US3621293A - Electronic dryer control - Google Patents

Abstract

A dryer control including sensing means to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging the sensing means; switch means to terminate operation of the dryer; and circuit means, including a capacitor, interconnecting the sensing means and the switch means. The circuit means include an insulated gate field-effect transistor. The transistor operates the switch means to terminate operation of the dryer when the charge on the capacitor reaches a predetermined level.

Description

I United States Patent [1113,621,293

[72] inventor Donald S. l-leidtmann [56] References Cited Louisville, 9- UNITED STATES PATENTS [21] P 3,331,139 7/1967 Finnegan et al. 34/48 x [221 3,436,838 4/1969 l-lelfrich 317/1485 B x [451 Paemed 1971 3 471 93:; 10/1969 Elders 34/45 Assignee General p y Primary ExaminerDonald D. Forrer Assistant Examiner-L. N. Anagnos Attorneys-James E. Espe and F. H. Boos,.lr., Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [54] ELECTRONIC DRYER CONTROL ABSTRACT: A dryer control including sensing means to con- 12 Claims 4 Drawing Figs tact items being dried to establish a current path therethrough [52] US. Cl. 307/252 N, having a resistance which is a function of the moisture content 34/44, 34/45, 34/48, 307/252 W, 317/ 148.5 E, of the items bridging the sensing means; switch means to ter- 307/304 minate operation of the dryer; and circuit means, including a [51] l t.(l .H03kl7/64, capacitor. interconnecting the sensing means and the switch F26b 13/ 10 means. The circuit means include an insulated gate field-efi'ect [50] Field of Search 34/43, 44, transistor. The transistor operates the switch means to ter- 45, 48, 134; 307/252 R, 252 A, 252 H, 252 N, 252 .l, 252 W, 279, 304, 284; 317/l48.5 B

minate operation of the dryer when the charge on the capacitor reaches a predetermined level.

as HF 6'5 I 82' 93 t. 73 7| un- "55 SS 75 76 4,2 I 5% i fi- 7 97 78 77 L L/J e4 8o 4 63 3e 67 PATENTEnuuv 16 197i SHEET 1 UF 3 mvsmon. DONALD s. HEIDTMANN PIC-ll H is ATToRwEY BACKGROUND OF THE INVENTION Over the past several years, clothes-dryer controls which use a measurement of the resistance of fabrics being dried as the control basis have been developed. In the more popular approach to this type of control, sensors in the dryer drum are positioned so as to be directly contacted by the fabrics being dried. In this manner, the fabrics become a part of the control circuit and, in effect, serve as a variable resistance. The resistance of the fabrics increases as the moisture content of the fabrics decreases. The sensors are connected across the capacitor and control the voltage to which the capacitor charges. A neon lamp, or similar device, is associated with the capacitor such that when the charge reaches the breakdown voltage of the lamp, means are initiated to terminate dryer operation. In such a control circuit, the neon lamp is the least stable and least reliable component and it becomes quite expensive to obtain a neon lamp having the required close tolerances.

Moreover, other approaches to resistance-measuring controls have included a relatively expensive timer mechanism. In these arrangements, the timer is energized when the fabrics reach some predetermined moisture content and continues to operate for several minutes to bring the moisture content down to the desired level. In arrangements heretofore proposed, it was necessary to utilize the timer in order to pro vide a sufficient time delay, between charging of the capacitor and deenergization of the dryer, to bring the moisture content of the fabrics down to the desired level.

Accordingly, it would be advantageous to provide a resistance-sensing dryer control which obviates the neon lamp but retains the desirable aspects of former control. It would also be advantageous to provide a dryer control of the type referred to which obviates the relatively expensive timer mechanism but still provides the necessary delay between the time the fabrics reach the predetermined moisture content and the dryer is shut off.

An object of this invention is to provide an improved dryer control of the resistance-sensing type.

Another object of this invention is to provide such an improved control which obviates the heretofore utilized neon lamp.

Another object of this invention is to provide such an improved control which obviates the timer mechanism.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspect of this invention, there is provided a control for a dryer which includes switch means effective to terminate operation of the dryer in response to an electrical signal. Sensing means are positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging the sensing means. Insulated gate field-efiect transistor means are provided having a source connection, a drain connection and a gate connection. Circuit means are provided to establish a voltage at the source connection. Other circuit means, including the sensing means, provides a voltage at the gate connection which varies with the resistance of the item bridging the sensing means. Circuit means interconnect the transistor means and the switch means. With this arrangement, the transistor means will provide an electrical signal to said switch means when the sensing means provides a resistance which causes a voltage at the gate connection approaching the-value of the voltage at ,the source connection.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed the invention will be better understood from the following description of the preferred embodiments taken in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of a clothes dryer with which my improved dryer control may be used, the view being partly broken away and partly in section to illustrate details;

