US3505828A

US3505828A - Control for refrigeration apparatus

Info

Publication number: US3505828A
Application number: US738609A
Authority: US
Inventors: Donald R Vander Molen; Dana E Whitlow
Original assignee: Whirlpool Corp
Current assignee: Whirlpool Corp
Priority date: 1968-06-20
Filing date: 1968-06-20
Publication date: 1970-04-14
Anticipated expiration: 1987-04-14

Description

April 14, 1970' D. R. VAND ER MOLEN ETAL 3,505,323

- CONTROL FOR REFRIGERATION APPARATUS I Filed June 20, 1968 2 Sheets-Sheet 1 FIG. I CONDENSER coMPREssoR 23 CQNTROL i A R 25 COMPRESSOR ON A OFF FAN SPEED HIGH LOW I g I =TEMP. 1. l 1, i 1 l l I I 46 coouws 1 CAPACITY m 44 I I) '45 c 1 /l i I I o v TEMP.

SELECTED TEMPERATURE INVENTORS DONALD RVANDER MOLEN DANA E.WH|TLOWv ATTORNEYS.

FIGZ

April 14, 1970 'ID.-R. VANDER 'MOLEN E -x 1 0 8 7 CONTROL FOR REFRIGERATION APPARATUS Filed June 20, 1968 2 sheets-sheaf 2 United States Patent 3 505,828 CONTROL FOR REFRIGERATION APPARATUS Donald R. Vander Molen, Stevensville, and Dana E. Whitlow, St. Joseph, Mich., assignors to Whirlpool Corporation, a corporation of Delaware Filed June 20, 1968, Ser. No. 738,609 Int. Cl. F25d 17/00 US. Cl. 62180 9 Claims ABSTRACT OF THE DISCLOSURE An air conditioning control having a two stage temperature compensated transistor amplifier connected to a thermistor temperature sensing bridge for'cycling the compressor on and off. The speed of a fan for the evaporator is varied above a minimum speed in accordance with temperature when the compressor is operating.

This invention relates to a control for: refrigeration apparatus, and more particularly to a refrigeration control for regulating the speed of a fan and the operation of a compressor unit.

A refrigeration apparatus, such as a conventional room type air conditioner, has an evaporator through which air is passed by a fan to effect a heat exchange for cooling the air. A refrigerant fluid is circulated through the evaporator by a compressor which is cycled on and off by a control unit. The control unit is connected to a temperature sensor arranged to initiate operation of the compressor when the room air temperature rises above a preselected high value, and to discontinue operation of the compressor when the room air temperature falls below a preselected low value.

It has been suggested that the speed of operation of the fan for the evaporator may be varied in proportion to temperature, independent of the operation of the compressor circuit to vary system capacity. While such a control represents an improvement over prior controls, it does not produce a system having the desired maximum efiiciency at varying control settings.

In accordance with the invention, control of the evaporator fan speed is synchronized with control of energization and deenergization of the compressor unit, to produce a refrigeration system having improved cooling and dehumidifying efliciency over a wide range of control settings. The speed of the fan is preferably increased subsequent to the energization of the compressor to increase the cooling capacity of the refrigeration apparatus. A desired temperature is maintained by varying the speed of the fan and thus the system cooling capacity within a preselected controlled range. Since the compressor is continuously energized when the control calls for cooling, the evaporator remains cool in order to condense water vapor and hence dehumidify the air.

A principal object of this invention is to provide an improved refrigeration apparatus control which synchronizes control of the operation of the evaporator fan with control of the operation of the compressor.

One of the features of the invention is'the provision of an air conditioner control in which evaporator fan speed is varied while the compressor is operating. While the compressor'is deenergized, the evaporator fan is maintained at a constant minimum speed.

Another feature of this invention is the provision of an air conditioner control having a temperature sensor circuit for switching a compressor on and off. The temperature sensor circuit also controls the firing angle of a switching device connected between a fan motor and a source of alternating current.

