US3439431A - Microwave dryer control circuit - Google Patents

Description

April 22, 1969 DCIS. HEIDTMANN 3,439,431

MICROWAVE DRYER CONTROL CIRCUIT Filed Dec. 15, 1967 Sheet of 2 F" DONALD s. S E ITT Ia ANN ms ATTORNEY Apfil 22, 1969- 5, am-M 3,439,431

MICROWAVE DRYER CONTROL CIRCUIT Filed Dec. 15, 1967 Sheet 3 of 2 FGc 3 4-2 7 R4 R D5 JSIZAH 4Z gs- 3 D "e 12.. .1 1 C5 Ru rCv i R 5 s Ci T INVENTOR. DONALD s. HEmTMArQN H \s ATTORNEY United States Patent Int. Cl. F26b 3/34 US. Cl. 34-1 Claims ABSTRACT OF THE DISCLOSURE In a dryer of the type having means providing a signal responsive to the pattern of reflection of high frequency electromagnetic waves emitted into the drying chamber, a control including timing means to terminate operation of the dryer, an R-C charging circuit connected to the timing means to energize the timing means in response to a predetermined charge on the capacitor and a transistor, having its collector-emitter path connected across the capacitor and its base coupled to the signal means to dissipate the charge whenever the signal indicates wet items in the dryer.

Background of the invention This invention relates to drying machines of the type having a microwave control, that is, a control which utilizes the pattern of reflection of high frequency electromagnetic waves emitted into the drying chamber to control the operation of the dryer. Two machines of this general type are described and claimed in US. Patents 3,192,642, Hughes; and 3,290,587, Chafee; both of which are assigned to General Electric Company, assignee of the present invention.

Summary of the invention It is an object of this invention to provide an improved dryer control of the microwave type which more accurately responds to the true condition of the total load.

It is another object of the present invention to provide an improved dryer control of the microwave type which continues to be responsive to the reflected signal even after that signal first indicates the fabric load has reached the desired degree of moisture retention.

In accordance with one embodiment of this invention there is provided a control for a fabric drying machine having a chamber to receive fabrics to be dried, heating means arranged to heat fabrics in the chamber, and cyclically moving means for tumbling fabrics within the chamber. The control includes signal means for providing a signal responsive to the pattern of reflection of high frequency electromagnetic waves emitted into the chamber, the pattern of reflection being an indication of the moisture content of fabrics in the chamber. The control also includes shut-01f control means effective, after energization, to terminate operation of the fabric drying machine and time delay means connected to the control means for normally causing energization of the control means after a predetermined time delay. Shunt means interconnects the signal means and the time delay means and shunts the time delay means in response to signals indicating at least a predetermined moisture content of the fabrics, whereby energization of the control means is prevented when the moisture content of the fabrics is at least as high as said predetermined value.

Brief description of the drawings FIGURE 1 is a schematic side elevational view of a clothes dryer suitable for incorporation of my improved 3,439,431 Patented Apr. 22, 1969 dryer control, the View being partly broken away and partly sectionalized to illustrate details;

FIGURE 2 is a circuit diagram of an oscillator and detector which may be employed in the dryer control of the present invention;

FIGURE 3 is a circuit diagram of other portions of a control in accordance with one embodiment of this invention; and

FIGURE 4 is a circuit diagram of other portions of a control in accordance with another embodiment of this invention.

Description of the preferred embodiments Referring now to FIGURE 1, the machine illustrated is a domestic clothes dryer generally indicated by the numeral 1. Dryer 1 is provided, in the usual way, with a cabinet 2 having a front door 3 to provide access to the interior of the cabinet. Within cabinet 2 there is provided a fabric tumbling chamber or drum 4, mounted for rotation on a substantially horizontal axis. The drum 4 is generally cylindrical in shape having an outer cylindrical wall 5, a front wall '6 and a back wall 7. The cylindrical wall 5 is imperforate so that the outer shell of the basket is imperforate. A plurality of clothes tumbling ribs, such as that illustrated at 8, are provided on the interior of the wall 5 to lift fabrics as the drum rotates, and then allow them to thumble back to the bottom of the drum.

The front of the drum is rotatably supported within the cabinet 2 by suitable idler wheels, one of which is shown at 9 These wheels are mounted on a member 10 which extends up from the base 11 of the cabinet. The wheels 9 aredisposed beneath the drum and in contact with the cylindrical wall 5 so as to provide support on each side of the front portion of the drum. The rear end of the drum receives support by means of a stub shaft 12 which extends from the center of the wall 7 and is secured within a bearing 13 which, in turn, is rigidly secured to the back wall 14 of the cabinet.