FIG. 2 is a schematic electric circuit diagram of one embodiment of my invention;

FIG. 3 is a schematic electric circuit diagram of another embodiment of my invention; and,

FIG. 4 is a schematic electric circuit diagram of another embodiment of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is illustrated a domestic clothes dryer 10 which is provided in the usual way with a cabinet 11 having a front door 12 to provide access to the interior of the cabinet for loading or unloading fabrics. Provided on the top wall 13 of cabinet 11 isa control panel 14 which may include a suitable control knob 15. By manual manipulation of control knob 15, the machine may be caused to start, and automatically proceed through a cycle of operation. X

Within cabinet 11, there is provided a fabric-tumbling container, or drum 16, mounted for rotation about a substantially horizontal axis. Drum 17 is substantially cylindrical in shape, having'a first outer cylindrical wall portion 18; second and third, outer, cylindrical wall portions 19 and 20,. located respectively adjacent the front and back of the drum; a front wall 2I and a backwall 22. Outer wall portions 18, 19 and 20 are imperforate over their entire length so that the outer-shell of the basket is imperforate. A plurality of clothes-tumbling ribs 23 are provided ontheinterior of wall portion 18 so that fabrics are lifted'up by the-ribs when the drum rotates, and then tumbled back down to the bottom of the drum.

The front drum 1? may be rotatably supported within cabinet 11 by suitable idler wheels 24,"one of which is shown in FIG. 1. These wheels are rotatably secured to the top of a member 25 which extends up from base 26' of the machine. The wheels 24 are disposed beneath the drum in contact with portion 19 thereof so as to support portion 19 on each side to provide a stable support. The rear end of drum [7 receives support by means of a stub shaft 27 extending from the center of wall 22. Shaft 27 is secured within a bearing 28 supported by a baffle 29 which, in turn, is rigidly secured to the backwall 30 of cabinet 11 by any suitable means such as welding at a number of points 31. Withlthe arrangement shown, the drum may rotate on a horizontal axis, with rollers 24 providing the front support and stub shaft 27 within bearing 28 providing the rear support.

In order to provide for the flow of a stream through the drum, ture 32in its front wall 21 and-with an opening in the form of a plurality of perforations 33 in its rear wall 22, the perforations in the present case being fonned to extend around the rear wall in an annulus.

Bafile member29 also serves -to support heating means 34 which, in the machine illustrated, includes two electrical-resistance heating elements 34a' and 34b, appropriately insulated from the baffle member. "Elements 34a and 345 may be annular in shape so as to be generally coextensive with perforations 33 in wall 22. A second bafi'le member 35 is rigidly secured to the backwall 22 of the drum outside the ring of perforations 33 and within the stationary baflle 29, so that an anof drying air nular air inlet 36 is, in effect, formed by baffles 29 and 35. In

this manner a passageis formed for air to enter annular inlet opening 36 between the battles, pass over the heating-means 34, then pass through openings 37, formed in baffle'35, and then through the perforations 33 to the interior of drum 17.

The front opening 32 of the drum is substantially closed by means of a stationary bulkhead 38. Bulkhead 38 is made up of a number of adjacent members including the innerwall 39 of access door 12; a stationary frame 40 for the door, fbrmed as a flange of the front wall .41 of the cabinet; the inner surface member-42 of an exhaust duct which is formedby cooperation of member 42 with the front wall 41 of the cabinet; and an annular flange 43 mounted on frame 40 and on the duct wall. It

the drum is provided with a central aper-.

will be noted that a suitable clearance is provided between the inner edge of drum opening 32 and the edge of bulkhead 38 so that there is no rubbing between the drum and bulkhead during rotation of the drum. In order to prevent any substantial air leakage through opening 32 between the interior and exterior of the drum, a suitable ring seal 44, preferably formed of feltlike material, is secured to flange 43 in sealing relationship with the exterior surface of drum wall 21.

Front opening 32 also serves as a means whereby clothes may be loaded into and unloaded from the drum. Door 12, whose inner wall forms part of the bulkhead closing the opening, is mounted on cabinet 11 so that, when the door is opened, clothes may be inserted into or removed from the drum through the doorframe 40. It will be noted that the door includes an outer, flat, imperforate section 45 and an inwardly extending hollow section 46 mounted on the outer section. Hollow section 46 extends into the doorframe 40 when the door is closed, and the inner wall 39 which comprises part of the combination bulkhead 38 is actually the inner wall of the hollow section.

