Abstract:
A control circuit for a blender provides low-cost power conditioning through the use of a high resistance which provides temporary power for operation of low-voltage logic circuitry and low-voltage switches for a time sufficient to switch the motor on, and a lower resistance which provides sufficient power for maintaining the motor on state indefinitely as instructed by the low-voltage logic circuitry. Low average power dissipation is provided by powering the low-voltage logic circuitry and low-voltage switches using the high resistance in a standby mode and switching in the lower resistance only when the motor is activated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is national phase of PCT/US2009/067262 filed Dec. 9, 2009, and claims the benefit of U.S. provisional application 61/142,815, filed Jan. 6, 2009. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to household appliances and, in particular, to control circuitry for small appliances such as blenders and the like. 
     Electrical controls for food blenders of the type used for preparation of meals in a kitchen typically include a set of electrical switches or buttons on a front panel that may be used to control the blender motor. Such switches normally provide at least three operating modes: a “pulse” mode in which the blender operates only while a “pulse” button is pressed, an “on” mode in which the blender operates continuously after the “on” button is pressed and until released by pressing of an “off” button, and an “off” mode which cancels the “on” mode when the “off” button is pressed. 
     These electrical controls may be implemented using “electro-mechanical logic” that employs mechanical features to implement the above mode logic. In such electro-mechanical logic, the pulse button is spring-loaded to return after it is pressed and the on and off buttons are joined with a linkage so that the pressing of the off button releases the on button. 
     Alternatively, the electrical controls may be implemented using “electronic logic” in which each of the buttons is a momentary contact pushbutton and integrated logic circuitry implements the above modes. An advantage of electronic logic is that it works with low-voltage membrane switches requiring lower actuation forces and providing better sealing against contamination. Electronic logic and low-voltage membrane switches also make it easier to provide feedback using LEDs that can shine through transparent windows in the switch membrane. In contrast, electromechanical logic can result in buttons that are relatively hard to push, require substantial actuation distance, and are hard to seal against environmental contamination. 
     Low-cost blenders often cannot support electronic logic, principally because of the cost of circuitry necessary to convert 120 VAC power used for the blender motor to regulated, low-voltages required for typical logic circuits, and because of the cost of integrated circuits to implement the logic. 
     SUMMARY OF THE INVENTION 
     The present invention provides circuitry implementing electronic logic that is suitable for use with low-cost blenders having low-voltage membrane switches and the like. Low-cost power conversion necessary for such electronic logic is obtained by using two different regulation circuits. One circuit provides low power dissipation and low power suitable for standby use and for initial switching of power to the motor. The second circuit provides much higher power dissipation but also higher power suitable to maintain switching of power to the motor. The second circuit is connected to receive power only when the motor is connected. In this way, average power dissipation of power conversion is kept low by the intermittent motor cycling typical of a blender. 
     Specifically, the present invention provides a blender control circuit for a blender having a motor switchably connectable to a power line voltage. The blender control circuit includes an electrically controllable switch having a first terminal receiving the power line voltage and a second terminal providing the power line voltage to the motor and low-voltage logic circuitry receiving a low-voltage for controlling the electrically controllable switch. A first power conditioning circuit receives the power line voltage and provides the low-voltage to the low-voltage logic circuitry at a power level insufficient for continuous operation of the low-voltage logic circuitry and a second power conditioning circuit receives power line voltage from the second terminal of the electrically controllable switch and provides the low-voltage to the low-voltage logic circuitry at a power level sufficient for continuous operation of the low-voltage logic circuitry. 
     It is thus a feature of at least one embodiment of the invention to provide low-cost power conditioning with low standby power dissipation. 
     The low-voltage logic circuitry may communicate with membrane switches having contacts formed from printed circuit traces with limited current handling capacity. 
     It is thus a feature of at least one embodiment of the invention to permit the use of membrane switches in a low-cost blender. 
     The first power conditioning circuit and second power conditioning circuit may be voltage dropping resistors reducing the voltage of line power to the low-voltage. The resistors of the second power conditioning circuit may dissipate more power than the resistors of the first power conditioning circuit. 
     It is thus a feature of at least one embodiment of the invention to convert line voltage to low-voltage through simple resistive voltage reduction without excess power dissipation. Because the second powered conditioning circuit is only active when the motor is active, higher power dissipation and thus higher power production is possible in the second power conditioning circuit while maintaining a low average power dissipation. 
     The power dissipated by the second power conditioning circuit may be greater than a watt and the power dissipated by the first power conditioning circuit may be less than one-tenth of a watt. The electrically controllable switch may be an electromechanical relay having a coil acting on a magnetic armature to open and close contacts across the first terminal and second terminal and, in the low-voltage logic circuitry, provide power for activation of the coil from the low-voltage. 
