Two stage control circuit for reversible air cycle heat pump

A two-stage circuit for controlling all operational functions of a reversible air cycle heat pump unit, including room thermostat functions and defrosting functions. All user controllable functions are controlled via a remotely locatable thermostat-like control unit mounted on an inside wall of the room separate from the heat pump unit, and connected to the heat pump unit with only five conductors. A number of desirable functions and features are provided. For example, for optimum energy usage the control provides two stages in both heating and cooling modes of operation, with almost constant temperature differential between the two stages regardless of temperature setting, and almost constant hysteresis for each stage. To facilitate compliance with Government-imposed regulations where applicable, heating and cooling limits are independently adjustable without any modification whatsoever for use in those public buildings where heating and cooling must be limited.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a two stage control circuit for 
air conditioning applications and, more particularly, to a circuit for 
controlling room temperature thermostat and demand defrost functions of a 
reversible air cycle heat pump. 
While various aspects and features of the present invention are adaptable 
to a variety of air conditioner and heat pump units, the invention is 
particularly adapted for the control of an air valve heat pump wherein, to 
alternately provide heating and cooling modes of operation, indoor and 
outdoor airflow is redirected across the condensor and evaporator, rather 
than by reversing the function of the evaporator and condensor heat 
exchangers as is more conventional in heat pump practice. In particular, 
while the unit is operating in the cooling mode, outdoor air circulates in 
heat exchange relationship with the condensor, and indoor air circulates 
in heat exchange relationship with the evaporator. Conversely, during 
heating mode operation, outdoor air circulates in heat exchange 
relationship with the evaporator, and indoor air circulates in heat 
exchange relationship with the condensor. A commonly-assigned U.S. patent 
application Ser. No. 144,796, filed Apr. 28, 1980, by McCarty et al, and 
entitled "AIR VALVE HEAT PUMP" is directed to such a heat pump, to which 
reference may be had for further information. 
While various arrangements may be employed to defrost the evaporator of 
such a heat pump, a presently prefered arrangement involves a passive 
defrosting system wherein, when defrosting is required, operation of the 
refrigerant compressor is interrupted, and refrigerant pressure within the 
system is allowed to equalize, with attendant equalization of temperature. 
In this way, heat from various elements of the system is allowed to reach 
the evaporator, melting the frost therefrom. Various valving arrangements 
may be employed to hasten and augment this process. Such a defrosting 
arrangement is the subject matter of another commonly-assigned U.S. patent 
application Ser. No. 144,795, filed Apr. 28, 1980, by McCarty, now U.S. 
Pat. No. 4,285,210 and entitled "SELF-CONTAINED HEATING AND COOLING 
APATUS", to which reference may be had for further information. 
By way of example, a reversible heat pump for which the circuit of the 
present invention is particularly intended comprises a single room-sized 
unit mountable in an opening through an outdoor wall of a building, the 
single unit including all major components, namely the evaporator, 
condenser, compressor, fans, auxillary electric resistance heaters, as 
well as a major portion of the present control circuitry. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a highly-effective low 
cost control circuit for air conditioning applications generally, and more 
particularly, for a reversible air cycle heat pump of the type described 
generally above. 
It is another object of the invention to provide such a control circuit 
which accomplishes a variety of functions, but which is relatively low in 
cost. 
Briefly, by the present invention there is provided a circuit for 
controlling all operational functions of such a heat pump unit, including 
room thermostat functions and defrosting functions. All user controllable 
functions are controlled via a remotely locatable thermostat-like control 
unit mounted on an inside wall of the room separate from the heat pump 
unit, and connected to the heat pump unit with only five conductors. A 
number of desirable functions and features are provided. For example, for 
optimum energy usage the control provides two stages in both heating and 
cooling modes of operation, with almost constant temperature differential 
between the two stages regardless of temperature setting, and almost 
constant hysteresis for each stage. To facilitate compliance with 
Government-imposed regulations where applicable, heating and cooling 
limits are independently adjustable without any modification whatsoever 
for use in those public buildings where heating and cooling must be 
limited. 
Additional features of the subject control circuit are an inhibiting of all 
power functions either when the user operable mode switch is between the 
"heat" and "cool" positions, the ON/OFF switch on the remotely locatable 
unit is OFF, or the remotely locatable unit is disconnected. 
Further, evaporator demand defrost for both heating mode and cooling mode 
of operation is provided. During heating mode defrost when the evaporator 
is exposed to outside air, the evaporator fan is allowed to operate if the 
outside air temperature is above freezing, thus allowing the outside air 
to aid in the defrosting operation. Operation of the evaporator fan is 
inhibited during heating mode defrosting in the event the outside air 
temperature is below freezing, thereby avoiding additional cooling of the 
evaporator which would otherwise slow or even prevent defrosting of the 
evaporator. 
The remotely-locatable control unit, through the only five wires mentioned 
above, allows control over a number of functions, including ON/OFF, and 
independent setting of heating and cooling limits where required. 
Additionally, the remote control unit has a pair of lamps which illuminate 
to indicate whether the heat pump unit is in the heating or the cooling 
mode of operation, the mode switch itself being on the main unit as a part 
of the air valve arrangement. 
Briefly stated, and in accordance with a more particular aspect of the 
invention, there is provided a two stage control circuit for a reversible 
air cycle heat pump unit of the type having a heating mode and a cooling 
mode of operation and which includes a closed circuit refrigeration system 
having a compressor, an evaporator, and a condenser. The heat pump unit 
further includes a pair of two-speed motor driven fans for moving air over 
the evaporator and condenser respectively, and an air valve arrangement 
for alternatively circulating outdoor air flow over the evaporator and 
indoor air flow over the condenser during heating mode operation, and 
circulating indoor air flow over the condenser and outdoor airflow over 
the evaporator during cooling mode operation. The heat pump additionally 
includes at least one controllable supplemental electric resistance heater 
for warming indoor air during heating mode operation. 
In particular, the control circuit of the present invention includes a mode 
switch, preferably ganged with the air valve arrangement of the heat pump 
unit, for making predetermined electrical connections depending upon 
whether heating mode or cooling mode operation is selected. The circuit 
includes a first stage controlled switching device, such as a relay, 
connected, when activated, to energize the compressor and the fans, and a 
second stage controlled switching device, for example another relay, 
connected, when activated, to condition the fans for relatively higher 
speed operation and to activate the supplemental electric heater if 
heating mode operation is selected. 
Thus, for the first stage of either heating or cooling, the refrigerant 
compressor is energized and the evaporator and condenser fans are operated 
at low speed. When additional heating or cooling is required to maintain a 
desired temperature, the second stage of heating or cooling is activated. 
The second stage for heating mode operation involves energizing the 
supplemental electric resistance heater preferably positioned in the path 
of warm air discharged back into the room, along with switching the fans 
to their relatively higher speed of operation. The second stage for 
cooling mode operation involves switching both fans to their higher speed 
of operation. 
The control circuit additionally includes thermostatic control circuitry 
for comparing sensed indoor temperature with a temperature setting and for 
activating the first and second stage controlled switching devices 
depending upon the difference between sensed indoor temperature and the 
temperature setting. In particular, the thermostatic control circuitry is 
operable when the heating mode is selected to activate the first stage 
switching device if sensed indoor temperature is below the temperature 
setting, and to additionally activate the second stage switching device if 
sensed indoor temperature is below the temperature setting by a 
predetermined additional amount, termed the first and second stage 
"temperature differential." The thermostatic control circuitry is operable 
when the cooling mode is selected to activate the first stage switching 
device if sensed indoor temperature exceeds the temperature setting and to 
additionally activate the second stage switching device if sensed indoor 
temperature exceeds the temperature setting by the first and second stage 
differential temperature. 
In order to avoid unnecessary operation of the supplemental electric 
resistance heater during heating mode operation with attendant unnecessary 
operating expense, circuitry is included for sensing outdoor air 
temperature and inhibiting activation of the supplemental electric heater 
during heating mode operation if outdoor air temperature exceeds a 
predetermined temperature. This predetermined temperature is selected 
according to the mechanical and thermodynamic characteristics of the 
particular heat pump unit. A typical predetermined temperature is 
36.degree. F. One particular way in which activation of the supplemental 
electric heater may be inhibited is simply by inhibiting activation of the 
second stage controlled switching device during heating mode operation if 
outdoor temperature exceeds the predetermined temperature. 
The present control circuit preferably includes cooling mode demand defrost 
circuitry for recognizing excessive frost accumulation on the evaporator 
and for interrupting operation of the compressor in response to allow 
defrosting. The cooling mode demand defrost circuitry includes a sensor 
for sensing the temperature of a portion of the evaporator, and circuitry 
responsive to the evaporator sensor for inhibiting activation of the first 
stage controlled switching device when sensed evaporator temperature falls 
below a predetermined temperature, for example 32.degree. F., indicative 
of excessive evaporator frost. The circuitry again permits activation of 
the first stage controlled switching device when sensed evaporator 
temperature again rises above the predetermined temperature. 