The air outlet from the drum is provided by a perforated opening 47 formed in the inner wall 39 of the hollow door section 46. The bottom wall section of the door 12 and the adjacent wall of door frame 40 are provided with aligned openings 48 and 49. Opening 49 provides the entrance to a duct 50 formed by the cooperation of member 42 with front wall 41. As shown, a lint trap 51 is positioned in the exhaust duct 50 at opening 49, the trap being supported by the doorframe 40.

Duct 50 leads downwardly to an opening 52 formed in the member 25 which supports the wheels 24. Opening 52 constitutes the inlet to a blower member 53 contained within a housing 54 and directly driven by an electric motor 55. An inlet, such as the opening 61, is provided so that the blower means draws ambient air in through opening 61, over the heating means 34, then through the basket, then through the door 12 and duct 50, and then into the blower. From the blower the air passes through an appropriate duct (not shown) out of cabinet 11 so as to be exhausted from the machine.

In addition to driving blower 53, motor 55 constitutes the means for effecting rotation of drum 17. In order to effect this rotation, motor 55 is provided with a shaft 56 having a small pulley 57 formed at the end thereof. A belt 58 extends around pullet 57 and also entirely around the cylindrical wall section 18 of drum 17. The relative circumferences of pulley 57 and wall section 18 cause the drum to be driven by the motor at a speed suitable to effect tumbling of the clothes therein. in order to effect proper tensioning of belt 58, there may be provided a suitable idler assembly 59 secured on the same support 60 which supports one end of the motor. Thus, air is pulled through the drum and at the same time the fabrics in the drum are tumbled. When the air is heated by heating elements 340 and 34b, the heated air passing through the drum causes vaporization of moisture from the clothes and the vapor is carried off with the air as it passes out of the machine.

In order to sense the amount of moisture in the fabrics being dried in the drum 17, and thus obtain a suitable signal for controlling the operation of the dryer 10, sensing means are provided to contact and be bridged by the items being dried. In the machine of FIG. 1 the sensing means is in the form of a pair of spaced electrodes 62 and 63 which are mounted on an inwardly extending portion 64 of member 42. The extension 64 is formed so that electrodes 62 and 63 are exposed to the interior of the drum and positioned to be readily contacted and bridged by the fabrics as they tumble within the drum 17. A temperature-sensitive switch or thermostat 65 is positioned in duct 50 just below opening 49 to sense the temperature of the air leaving drum 17 and passing through openings 48 and 49.

The operation of dryer is controlled by the new and improved control arrangement schematically illustrated in the circuit diagram of FIG. 2. Terminals 66, 67 and 68 are connected to a source of electrical power, specifically, 230-volt AC power. Terminal 67 is connected to the neutral conductor of the three wire supply so that a ll5-volt AC supply is provided across terminals 66 and 67 while a 230-volt AC power supply is provided across terminals 66 and 68. Terminal 66 is directly connected to a conductor 69 while terminal 67 is directly connected to a conductor 70. Motor 55 is connected between conductors 69 and 70 while heating means 34 is connected between conductor 69 and terminal 68. A switch 71 is connected in series with motor 55 and switch 72 is connected in series with heating means 34 such that when either of these switches is open, the component connected in series therewith is deenergized. Switches 71 and 72 form a part of a relay further including coil 73. Switches 71 and 72 are normally open and will remain open until coil 73 is energized to move the switches 71 and 72 to their closed position.

Coil 73 is connected between conductors 69 and 70 in series with a resistor 74 and a controlled rectifier 75. Controlled rectifier 75 may be a conventional silicon controlled rectifier having an anode 76, cathode 77 and gate 78. A capacitor 79 shunts coil 73 so that when controlled rectifier 75 is conducting, in response to a suitable signal applied to gate 78, coil 73 will maintain switches 71 and 72 closed despite the fact that rectifier 75 is providing only half-wave power to coil 73. Thus, whenever controlled rectifier 75 is conducting, coil 73 will maintain switches 71 and 72 in their closed positions. A capacitor 80 may be connected across the anode to cathode terminals of rectifier 75 to suppress any transient signals received from the power source.

Thus, the portion of the control circuit thus far described comprises a switch means effective to tenninate the operation of the dryer in response to an electrical signal applied to gate 78 of controlled rectifier 75. This switch means includes controlled rectifier 75, coil 73, and switches 71 and 72.