In order to provide for the flow of a stream of drying air through the drum, the drum is provided with a central aperture 15 in its front wall 6 and an opening in the form of a plurality of perforations 16 in its rear wall 7. Electrical heating elements, such as those schematically illustrated at 17 and 18, are mounted between the rear wall 7 of the drum and the rear wall 14 of the cabinet. The front opening 15 of the drum is substantially closed by means of a stationary bulkhead structure made up of a number of adjacent members including the upper portion of member 10 and the inner member 19 of the door 3. A suitable clearance is provided between the edge of the drum providing opening 15 and the edges of the members constituting the bulkhead so that there will be no rubbing between the drum and the bulkhead during rotation of the drum.

The front opening 15 serves as a means whereby the clothes may be loaded into and unloaded from the drum. Door 3 includes an outer, flat, imperforate member 20 and the inner member 19, which is formed to be hollow and inwardly extending so as to extend into the opening 15 when the door is closed. The door is mounted on the cabinet 2 so that, when the door is open it exposes opening 15 so that clothes may be inserted into or removed from the drum through the opening 15 and when the door is moved to its closed position it closes opening 15.

The air inlet to the cabinet is provided by perforated openings 21 provided in the back wall 14 so that air may flow in through the openings 21 then upwardly over the heating elements 17 and 18 into the drum through the openings 16. The air outlet from the drum is provided by perforated openings 22 formed in the inner member 19 of the door. The bottom wall section of the door and the adjacent wall of the cabinet are provided with aligned openings 23 and 24 so that the air may travel from the door to a duct 25, formed by the cooperation of member with the front wall 26 of the cabinet. A lint trap may be positioned in the duct 25 at the opening 24 and supported by the wall around the opening. The duct 25 leads downwardly to an opening 26a formed in the member 10. The opening 26a constitutes the inlet to a blower member 27 contained in a housing 28 and directly driven by an electric motor 29. From the blower the air passes through an appropriate duct 30 and out of the machine through the opening 31 in the rear wall l4.

In addition to driving blower 27, motor 29 constitutes a cyclically moving means for rotating the drum to tumble fabrics within the drum. To this end, the motor 29 .is provided with a shaft 32 having a small pulley 33 formed at the end thereof. A belt 34 extends around the pulley and completely around the cylindrical wall 5 of the drum. The relative circumferences of the pulley 33 and drum wall 5 cause the drum to be driven at a speed suitable to effect tumbling of the fabrics within the drum.

A dryness sensor for use with the present invention is positioned within the door 3'. The dryness sensor comprises a circuit board 35 having a ground plane affixed to one surface thereof and upon which there is positioned an oscillator for generation of high frequency oscillations and a detector for the detection of waves reflected from the fabric chamber. An antenna 36 is aflixed to the circuit board 35, the antenna 36 communicating with the chamber 4 through an electromagnetic wave transparent window 37, which forms a part of the inner member 19 of the door.

Referring now to FIGURE 2, there is shown a circuit diagram of a dryness sensor or signal means for providing signals responsive to the pattern of reflections of high frequency electromagnetic waves emitted into the chamber 4. The oscillator of the sensor comprises a transistor T having a microstrip line 38 connected between its collector and a supply conductor 39. The microstrip line comprises a metal strip on the opposite side of the circuit board from the ground plane and is formed in accordance with well known printed circuit techniques. A second microstrip line is connected between the intermediate point of the microstrip line 38 and the antenna 36 to couple the generated oscillations to the antenna. The base of the transistor T is connected to the intermediate point of a voltage divider comprising resistances R and R connected between the supply conductor 39 and a neutral conductor 41 to establish a fixed bias voltage at the base of transistor T A current limiting resistance R is connected between the emitter of transistor T and the neutral conductor. An A.C. bypass capacitor C is connected between the supply conductor 39 and the base of the transistor T The detector portion of the sensor comprises a diode D capacity coupled to an intermediate point of the microstrip 40 through a capacitance C and connected to the neutral conductor 41 through a resistance R The detector serves to provide an A.C. signal between a signal conductor 42 and the neutral conductor 41 which is responsive to the pattern of reflection of the high frequency electromagnetic waves which are emitted into the chamber or drum formed by the antenna 36. A capacitor C is connected between signal conductor'42 and neutral conductor 41. This capacitor filters out the high frequency signal but passes the low frequency signal associated with the pattern of reflection.