During this cooling mode defrosting operation, operation of the second 
stage is not inhibited, and the evaporator and condensor fans are 
permitted to operate at their relatively higher speed in the event the 
room temperature thermostat control circuitry is calling for the second 
stage of cooling. The resultant circulation of indoor air over the 
evaporator aids in the defrosting operation, and hastens return to normal 
operation. 
Demand defrost circuitry for the heating mode is also provided. Since, 
during heating mode operation, the evaporator is exposed to cold outside 
air which may be well below 32.degree. F., the relatively simple demand 
defrost scheme described above for cooling mode operation cannot be 
employed. Instead, the present control circuit implements a demand defrost 
system wherein the heat exchange efficiency of the evaporator is monitored 
by determining differential temperature between the cold outside air 
entering the evaporator and the temperature of the evaporator itself. 
During operation, as a layer of frost develops on the evaporator, the 
temperature of the evaporator decreases, even though the temperature of 
the outdoor air may remain constant. 
Specifically, sensors are provided for sensing the temperature of outdoor 
air entering the evaporator and for sensing the temperature of a portion 
of the evaporator. The sensor for sensing the temperature of a portion of 
the evaporator preferably is the same one employed for cooling mode demand 
defrost initiation and termination as described above. Circuitry is 
responsive to these two sensors for generating a signal to initiate 
defrosting when sensed evaporator temperature falls a predetermined amount 
below sensed outdoor air temperature, and additional circuitry is provided 
for inhibiting activation of the first stage controlled switching device 
in response to the signal to initiate defrosting. 
This inhibiting of the first stage prevents operation of the compressor, 
allowing equalization of refrigerant system pressure and temperature, thus 
providing heat for defrosting the evaporator. In the event, outdoor air 
temperature is above 32.degree. F., the evaporator fan is allowed to 
operate provided the room thermostat control is calling for the second 
stage of heating, and the above-freezing temperature outside air thus is 
permitted to aid in the defrosting operation. The same outdoor air 
temperature sensor employed for initiating heating mode demand defrost 
operation preferably is also used for this purpose. If, however, outside 
air temperature is below 32.degree. F., then operation of the evaporator 
fan is inhibited, preventing outdoor air from further cooling the 
evaporator. 
During heating mode defrost operation it is possible that the supplemental 
electric resistance heater which normally is energized during heating mode 
second stage operation may not be sufficient to warm the room. A second 
supplemental electric heater is provided and is energized during heating 
mode operation in the event activation of the first stage switching device 
is inhibited, thus compensating for the absence of heat pump operation 
during defrosting. 
While a variety of approaches may be taken for terminating heating mode 
demand defrost operation, the one which is presently preferred involves a 
sensor for sensing the presence of cold defrost water draining from the 
evaporator, and which is connected for terminating the heating mode 
defrosting operation when the flow of evaporator drain water ceases. In 
particular, this sensor is a "stopper" temperature sensor positioned in 
the path of evaporator drain water such that the temperature sensor is 
maintained at a temperature of approximately 32.degree. F. so long as 
water is flowing. When the evaporator is completely defrosted and the flow 
of cold defrost water draining therefrom accordingly ceases, this 
"stopper" temperature sensor increases in temperature, and this increase 
is recognized by the circuitry to terminate the defrosting operation. 
A number of important features and aspects of the invention involve the 
thermostatic control circuitry including the remotely locatable user 
control unit. In particular, the user control unit comprises a portion of 
the thermostatic control circuitry and is locatable remotely from a main 
portion of the control circuit, which main portion is physically located 
with the main heat pump unit. It will be appreciated, however, while more 
accurate control of room temperature results when the thermostat unit is 
remotely located from the heat pump unit, in certain applications it may 
be desired to locate the remotely locatable control unit on the unit main 
heat pump itself, and the operational characteristics remain unchanged. 
Power is supplied to the remotely locatable user control unit from the 
control circuit main portion via a pair of supply conductors, with one of 
the pair of supply conductors further subdivided into a heating mode 
select conductor and a cooling mode select conductor alternately selected 
for continuity by the mode switch ganged with the air valves of the heat 
pump unit. The remotely locatable user unit recognizes which of these 
heating and cooling mode select conductors is selected in order to vary 
the configuration of the temperature control circuitry as appropriate for 
heating or cooling mode. Additionally, heating and cooling mode indicator 
lamps located in the user control unit are operated depending upon which 
of the select conductors is selected. 
More particularly, the remotely locatable user control unit has a 
temperature setting potentiometer connected in adjustable voltage divider 
configuration across the pair of supply conductors to provide a voltage 
representative of the desired degree of heating or cooling. In the 
particular control circuit described herein, this user control bears the 
legend "More" and the setting thereof is increased in the same direction 
irrespective of whether "More" heating or "More" cooling effect is 
desired. While it will be understood and appreciated that particular 
voltage magnitudes, polarities, and directions of change are merely 
matters of design choice, in the exemplary embodiment disclosed herein, 
this voltage representative of the desired degree of heating or cooling 
increases in a positive sense when either "More" heating or "More" cooling 
is desired. 
The temperature setting potentiometer is connected in combination with 
independent heat and cool trimmers respectively selected in response to 
the heating and cooling mode select conductors and arranged to 
controllably limit user selection of the voltage representative of the 
desired degree of heating or cooling, thereby to facilitate compliance 
with Government-imposed building temperature regulations where applicable. 
The remotely locatable user control unit additionally has an indoor 
temperature sensor connected in a circuit to provide a voltage 
representative of the actual degree of heating or cooling. The indoor 
temperature sensor circuit is responsive to the heating and cooling mode 
select conductors to selectively cause the voltage representative of the 
actual degree of heating or cooling to vary either directly or inversely 
with sensed temperature to match the characteristic and configuration of 
the temperature setting potentiometer. In the particular embodiment 
described in detail herein, for heating mode operation, the voltage 
representative of the actual degree of heatin or cooling varies directly 
with sensed room temperature, and, during cooling mode operation, varies 
inversely with sensed room temperature. 
Comparator circuitry is located in the main portion of the control 
circuitry and is responsive to both the voltage representative of the 
desired degree of heating or cooling and to the voltage representative of 
the actual degree of heating or cooling to activate the first and second 
stage controlled switching devices depending upon the amount by which the 
voltage representative of the actual degree of heating or cooling falls 
short of the voltage representative of the desired degree of heating or 
cooling. It is a particular feature of the invention that, since the 
reversal in direction of voltage change is accomplished within the 
remotely locatable control unit, the comparator circuitry located in the 
main portion of the control circuit need not be reconfigured when 
switching between heating mode and cooling mode operation. In particular, 
the first and second stage switching devices are activated in the same 
manner depending upon the amount by which the voltage representative of 
the actual degree of heating or cooling falls short of the voltage 
representative of desired degree of heating or cooling, without respect to 
whether the unit is in the heating or cooling mode of operation. 
More particularly, this comparator circuitry comprises first and second 
stage comparators each having a reference input connected to receive the 
voltage representative of the desired degree of heating or cooling, and a 
comparison input connected to receive the voltage representative of the 
actual degree of heating or cooling. The first and second stage 
comparators have outputs respectively connected to the first and second 
stage controlled switching devices. The comparator circuitry additionally 
includes a biasing circuit arrangement for shifting the switching 
threshold of the first and second stage comparators with respect to each 
other to provide a temperature difference (first and second stage 
temperature differential) between the switching points of the first and 
second stage controlled switching devices. 
In an even more particular arrangement, each of the first and second stage 
comparators has a pair of series input resistors connected to the voltage 
representative of the desired degree of heating or cooling and the voltage 
representative of the actual degree of heating or cooling, and the biasing 
circuit arrangement includes relatively high resistances connected to 
cause a biasing current to flow between positive and negative supply 
conductors through an input resistor of each of the first and second stage 
comparators. This arrangement results in a relatively constant current 
flowing through the input resistors, causing the same differential voltage 
between the two stages to be present regardless of the temperature 
setting. 