     It is thus a feature of at least one embodiment of the invention to provide a low standby power and yet sufficient power during motor operation to hold closure on an electro-mechanical relay. 
     The blender control circuit may further include a voltage regulator limiting the voltage of the low-voltage. The voltage regulator may be a zener diode attached between the low-voltage and a ground point. 
     It is thus a feature of at least one embodiment of the invention to permit to a resistive voltage regulation to provide for substantially constant low-voltage supply using low-cost circuitry. 
     The blender control circuit may further include an energy storage element storing energy from the first power conditioning circuit to provide temporarily greater power to the low-voltage logic circuitry. The energy storage element may be a capacitor attached between the low-voltage and a ground point. 
     It is thus a feature of at least one embodiment of the invention to permit the standby power of the first power conditioning circuit to be lowered to a point where it is insufficient for continuous operation of the low-voltage logic circuitry by allowing power to be stored for momentary operation. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a blender suitable for use with the present invention providing on, off, and pulse control buttons; 
         FIG. 2  is a block diagram of the principal components of the present invention; and 
         FIG. 3  is a schematic of the electronic control circuit of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a blender  10  may include a glass, plastic or metal blender container  12  typically having a removable lid  14  permitting foods (not shown) to be inserted in the container  12  and blended by an internally contained blender knife  16 . 
     The blender container  12  may sit on top of a blender power unit  18  having a housing  20  containing a motor  15  and control electronics  17  to be described in more detail below. The front of the housing  20  may present a control panel  22  having a set of switches  24 , preferably “tactile” type membrane switches, each providing momentary contact, single pole single throw operation and suitable for low-voltage control. As is understood in the art, such membrane type switches may include an outer membrane providing a hermetic seal against environmental contamination and may use contacts formed from printed circuit traces typically on at least one flexible membrane. Such membrane switches are low-voltage devices operating, for example, at 24 V and voltages much less than line voltage of 110-120 VAC. 
     In a preferred embodiment of the invention, the switches  24  include an “on” switch  24   a , an “off” switch  24   b , and a “pulse” or “momentary” switch  24   c , providing a standard functionality described above. One of the switches may be associated with an LED indicator  26  visible through the membrane of control panel  22 . The blender  10  receives line voltage  28  through a cord communicating with the control electronics  17 . 
     Referring now to  FIG. 2 , the control electronics  17  may include a low-voltage logic circuit  19  receiving low-voltage DC power (e.g. 24 V) at a power terminal  34  to provide low-voltage power to power its circuitry and to provide power to switches  24   a - c  and receive low-voltage signals from the switches  24   a - c.    
     The low-voltage logic circuit  19  uses the power from the power terminal  34  to develop a motor control signal  21  that may activate an electrically controllable switch  31 . The electrically controllable switch  31  may provide a single pole, single throw contact set having a first terminal  23   a  connected to a source of line voltage  28  and a second terminal  23   b  connected to the motor  15  to provide power thereto. The remaining terminal of the motor  15  is connected to line ground  25 . Accordingly, and as will be described in greater detail below, the low-voltage logic circuit  19  responding to signals from switches  24  controls the application of power to the motor  15  by providing a motor control signal  21  to the electrically controllable switch  31 . 
     During a standby mode, when the blender  10  is plugged in but the motor  15  is not running and none of the switches  24  is pressed, low voltage power is provided to the low-voltage logic circuit  19  through a first power conditioning circuit  27 . This first power conditioning circuit  27  has relatively low power dissipation (less than 0.03 W in one embodiment) and provides limited power to the low-voltage logic circuit  19 . 
     Generally the power provided to the low-voltage logic circuit  19  by the first power conditioning circuit  27  is insufficient for continuous activation of the electrically controllable switch  31  providing, for example, several milliamps of current flow in contrast to tens of milliamps required by the motor control signal  21  to activate the electrically controllable switch  31 . Nevertheless, the first power conditioning circuit  27  provides sufficient power from for the logic circuitry of low-power logic circuit  19  before activation of the electrically controllable switch  31  and, with energy storage, can provide a temporary activation of the electrically controllable switch  31 . 
     For this purpose, an energy storage element  33  is interposed between the first power conditioning circuit  27  to store energy during the standby mode to provide the low-voltage logic circuit  19  sufficient power reserves to temporarily activate the electrically controllable switch  31  in response to activation of either switch  24   a  or switch  24   c.    