The operation of the oscillator in FIGURE 2 is such that the internal collector-to-emitter capacitance of the transistor T causes positive feedback from the collector to the emitter, thereby developing oscillations. These oscillations appear across the microstrip 38, which serves as a collector tuned circuit and which is resonant at the desired frequency. The resonant oscillations across the microstrip 38 are coupled to the antenna 36 through the microstrip 40.

The antenna 36 transmits electromagnetic waves into the fabric chamber or drum 4 and which are reflected from the container. The pattern of reflection is an indication of the moisture content of the fabrics in the chamber, which enables the reflected waves to be utilized to terminate the operation of the machine.

The detection may be achieved in several ways but can be expediently effected by measuring the standing wave ratio of the microstrip 40. The diode D in conjunction with the R-C network comprised of the resistance R; in capacitance C serves to detect the standing ratio and thus provide a signal responsive to the degree of dryness of the fabrics within the chamber. The detected signal, appearing between the conductors 42 and 41 is used to terminate operation of the machine. The dryness sensor or signal means described above is substantially similar to that shown in US. Patent No. 3,290,587, Chafee, assigned to General Electric Company, assignee of the present invention. v

Referring now to FIGURES 3 and 4 there is shown therein, in circuit diagram form, other portions of the control, in accordance with two embodiments of the present invention. It will be noted from the use of the reference numerals 39, 41 and 42 for the supply conductor, neutral conductor and signal conductor in FIG- URES 3 and 4 that the circuits illustrated in these figures are designed to be connected to the ends of the conductors 39, 41 and 42, as illustrated in FIGURE 2, so as to form a complete control circuit.

Conductor 41 and a conductor 39A are adapted to connect the control to a suitable source of electrical energy such as the usual volt, A.C. domestic power supply with conductor 39A being the power conductor and conductor 41 the neutral conductor.

Referring now particularly to FIGURE 3, shutoff control means are connected between the conductors 39A and 41 and include timing means such as timer motor 42, which is effective after a predetermined amount of run time to terminate operation of the machine. Many ways in which timer motors may be used to terminate the operation in fabric dryers are well known and specific connections between the timer motor and other operative elements of the machine, such as heaters 17 and 18 and drive motor 29, do not form a part of this invention and have been omitted for the sake of simplicity.

The timer motor is connected on one side to the power conductor 39A and one the other to the neutral conductor 41 through a parallel path including the anode to cathode circuit of a controlled rectifier 43 and a diode D It will be seen that the controlled rectifier and diode D are connected in the circuit in opposite polarity. Thus the diode D will conduct when power conductor 39A is negative with respect to the neutral conductor 41 and the controlled rectifier will conduct when the power conductor 39A is positive with respect to the neutral conductor, assuming a suitable signal has been applied to its gate. The timer motor may be of the induction type and it will not run on the half wave power provided by the diode D alone, so that operation of the motor 42 is possible only when the controlled rectifier 43 conducts for at least a portion of the half cycles when the power conductor 39A is positive. A capacitance C is connected across the parallel connection of controlled rectifier 43 and diode D primary for suppression of any transient signals received from the source.

Down circuit from the shut-01f control means connections, the control circuit is provided with a regulated half wave D.C. power supply. The series connection of a dropping resistor R and a diode D is connected to the power conductor 39A. A- capacitance C; is connected,

On one side, to the diode D and, on the other side, to the neutral conductor 41. This provides a filtered, halfwave DC. signal. A resistance R is connected, on one side, to the junction of D and C and, on the other side,to supply conductor 39. A Zener diode Z is connected between neutral conductor 41 and supply conductor 39 'at the other side of R This provides a regulated DC. power supply of a type well known in the art. which establishes a stable DC. potential between the conductors 39 and 41 with the voltage being established by the rating of the Zener diode.

Conduction of the controlled rectifier 43 is controlled through a time delay means including a voltage divider comprising a pair of serially connected resistances R and R which are connected between the conductors 39 and 41. An R C charging circuit, consisting of a serially connected capacitance C and resistance R is also connected between the conductors 39 and 41. A transistor T is provided with its emitter connected to the junction of the resistances R and R its base connected to the'junction of the capacitance C and resistance R and its collector connected to the conductor 41 through a resistance R The gate of the controlled rectifier is connected by a conductor 44 to the junction between the collector of the transistor T and the resistance R With this arrangement the transistor will conduct only when the voltage at its base, that is the voltage across the capacitance C has reached a level which is slightly greater than the voltage at the junction at the resistances R and R When this occurs the transistor begins to conduct and the collector current gates the controlled rectifier 43. The resistance R serves to prevent leakage current from gating the controlled rectifier.

