output of 
the voltage comparator and the feedback resistor. In order to correct 
polarity, the feedback is connected in this configuration to the negative 
input port of the voltage omparator. The voltage comparator output will 
serve as one input to this NOR gate and the external control signal, 
typically from a time delay circuit, will serve as the second input to 
this gate. The gate's output will then register "on" only when both the 
external control signal and the control signal generated by the 
temperature bridge in combination with the voltage comparator are present. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an electronic 
thermostat. 
A further object of the present invention is to provide an electronic 
thermostat using no moving parts. 
Another object of the present invention is to provide an electronic 
thermostat capable of withstanding shock, vibration, and aging. 
Still another object of the present invention is to provide an electronic 
thermostat that will provide a digital control signal that will latch "on" 
when the temperature being measured exceeds a predetermined value. 
Yet another object of the present invention is to provide an electronic 
thermostat, the control signal from which can be combined with a control 
signal generated externally.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference characters designate 
like or corresponding parts throughout, FIG. 1 is a schematic diagram of 
the electronic thermostat circuit according to the present invention. The 
thermostat contains a temperature sensing means 10 with a positive 
feedback voltage comparator circuit 12 for generating and latching "on" a 
digital control signal in response to a temperature rise above a 
preselected value. 
Temperature sensing means 10 is an electrical resistance bridge having four 
legs 22, 24, 26, 28 and four nodes 14, 16, 18, 20. Node 14 joins leg 26 
with 28; node 16 joins leg 22 with 24; node 18 joins leg 24 with 26; and 
node 20 joins leg 28 with 22. Input voltage is applied to the bridge at 
nodes 14 and 16 and bridge output is taken from nodes 18 and 20. 
Temperature sensing occurs by virtue of the negative temperature 
coefficient property of the silicon junction diode in leg 22 of bridge 10. 
Current through this diode is maintained at approximately constant value 
by supplying this current from a voltage source and series resistor (not 
shown), the voltage source being of much greater potential than the 
voltage drop across the diode. Current is supplied so that the diode will 
be forward biased to take advantage of its negative temperature 
coefficient. The voltage drop across the diode in leg 22 of branch 10 will 
be an inverse function of the temperature of its junction. 
After the voltage-temperature characteristic range of the diode at the 
operating current is selected, the values of the three resistors in legs 
24, 26, and 28 are selected to make the output voltage Vb1 at node 18 
equal to the output voltage Vb2 at node 20 at the desired switching 
temperature. These resistor values are calculated so that a supply voltage 
V is applied to node 14 and one half of the supply voltage (V/2) is 
applied to node 16 to produce equivalent voltage outputs at nodes 18 and 
20 at the desired switching temperature. 
A voltage comparator circuit 12 coupled to the output ports of temperature 
bridge 10 provides a digital control signal output suitable for 
interfacing with external circuitry machinery or equipment. The heart of 
voltage comparator circuit 12 is a voltage comparator 30 having positive 
and negative input port. The negative input port is coupled to node 20 of 
bridge 10 such that output voltage Vb2 will appear at this negative input 
port to the comparator. The positive input port of voltage comparator 30 
is coupled to node 18 of bridge 10 such that output voltage Vb1 will 
appear at this positive input port. A resistor 32 coupled from the output 
34 of voltage comparator 30 back to its positive input port forms a 
positive feedback path for voltage comparator circuit 12. Careful 
selection of the value of this resistance controls the hysteresis 
associated with on-off switching. The voltage comparator is powered by the 
same power supply voltage V that powers input port 14 of temperature 
bridge 10. 
In voltage comparator circuit 12 as described and depicted in FIG. 1, its 
output 34 will be high, nearly at the supply voltage V, when its positive 
input terminal is at a more positive voltage than that at its negative 
input terminal. When the voltage at its negative input port is more 
positive than at its positive input terminal, the comparator output 34 is 
at or near 0 volts. Positive feedback resistor 32 produces a fast 
comparator switching action thereby simulating the closing of the switch 
contact of a conventional bimetallic thermostat. It also provides a 
hysteresis switching effect for the combined temperature sensing bridge 10 
and comparator circuit 12. 
Referring now to FIG. 2, the configuration of the basic electronic 
thermostat is slightly altered so that its digital control output 34 can 
be combined with a digital control signal from an external source (not 
shown). A NOR gate is inserted between the output of voltage comparator 30 
and the feedback resistor 32 such that the output of voltage comparator 30 
serves as one of the two inputs 40 to NOR gate 36. The other input 38 to 
NOR gate 36 serves as the input for an external control signal typically 
from a digital time delay circuit. The output of the electronic thermostat 
in this configuration becomes the output of NOR gate 36 and feedback 
resisitor 32 couples the output of NOR gate 36 to the negative input port 
of voltage comparator 30. This more sophisticated configuration provides 
an inverted output from the configuration shown in FIG. 1 which is useful 
in implementing logic functions such as an "AND" gate made from positive 
logic "NOR" gate. 
Therefore, it is apparant that there has been provided an electronic 
thermostat capable of delivering a digital control signal whenever the 
temperature being sensed rises above a certain preselected value. The 
combination of temperature sensing bridge 10 with voltage comparator 
circuit 12 provides not only a thermostat having no moving parts, but one 
that reliably provides a fast switching hysteresis type output. 
Obviously, other embodiments and modifications of the present invention 
will readily come to those of ordinary skill in the art having the benefit 
of the teachings presented in the foregoing description and the drawings. 
It, therefore, to be understood that this invention is not to be limited 
thereto and that said modifications and embodiments are intended to be 
included within the scope of the appended claims.