User ON/OFF control functions are provided by a user ON/OFF switch included 
in the remotely locatable user control unit and connected for interrupting 
the supply conductors when switched to the OFF position. The main control 
circuit additionally includes a current detector for detecting an absence 
of current through the supply conductors to the remotely locatable user 
control unit when the user ON/OFF switch is in the OFF position, and for 
disabling operation of the first and second controlled switching devices 
in response. This particular current detecting arrangement also serves the 
desired function of inhibiting any operation if circuit continuity is 
interrupted for any reason, such as the remote unit being disconnected or 
a failed switch contact. Additionally, if the mode switch located in the 
main unit and ganged with the air valves is positioned intermediate 
between the heating mode and cooling mode positions, the same current 
detecting circuitry also desirably inhibits operation of the unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, a reversible air cycle heat pump unit generally 
designated 10 includes a refrigerant compressor 12, a refrigerant 
condenser 14 for cooling and condensing hot compressed gaseous refrigerant 
received from the compressor 12 to warm liquid refrigerant, thereby 
producing heating, and an evaporator 16 within which liquid refrigerant 
vaporizes to produce cooling. It will be appreciated that the compressor 
12, condensor 14 and evaporator 16 are all conventional elements of a 
closed circuit refrigeration system, the remaining details of which, 
including interconnections and an expansion valve, are not necessary to an 
understanding of the present invention and are not shown. Also shown are 
two two-speed motor driven fans 18 and 20 driven by respective two-speed 
electric motors 22 and 24 for respectively drawing air over the evaporator 
16 and the condensor 14. 
A main portion of the circuit of the present invention is contained within 
a box or suitable containment generally designated 25. 
The heat pump unit 10 is shown mounted through an exterior wall 26 of a 
room for which the heat pump unit 10 is provided to selectively heat and 
cool. In the particular orientation illustrated, the right hand side 28 of 
the heat pump unit 10 faces the indoors, and the left hand side 30 of the 
heat pump unit 10 faces the outdoors. 
In order to alternatively select between heating and cooling mode 
operation, an air valve arrangement alternatively circulates outdoor air 
flow 32 over the evaporator 16 and indoor air flow 34 over the condenser 
14 during heating mode operation, and circulates indoor air flow over the 
condenser 14 and outdoor air flow over the evaporator 16 during cooling 
mode operation. While it will be appreciated that a variety of air valve 
arrangements might be employed, including arrangements involving extensive 
ducting, the presently preferred form of the invention involves a pair of 
sliding doors or panels 36 and 38 mechanically interconnected such as by a 
pulley and cable arrangement (not shown) to redirect the air flow as 
required. In FIG. 1, the solid line representation of the panels 36 and 38 
shows their positioning during heating mode operation, during which the 
panels 36 and 38 permit circulation at the upper left of FIG. 1 of outdoor 
air flow 32 over the evaporator 16, and circulation at the lower right of 
FIG. 1 of indoor air flow over the condenser 14. For cooling mode 
operation, the movable panels or doors 36 and 38 are repositioned as shown 
in phantom lines so as to permit indoor air flow at the upper right of 
FIG. 1 to circulate over the evaporator 16, and outdoor air flow at the 
lower left of FIG. 1 to circulate over the condensor 14. 
This particular reversible air valve heat pump arrangement comprises at 
least a portion of the subject matter of the above-mentioned 
commonly-assigned McCarty et al patent application Ser. No. 144,796, 
entitled "AIR VALVE HEAT PUMP", the entire disclosure of which is hereby 
expressely incorporated herein by reference. The defrosting aspects of the 
reversible air cycle heat pump unit 10 are described in greater detail in 
the above-mentioned commonly-assigned U.S. patent application Ser. No. 
144,795 of McCarty, entitled "SELF-CONTAINED HEATING AND COOLING 
APATUS", the entire disclosure of which is also hereby expressely 
incorporated by reference. 
In order to provide signals to the control circuitry, hereafter described 
in detail, concerning whether the movable panels 36 and 38 are in position 
for heating mode or cooling mode operation, a mode switch, generally 
designated 40, makes predetermined electrical connections depending upon 
whether heating mode or cooling mode operation is selected. While it will 
be appreciated that a variety of electrical and mechanical switching 
arrangements may be provided, FIG. 1 in highly schematic form illustrates 
a pair of switches 42 and 44, for example of the microswitch type, 
mechanically arranged and positioned so as to be actuated when the movable 
panels 36 and 38 are positioned fully home for the respective heating mode 
and cooling mode positions. For heating mode operation as depicted in FIG. 
1, the switch 44 is actuated by the indoor panel 38, and the switch 42 is 
not actuated. For cooling mode operation, the converse is true. The 
advantage of this particular arrangement is that the circuitry may be 
arranged to completely inhibit all operation of the controlled components 
whenever the panels 36 and 38 are in an intermediate position and neither 
switch 42 or 44 is actuated. 
Also shown in FIG. 1 is at least one controllable supplemental electric 
resistance heater 46 for use during the second stage of heating mode 
operation when heat pump operation alone is insufficient to supply the 
heating needs of the room. Preferably, an additional electrical resistance 
heater 48 is provided for use during heating mode defrosting operations 
when the compressor 12 is not operating. The electrical resistance heaters 
46 and 48 are positioned for warming indoor air, preferably on the 
discharge side of the recirculating indoor air stream 34. 
Referring now to FIG. 2, there is shown power control switching circuitry, 
generally designated 50, for appropriately energizing the various 
electromechanical components shown in FIG. 1 from a pair of 120 volt AC 
supply conductors L and N. Specifically, in FIG. 2 may be seen the 
compressor 12, the evaporator fan motor 22, the condensor fan motor 24, 
and the two supplemental electric resistance heaters 46 and 48. An 
additional component shown in both FIGS. 1 and 2 is the mode switch 40. 
Specifically, in FIG. 2 may be seen a contact 44' of the FIG. 1 switch 44, 
which contact is closed during heating mode operation as illustrated for 
the purpose of enabling activation of the supplemental electrical 
resistance heaters 46 and 48 during heating mode operation. 
Two additional switches shown in FIG. 2 are a main AC power switch 52 
which, when closed, energizes an L' line from the L conductor. 
Additionally, and SPDT fan switch 54 is provided to allow the user to 
select between continuous operation of the fans 18 and 20 and automatic 
control of fan operation. 
The subject control circuit provides two stage operation for both heating 
and cooling. Correspondingly, there are provided first and second stage 
controlled switching devices which, in the illustrated embodiment, 
comprise relays having contacts K1 and K2 (the coils which actuate the 
relay contact K1 and K2 are described hereinbelow with particular 
reference to FIG. 6.) It will be appreciated, however, that other forms of 
controlled switching devices may equally well be employed, for example 
semiconductor switching elements such as thyristors. 
Each of the relay contacts K1 and K2 comprises a 4PDT contact having 
individual sections denoted A, B, C and D. The relays are shown in their 
de-energized position, with their normally-closed contacts closed and 
their normally-open contacts open. 
More particularly, the first stage controlled switching device is 
connected, when activated, to energize the compressor 12 through relay 
contacts K1-A and the fan motors 22 and 24 through the relay contacts K1-B 
and K1-C. The fan motors 22 and 24 preferably are each capable of rotation 
at both a relatively lower speed and a relatively higher speed, depending 
upon which windings are energized. Accordingly, evaporator fan motor 22 
has a winding 56 representative of those motor windings energized for the 
relatively lower speed of rotation, and a winding 58 representative of the 
motor windings energized for the relatively higher speed of rotation. 
Similarly, the condenser fan motor 24 has a relatively lower speed 
representative winding 60 and a relatively higher speed representative 
winding 62. 
The second stage controlled switching device is connected, when activated, 
to condition the fans 18 and 20 for relatively higher speed operation by 
energizing the representative windings 58 and 62 through relay contacts 
K2-A and K2-B. Additionally, the second stage relay energizes at least the 
supplemental electric resistance heater 46 through relay contact K2-D when 
activated during heating mode operation. 
In order to inhibit operation of the evaporator fan motor 22 if outdoor air 
temperature is below approximately 32.degree. F. during heating mode 
defrost operation, a relay having contacts K3 is provided. (During heating 
mode defrost operation, the relay K1 is not energized, and power for the 
evaporator fan motor 22 must come through relay contacts K3.) The coil of 
the relay K3 is described hereinafter with particular reference to FIG. 7. 
The control circuit of the present invention includes thermostatic control 
circuitry for comparing sensed indoor temperature with a temperature 
setting and for activating the first and second stage controlled switching 
devices comprising relay contacts K1 and K2 depending upon the difference 
between sensed indoor temperature and the temperature setting. 
While the detailed thermostatic control circuitry is described hereinafter 
with particular reference to FIGS. 5 and 6, it is believed that this 
circuitry and its operation will better understood in light of the 
following description of the thermostatic control characteristics with 
particular reference to FIGS. 3 and 4. 
Referring first to FIG. 3, heating characteristic curves of the two-stage 
thermostat are shown as a function of both the user operable "Setting" 
control, and as a function of a hidden trimmer which serves to limit the 
maximum temperature which may be selected by a user having access only to 
the "Setting" control. In particular, with the trimmer set for minimum 
limiting, a full control range for the heating function is available. 
However, with the trimmer set for maximum limiting, the room temperature 
setting is limited as required by Government regulations for certain 
buildings. This feature of the subject control circuit permits a single 
type of unit to be manufactured which can then readily be adjusted in the 
field to suit the particular application desired, i.e., either in a 
private home or a public building. 