With the circuit as thus far described the operation of the timer motor 42 would be delayed only for a few seconds after the dryer begins operation, that is until the voltage on the capacitance C has built to a suflicient level. Thereafter the controlled rectifier would be gated so that the timer motor would run and cause termination of the operation of the machine. In order to prevent energization of the timer motor 42 until the pattern of reflection of the high frequency electromagnetic waves emitted in the chamber 4 indicates that the moisture content of the fabrics in the chamber have reached a predetermined desired level, generally corresponding to suitably dry fabrics, there is provided shunt meants interconnecting the signal means and the time delay means. This shunts the time delay means in response to the pattern of reflection indicating a moisture content of the fabrics which is above the predetermined level.

To this end a resistance R is connected between the signal conductor 42 and the neutral conductor 41 and includes an adjustable center tap 45 so that the percentage of the signal responsive to the pattern of reflection appearing at the tap 45 may be varied to calibrate the control.

A transistor T has its collector connected to conductor 39 through a limiting resistance R its emitter connected to neutral conductor 41 through a limiting resistance R and its base coupled to the center tap through a capacitance C A bias resistance R is connected between the collector and base of transistor T and provides a fixed current to the base to set the zero signal operating point of the transistor.

The signal at center tap 45 is a DC. voltage with a low frequency AC. voltage superimposed on it. The coupling provided by capacitance C removes the DC. portion of this signal so that only the AC. component is fed into the base of the transistor T In the usual manner, the transistor T operates as an amplifier and a larger but proportional collector-emitter current flows from conductor 39 through R T and R to conductor 41 in response to the A.C. base current.

A transistor T; has its emitter connected to conductor 39 just down circuit of capacitance C and its collector connected to the junction of the capacitance C and resistance R of the charging circuit. The base of transistor T is coupled to the collector of the transistor T through a capacitance C to remove the DC. level of the amplified signal so that only the AC. component of the amplified signal is applied to the base of transistor T A resistance R is connected between the conductor 39 and the junction of C and the base of T to provide a discharge path for C when the base of T is positive with respect to conductor 39 and to prevent leakage current from turn- 1ng on T With this arrangement of parts for the shunt means, the signal produced by the signal means in response to the pattern of reflection of high frequency electromagnetic waves emitted into the drying chamber 4 is amplified and fed to the base of the transistor T so as to control the flow of current therethrough. When current flows through the transistor T it will discharge the voltage which is tending to build on the capacitance C The elements are sized so that, as long as the pattern of reflection indicates that the degree of moisture in the fabrics being dried is above a predetermined level, the transistor T conducts. This prevents capacitor C from being charged to a level sufficient to cause the transistor T to conduct and the timer motor 42 is prevented from running. Once the pattern of reflection indicates the degree of moisture in the fabrics has fallen to the predetermined level the signal from the dryness sensor or signal means will be insufficient to cause the transistor T to conduct. Thereafter, within a few seconds, the capacitor C will have charged to the firing voltage for transistor T which will conduct and gate the controlled rectifier 43. The timer motor then begins to run and, after a predetermined amount of run time, will cause the operation of the dryer to be terminated. The manner of control of the dryer by timer motors is well known in the art, such as for instance operating a number of cams which open and close switches controlling the various components. Therefore no particular cam and switch arrangement will be illustrated. The timer motor could, for instance, first cause the heating elements '17 and 18 to be de-energized while con inuing operation of the motor 29 so as to provide a period of cooling of fabrics within the dryer and then interrupt the main power to the machine so as to tie-energize both the main motor 29 and the control circuit itself.