Also connected between conductors 69 and 70 is a resistor 81 and a Zener diode 82 in electrical series relationship with each other. Connected between resistor 81 and Zener diode 82 is a conductor 83. Resistor 81 reduces the voltage applied to conductor 83 and diode 82, while diode 82 limits the voltage to which conductor 83 may rise due to the breakdown characteristics of Zener diode 82 which are well known in the art. Thus, half-wave rectified voltage is applied to conductor 83, due to the rectifying characteristics of diode 82 when conductor 70 is positive with respect to conductor 69, with the peak voltage of conductor 83, during its positive half cycle, being limited by the breakdown characteristics of diode 82.

A transistor 84 is provided and is of the insulated gate fieldeffect type sometimesreferred to as a metal-oxide-semicom ductor transistor. Transistor 84 has a gate 85, source 86 and drain 87. The insulated gate field-effect transistor is a voltageoperated device requiring little or no gate current. Moreover, the input resistance is exceptionally high because the gate behaves as a capacitor with very low leakage. In the control circuit of FIG. 2, transistor 84 is a P-channel-type wherein when the gate voltage is more negative than the source voltage, the transistor conducts from source to drain. As the gate voltage approaches the source voltage, the transistor discontinues to conduct from source to drain. in most insulated gate field-effect transistors presently commercially available, the transistor shuts off or discontinues conduction from source to drain when the gate voltage approaches within 2 to 5 volts of the source voltage. Thus, in a control circuit of FIG. 2, whenever the voltage of gate 85 is more than 5 volts more negative than the voltage of source 86, transistor 84 will conduct from source 86 to drain 87.

The voltage of source 86 is provided by a center tap variable resistor 88 connected between conductors 83 and 70 such that the center tap 89 provides a voltage-divider effect for source 86.

The voltage at gate 85 is established through a network including a variable resistor 90 which, in conjunction with a resistor 91, serves as a voltage divider to establish a voltage across the series-connected resistor 92, diode 93, and capacitor 94. The sensing means, comprising electrodes 62 and 63, is

connected in parallel with capacitor 94 along with resistors 95 and 96. Drain 87 is connected to conductor 70 through a re sistor 97 and a capacitor 98 and to gate 78 of controlled rectifier 75. With this arrangement, whenever transistor 84 is conducting from source 86 to drain 87, a signal is applied to gate 78 to render rectifier 75 conductive so that coil 73 is energized and switches 71 and 72 remain closed.

The function of the control circuit is, as mentioned above, to terminate operation of the dryer in response to the moisture content-of the items being dried. When the items reach the desired moisture content, motor 55 and heating means 34 are tobe deenergized, although these two components may not become deenergized at the same instant. Thermostat 65, as shown in FIG. 2, shunts switch 71 so that motor 55 will be energized as long as either of thermostat 65 or switch 71 is closed. As mentioned above, thermostat 65 opens and closes in response to the temperature of the air leaving drum 17. in the embodiment of FIG. 2, thermostat 65 is a normally open thermostat and will open whenever the temperature it is sensing drops below some predetermined value. Thus, when the predetermined moisture content of the items being dried reaches that level wherein coil 73 is deenergized and switches 71 and 72 are opened, motor 55 will continue to move air through drum 17 and continue to tumble the items in drum 17 until the temperature of the air passing thermostat 65 drops below some predetermined value. Heating means 34 will, however, be deenergized at the time that coil 73 is deenergized.

As the circuit in FIG. 2 is initially energized, the half-wave rectified power provided by diode 82 establishes a source voltage on source86 of transistor 84 by virtue of resistor. 88. At the time, a varying gate voltage is applied to the gate 85 of transistor 84 through the voltage-divider network comprising resistors 90, 91, 95 and 96 and the resistance of the items bridging electrodes 62 and 63. The gate voltage of gate 85 is variable in that the resistance of the items bridging electrodes 62 and 63 varies with the moisture content of those items. It is the relationship of the gate voltage, applied to gate 85, to the source voltage applied to source 86 which determines whether transistor 84 will conduct to provide a suitable signal to gate 78 of rectifier 75.

The source voltage at source 86' is established by diode 82' and resistor 88. The gate voltage at gate 85 remains more negative than the source voltage because the resistance of the items bridging electrodes 62 and 63 will be relatively low as compared to the resistance of resistor 92. Capacitor 94 will charge to a voltage representative of the voltage across the series-connected resistors 95 and 96 and the items bridging electrodes 62 and 63. As the dryer continues to operate, and the items become drier, the resistance of the items increases and the voltage on gate 85 becomes less negative with respect to source 86. This process continues until such time as the voltage on gate 85 approaches to within 2 to 5 volts of the voltage at source 86 whereupon the transistor means 84 "shuts ofi" or discontinues to conduct.