In FIG. 3, the ON switching thresholds of the first and second stages of 
heating are respectively denoted by solid lines 64 and 66, and the first 
and second stage OFF switching thresholds are denoted by dash lines 68 and 
70 respectively. The characteristic lines at the high end of the 
temperature setting (towards the "More" heat end) are denoted by plain 
reference numerals, while those at the lower end of the temperature 
setting (energy save) are denoted by primed reference numerals. 
Assuming first that the trimmer is set for minimum temperature limiting, 
and the user operable "Setting" control is set at its maximum, i.e., 
calling for "More" heat, then the set of four temperatures at the upper 
left of the graph apply. 
In one example of the operation, if the room temperature is above 
91.degree. F., then no heat is required, and the heat pump unit does not 
operate. As room temperature falls, eventually the line 64 is reached 
(90.2.degree. F.) at which the first stage comes on and the relay K1 is 
activated, energizing the compressor 12 (FIGS. 1 and 2) and the fan motors 
22 and 24. Since at this point relay K2 is not yet activated, the fan 
motors 22 and 24 run at their relatively lower speed. If room temperature 
nevertheless continues to fall, eventually the line 66 (86.2.degree. F.) 
is reached, at which the second stage of heating is activated. In 
particular, the relay K2 is activated, energizing the high speed windings 
58 and 62 of the evaporator and condensor fan motors 22 and 24. 
Additionally, the supplemental electric resistance heater 46 is activated. 
At this point, assuming adequate capacity, the room temperature is 
increasing. When the temperature denoted by line 70 is reached (88.degree. 
F.), the second stage deactivates, and the circuit reverts to frist stage 
operation. Finally, when the temperature denoted by the line 68 is reached 
(91.degree. F.), the first stage is deactivated, and the system reverts to 
standby mode. 
The temperature spread between the point at which a given stage (first or 
second) switches on and switches off is known as hysteresis, and is 
desirable for control stability and necessary to prevent "short cycling" 
of the compressor 12, as is known in the art. The hysteresis between the 
first stage ON and OFF lines 64 and 68 is approximately 1.5.degree. F. The 
hysteresis between the second stage ON and OFF lines 66 and 70 is 
approximately 1.5.degree. F. 
Another system characteristic shown in the graph of FIG. 3 is the 
temperature differential between the first and second stages. During turn 
on of these stages, the differential temperature is represented by the 
distance between the first stage ON line 64 and the second stage ON line 
66, for example, 3.degree. to 4.degree. F. After the desired temperature 
is reached, the turn off temperature differential between the two stages 
is represented by the distance between the second stage OFF line 70 and 
the first stage OFF line 68, also approximately 3.degree. to 4.degree. F. 
From FIG. 3, it may be seen that as the user "Setting" control is varied 
between the limits represented by the legend "More" and the legend "Energy 
Save", the entire characteristic curve is shifted generally vertically, 
with the extreme lower limit being the point on the characteristic curves 
where the first stage OFF line 68' is at 44.degree. F. It is feature of 
the present thermostatic control that the hysterisis of each stage and the 
temperature differential between the two stages both remain relatively 
constant throughout the control range. 
The remaining variable shown in FIG. 3 is the setting of the "Trimmer" 
control which is a part of the remote user operable control unit, but 
inaccessible without removing the cover. If the "Trimmer" control is 
turned all the way to the maximum setting, meaning there is maximum 
temperature limiting, the warmest temperature which may be selected by a 
user, even with the "Setting" control turned all the way to the "More" 
limit, is approximately 69.degree. F., above which the entire system is in 
standby. The timmer, however, does not significantly affect the lower end 
of the user control setting, as may be seen. 
FIG. 4 is a similar graph depicting the thermostat cooling characteristics, 
differing primarily in that the order of temperatures along the vertical 
axis is reversed. Comparing FIGS. 3 and 4, it will be seen that a user 
setting of "More" reflects a request for either more heating or more 
cooling, as the case may be depending on whether heating mode or cooling 
mode operation is selected. 
Thus the user setting control action is quite different than is 
conventionally employed. Specifically, conventionally the user is provided 
with an adjustment lever calibrated in terms .degree.F., the temperature 
increasing as the lever is moved to the right and decreasing as the lever 
is moved to the left. During heating mode operation, the user moves the 
lever to the right where more heat is required; during cooling mode 
operation, the user moves the lever to the left when more cooling is 
desired. In contrast, in the subject system, the lever is moved in the 
same direction whether it is "more" heating or "more" cooling which is 
desired. It will be appreciated that in the particular thermostatic 
control system described herein, it is not readily convenient (although 
certainly not impossible) to calibrate the user setting control in terms 
of temperature per se. Rather, it is contemplated that the user simply 
adjust the setting control to what is comfortable. 
Referring now more particularly to FIG. 4, first stage ON and OFF lines are 
designated 72 and 74 respectively, and second stage ON and OFF lines are 
designated 76 and 78, respectively. Consistent with FIG. 3, the curves for 
the extreme "Energy Save" end of the "Setting" control range are 
designated by primed reference numerals. 
Considering, for purposes of example, operation of the thermostat control 
when the user "Setting" control is at the extreme "More" end, and the 
"Trimmer" is set for maximum limiting, the set of four temperatures at the 
upper right corner of the characteristic curves apply. If sensed room 
temperature below 76.5.degree. F., then the system is idle. If room 
temperature warms to 78.degree. F., reaching the line 72 at which the 
first stage is energized, the relay contacts K1 are activated, energizing 
the compressor 12 and the fan motors 22 and 24 on low speed. If room 
temperature continues to rise and reaches the line 76 (81.degree. F.) the 
second stage is activated and the K2 relay contacts actuate. The 
evaporator and condenser fan motors 22 and 24 are then switched to higher 
speed operation. (During cooling mode operation, neither of the 
supplemental electric resistance heaters 46 and 48 operate because the 
FIG. 2 switch contacts 44' are in the cool position and the N return for 
the heaters 46 and 48 is interrupted.) As indoor air circulates over the 
evaporator 16 to be coold and returned to the room, room temperature goes 
down. When the line 78 is crossed (79.5.degree. F.), the second stage goes 
off, returning the evaporator and condenser fan motors 22 and 24 to low 
speed operation. Finally, as the line 74 is reached (76.5.degree. F.), the 
first stage turns off, and the system again is in a idle condition. 
The remaining FIGS. 5, 6 and 7 together are an electrical schematic diagram 
of a preferred form of control circuit in accordance with the invention, 
including relay coils corresponding with the relay contacts illustrated in 
FIG. 2. 
Referring initially to FIG. 5, a low voltage power supply for the circuitry 
includes a transformer (not shown) energized from the L' and conductors of 
FIG. 2 and supplying twenty-four volts AC to a bridge rectifier 80 having 
positive and negative DC output terminals 82 and 84. The negative output 
terminal 84 is connected to a (-) line which serves as a common reference 
line for the circuitry. The positive output terminal 82 directly supplies 
a +24 VDC line for energizing the relay coils with pulsating DC. 
A low voltage regulated power supply comprises a series isolation diode 86, 
a filter capacitor 88, and an 8.2 volt Zener diode 90 with a series 
voltage dropping resistor 92 and a parallel high frequency bypass 
capacitor 94. This power supply circuit supplies +8.2 VDC to the remainder 
of the circuitry along the line so designated. 
In overview, the active portion of circuit employs a plurality of 
integrated circuit comparators 96, 98, 100, 102, 104, 106, 108, and 110. 
While a variety of integrated circuit comparators may be suitable, 
preferred comparators are those included in Motorola Type No. MC3302 
single-supply comparator integrated circuit packages, National 
Semiconductor Type No. LM3302, or equivalent. These comparators have open 
collector outputs such that the output of a plurality of individual 
comparators may be connected in parallel, sharing a single pull-up 
resistor. For clarity of illustration, the supply voltage connections to 
these comparators are not shown; it will be understood that they are 
supplied from the +8.2 VDC and the (-) lines. 
Although operated as generally as comparators and not as operational 
amplifiers, to enable positive feedback to be applied each of the 
comparators includes a pair of series input resistors connected 
respectively to the inverting (-) and non-inverting (+) inputs. An 
exemplary value for each of the input resistors throughout the entire 
circuit is 150 K ohms. Specifically, the FIG. 5 comparator 96 has input 
resistors 112 and 114, the comparator 98 has input resistors 116 and 118, 
the FIG. 6 comparator 100 has input resistors 120 and 122, the comparator 
102 has input resistors 124 and 126, the comparator 104 has input 
resistors 128 and 130, the comparator 106 has input resistors 132 and 134, 
the comparator 108 has input resistors 136 and 138, and the comparator 110 
has input resistors 140 and 142. 