It will be understood that the signal from the transistor T could be used to operate other shut-ofl? control means such as, for instance, a relay which would immediately de-energize the entire dryer upon conduction by the transistor T It will be obvious to those skilled in the art that the transistors T and T are PNP types. Similar controls may be provided in accordance with the present invention wherein these transistors are of the NPN type and one such control is shown in FIGURE 4. Elements in FIGURE 4 which are the same as FIGURE 3 are identified with the same number. The timer motor control circuit is essentially the same as with FIGURE 3 and the power supply circuit is the same. The amplifier portion of the shunt means, that is those elements associated with the transistor T is the same. The modification of the circuit essentially is in the time delay portion. The R-C charging circuit is reversed, that is the resistance R is connected to the conductor 39 and the capacitance C is connected to the neutral conductor 41. The transistor T which controls the gating of the controlled rectifier 43 has its collector connected to the conductor 39 through a resistance R its emitter connected to the conductor 41 through a resistance R and its base connected to the junction between resistance R and capacitance C The shunt transistor T has its collector connected to the junction between the resistance R and the capacitance C its emitter connected to the conductor 41, and its base coupled to the collector of the transistor T through the capacitance 0,. A resistor R is connected between the neutral conductor 41 and the base of transistor T to provide a discharge path for C when the base of T is negative with respect to conductor 41 and to prevent leakage current from turning on T The mode of operation with the NPN transistors T and T is slightly different than the mode of operation of the circuit shown in FIGURE 3. In the circuit of FIGURE 3, the transistor T will not turn on until the capacitor C has a charge which is slightly higher than the voltage at the junction between R and R However, when this voltage is reached the transistor T is essentially turned full on to provide a gate signal for the controlled rectifier 43. In the circuit of FIGURE 4, the transistor T acts as an amplifier and, once the charge on the capacitor C has built to some minimum value higher than the internal emitter to base voltage drop of the transistor, T will begin to conduct, acting as an amplifier. This will provide a voltage across the resistance R which tries to gate the controlled rectifier 43. The transistor T shunts the capacitor C and thus controls the size of the signal at resistance R16, however, T will conduct to some extent before the fabrics are dry. In order to prevent the controlled rectifier from being gated until after the refiected wave pattern causes the transistor T to be turned off, a voltage breakdown device such as Zener diode Z is connected in the line 44. Thus the signal appearing across the resistance R must reach the breakdown voltage of the device Z before the controlled rectifier is gated. The components are sized so that this will not occur until the charge on capacitance C builds to a level that is impossible so long as transistor T is conducting. A resistance R is connected between the neutral conductor 41 and the gate of the controlled rectifier to prevent leakage current from turning on the rectifier.

With a control built in accordance with the present invention the termination of the operation of the dryer is more truly responsive to the average moisture content of the load rather than any individual item. Normally the signal provided by a signal means such as that shown in FIGURE 2 will be responsive to the average degree of dryness of the fabrics in the dryer; however, if the volume of the items being dried is very small as compared to the volume of the container 4, there may be instances of time when none of the items are in the path of the high frequency waves emitted by the antenna 36. In this event the pattern of reflection will indicate the container is empty, which pattern is very similar to the one when the items are dry. Therefore, with very small loads, the signal may indicate that the load is dry even though the items in the container are still wet. The elements of the present control may be sized so that there is a delay of at least several seconds after the shunt transistor ceases to conduct before the controlled rectifier will be gated. This provides ample opportunity for items of a small load to move into the path of the high frequency waves and the shunt transistor to start conducting again and bleed the charge off the storage capacitor before the timer motor is energized.

Also, by using a timing means such as a timer motor to cause the operation of the dryer to continue for some predetermined period of timer run time after a dry signal is received, such as for a cool down opeartion; a circuit built in accordance with the present invention has the added advantage of continuing to monitor the moisture content of the load even after the pattern of reflection indicates dryness and the controlled rectifier is gated. Even after the timer motor begins to run it will continue to run until it causes termination of the operation of the dryer only so long as the shunt transistor, T or T does not conduct. Thus, if the timer motor starts and then the reflection pattern changes to show that the load is not dry, the shunt transistor will be turned on and the timer motor will turn off until the load is dry.

8 I have found that controls as illustrated in FIGURES 2, 3, and 4 for present day fabric dryers may be made with the below listed components:

R ohms 8200 R do 1500 R do 390 R do 330 R H R kilohms 10 R do 150 R9, & R 10 d0 R11 .....do R ohrns 470 R megohm 1 R14 ki10hmS R15 ...0hm R kilohms 10 R17 ...dO- R18 d0 1 1 microfarads .0001 0 picofarads 25 C do C mi ofara .05 C ..do.. C do.. 15 C do 10 C ....dO T Fairchild semiconductor SE3005 T 2N3638 T 2N2926 T 2N3638 T 2N2926 T 2N2926 D 1N82A D 1N5059 D 1N5059 Z Fairchild semiconductor FZ903 Z Fairchild semiconductor FZ902 Controlled rectifier 43 General Electric C10=6B What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a fabric drying machine having a chamber to receive fabrics to be dried, heating means arranged to heat fabrics in the chamber, and cyclically moving means for tumbling fabrics within the chamber; a control including:

(a) signal means for providing signals responsive to the pattern of reflection of high frequency electromagnetic waves emitted into the chamber, said pattern of reflection being an indication of the moisture content of fabrics in the chamber;

(b) shut-off control means effective, after energization, to terminate operation of the fabric drying machine;

(c) time delay means connected to said control means for normally causing energization of said control means after a predetermined time delay;

(d) shunt means interconnecting said signal means and said time delay means to shunt said time delay means in response to signals indicating at least a predetermined moisture content of the fabrics, whereby energization of said control means is prevented when the moisture content of the fabrics is at least as high as said predetermined value.

2. The invention as set forth in claim 1 wherein said time delay means includes an R-C charging circuit effective to cause energization of said control means in response to a predetermined charge on said capacitor and wherein said shunt means is effective to dissipate the charge on said capacitor when the signal indicates at least a predetermined moisture content of the fabrics.

3. The invention as set forth in claim 1 wherein said control means includes timing means effective to terminate operation of the dryer after a predetermined amount of timing means operation.

4. The invention as set forth in claim 3 wherein said time delay means includes an R-C charging circuit and is effective to cause energization of said timing means in response to a predetermined charge on said capacitor and wherein said shunt means is effective to dissipate the charge on said capacitor when the signal indicates the moisture content of the fabrics is at least said predetermined value; whereby said timing means is energized only when the signal indicates the moisture content of the fabrics is below said predetermined value.

5. In a fabric drying machine having a chamber to receive fabrics to be dried, heating means arranged to heat fabrics in the chamber, and cyclically moving means for tumbling fabrics within the chamber; a control including:

(a) signal means for providing signals responsive to the pattern of reflection of high frequency electromagnetic waves emitted into the chamber, said pattern of reflection being an indication of the moisture content of fabrics in the chamber;

(b) an energizing circuit for connection to a source of electrical energy;

(c) shut-off control means effective, after energization, to terminate operation of the fabric drying machine;

(d) a first transistor, including a collector, an emitter and a base, said control means being connected in said energizing circuit through the collector-emitter circuit of said first transistor;

(e) time delay means connecting said base in said energizing circuit and effective for normally causing conduction of said first transistor to energize said control means after a predetermined time delay;

(f) shunt means interconnecting said signal means and said time delay means to shunt said time delay means in response to signals indicating at least a predetermined moisture content of the fabrics, whereby energization of said control means is prevented when the moisture content of the fabrics is at least as high as said predetermined value.

6. The invention as set forth in claim 5 wherein said time delay means includes an R-C charging circuit effective to cause conduction of said first transistor to energize said control means in response to a predetermined charge on said capacitor and wherein said shunt means includes a second transistor having its collector-emitter circuit connected across said capacitor and its base coupled to said signal means to dissipate the charge on said capacitor when the signal indicates the moisture content of the fabrics is at least said predetermined value, whereby said control means is energized only when the signal indicates the moisture content of the fabrics is below said predetermined value for a predetermined time delay.

7. The invention as set forth in claim 6 wherein a third transistor is connected in said energizing circuit with its collector-emitter circuit coupled to said base of said second transistor and its base coupled to said signal means thereby to amplify said signal.

8. The invention as set forth in claim 5 wherein said control means includes timing means effective to terminate operation of the dryer after a predetermined amount of timing means operation.

9. The invention as set forth in claim 8 wherein said time delay means includes an R-C charging circuit effective to cause conduction of said first transistor to energize said timing means in response to a predetermined charge on said capacitor and wherein said shunt means includes a second transistor having its collector-emitter circuit connected across said capacitor and its base coupled to said signal means to dissipate the charge on said capacitor when the signal indicates the moisture content of the fabrics is at least said predetermined value; whereby said timing means is energized only when the signal indicates the moisture content of the fabrics is below said predetermined value for a predetermined time delay.

10. The invention as set forth in claim '9 'wherein a third transistor is connected in said energizing circuit with its collector-emitter circuit coupled to said base of said second transistor and its base coupled to said signal means thereby to amplify said signal.

References Cited UNITED STATES PATENTS 3,290,587 12/1966 Chafee 34-l XR 3,192,642 7/1965 Hughes -34-48 XR 3,331,139 7/1967 Finnegan et a1. 34-48 XR KENNETH W. SPRAGUE, Primary Examiner.

US. Cl. X.R.

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