Patented Apr. 14, 1970 Yet another feature of this invention is the provision of an air conditioner control in which a conduction device connected to a fan motor is controlled by a circuit including a voltage divider for maintaining a desired minimum voltage. The circuit is controlled by the same temperature sensor which cycles a compressor between energized and deenergized states.

Still another feature of the invention is the provision of a control for refrigeration apparatus employing temperature compensation for semiconductor devices used in the control.

Other features and advantages of the invention will be apparent from the following description of one embodiment taken in conjunction with the accompanying drawings. Of the drawings:

FIGURE 1 is a block diagram of refrigeration apparatus controlled by the invention;

FIGURE 2 is a schematic diagram of the control shown in block form in FIGURE 1; and

FIGURE 3 illustrates curves of the operating characteristics of the control over a limited range of temperatures.

While an illustrative embodiment of the invention is shown in the drawings and will be described in detail herein, the invention is susceptible of embodiment in many different forms and it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. Throughout the specification, values and type designations will be given for certain of the components in order to disclose a complete, operative embodiment of the invention. However, it should be understood that such values and type designations are merely representative and are not critical unless specifically so stated. The scope of the invention will be pointed out in the anpended claims.

Turning to FIGURE 1, refrigeration apparatus in the form of an air conditioner 10, which may be of the room air conditioner type adapted for use in a window or through-the-wall installation, is controlled by a unit 11 constructed in accordance with the invention. The air conditioner 10 is enclosed in a suitable housing (not shown) and has an evaporator 13 with refrigerant conducting tubes (not illustrated) which may have heat transfer fins thereon through which room air is circulated by means of a fan or blower 14. A compressor 16 is connected with evaporator 13 for circulating a refrigerant fluid through the evaporator 13 and through a condenser 17 connected in a closed fluid path with a compressor 16 enclosed in a housing including a compressor motor. The refrigerant line connecting condenser 17 to vaporator 13 includes the usual restrictor, illustrated in the form of a capillary 9. A second fan 19 is disposed adjacent condenser 17 for circulating cooling air thereto. Fan 19 is located behind a transverse partition 20 which separates fan 19 and condenser 17 from fan 14 and evaporator 13 in the air conditioner housing.

Control unit 11 maintains room air at the temperature selected by the user of the apparatus through control of the operation of both compressor motor 16 and a fan motor 22, which may be arranged to drive both fans 14 and 19 as illustrated in FIGURE 1. It will be understood that fan 19 could also be operated by another control and another motor independent of control 11 and motor 22. A temperature sensor illustrated in the form of a thermistor 23 is located in the path of air entering evaporator 13 for sensing the temperature of room air drawn through the air conditioner by fan 14. When the sensed air temperature exceeds a predetermined value, control unit 11 energizes compressor motor 16. The apparatus discussed above, except for the structure and operation of control unit 11 discussed in detail hereafter, is generally known and will not be described in detail.

Turning now to FIGURE 2, control unit 11 is shown in schematic form. A pair of lines from a conventional AC power source (not illustrated) provide power through a transformer 50 to a DC supply circuit which provides operating power to a thermostat circuit 31, which includes temperature sensor 23, and a fan motor speed circuit 32. Thermostat circuit 31 cycles compressor motor 16 between on and off states in accordance with a set point temperature selected by adjusting a control knob (not shown) associated with control unit 11, and the relationship between set point temperature and the room temperature as monitored by sensor 23. Fan speed circuit 32, which is responsive to signals from thermostat circuit 31 when the compressor is energized, passes a variable amount of AC current to fan motor 22 for varying the speed of the fans driven thereby. An understanding of the general operation of control unit 11 may be facilitated by referring to FIGURE 3, in which curves are illustrated of the operating characteristics of compressor 16 (FIGURE 3A), fan speed circuit 32 (FIGURE 3B), and the overall system (FIG- URE 3C). The three curves have a common base which represents a limited range of temperatures near the temperature selected by the user.