For stability and noise immunity, each of the comparators of the system has 
a relatively high resistance positive feedback resistor connected between 
the comparator output and the non-inverting (+) input. Additionally, the 
hysterisis-introducing positive feedback applied to the FIG. 6 comparators 
98 and 100 provides the temperature hysterisis for each stage of the first 
and second stage thermostat switching. 
Different values of positive feedback resistors are employed for the 
various comparators in the system. The largest degree of positive feedback 
is applied to the above-mentioned thermostatic control comparators 98, in 
the form of 7.5 megohm resistors 144 and 146. A positive feedback resistor 
148 connected to the FIG. 7 comparator 108 is also 7.5 megohms. Positive 
feedback resistors 150, 152 and 154 for the FIG. 5 comparator 96 and the 
FIG. 7 comparators 104 and 106, respectively, are each 22 megohms. Lastly 
the FIG. 7 comparators 102 and 110 have 2.2 megohm positive feedback 
resistors 156 and 158. 
For maximum noise immunity, each of the comparators has a capacitor 
connected directly across its inputs. The FIG. 5 comparator 96 has an 
exemplary 0.1 MFD capacitor 160, while the remaining comparators have 1.0 
MFD capacitors collectively designated 162. 
Considering in detail the two-stage thermostatic control circuitry, the 
thermostatic control circuitry comprises a user control unit 164 (FIG. 5) 
locatable remotely from the main portion of the control circuit, and 
comparator circuitry generally designated 166 (FIG. 6) located in the main 
portion of the control circuit in the FIG. 1 box 25. 
The FIG. 6 comparator circuitry 166 is responsive to a reference voltage 
representative of the desired degree of heating or cooling conveyed along 
a SET line from the FIG. 5 remote control unit 164, and to a voltage 
representative of the actual degree of heating or cooling conveyed along a 
TEMP line from the FIG. 5 remote control unit 164. In particular, the SET 
line is applied through input resistors 118 and 122 to reference inputs of 
the two comparators 98 and 100, in this particular circuit embodiment the 
reference inputs being the comparator non-inverting (+) inputs. 
Correspondingly, the TEMP line is applied through input resistors 116 and 
120 to comparison inputs of the comparators 98 and 100 which, in this 
particular embodiment, are the comparator inverting (-) inputs. 
The outputs of the first and second stage comparators 98 and 100 have 
respective pull-up resistors 168 and 170 and are connected through 
respective current limiting resistors 172 and 174 to the bases of 
respective NPN first and second stage relay driver transistors 176 and 
178. 
The first stage switching transistor 176 drives the coil of relay K1, which 
has a freewheeling diode 180 connected in parallel, and the second stage 
switching transistor 178 drives the coil of relay K2, which has a 
freewheeling diode 182 connected in parallel therewith. The transistor 176 
and 178 bases additionally have biasing resistors 184 and 186 connected to 
the (-) common circuit reference line. 
In order to disable operation of the relay coils K1 and K2 when the control 
system is turned OFF, but AC power is still applied, the emitters of the 
first and second stage switching transistors 176 and 178 are returned to a 
switched negative conductor, denoted "-, SWITCHED", controlled by current 
detector circuitry 188 described hereinafter with particular reference to 
FIG. 5. 
One feature of the invention which may be seen from FIG. 6 is the manner in 
which nearly constant temperature differential is maintained between the 
two stages for both heating mode and cooling mode operation over a wide 
range of possible temperature settings. With the comparator circuitry 166 
as thus far described, the differential inputs (connected to the TEMP and 
SET lines) of each of the first and second stage comparators 98 and 100 
vary in common mode fashion over a wide range between +8.2 volts DC and 
the negative (-) line. In order to establish a relatively constant 
switching voltage differential between the comparators 98 and 100 and 
therefore a relatively constant temperature differential between the two 
stages, a biasing circuit arrangement, generally designated 190, is 
provided for shifting the switching thresholds of the first and second 
stage comparators 98 and 100 with respect to each other. More 
particularly, the biasing circuit arrangement 190 includes a relatively 
high resistance, for example resistors 192 and 194, connected to cause a 
biasing current to flow between the positive and negative supply 
conductors through an input resistor each of the first and second stage 
comparators 98 and 100. Specifically, the resistor 192 is connected 
between the +8.2 VDC conductor and the junction of the second stage 
comparator 100 inverting (-) input with the input resistor 120, and the 
resistor 194 is connected between the negative (-) supply conductor and 
the junction of the first stage comparator 98 inverting (-) input with the 
input resistor 116. In order to approximate a constant current source and 
to provide minimal disturbance, other than the differential, to the 
accuracy of the thermostatic temperature control, the resistors 192 and 
194 have relatively high resistance, for example, 10 megohms, which may be 
contrasted with the 150 K ohm resistance of the input resistors 116, 118, 
120 and 122. 
The current flow path as a result of the biasing circuit arrangement 190 is 
from the +8.2 VDC line, through the resistor 192, through the input 
resistor 120, through the input resistor 116, and then through the 
resistor 194 to the negative (-) supply conductor. Current flow through 
the two comparison input resistors 116 and 120 is in opposite directions, 
thus providing the required differential. While a particular form of 
biasing circuit arrangement is illustrated and described herein, it will 
be appreciated that various modifications of the precise technique may be 
employed, for example, selecting different combinations of the input 
resistors 116, 118, 120 and 122 for current biasing, as well as applying 
current biasing to only one of these resistors. 
With reference again to FIG. 5, the circuitry of the remotely locatable 
unit 164 which provides the TEMP and SET voltages to the FIG. 6 comparator 
circuitry 166 will now be described in greater detail. 
In FIG. 5 it will be seen that there are only five electrical connections 
between the remote control unit 164 and the remainder of the circuitry. 
These five connections are labeled A, B, C, D and E, and will be 
understood to be any suitable form of terminal arrangement and comprise a 
cable of sufficient length to extend between the box 25 in the main heat 
pump unit 10 and the remote control unit 164. 
Power is supplied to the remote control unit 164 from the main portion of 
the control circuit via a pair of supply conductors 196 and 198. The 
supply conductor 196 is a positive supply conductor, and is connected 
directly to the +8.2 VDC supply line. The supply conductor 198 is a 
negative supply conductor and is connected through a current-sensing 
resistor 200 to the negative (-) supply conductor. A typical value for the 
current sensing resistor 200 is 24 ohms. 
One of the pair of supply conductors 196 and 198, in this embodiment the 
negative supply conductor 198, is further subdivided into a heating mode 
select conductor HEAT and a cooling mode select conductor COOL 
alternatively selected for continuity by the FIG. 1 mode switch 40. 
Preferably, as illustrated in FIG. 5, this particular portion of the mode 
switch 40 is in effect an SPDT switch, but comprises a contact 44" of the 
FIG. 1 switch 44 and a contact 42" of the separate FIG. 1 switch 42. This 
particular arrangement ensures that when the FIG. 1 movable panels 36 and 
38 are intermediate the heating and cooling positions, neither of the 
contacts 42" and 44" is closed, and no power is applied to the remote 
control unit 164. More importantly, no current flows through the current 
sensing resistor 200 during this condition. 
In the particular circuit illustrated herein wherein it is the negative 
supply conductor 198 which is subdivided into select conductors, the 
selected conductor HEAT or COOL is pulled to the negative supply through 
the current sensing resistor 200. Thus these two conductors HEAT and COOL 
are active-low digital logic lines. The circuitry within the remote 
control unit 164 responds to the heating and cooling mode select 
conductors HEAT and COOL to effect indications and operational connections 
appropriate to the particular mode selected. 
Internally of the remote control unit 164, the positive supply conductor 
196 is connected through the terminal A and then to a user ON/OFF switch 
202, in turn connected to a positive supply conductor 204. The HEAT and 
COOL select conductors similiarly enter through the terminals B and C. 
To indicate to the user which mode has been selected, a pair of indicator 
lamps, preferably light emitting diodes (LED's) 206 and 208 are connected 
through a common current limiting resistor 210 to the positive supply 
conductor 204, and through individual isolation diodes 212 and 214 to the 
HEAT and COOL select conductors, respectively. 
In order to sense room temperature, a thermistor circuit, generally 
designated 216 includes a negative temperature coefficient thermistor 218 
connected in series with a resistor 220 in voltage divider configuration, 
with the TEMP line supplied from the voltage divider tap point via the 
Terminal D. In the particular circuit described herein, it is desired that 
the thermistor circuit 216 provide a voltage (on the the TEMP line) 
directly representative of the actual degree of heating or cooling, 
regardless of whether heating mode or cooling mode operation is selected. 
In other words, during heating mode operation, the voltage on the TEMP 
line increases as the room gets warmer, and during cooling mode operation 
the voltage on the TEMP line increases as the room gets cooler. 
