its opening before moisture is reflected at the sensing element 54 in the dryer exhaust. It should be recognized, however, that a more sensitive sensing element 54 will preclude the necessity for this time delay in the start switch.
The dryer termination control includes an improved amplifier 56 having a direct current relay coil or winding 82 for actuating a switch 84 in series with the holding circuit coil 78. The switch 84 is normally open and is closable so long as the relay coil 82 is energized. By-passing the relay coil 82 is a transistor 86 having its emitter 88 connected to one side of the coil 82 and its collector SO connected to the other side of the relay coil. Since the relay coil 82 is actuatable by direct current, a diode 92 is included in parallel relationship with the coil to pass current in one direction only. A capacitor 94 may also be included for relays which have a tendency to hum when used on A.C. or pulsating D.C A resistance 96, in series with the relay coil 82, must be small enough to permit the energization of the coil 82 with the available power supply. Grouping the foregoing components, the only electrical connections to the amplifier 56 are a conductor 98 to one side of the domestic 115 v-230 v. power supply line Li and a conductor 100 to the neutral side of the line N. The base connection 102 for the transistor 86 extends out of the amplifier 56 and is connected to a voltage divider circuit which includes the humidity sensing element 54 and a motorized resistance unit 104 in juxtaposition to the amplifier in the console. The unit 104 has a variable resistance 108 and a positioning motor 106 which is thermally responsively energizable for progressively increasing the resistance 108. Since the resistance 108 is gradually and progressively increased during the drying cycle whenever the motor 106 is energized, a mechanical connection (shown schematically at 110) is provided between the positioning motor shaft and the start button 76 so that the variable resistance 108 may be mechanically repositioned to its lowest value each time a new drying cycle is initiated.
The resistances 108 and 54 in the voltage divider circuit are proportioned such that the resistance 108 is much larger, on the order of 100 to 1, than the sensing element resistance 54. Insofar as the voltage divider circuit is concerned, therefore, when the motor 106 runs, the increase in resistance 108 tends to cancel out or offset a portion of the increased voltage drop across the sensor 54 due to the drying of the clothes load (resistance 54 increases as the clothes become dry). The partial offsetting action tends to delay the point at which a sufficient voltage potential exists, base 102 to emitter 88, to fire the transistor 86 and terminate the drying cycle. In effect, the gradually increasing resistance 108 acts to continually decrease the percent relative humidity requirement of the exhaust air necessary to terminate the drying cycle.
Referring now to FIGURES 1 and 2, the dryer control circuit includes such normal safety features as a high limit thermostat 112 which opens upon an abnormal rise in temperature within the heater chamber and a motor switch 114 which is closed by a centrifugally actuated mechanism (not shown) in the main motor 62 to insure that the heater 60 will not be energized unless the motor 62 is operating to rotate the tumbling drum 20. In addition to the safety controls, this circuit includes a termination conrol thermostat 116 responsive to the temperature of air leaving the tumbling drum. The thermostat 116 has a switch blade 118 which closes on a heater circuit contact 120 when the temperature of the exhaust air is between 130° and 140° F. or below. Above this predetermined temperature range, the switch blade moves to the contact 122 in which position it energizes the positioning motor 106 to increase the resistance 108 in the voltage divider circuit. Note that the termination control thermostat 116 is first moved to its positioning motor contact 122 when the sensible temperature of the exhaust air increases suddenly over the said predetermined temperature range. This generally occurs when the clothing is reaching a dry condition as reflected by the moisture content of the dryer
exhaust air. This dry bulb temperature jump has been used repeatedly in prior art devices to terminate the drying cycle. However, such prior art systems fail to compensate for the occasional or LIGHT loads, such as a shag
6 rug or four to five pounds of assorted clothing and hardto-dry items wherein fabric seams resist giving up moisture evenly. In these LIGHT loads, the point at which the sensible temperature jumps above the predetermined temperature range is in advance of the proper dryness
10 end point for the clothes load and, thus, this invention seeks to prolong the cycle by operating the motor 106 to gradually reduce the percent relative humidity at which the cycle will terminate. As has been explained hereinbefore, this is accomplished by increasing the resistance
15 108 in the voltage divider circuit to delay the point at which the humidity sensing element 54 is effective to bias transistor 86 to by-pass the power and shunt the termination relay coil 82. An operating drying cycle will be explained with refer
90 ence to the control circuit of FIGURE 2 and the graph in FIGURE 3. Note that the graph in FIGURE 3 takes into account only the last portion of the drying cycle which shows the manner in which the various type loads terminate. Consider first a NORMAL eight pound load which
2g is placed in the tumbling drum 20 and the dryer energized by pushing the button 76. At the start, the temperature in the tumbling drum is below 130° F. and the thermostat 116 is positioned to energize the heater 60 and the prime mover or main motor 62. The amplifier 56 is actuatabie
30 by humidity sensed by the sensing element 54 in the exhaust duct 52. It is possible that the system may terminate the NORMAL or regular eight pound load cycle before the motor 106 is even energized—assuming that the sensor 54 fired the transistor 86 in advance of the point at which
35 the sensible temperature jumps to actuate the thermostat 116. In this situation, the NORMAL drying cycle will terminate when the resistance of the element 54 is increased to the point at which the transistor 86 is biased to shunt the power from the relay coil §2 to cause the switch 84
40 to open. However, most loads do permit the temperature to increase enough to actuate the thermostat 116 and run the positioning motor 106 some. In graphic form, this more usual condition is represented by curve 136 which is shown dropping off to about to 35% R.H. mark at which point 138 the relay coil 82 is deenergized and the
45 cycle terminated before the clothes are overdried. Thus, at point 138 a NORMAL eight pound load of clothes is dry when the resistance of the sensor 54 increases to create a voltage at the base 102 which wiil fire the transistor.
50 Let us assume now that a LIGHT load, such as a shag rug or a small load (under five pounds) of light fabrics, is being dried. The moisture will be very quickly vaporized from the exposed fabric surface in the LIGHT load. However, in the depths of the shag rug and in the folds
5a and seams of the light fabrics, the moisture will still be retained so that an exhaust air indication of thirty-five percent relative humidity would be a premature indication that the load is dry. Consequently, and in accordance with this invention, when the sensible temperature
00 first jumps and moves above the predetermined setting of thermostat 116, say 140° F., the switch blade 118 will move to the contact 122 to energize the positioning motor 106. The motor then starts to gradually increase the resistance 108 to hold off the point at which the
G5 sensing element 54 would normally fire the transistor 86 to terminate the drying cycle. This, in effect, lowers the relative humidity setting which is necessary to terminate the drying cycle.
_0 A load controlled in accordance with this invention
'would exhibit three curves of interest. The drying rates of a typical LIGHT load and NORMAL load are evidenced on the drying curves 140 and 136 respectively, and the rate of depressing termination point relative
75 humidity is shown by the control curve 142. The re