     Upon closure of the electrically controllable switch  31 , line voltage  28  is applied to the motor  15  and also to a second power conditioning circuit  29 . This second power conditioning circuit  29  also provides power to the low-voltage logic circuit  19  through the energy storage element  33  but has relatively high power dissipation (on the order of 3 W in one embodiment). Nevertheless the second power conditioning circuit  29  provides greater power to the low-voltage logic circuit  19 , power on the order of tens of milliamps and sufficient for continuous activation of the electrically controllable switch  31 . This second power conditioning circuit  29 , if operated continuously, would produce undesirable power drain and heating, but is operated only while the motor  15  is running and thus is acceptably limited. The second power conditioning circuit  29  may also benefit from a cooling airflow of a fan optionally associated with motor  15 . In this way, continuous power is provided to the low-voltage logic circuit  19  during motor operation without reliance on the energy storage element  33 . Because the energy storage element  33  must provide only sufficient energy storage for a brief period of time for closure of the electrically controllable switch  31 , the energy storage element  33  may be modestly sized. 
     Referring now to  FIG. 3 , line voltage  28  may be received by a first power conditioning circuit  27  comprised of a diode  30  and series connected resistors  32 . The line voltage  28  is received at an anode of a diode  30  to be rectified and have its voltage dropped by series connected resistors  32 . These resistors  32  are, in turn, connected to a 24 V power terminal  34  which also connects to the cathode of the zener diode  36  (having a 24 V breakdown voltage) and a capacitor  38  providing for energy storage. The remaining terminals of the zener diode  36  and capacitor  38  are grounded. 
     As noted, the diode  30  and resistors  32  provide a first power conditioning circuit  27  for standby power for the remainder of the circuit to be described. In this case, the resistors  32  provide a steady state current draw of less than one milliamp and thus a very low power dissipation both in the resistors  32  and in the zener diode  36 . During operation of the first power conditioning circuit, 24 V power is stored up over a period of time in capacitor  38 . 
     Once switch  24   a  is pressed, a first NPN transistor  40  is turned on by means of 24 V power connected through the switch  24   a  then through a resistor  42  to the base of the transistor  40 . The emitter of the transistor  40  is grounded. Noise suppression resistor  44  and capacitor  46  are placed in parallel between ground and the base of the transistor  40  to prevent false triggering from coupled of electrical noise. 
     The collector of the transistor  40  leads to a resistor  48  which in turn connects to a pull-up resistor  50  connected to the terminal  34  and to the base of a second transistor  52 . The second transistor  52  is a PNP transistor normally biased off by pull-up resistor  50  but turned on when transistor  40  pulls the base voltage down upon activation of switch  24   a . In the on state, current flows into the emitter of transistor  52  from the terminal  34  and out of its collector through a diode  54  and LED  26  to the coil  56  of a relay  58  which makes up the electrically controllable switch  31 . The signal through the diode  54  is the motor control signal  21 . A flyback diode  59  is placed across the terminals of the coil  56 , as is understood in the art, to suppress inductive electrical spikes. The voltage of the collector of transistor  52  is also connected to the junction between switch  24   a  and resistor  42  to provide “latching” of the transistor  40  even when switch  24   a  is released. Together the circuitry associated with transistors  40  and  52  makes up the low-voltage logic circuit  19 . 
     The relay  58  includes contacts  60  that are normally open and connect to line voltage  28  on one side and to a motor  15  of the blender  10  on the other side. The remaining terminal of the motor  15  returns through ground  25  which may be shared with line voltage ground. The contacts  60  that are directly connected to the motor  15  also connect to a secondary power conditioning circuit including rectifier diode  64  and resistor  66 . Resistor  66  has substantially lower resistance than resistors  32 , for example thirty times lower, to provide for higher current necessary to hold the relay coil  56  actuated after depletion of power from capacitor  38  at the expense of substantially increased power dissipation. This higher current from the secondary power conditioning circuit does not overwhelm the wattage rating of the zener diode  36  because the load provided by the coil  56  draws current away from the zener diode  36 . This secondary power conditioning circuit of diode  64  and resistor  66  dissipates substantially more energy than the first power conditioning circuit, on the order of 1 W, but only while the motor  15  is running. 
     When switch  24   b  is pressed, it connects terminal  34  to the base of transistor  52 , turning off transistor  52  and transistor  40  and relay  58  and motor  15 . As a result, power from rectifier diodes  64  and resistor  66  is also turned off and power is again received primarily through rectifier diode  30  and resistors  32 . 
     Momentary operation of the motor  15  is obtained by pressing switch  24   c  which connects terminal  34  to a diode  68  connecting through LED  26  to relay coil  56 . 
     Generally, it will be understood that the relay  58  may be replaced with a triac, for example, with the gate triac receiving the motor control signal  21 . The resistors  32 , for example, may be in a single integrated package for low-cost installation. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.