To achieve this result, the thermistor 218 and resistor 220 are connected 
to an electronic DPDT switching arrangement 222 comprising a pair of 
inverters 224 and 226 having their inputs connected through appropriate 
biasing networks to the HEAT select conductor and the COOL select 
conductor, respectively. In particular, the input network for the inverter 
224 includes resistors 228 and 230, and the input network for the inverter 
226 includes resistors 232 and 234 and an isolation diode 236. Although 
conventional supply voltage connections to integrated circuit devices are 
not generally shown and described herein, in FIG. 5 the manner in which 
the inverters 224 and 226 are supplied during both heating mode and 
cooling mode operation is illustrated. In particular, the inverter 224 
includes a positive supply voltage line 238 and a negative supply line 240 
connected through a pair of isolation diodes 242 and 244 to the HEAT and 
COOL select conductors, respectively. It will be understood that the other 
inverter 226 is included within the same integrated circuit package as the 
inverter 224, and is accordingly supplied simultaneously. Preferably, the 
inverters 224 and 226 comprise CMOS digital logic devices. Suitable 
inverters are RCA Type No. CD4001 NOR Gates, with both NOR gate inputs 
tied together. 
The electronic DPDT switch functions, during operation, to reverse the 
positive and negative supply voltage connections to the thermistor 218 and 
the resistor 220, depending on whether heating mode or cooling mode 
operation is selected. Specifically, during heating mode operation, the 
HEAT line is low, activating the inverter 224, the output of which is then 
high. The input to the inverter 226 is high, this inverter is therefore 
not activated, and its output is low. Thus the free end of the thermistor 
218 (the end away from the midpoint connection to the TEMP line) is 
effectively connected to the positive supply conductor 204, and the free 
end of the resistor 220 is effectively connected to the negative supply 
conductor via the negative supply connection to the inverter 226. Since 
the thermistor 218 has a negative temperature coefficient, as temperature 
goes up, the thermistor 218 resistance goes down. Voltage on the the TEMP 
line goes up. Thus TEMP voltage is is directly related to actual 
temperature, as is desired for proper control action during heating mode 
operation. 
On the other hand, when cooling mode operation is selected, the inverter 
226 is activated, and the inverter 224 is not activated. Thus, the free 
end of the resistor 220 is in effect connected to the positive supply 
conductor 204, and the free end of the thermistor 218 is effectively 
connected to the (-) supply conductor. Therefore, as sensed temperature 
goes down, and the resistance of the thermistor 218 increases, the voltage 
on the TEMP line increases. Accordingly, the TEMP voltage is inversely 
related to actual temperature, as is desired for proper function during 
cooling mode operation. 
It is this automatic switching of the connections to the thermistor circuit 
216 in response to the selecting of either the HEAT select conductor or 
the COOL select conductor which enables the FIG. 6 comparator circuitry 
166 to function identically during both heating and cooling mode 
operation. 
In order to provide the SET reference voltage representative of the desired 
degree of heating or cooling, a temperature setting circuit 246 includes a 
temperature setting potentiometer 248 comprising the user "Setting" 
control connected in adjustable voltage divider configuration across the 
+8.2 VDC and negative (-) supply conductors. The SET line is supplied from 
the potentiometer 248 movable wiper 250 through the terminal E. 
In this particular arrangement the left end of the potentiometer 248 is 
positive with respect to the right end, and these two ends are accordingly 
designated (+) and (-). Thus, as the user "Setting" control comprising the 
potentiometer is adjusted towards "More," the SET line voltage increases 
in a positive sense, whether for more heating or more cooling. 
Although the (+) and (-) ends of the potentiometer 248 might simply be 
connected directly to the positive and negative supply lines, or connected 
to these supply lines through simple range-limiting resistors, the 
particular form of temperature setting circuit 246 illustrated provides 
additional refinements for the dual purposes of slightly varying the 
calibration between heating mode and cooling mode operation to correspond 
with similar changes in the characteristics of the thermistor circuit 216 
and, secondly, to provide independent limiting of the maximum temperature 
during heating and the minimum temperature during cooling to correspond 
with the effect of the "Trimmer" illustrated in the graphs of FIGS. 3 and 
4. 
Specifically, the (-) end of the potentiometer 248 is connected to the tap 
point 252 of a selectable voltage divider 254 comprising a resistor 256 
connected to the positive supply conductor 204, and a pair of resistors 
258 and 260 connected through respective isolation diodes 262 and 264 to 
the HEAT and COOL lines, to be selectively connected in circuit depending 
upon whether heating mode or cooling mode operation is selected. This 
permits the characteristics of the desired temperature reference to be 
closely tailored to the characteristics of the thermistor circuit 216 for 
both heating and cooling mode operation. 
Similarly, the (+) end of the temperature setting potentiometer 248 is 
connected to the tap point 266 of another selectable voltage divider 268 
comprising a resistor 270 connected to the positive supply line 204, and a 
pair of alternately selected resistances 270 and 272 connected through 
respective isolation diodes 274 and 276 to the HEAT and COOL lines for 
selection according to whether heating mode or cooling mode operation is 
selected. 
More particularly, the resistance 270 may be seen to comprise a variable 
heating mode trimmer variable resistor 278 in series with a fixed resistor 
280, and paralleled by a fixed resistor 282. Similarly, the resistance 272 
more particularly may be seen to comprise a cooling mode trimmer variable 
resistor 284 connected in series with a fixed resistor 286. 
While there is no intention to limit the present invention to particular 
component values, the following TABLE of resistance values is provided by 
way of example for the purpose of more clearly explaining the operation of 
the temperature setting circuitry and the manner in which the temperature 
setting circuitry 246, together with the FIG. 6 biasing circuit 
arrangement 190, provide the characteristic curves of FIGS. 3 and 4: 
TABLE 
______________________________________ 
Setting Potentiometer 248 
50K Ohms 
Resistor 256 6800 Ohms 
Resistor 258 3000 Ohms 
Resistor 260 3300 Ohms 
Heat Trimmer Resistor 278 
5000 Ohms, .+-. 20% 
Resistor 280 4420 Ohms 
Resistor 282 11K Ohms 
Cool Trimmer Resistor 284 
5000 Ohms, .+-. 20% 
Resistor 286 4120 Ohms 
______________________________________ 
Preliminarily, with the above component values connected as shown in FIG. 5 
it will be noted that the (+) end of the temperature setting potentiometer 
248 is always positive with respect to the (-) end, regardless of the 
settings of the heat and cool trimmer resistors 278 and 284 and regardless 
of whether heating or cooling mode operation is selected. Thus, for either 
heating or cooling mode of operation, the voltage on the SET line 
increases as the setting of the potentiometer 248 comprising the user 
"Setting" control is moved towards "More". 
During heating mode operation, the heat trimmer resistor 278 operates to 
increasingly limit the maximum temperature which may be user selected as 
its resistance is decreased. As the trimmer 278 resistance is decreased 
(assuming the HEAT select conductor is low and the isolation diode 274 is 
conducting), the tap point 266 voltage decreases, thus limiting the 
maximum possible magnitude of the SET line voltage, corresponding to the 
effect of the "Trimmer" adjustable shown in the graph of FIG. 3 towards 
the MAX limit. 
It is a characteristic of this circuit arrangement that the precision of 
the maximum temperature limiting is not affected by the tolerance of the 
trimmer resistor 278. In particular, the trimmer resistor 278 may have a 
20% tolerance, without affecting the accuracy of the ultimate limit. The 
reason for this is that maximum limiting occurs at the zero end of the 
variable resistance range, where trimmer resistor tolerance is not a 
factor. 
Operation of the temperature setting circuit 246 during cooling mode 
operation is essentially similar, with only a slight change in SET voltage 
magnitudes due to the different-valued components which are switched in. 
Specifically, as the resistance of the cool trimmer resistor 284 is 
decreased, the limiting effect increases, limiting the amount of cooling a 
user can request by adjusting the setting potentiometer 248 towards 
"More". As in the case of heating mode limiting, the accuracy of the 
minimum temperature limiting is not affected by the tolerance of the 
trimmer resistor 284, but rather is determined by the resistances of the 
other resistors in the network. 
The remaining circuitry shown in FIG. 5 is that of the current detector 188 
comprising the twenty four Ohm current sensing resistor 200 and the 
comparator 96. A reference voltage divider comprising resistors 288 and 
290 is connected through the input resistor 112 to the comparator 96 
inverting (-) input, and the voltage drop across the current sensing 
resistor 200 is applied through the input resistor 114 to the comparator 
96 non-inverting (+) input. The comparator 96 output has a pull-up 
resistor 292, and drives the base of an NPN switching transistor 294 
through an input divider comprising resistors 296 and 298. The emitter of 
the switching transistor 294 is connected directly to the negative (-) 
supply conductor, and the transistors 294 collector is connected to drive 
the "-, SWITCHED" conductor. 