In operation, thermostat circuit 31 turns the compressor on at a set point temperature indicated at 35, as seen in FIGURE 3A. The compressor remains on for all temperatures in excess of temperature 35, and will not turn ofi until a lower temperature indicated at 37 is reached. Once compressor 16 is off, the room temperature as monitored by sensor 23 must rise to the higher value indicated at 35 before the compressor will again be turned on. In the embodiment of FIGURES 1-3, the temperature diiterential between

temperatures

35 and 37 is approximately 1 F.

Fan speed control circuit 32 normally maintains the fan at a low speed indicated at 39, FIGURE 3B, for all temperatures below the temperature indicated at 35 at which the compressor is turned on. The operation of fan speed control circuit 32 is synchronized with the operation of thermostat circuit 31 so that when the compressor is turned on at temperature 35, the speed of the fan may be modulated by being increased continuously over a range indicated at 40 until a maximum speed indicated at 41 is reached for all temperatures in excess of a temperature indicated at 42, higher than the set point temperature indicated at 35. As shown in FIG- URE 3B, a temperature indicated at 36 falls within the range of continuous speed control of the fan motor 22. If the air conditioner capacity has been properly matched with the required cooling load, fan speed control 32 will respond to maintain the room temperature at the temperature indicated at 36. The temperature indicated at 36 may be called the control point temperature. The temperature differential between the temperature indicated at 42, at which maximum fan speed indicated at 41 occurs, and the temperature indicated at 35, at which 'theicompressor is first energized, is approximately 1 F.

The resulting cooling capacity of the air conditioner when operated in the above manner can be seen in FIGURE 3C. The cooling capacity of the air conditioner has a constant value indicated at 44 when the compressor is on and the fan is driven at low speed. For temperatures in excess of the temperature indicated at 35, however, the increase in fan speed has the effect of increasing the cooling capacity of the air conditioner, along a curve 45, until a maximum cooling capacity indicated at 46 is reached which represents the compressor on and the fan operating at maximum speed. The control unit is efiective to vary the fan speed to maintain a room temperature at a control point temperature indicated at 36 which falls within the variable cooling capacity curve of the air conditioner. Depending on the cooling load,

the control pointtemperature may not fall within the variable cooling capacity range. Thus under high load conditions, the control point temperature may be in excess of the temperature indicated at 42.

By synchronizing the start of the fan speed modulation with the initial energization of the compressor, and contlnuously varying fan speed between minimum and maxlmum values over a narrow range of temperatures, several important operating advantages are achieved. The full range of fan speeds is available over a narrow temperature ditferential in order to control more precisely the capacity necessary to maintain a selected temperature. Furthermore, the compressor is maintained on for a maxll'Illll'Il time, keeping the evaporator coils cool to condense water vapor and hence control the humidity of the air over a maximum period of time. Also, when the compressor is first turned on, indicating sensing of a rising temperature, a variable rather than a fixed cooling capac- 1ty is available to interrupt the sensed temperature rise. Furthermore, the air conditioner will always operate at the lowest fan speed consistent with the cooling load and thus provide quieter operation. This is particularly advantageous for room air conditioners used for cooling bedrooms where the night cooling load is typically low and where undue noise will interfere with sleeping. Also, legislation now in force establishing permissible noise levels for air conditioners sets lower levels for nighttime operation. Thus, it is desirable for the. air conditioner to be quieter in operation during the nighttime hours while achieving the desired cooling capacity.

Returning to FIGURE 2, the control unit 11 which produces the above results will now be described in detail. Power supply circuit 30 consists of a power transformer 50 which drops the AC voltage input thereto from power lines 25 to 24 volts, which is coupled to a full wave diode rectifier bridge 52. The output of bridge 52 is connected through a ohm resistor 53 to a 20 volt Zener diode 54 for clipping the full wave AC voltage. The voltage across Zener diode 54 is coupled directly to fan speed circuit 32 and to a blocking diode 56 and a microfarad smoothing capacitor 57 for powering thermostat circuit 31.