As mentioned above, while transistor 84 is conducting, a signal isapplied to gate 78 of. controlled rectifier 75 causing rectifier 75 to conduct during those half cycles when the polarity of the signal applied to terminals 66 and 67 is proper. During the time that the gate 78 is receiving the electrical signal and rectifier 75 is conducting, coil 73 remains energized and switches 71 and 72 remain closed. When the voltage on gate 85 of transistor 84 approaches to within 2 to 5 volts of the voltage on source 86, and transistor 84 discontinues conduction from source 86 to drain 87, the signal to gate 78 of rectifier 75 discontinues and coil 73 is deenergized. Upon deenergization of coil 73, switches 71 and 72 open. The opening of switch 72'immediately deenergizes heating means 34 while the opening of switch 71 makes the continued energization of motor 55 dependent upon the condition of thermostat 65.

l have found that a control as illustrated in FIG. 2 for present-day fabric dryers may be made with the following components:

Resistor 74 600 ohms Controlled Rectifier 75 CIO6B Capacitor 79 20 microfarads Capacitor 80 0.05v microfarad Resistor 81 27 K ohms Zener Diode 82 IN968B Transistor 84 MEMSI lC Resistor 88 50 K ohms Resistor 90 50 K ohms Resistor 9l 6!! K ohms Resistor 92 I00 megohms Diode 93 GP2-3 l 2 Capacitor 94 3 microfurnds Resistor 95 v 470 K ohms Resistor 96 470 K ohms Resistor 97 I0 K ohms Capacitor 98 0.! microtarad It can be seen from the=values listed above for resistor 92 and capacitor 94 that the charging time constant is approximately 300 seconds, this representing the product of the resistance of resistor 92, expressed in megohms, and the capacitance of capacitor 94, expressed in microfarads. As long as items of relatively high moisture content, and concurrently relatively low resistance, bridge electrodes'62 and 63, capacitor 94 will discharge through the circuit including resistors 95 and 96 and the electrodes 62 and 63 to prevent a full charge from-being established on capacitor 94 untilthe fabrics or items-being dried reach the proper resistance. Capacitor 94 provides a timelag between the creation of high resistance across electrodes 62 and 63 and the deenergization of rectifier 75. With such timelag, rectifier 75 will continue to conduct even though items momentarilyfail to contact electrodes 62 and 63. if, as the fabrics tumble, there are momentary lapses of contact with electrodes 62 and 63, the charging time of capacitor 94, resulting from the large time constant of the charging circuit, will provide a period during which the items may reestablish contact before rectifier 70 is deenergized.

Moreover, because the time constant is in the magnitudeof several minutes, the charging time of capacitor 94 also provides a period to bring the moisture content of the items being dried down to a substantially lower level than the level at which their resistance allows capacitor 94 to begin charging. In a normal resistance-measuring dryer control, the resistance of fabrics being dried reaches a magnitude of 5 to l0 megohms when the moisture content-is approximately l5 percent. It is difficult to measure a change in resistance when the items have a moisture content less than 15 percent. However, the moisture content of 15 percent may not be as low as the desired moisture content and, for this reason, in systems heretofore proposed, a timing device was incorporated in the control circuit which wouldbe energized at approximately l5 percent moisture content and then time out an additional drying period to bring the moisture content of the clothes down to the desired level. The relatively high time constant of the charging circuit ofcapacitor 94,provides this additional drying period without requiring the relatively expensive timer mechanism.

Resistor 90 is actual practice comprises a manually adjustable potentiometer-which has its control knob accessible to the operator of the machine so that the operator, by adjusting resistor 90, can adjust the ultimate dryness of the items being I dried by varying the charging rate of capacitor 94 since capacitor 94 charges through resistor 90. Varying the re sistance of resistor will not vary the time constant established by resistor 92 and capacitor 94 but it will vary the voltage to which capacitor 94 charges and, since resistor 90 is positioned between capacitor 94 and the path through which the voltage at source 86 is established, it does vary the voltage to which capacitor 94 will be charged relative to source 86. Accordingly, when the resistance of resistor 90 is lowered, capacitor 94 will charge to aneffective voltage relative to source-86 in a shorter time. Center justable so that a factory adjustment can be made to calibrate the voltage established at source 86.

In many applications of an automatic clothes dryer, it is desired to have a cool-down period following the removal of tap resistor 88-is made admoisture from the clothes to the desired level so that the clothes will be cool when removed from the dryer. Experience has also shown that with permanent press items, it is especially desirable to have the drying period followed by a cool-down period during which the clothes are tumbled without the application of heat. In order to achieve this in the circuit of FIG. 2, thermostat 65 shunts switch 71 so that motor 55 will continue to tumble the clothes within drum 17 after heating means 34 has been deenergized. This tumbling action will continue until the temperature of the air passing from drum 17 into duct 50 drops to some predetermined temperature. When this occurs, thermostat 65 opens and motor 55 becomes deenergized.