In the operation of the current detector circuit 188, whenever the remote 
control unit 164 is connected, energized, and turned on, with the mode 
switch 40 in either the heating or cooling position, but not intermediate, 
current drawn by the various networks of the remote control unit 164 
produces a voltage drop across the current sensing resistor 200. In this 
condition, the comparator 96 non-inverting (+) input is more positive than 
the inverting (-) input, and the comparator 96 output goes high, biasing 
the switching transistor 294 into conduction. this completes the negative 
supply return for those components, such as the FIG. 6 relay driver 
transistors 176 and 178, which are connected to the "-, SWITCHED" line. In 
particular, the relays K1, K2 and K3 are allowed to operate so that the 
various load devices of FIGS. 1 and 2 may be energized. 
Conversely, if current for any reason does not flow throught the current 
sensing resistor 200, for example when the user ON/OFF switch 202 is OFF 
or the FIG. 1 movable panels 36 and 38 are in an intermediate position, 
the comparator 96 output is low, and the switching transistor 294 does not 
conduct. All loads, particularly drivers for the relays K1, K2 and K3, 
having their negative supply connection returned through the "-, SWITCHED" 
conductor cannot operate. 
Referring lastly to FIG. 7, the remainder of the circuitry is concerned 
primarily with the automatic demand defrost functions for both heating and 
cooling. Additionally a portion of the FIG. 7 circuit is concerned with 
inhibiting activation of the supplemental electrical resistance heater 46 
(FIG. 1) during heating mode operation in the event outdoor air 
temperature exceeds a predetermined temperature, for example 36.degree. F. 
The FIG. 7 circuitry is automatically controlled in response to 
temperature sensed at three points in the heat pump unit 10 of FIG. 1. In 
the preferred embodiment illustrated, three negative temperature 
coefficient thermistors are employed for the sensing. It will be 
appreciated, however, that various other forms of temperature sensor may 
be employed. 
In particular, an outside thermistor 300 (FIGS. 1 and 7) is positioned in 
the incoming path of the air stream 32 which circulates through the 
evaporator 16 from the outdoors during heating mode operation. An 
evaporator thermistor 302 is mounted in heat exchange relationship with a 
portion of the evaporator 16 so as to sense the temperature thereof. 
Lastly, a stopper thermistor 304 is positioned to sense the presence of 
cold defrost water draining from the evaporator 16 during a defrosting 
operation. In the particular arrangement illustrated, an evaporator drain 
pan 306 (FIG. 1) is positioned below the evaporator 16 so as to catch and 
appropriately direct cold defrost water draining from the evaporator 16 to 
a drain or discharge. The "stopper" thermistor 304 is preferably 
positioned in the drain pan 306 at a low point thereof. 
In order inhibit activation of the supplemental electric resistance heater 
46 during heating mode operation if outdoor air temperature exceeds a 
predetermined temperature, for example 36.degree. F., thermostat stage two 
operation is inhibited under such conditions through operation of the 
comparator 110 and the outside thermistor 300. The outside thermistor 300 
comprises an element of a voltage divider additionally comprising series 
resistors 308 and 310, and a resistor 312 in parallel with the thermistor 
300. The junction of the thermistor 300 and the resistor 310 is connected 
through the input resistor 142 to the comparator 110 non-inverting (+) 
input, which functions as a comparison input. To establish a reference 
voltage for the comparator 110, a fixed voltage divider comprising 
resistors 314 and 316 is connected through the input resistor 140 to the 
comparator 110 inverting (-) input. 
The output of the comparator 110 supplies an INHIBIT STAGE 2 line, which is 
connected in parallel with the output of the FIG. 6 second stage 
comparator 100. Thus, when the INHIBIT STAGE 2 line is low, conduction of 
the stage two switching transistor 178 and operation of the stage relay K2 
are positively inhibited. 
The various resistance values involved are selected such that, during 
heating mode operation, if the temperature of the outside thermistor 300 
is sufficiently high, and its resistance correspondingly sufficiently low, 
the voltage on the comparator 110 non-inverting (+) input goes below the 
reference voltage applied to the comparator 110 inverting (-) input, and 
the comparator output goes low. 
This particular inhibiting of stage two operation is prevented during 
cooling mode operation by a connection of the COOL line (from FIG. 5) 
through an isolation diode 318 to the midpoint of the voltage divider 
comprising the resistors 314 and 316. Specifically, during cooling mode 
operation, the COOL line is low, the isolation diode 318 conducts, pulling 
the comparator 110 inverting (-) input lower than the comparator 110 
non-inverting (+) gets under any circumstance. Therefore, the output of 
the comparator 110 and the INHIBIT STAGE 2 line remain high. 
Circuit elements in FIG. 7 which control cooling mode demand defrost 
operation are the evaporator thermistor 302, which is connected in series 
with a resistor 320 in voltage divider configuration, and the comparator 
102. The comparator 102 inverting (-) input functions as a comparison 
input and is connected through the input resistor 124 to the evaporator 
thermistor 302 voltage divider. The comparator 102 non-inverting (+) input 
functions as a reference input and is connected through the input resistor 
126 to a fixed voltage divider comprising resistors 322 and 324. The 
comparator 102 additionally has an output pull-up resistor 326. 
In operation, during cooling mode, whenever the evaporator temperature 
falls below a temperature predetermined by the relative resistances of the 
various resistors and the thermistor 302 involved, the resistance of the 
evaporator thermistor 302 becomes sufficiently high to increase the 
voltage on the comparator 102 inverting (-) input above the reference 
voltage maintained on the comparator 102 non-inverting (+) input, and the 
comparator 102 output goes low. 
The output of the comparator 102 drives an INHIBIT STAGE 1 line, which is 
connected directly to the base of the FIG. 6 first stage switching 
transistor 176 through an isolation diode 314. Thus when the INHIBIT STAGE 
1 line is low, the first stage relay K1 is deactivated. 
At this point, the compressor 12 is deenergized, and evaporator 16 
defrosting is effected by temperature equalization throughout the 
refrigeration system. As described in the above-referenced 
commonly-assigned McCarty application Ser. No. 144,795, various automatic 
check valves may be employed to hasten this process. 
During cooling mode defrost operation, the first stage relay K1 is not 
activated, and the compressor 12 does not run. However, the second stage 
relay K2 is free to operate under control of the FIG. 6 second stage 
comparator 100 in the event measured room temperature is sufficiently 
high. 
When sensed evaporator temperature subsequently increases, the voltage 
applied to the comparator 102 inverting (-) input begins to decrease. When 
this voltage is sufficiently low, as determined by the range of hysteresis 
introduced by the positive feedback resistor 156, the output of the 
comparator 102 and the INHIBIT STAGE 1 line again go high. Normal cooling 
operation resumes. 
This particular defrost operation is inhibited during heating mode when the 
COOL line floats high due to the pull-up effect of the resistor 210, the 
LED 208, and the isolation diode 214 of the FIG. 5 remote control unit 
164. The COOL line, connected to the comparator 102 non-inverting (+) 
input through an isolation diode 330, high, biases the comparator 102 
non-inverting (+) input higher than the voltage on the inverting (-) input 
can ever get as a result of decreases in temperature of the evaporator 
thermistor 302. 
The heating mode defrost circuitry is somewhat more complex in that two 
thermistors are used for initiating defrosting, and a third thermistor is 
used for terminating heating mode defrost operation. Specifically, heating 
mode defrosting operation is initiated by a decrease in evaporator 16 heat 
exchange efficiency as indicated by an increase in temperature 
differential between the outside thermistor 300 and the evaporator 
thermistor 302, recognized by means of the comparator 104. In particular, 
the midpoint of the resistors 308 and 310 comprising the elements of the 
outside thermistor 300 voltage divider is connected through the input 
resistor 128 to the comparator 104 inverting (-) input, and the midpoint 
of voltage divider comprising the resistor 320 and the evaporator 
thermistor 302 is connected through the input resistor 130 to the 
comparator 104 non-inverting (+) input. The comparator 104 also has an 
output pull-up resistor 332. 
When the evaporator temperature as sensed by the thermistor 302 is 
sufficiently below the outside air temperature as sensed by the thermistor 
300, the precise differential required being a function of the various 
resistance values involved, the comparator 104 non-inverting (+) input 
voltage becomes higher than the inverting (-) input voltage, and the 
comparator 104 output goes high. An exemplary temperature differential at 
which this defrost initiation occurs is 31.degree. F. 
The heating mode demand defrost circuitry of FIG. 7 also includes 
comparator circuitry generally designated 334, comprising the "stopper" 
thermistor 304 and the comparator 106, which circuitry 334 is connected 
through an isolation diode 336 so as to be initially energized when the 
output of the comparator 104 goes high at the beginning of a heating mode 
defrosting operation. 