Temperature sensor 23 in thermostat circuit 31 is a negative temperature coefficient thermistor, having 1,000 ohm resistance at 77 F., which is connected in a bridge type circuit across DC output capacitor 57. One side of the bridge consists of a 10 kilohm potentiometer 60, a 4.7 kilohm resistor 61, thermistor 23, and a diode 6-3 connected in series across the DC supply. The other side of the bridge consists of a 10 kilohm resistor 65 and a 1 kilohm resistor 66 connected in series across the DC supply. The output from the thermistor bridge is available at a junction 68 between resistor 61 and thermistor 23, and a second junction 69 between resistor 65 and resistor 66. The resistance of potentiometer 60 is varied by the user of the air conditioning system to select the desired set point temperature, as may be indicated by a suitably calibrated control knob (not illustrated) associated with control unit 11.

A two stage transistor amplifier using NPN transistors 71 and 72 is connected between the thermistor bridge and a relay coil 73 for energizing compressor motor 16. Transistor 71 has its base directly connected to junction 68. The collector of transistor 71 is directly connected to the base of transistor 72, and is connected to the DC power supply through a 100 kilohm resistor 75. A 2 kilohm potentiometer 76 connects the emitter of transistor 71 to the junction 69 on the opposite side of the thermistor bridge.

The collector of transistor 72 is connected through relay coil 73 with the DC power source. Relay coil 73 has a 2.0 kilohm resistance with a 4.9 milliampere pullin and a 2.9 milliampere drop-out rating. When the value of pull-in current is reached or exceeded, a normally open contact switch 73-1 is closed to energize compressor motor 16. The emitter of transistor 72 is connected to a voltage divider consisting of a 3.3 kilohm resistor 77 and a 680 ohm resistor 78 connected in series across the DC supply, with the junction therebetween being directly connected to the emitter of transistor 72.

Compressor motor 16 is connected across the AC power lines 25 through a normally nonconducting semiconductor bidirectional triode switch or triac 80. Triode switch 80 is shunted byrelay contact 73-1 in series with a pair of

resistors

81 and 82. The junction between

resistors

81 and 82 is connected to a gate terminal 83 of the triode switch.

In operation, transistor 71 is driven toward saturation at temperatures'below that selected by potentiometer 60. Transistor 71 diverts base current away from transistor 72, tending to drive transistor 72 more nonconductive as transistor 71 becomes more conductive. As the temperature sensed by thermistor 23 increases, the thermistor resistanceand corresponding voltage drop decreases, driving transistor 71 towards cut-01f and increasing the conduction of transistor 72. At the temperature indicated at 35 in FIGURE 3A, transistor 72 passes sufficient current to pull in relay 73, thereby closing relay contact 73-1 and gating triac 80 into conduction. Triac 80 thereupon passes both half cycles of AC power to compressor motor 16 causing operation of the compressor. Since relay 73 has a smaller drop-out current than that required for pull-in, compressor motor 16 will not be turned off until a lower temperature 37 is sensed by temperature sensor 23.

The operation of fan speed circuit 32 is synchronized with the operation of thermostat circuit 31. More particularly, a 25 kilohm resistor 90 is connected in parallel with transistor 72 and resistor 78' so that voltage thereacross is approximately inversely proportional to the conduction of transistor 72. Resistor 90-is in the form of a potentiometer having a variable tap 91 connected to a ramp and pedestal circuit 92 for controlling the firing angle of a second triac 93 whichcontrols the current to fan motor 22. Ramp and pedestal circuit 92, to the extent described herein, operates in a known manner for circuits of this type, and reference may be made to a standard semiconductor control device manual for a more detailed description, as the General Electric Company SCR Manual, third edition, 1964, pages 130 to 137.