Referring now to FIG. 3, another embodiment of the present invention, is shown. Those components which are substantially identical to the components of the embodiment illustrated in FIG. 2 have been given identical reference numerals. The operation of the switching means of the embodiment of FIG. 3 is the same as the operation of the switching means of the embodiment of FIG. 2. That is, as long as controlled rectifier 75 is conducting, coil 73 is energized and switches 71 and 72 are maintained in their closed positions to energize heating means 34 and motor 55. Similarly, thermostat 65 shunts switch 71. It will be noted that gate 78 of controlled rectifier 75 is connected to the collector connection 99 of a transistor 100. Transistor 100 also has an emitter 101 and a base 102. Collector 99 of transistor 100 is connected to conductor 83 through a resistor 103.

An insulated gate field-effect transistor 104 has a source 105, a drain 106 and a gate 107. Source 105 is connected to a voltage-divider network comprising resistors 108 and 109, with resistor 109 being a variable resistor to provide calibration of the voltage applied to source 105. Drain 106 is connected to conductor 70 through a resistor 110 and to base 102 of transistor 100. Gate 107 is connected into a voltage-divider network comprising resistors 111, 112, 113 and 1 14 as well as the items bridging electrodes 62 and 63. A capacitor 115 interconnects gate 107 with conductor 83 and shunts the leg of the voltage divider comprising resistors 113 and 114 and the items bridging electrodes 62 and 63.

The operation of the circuit of FIG. 3 is somewhat similar to that described above in connection with FIG. 2 in that as the resistance of the items bridging electrodes 62 and 63 increases, capacitor 115 is allowed to charge to a higher level to establish some predetermined voltage at gate 107 of transistor 104 to afi'ect the source to drain current flow through transistor 104. The change in this current flow in turn affects the signal applied to gate 78 of controlled rectifier 75. However, whereas in the circuit of FIG. 2 the transistor 84 conducted current from source to drain until such time as the items became relatively dry, in FIG. 3 transistor 104 does not conduct until such time as the items become relatively dry.

Specifically, the operation of the circuit of FIG. 3 is as follows. As the circuit is initially energized, and items of relatively low resistance are bridging electrodes 62 and 63, capacitor 115 is prevented from reaching a substantial charge. As a result, the voltage at source 105 is such that the voltage at gate 107 is not sufficiently negative with respect to the source voltage for transistor 104 to conduct from source 105 to drain 106. During this condition, transistor 100 remains nonconductive from collector 99 to emitter 101. Accordingly, a signal is applied to gate 73 through resistor 103 to maintain coil 73 energized. As the items bridging electrodes 62 and 63 become more resistive, and the charge on capacitor 115 increases, the voltage at gate 107 of transistor 104 becomes more negative with respect to the voltage at source 105. Depending upon the calibration of variable resistor 109, at some predetermined moisture content of the items bridging electrodes 62 and 63, transistor 104 conducts from source 105 to drain 106 applying a signal to base 102 of transistor 100. As this signal is applied to base 102, transistor 100 conducts from collector 99 to emitter 101 shunting gate 78 and, in effect, removing the signal from gate 78 so that controlled rectifier 75 no longer conducts. As controlled rectifier 75 discontinues conducting,

coil 73 is deenergized and the eventuaj'sirutdbwn of the dryer takes place as it does in the circuit of FIG. 2.

Examples of appropriate components for the circuit of FIG. 3 are set forth below:

Controlled Rectifier 75 C I068 Capacitor 0.05 microfamd Resistor 81 22 K ohms Zencr Diode 82 IN96HB Diode 93 GP2-3 l 2 Transistor 100 2N5l72 Resistor 103 33 K ohms Transistor 104 TIXS67 Resistor 108 3300 ohms Resistor 109 20 K ohms Resistor H0 47 K ohms Resistor I11 20 megohms Resistor 112 $0 megohms Resistor 3 I00 K ohms Resistor 114 100 K ohms Capacitor 115 5 microfarads Diode I16 IN5059 It should be noted that in the embodiment of FIG. 3 a diode 116 is shown shunting coil 73 to prevent chatter of the relay during the half cycle of the applied power when the polarity of that power is such that controlled rectifier 75 is not conducting. Diode 116 serves the same purpose as capacitor 79 in the circuit of FIG. 2.