In particular, the comparator circuitry 334 includes a line 338 connected 
to the cathode of the isolation diode 336, and energized therethrough when 
the output of the comparator 104 is high to initiate heating mode 
defrosting operation. 
The comparator circuitry 334 includes a reference voltage divider having 
resistors 340 and 342 connected through the input resistor 132 to the 
inverting (-) input, and another voltage divider comprising the stopper 
thermistor 304 and a resistor 344 connected through the input resistor 134 
to the non-inverting (+) input. For the purpose of ensuring that the 
output of the comparator 106 is initially low, there is another voltage 
divider comprising series resistors 346, 348 and 350 connected between the 
+8.2 VDC supply conductor and the circuit negative (-) supply conductor. 
The midpoint of the resistors 348 and 350 is connected through an 
isolation diode 352 to the midpoint of the voltage divider resistors 340 
and 342 connected to the comparator 106 inverting (-) input. The 
comparator 106 has an output pullup resistor 354, and a latching diode 356 
is connected between the comparator 106 output and the line 338. The 
remaining connection associated with this particular portion of the 
circuit is a connection of the output of the comparator 106 through a 
control line 358, a current limiting resistor 360 and an isolation diode 
362 directly to the inverting (-) input of the comparator 102 to operate 
the comparator 102 as an inverter. 
In operation, when the output of the comparator 104 goes high to initiate 
heating mode defrost operation, voltage is applied to energize the line 
338. At this point experience has shown the temperature of the stopper 
thermistor 304 to be relatively low, and the resistance thereof relatively 
high. The values of the resistors 340, 342 and 344 together with the 
resistance of the stopper thermistor 304 is such that the comparator 106 
non-inverting (+) input is biased at a higher voltage than the inverting 
(-) input, with the result that the comparator 106 output goes high. This 
causes two things to occur: First, energization of the line 338 is 
maintained through the latching diode 356, even though the comparator 104 
output again goes low as defrosting operation proceeds. Second, the 
comparator 102 is operated as an inverter through its inverting (-) input, 
a relatively large positive voltage being applied through the isolation 
diode 362 to the inverting (-) input. Even though the COOL line applied 
through the isolation diode 330 to the comparator 102 non-inverting (+) 
input is floating high, the voltage applied to the comparator 102 
inverting (-) input is even higher. 
The comparator 102 output therefore goes low, activating the INHIBIT STAGE 
1 line, thus turning off the FIG. 6 first stage switching transistor 176 
and the first stage relay K1. Operation of the compressor 12 ceases, and 
defrosting operation commences. 
As discussed above, normally during heating mode operation, the second 
stage (supplemental electric resistance heating) is inhibited through 
operation of the outside thermistor 300 and the comparator 110 in the 
event outside temperature is above a predetermined temperature, for 
example 36.degree. F. However, during periods of defrosting, it is not 
desirable to so inhibit operation of the supplemental electric resistance 
heater 46 because room temperature could get too low. Accordingly, the 
INHIBIT STAGE 1 line is applied through an isolation diode 364 (FIG. 7) to 
the comparator 110 inverting (-) input, thus keeping the output of this 
comparator 110 high, and allowing the second stage to operate in the event 
the room temperature thermostat control circuitry cells for it. It will be 
appreciated that the comparator 110 is thus operated as an inverter via 
its inverting (-) input. 
As may be seen from the FIG. 2 circuit, to compensate for the loss of 
heating from heat pump operation during heating mode defrosting, the 
additional electric resistance heater 48 is allowed to operate. This 
additional heater 48 is energized through the de-energized first stage 
relay contacts K1-C and through the energized second stage relay contacts 
K2-C. Mode switch 40 contacts 44' provide the N or neutral return for the 
120 VAC supply line. 
While heating mode defrosting is proceeding, the temperature of the stopper 
thermistor 304 is maintained at approximately 32.degree. F. due to cold 
defrost water draining from the evaporator 16 (FIG. 1) passing thereover. 
However, when the evaporator 16 is completely defrosted, this flow of 
water ceases, and the temperature of the stopper thermistor 304 increases. 
The resistance thereof thus decreases, until the comparator 106 
non-inverting input (+) is below that applied to the comparator 106 
inverting (-) input, and the comparator 106 output goes low. 
This resets the comparator circuitry 334 by removing energizing voltage 
from the line 338, (assuming the output of the comparator 104 responsive 
to evaporator and outside air differential temperature by this time is 
low), and additionally allows the output of the comparator 102 to again go 
high, thus deactivating the INHIBIT STAGE 1 line and allowing normal 
thermostaticly controlled heating operation to resume. 
The remaining circuitry illustrated in FIG. 7 relates to a function active 
during heating mode defrost, and comprises the comparator 108. The 
comparator 108 inverting (-) input serves as a reference input, and is 
connected to the junction of voltage divider resitors 346 and 348. The 
comparator 108 non-inverting (+) input serves as a comparison input and is 
connected to the junction of resistors 308 and 310 in the voltage divider 
with the outside thermistor 300 which senses incoming evaporator airflow 
during heating mode operation. The comparator 108 has an output pull-up 
resistor 366, and has its output connected through a current limiting 
resistor 368 to the base of an NPN driver transistor 370, the collector of 
which is connected to drive the coil of relay K3. A free-wheeling diode 
372 is connected in parallel with the relay K3 coil, and a biasing 
resistor 374 is connected between the transistor 370 base terminal and the 
(-) circuit reference line. 
The emitter of the driver transistor 370 is returned to the "-, SWITCHED" 
line so that the relay K3 cannot be actuated when the FIG. 5 user ON/OFF 
switch 202 is off as sensed by the current detector circuitry 188. 
The specific function of the comparator 108 and the relay K3 is to prevent 
operation of the evaporator fan motor 22 (FIGS. 1 and 2) in the event 
outdoor air temperature as sensed by the thermistor 300 is below 
32.degree. F. In such event, the resistance of the thermistor 300 
increases to a point where the comparator 108 non-inverting (+) input 
voltage goes above the inverting (-) input fixed reference voltage. The 
comparator 108 output goes high, activating the driver transistor 370 and 
the relay coil K3. 
From the power circuitry of FIG. 2, it may be seen that, during heating 
mode defrost when the first stage relay K1 is not activated and the second 
stage relay K2 is activated, power to operate the fan motors 22 and 24 is 
supplied from the L' conductor through a circuit path comprising 
normally-closed contact K1-C, normally-open contact K2-C, normally-closed 
contact K1-D, and the user fan switch 54. (This assumes the user fan 
switch 54 is in the AUTO position. If the user fan switch 54 is in the MAN 
position the fan circuit including relay contact K3 is directly supplied 
from the L' conductor.) The evaporator fan motor 22 only is also supplied 
through the normally-closed contact of relay K3. However, when outdoor air 
temperature is below 32.degree. F., relay K3 operates and no power can be 
supplied to the evaporator fan motor 22. Thus below-freezing outdoor air, 
which otherwise would slow or even prevent the evaporator defrosting 
process, is not drawn over the evaporator 16. 
Conversely, when outdoor temperature is above 32.degree. F. during heating 
mode defrosting, the comparator 108 output is low, leaving both fan motor 
22 and 24 free to operate when the second stage relay K2 is activated. 
(However, due to hysteresis introduced by the positive feedback resistor 
148, if the comparator 108 output is initially high, an outdoor 
temperature of 35.degree. F. is required to switch the comparator 108 
output low.) The evaporator fan 18 then draws above-freezing temperature 
outdoor air over the evaporator 16, aiding in the defrosting operation. 
For the purpose of enabling those skilled in the art without undue 
experimentation, the following TABLE provides suitable values for various 
of the resistances for which exemplary values are not given hereinabove. 
It will accordingly be appreciated that these component specifications are 
given by way of example, and not limitation: 
TABLE 
______________________________________ 
Thermistors 
300, 302, 304 10 K Ohms at 77.degree. F. 
Resistors 
92 750 Ohm, 2 Watt 
168, 170, 292, 310, 366 
6.8 K Ohm 
172, 174, 296, 368 2.2 K Ohm 
184, 186, 298, 354, 374, 360 
4,7 K Ohm 
210 1500 Ohm 
220 11.5 K Ohm 
228, 232, 326 10 K Ohm 
230, 234 100 Ohm 
288 27 K Ohm 
290 360 Ohm 
308 49.9 K Ohm 
312 1 Meg Ohm 
314 86.6 K Ohm 
316 46.7 K Ohm 
320, 340 342 100 K Ohm 
322 196 K Ohm 
324 66.5 K Ohm 
332 16 K Ohm 
344 30.1 K Ohm 
346 140 K Ohm 
348 66.5 K Ohm 
350 34 K Ohm 
______________________________________ 
While a specific embodiment of the invention has been illustrated and 
described herein, it is realized that numerous modifications and changes 
will occur to those skilled in the art. It is therefore to be understood 
that the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.