Coils 100 and 100a, and capacitors 102 and 102a form a radio frequency interference filter for eliminating radio interference generated by triac 93. Triac 93 is in series with fan motor 22 in order to control the amount of each half cycle of AC which is passed to the motor. Triac 93 has a gate terminal 105 whch isconnected through a pulse transformer 106 to a unijunction transistor (UJT) 107. UJT 107, in turn, is under control of ramp and pedestal circuit 92 for controlling the firing angle of triode switch 93.

Ramp and pedestal circuit 92 includes an input NPN transistor'110 having its base directly connected to tap 91 of potentiometer 90. The collector of transistor 110 is connected through a 1.5 kilohm resistor 112 to power supply line 114 which is connected to Zener diode 54. As previously explained, line 114 provides a clipped or regulated full wave rectified DC voltage. The emitter of transistor 110 is connected through a one kiloohm resistor 116 to the opposite side of the DC power supply circuit 30. UJT 107 has its emitter 107 connected to a diode 117 and to one side of a 0.33 microfarad capacitor'118, and its B1 terminal 107 connected through the primary side of pulse transformer 106 to the opposite side of capacitor 118. UJT 107 is connected to power line 114 through a 22 kilohm resistor 120 connected with emitter 107 and a 1.5 kilohm resistor 122 connected with B terminal 107 When the voltage across capacitor 118 reaches a predetermined level, UJT 107 fires to discharge the capacitor, through pulse transformer 106. After UJT 107 is turned off by the momentary drop of DC voltage on line 114, capacitor 118 begins to charge again. The voltage across capacitor 118 increases rapidly to a pedestal level through the low resistance of resistor 112, and thereafter increases more slowly along a ramp shaped curve as the capacitor is charged through the relatively higher resistance of resistor 120. Thus, the firing voltage of UJT 107 is reached in a two step charging of capacitor 118. When the voltage reaches a predetermined fraction of the total voltage across the terminals 107 and 107 UJT 107 fires, discharging capacitor 118 through pulse transformer 106 and firing triac 93.

As transistor 72 in thermostat circuit 31 is driven toward saturation, the voltage drop across resistor decreases, causing transistor to shunt less of the current passing through resistor 112, and hence raising the pedestal; This advances the time of firing of UJT 107 thereby gating triac 93 sooner in each half cycle.

As previously explained, the fan motor 22 is main- I tained at'a constant low speed when the compressor is not energized. For this purpose, a 1.5 kilohm resistor 130, and a variable potentiometer 131, having a maximum 5 kilohm resistance, are connected in series across the DC source. The junction between voltage dividing resistors and 131 is connected through a diode 132 to the emitter 107 of UJT 107.

Capacitor 118 is charged to its initial pedestal level through resistor 112 and/or resistor 130. Thus, although transistor 110 may shunt the current available from resistor 112, capacitor 118 will rapidly charge to a fixed pedestal level determinedby variable potentiometer 131, establishing a minimum speed for the fan motor 101. Accordingly, variable potentiometer 131 is adjusted to set the desired minimum speed for the fan motor.

The tap 91 on potentiometer 90 is adjusted to establish the temperature diiferential between the initiation of variable speed fan operation and the initial energization of compressor motor 16. In FIGURE 3, the indicated temperature diiferential is zero, and fan motor speed increases with any increase in temperature above the set point temperature indicated at 35 at which compressor 16 is energized. However, potentiometer 90 may be set to initiate fan motor speed control at temperatures less than or in excess of the set point temperature indicated at 35. If initiation of fan motor speed control is not concurrent with compressor energization, it is preferred that fan speed control be initiated at temperatures in excess of the set point temperature. In FIGURE 3B, the broken line 4001 indicates the speed change curve when speed changes are initiated below the set point temperature indicated at 35, and the dotted line 40b indicates the speed change curve when speed changes are initiated above the set point temperature indicated at 35. The effect on capacity of changing the point at which fan speed change is initiated is indicated in FIGURE 3C. The broken lines 45a show the below set point curve and the dotted lines 45b the above set point curve. A comparison of the capacity curves will indicate the desirability of initiating fan speed changes at or above set point temperature indicated at 35. When fan speed changes occur below set point temperature indicated at 35, the air conditioner starts at an intermediate capacity, with a loss of modulation between the minimum capacity and the intermediate capacity when room temperature is rising. Full capacity modulation is available when the room temperature is decreasing because the compressor otf temperature is less than the temperature at which the fan motor 22 reaches minimum speed.