In FIG. 4 there is illustrated a circuit diagram of another embodiment of the present invention similar to the e'inbodiment of FIG. 3. Similar components have been given identical reference numerals. It will be noted that transistor 100 has been omitted and the output of transistor 104 is directly applied to the gate of controlled rectifier 75. Moreover, controlled rectifier 75 is in series circuit with the heater 117 of a thennally operated switch 118. Switch 118 includes switch elements 119 and 120, which are normally closed but which will open under the influence of heat generated by heater 117. Thus, switch elements 119 and 120 will remain closed, energizing motor 55 and heating means 34 until such time as controlled rectifier 75 conducts to energize heater 117.

The operation of the circuit in FIG. 4 is similar to that of FIG. 3 in that transistor 104 does not conduct until such time as the resistance of the items bridging electrodes 62 and 63 reaches some predetermined value corresponding to a degree of dryness of the items. When transistor 104 conducts, a signal is applied to gate 78 of rectifier 75 causing rectifier 75 to conduct. Conduction of rectifier 75 causes heater 117 to open switch elements 119 and 120. When switch element 120 opens, heating means 34 is deenergized. When switch element 119 is opened, energization of motor 55 becomes dependent upon the condition of thermostat 65.

Examples for appropriate components for the circuit of FIG. 4 are set forth below:

Controlled Rectifier 79 CIO6B Capacitor 80 0.05 microfarads Resistor 81 33 K ohms Zener Diode 82 IN968B Transistor I04 TIXS67 Resistor 108 3300 ohms Resistor I09 20 K ohms Resistor l0 K ohms Resistor 111 I0 mcgohms Resistor I12 20 megohms Resistor 113 100 K ohms Resistor I14 I00 K ohms Capacitor 5 microfarads It should now be apparent that each of the embodiments of FIGS. 2, 3 and 4 provide a resistance-sensing clothes dryer control which retains many of the advantages of the systems proposed heretofore, e.g., continued drying of the fabrics beyond the point where the moisture content triggers the control circuit to bring the fabrics down to the desired level of dryness, a cool down feature to allow the drum to continue to tumble with cool air passing therethrough, and the feature wherein if the circuit is triggered by momentary absence of fabrics bridging the electrodes, subsequent bridging of the electrodes by moist fabrics will reset the circuit so that the circuit is not irreversibly locked in to termination of the drying cycle by the momentary absence of fabrics bridging the electrodes. Moreover, these circuits obviate the relatively expensive timer mechanism and also obviate the troublesome neon lamp common in systems heretofore. Another advantage of the circuits of the present invention is that by employing an insulated gate field-effect transistor, very high resistances can be employed in the charging circuit of the capacitor shunting the electrodes to provide the very long time constant in the charging circuit of the capacitor. This is-achieved because the insulated gate field-effect transistor is a voltage operated device requiring virtually no current into the gate.

As will be evident from the foregoing description, certain aspects of the invention are not limited to the particular details of construction of the examples illustrated, and it is contemplated that other modifications, applications or variations will occur to those skilled in the art. it is therefore intended by the appended claims to cover such modifications, applications and variations as do not depart from the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is: I

1. A control for a dryer comprising:

switch means effective to terminate operation of the dryer upon deenergization of said switch means;

sensing means positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging said sensing means;

insulated gate field-effect transistor means having a source connection, drain connection and gate connection; circuit means establishing a voltage at said source connection; circuit means including said sensing means to provide a voltage at said gate connection which varies with the resistance of the items bridging said sensing means; and,

circuit means interconnecting said transistor means and said switch means;

whereby said transistormeans will deenergize said switch means when said sensing means provides a resistance which causes a voltage at said gate connection which approaches the value of the voltage at said source connection.

2. The invention of claim 1 wherein said switch means comprises a controlled rectifier in electrical series circuit with a switchmperating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

3. The invention of claim 2 wherein said switch-operating device is the coil of a relay.

4. The invention of claim 2 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

5. A control for a dryer comprising:

shutoff means effective to terminate operation of the dryer upon deenergization of said shutoff means;

sensing means positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging said sensing means;

insulated gate field-effect transistor means having a source connection, drain connection and gate connection; circuit means establishing a voltage at said source connection;

circuit means including said sensing means, a capacitor and through said resistance means and is discharged through said sensing means as long as items of sufficiently low resistance bridge said sensing means;

the capacitance of said capacitor and the resistance of said resistance means having values to provide an R-C time constant of at least 1 minute; and,

circuit means interconnecting said transistor means and said whereby said transistor means will deenergize said shutoff means to terminate operation of the dryer when said capacitor is charged to a voltage which approaches the voltage at said source connection.