When the fan speed changes are initiated at temperatures in excess of the set point temperature, the range of temperature between compressor OE and full capacity is increased. This does not affect the capacity modulation characteristics of the control but becomes undesirable from a comfort standpoint in that the control reacts more slowly to rising temperatures and may allow room temperature to reach levels in excess of 2 F. above the set point temperature. Some people are sensitive to temperature changes of as little as 1 F. and most people are sen sitive to temperature changes of 35 F. Thus if the control is caused to react too slowly to temperature riseabove set point temperature, some people will be uncomfortable in the room being cooled by the air conditioner.

For a temperature differential of zero and with pull-in current flowing through relay 73, tap 91 is adjusted to produce the same pedestal voltage at diode 117 as exists at diode 132. Thereafter, as the relay current increases beyond the pull in value, due to an increasing temperature, the pedestal voltage is increased in proportion to temperature, aligning the beginning of curve 40 of FIG- URE 3B with the temperature indicated at 35 at which the compressor is first energized. It will be readily understood that tap 91 may be adjusted to provide larger or smaller voltages at diode 117 and thus change the temperature at which initiation of fan motor speed changes occur. Once the tap 91 has been set, thus establishing the temperature differential between set point temperature indicated at 35 and the temperature at which fan motor speed change is initiated, the potentiometer 60 may be used to establish set point temperature indicated at 35 and synchronously the temperature at which fan motor speed change is initiated. The differential between these two temperatures will remain constant over the range of set point temperature settings permitted the user. In the illustrated embodiment of the invention the user can obtain any desired set point temperature within a range of 60-90 F.

Regardless of the setting of minimum speed potentiometer 131, UJT 107 is forced to fire at the end of each half cycle due to the momentary drop to zero of supply voltage on line 114. As UJT 107 fires, capacitor 118 is discharged; insuring uniform performance on all half cycles since capacitor 118 will always be charged from a fully discharged condition.

Control circuit 11 also includes temperature compensation for the semiconductor devices used therein to insure repeatable performance regardless of the air temperatures surrounding the components. Potentiometer 76, in series with the emitter of transistor 71, provides temperature stability by reducing the sensitivity of thermostat circuit 31 to beta variations of transistor 71, as well as providing a convenient adjustment of the sensitivity (deadband). As leakage currents through the transistor vary with changes in temperature, the voltage drop across potentiometer 76 is added as a feedback voltage to the fixed operating voltage established by resistors 65 and 66. This feedback voltage may be in a positive or negative direction. Resistor 78 serves the same purpose for transistor 72 as potentiometer 76 serves for transistor 71, thereby increasing temperature stability.

Diode 63 is preferably located in heat transfer relation with transistor ,71, as indicated by dashed lines 135. By using the same type semiconductor material for diode 63 and transistor 71, the voltage change in one device cancels the voltage change in the other for temperature variations. With the temperature compensation described herein, control unit 11 remains in calibration and synchronization regardless of the heating of the components used therein.

While we have shown and described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as defined in the appended claims.

The embodiment of the invention in which an exclusive property or privilege is claimed is defined as follows:

1. A control of refrigeration apparatus including an evaporator for chilling air, an evaporator fan for moving air over said evaporator, an evaporator fan motor for 8 driving said evaporator fan and a compressor for circulating refrigerant to said evaporator, comprising:

first circuit means connected to said evaporator fan motor for operating said evaporator fan motor at a first speed;

means for sensing temperature;

second circuit .means responsive to said means for sensing temperature for energizing said compressor when a predetermined temperature is sensed by said means for sensing temperature; third circuit means including said first circuit means,

connected to said second circuit means and responsive to said means for sensing temperature for varying the speed of said evaporator fan motor from said first speed in response to deviation of the temperature sensed by said means for sensing temperature from said predetermined temperature, thereby varying the cooling effect of said refrigeration apparatus by varying the rate of air flow moving over said evaporator;

said third circuit means further including a device switchable between conducting and nonconducting states under control of a gating signal, a source of alternating current for driving said fan motor, means connecting said device between said AC source and said fan motor, and firing angle control means for with respect to a phase of said alternating current to control the firing angle of said device in proportion to the sensed temperature deviation from said predetermined temperature; and

wherein said second circuit means includes a thermistor having resistance proportional to temperature, current operated switching means for energizing said compressor when a predetermined pull-in current is applied thereto, semiconductor means connected to said thermistor and to said current operated switching means for generating said pull-in current when said thermistor senses said predetermined temperature, said third circuit means includes an impedance device coupled to said second circuit means and having a voltage thereacross proportional to the current through said current operated switching means, and said firing angle control means is connected between said impedance device and said switchable device to vary the time of occurrence of said gating signal in proportion to the voltage across said impedance device.

'2. The control of claim 1 wherein said second circuit means includes a temperature responsive voltage divider having in series a variable resistor and said thermistor, said semiconductor means including an output transistor having a current proportional to the voltage across said thermistor, said current operated switching means comprising a relay coil in series with said output transistor for actuating said compressor when pull-in current flows through said output transistor, and said firing angle control means includes a transistor responsive to the voltage across said impedance device when current in excess of pull-in current flows through said relay coil for varying the firing angle of said switching device.

3. The control of claim 2 wherein the transistor in said firing angle control means is connected in a ramp and pedestal circuit, and said first circuit means generates a minimum pedestal for maintaining said fan at a minimum speed corresponding to said first speed condition.

4. The control of claim 1 wherein said second circuit means includes means temperature compensating said semiconductor means to changes in ambient temperature.

5. The control of claim 4 wherein said temperature compensating means includes a semiconductor device connected in an electrical path with said thermistor, said device being formed of the same semiconductor material as said semiconductor means and disposed in heat transfer association with said semiconductor means.

6. The control of claim 4 wherein said semiconductor means includes at least one transistor having base, collector and emitter electrodes, means connecting said base electrode to said thermistor, a source of DC power, means connecting said DC power source in an electrical path including said collector and emitter electrodes for gen erating said pull-in current, and said temperature compensating means includes resistive means located in said electrical path to reduce the sensitivity of said second circuit means.

7. The control of claim 1 wherein said second circuit means includes means coupling said AC source to said compressor when said predetermined pull-in current is applied to said current operated switching means, said circuit means having no current path from said thermistor to said AC source, thereby isolating said thermistor from said AC source.

8. The control of claim 7 wherein said circuit means solely includes air gaps with magnetic fields therein for signal transfer between said circuit means and the means connected to said AC source.

9. The control of claim 1 including a source of DC power, and means connecting said DC power source to said second circuit means for generating said pull-in current, thereby isolating said current operating switchin means from said AC source.

References Cited UNITED STATES PATENTS WILLIAM J. WYE, Primary 'Examiner US. Cl. X.R. 2-207; SIG-15? Patent No.

P0-1050 (s/ss) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated April 14, 1970 lnv nt fl Donald R. Vander Molen et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 73, in Claim 1, the word "of" should be --fOr-- Column 8, line 26, in Claim 1, after "for" should be said gating signal-- Column 10, lines 6-7, in Claim 9, the word "switchin" Atteat:

should be -switching-- SIGNED AND SEMI-YD SEP8-1970 EdwardMFletcherJr.

Attesting Officer