6. The invention of claim 5 wherein said shutoff means comprises a controlled rectifier in electrical series circuit with a switch-operating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

7. The invention of claim 6 wherein said switch-operating device is the coil of a relay.

8. The invention of claim 6 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

9. A control for a dryer comprising:

switch means effective to terminate operation of the dryer in response to an electrical signal;

sensing means positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging said sensing means;

a capacitor;

resistance means connected in series circuit with said capacitor whereby said capacitor is charged through said resistance means;

said capacitor being connected in parallel circuit with said sensing means whereby the voltage to which said capacitor is charged depends upon the moisture content of items bridging said sensing means;

said resistance means and said capacitor having resistance and capacitance values, respectively, to provide an RC time constant of at least 1 minute; and,

circuit means interconnecting said capacitor and said switch means to provide an electrical signal to said switch means in response to the charge on said capacitor reaching a predetennined level corresponding to a predetermined moisture content of items being dried;

said circuit means including an insulated gate field-effect transistor having its gate-drain path connected in parallel with said capacitor;

whereby said transistor will provide an electrical signal to said switch means when the charge on said capacitor reaches said predetermined level.

10. The invention of claim 9 wherein said switch means comprises a controlled rectifier in electrical series circuit with a switch-operating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

11. The invention of claim 10 wherein said switch-operating device is the coil of a relay.

12. The invention of claim 10 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

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Claims (12)

1. A control for a dryer comprising: switch means effective to terminate operation of the dryer upon deenergization of said switch means; sensing means positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging said sensing means; insulated gate field-effect transistor means having a source connection, drain connection and gate connection; circuit means establishing a voltage at said source connection; circuit means including said sensing means to provide a voltage at said gate connection which varies with the resistance of the items bridging said sensing means; and, circuit means interconnecting said transistor means and said switch means; whereby said transistor means will deenergize said switch means when said sensing means provides a resistance which causes a voltage at said gate connection which approaches the value of the voltage at said source connection.

2. The invention of claim 1 wherein said switch means comprises a controlled rectifier in electrical series circuit with a switch-operating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

3. The invention of claim 2 wherein said switch-operating device is the coil of a relay.

4. The invention of claim 2 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

5. A control for a dryer comprising: shutoff means effective to terminate operation of the dryer upon deenergization of said shutoff means; sensing means positioned to contact items being dried to establish a current path therethrough having a resistance which is a function of the moisture content of the items bridging said sensing means; insulated gate field-effect transistor means having a source connection, drain connection and gate connection; circuit means establishing a voltage at said source connection; circuit means including said sensing means, a capacitor and a resistance means to provide a voltage at said gate connection which varies with the resistance of the items bridging said sensing means; said sensing means, said capacitor and said resistance means being interconnected so that said capacitor is charged through said resistance means and is discharged through said sensing means as long as items of sufficiently low resistance bridge said sensing means; the capacitance of said capacitor and the resistance of said resistance means having values to provide an R-C time constant of at least 1 minute; and, circuit means interconnecting said transistor means and said shutoff means; whereby said transistor means will deenergize said shutoff means to terminate operation of the dryer when said capacitor is charged to a voltage which approaches the voltage at said source connection.

6. The invention of claim 5 wherein said shutoff means comprises a controlled rectifier in electrical series circuit with a switch-operating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

7. The invention of claim 6 wherein said switch-operating device is the coil of a relay.

8. The invention of claim 6 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

9. A control for a dryer comprising: switch means effective to terminate operation of the dryer in response to an electrical signal; sensing means positioned to contact items being dried to establish a current path therethrough having A resistance which is a function of the moisture content of the items bridging said sensing means; a capacitor; resistance means connected in series circuit with said capacitor whereby said capacitor is charged through said resistance means; said capacitor being connected in parallel circuit with said sensing means whereby the voltage to which said capacitor is charged depends upon the moisture content of items bridging said sensing means; said resistance means and said capacitor having resistance and capacitance values, respectively, to provide an R-C time constant of at least 1 minute; and, circuit means interconnecting said capacitor and said switch means to provide an electrical signal to said switch means in response to the charge on said capacitor reaching a predetermined level corresponding to a predetermined moisture content of items being dried; said circuit means including an insulated gate field-effect transistor having its gate-drain path connected in parallel with said capacitor; whereby said transistor will provide an electrical signal to said switch means when the charge on said capacitor reaches said predetermined level.

10. The invention of claim 9 wherein said switch means comprises a controlled rectifier in electrical series circuit with a switch-operating device whereby said switch-operating device is energized in response to energization of said controlled rectifier.

11. The invention of claim 10 wherein said switch-operating device is the coil of a relay.

12. The invention of claim 10 wherein said switch-operating device is an electric heater and the switch operated thereby is a thermally responsive switch.

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