Low cost blender control permitting low actuation force switches

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 1, a blender10may include a glass, plastic or metal blender container12typically having a removable lid14permitting foods (not shown) to be inserted in the container12and blended by an internally contained blender knife16.

The blender container12may sit on top of a blender power unit18having a housing20containing a motor15and control electronics17to be described in more detail below. The front of the housing20may present a control panel22having a set of switches24, 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 switches24include an “on” switch24a, an “off” switch24b, and a “pulse” or “momentary” switch24c, providing a standard functionality described above. One of the switches may be associated with an LED indicator26visible through the membrane of control panel22. The blender10receives line voltage28through a cord communicating with the control electronics17.

Referring now toFIG. 2, the control electronics17may include a low-voltage logic circuit19receiving low-voltage DC power (e.g. 24 V) at a power terminal34to provide low-voltage power to power its circuitry and to provide power to switches24a-cand receive low-voltage signals from the switches24a-c.

The low-voltage logic circuit19uses the power from the power terminal34to develop a motor control signal21that may activate an electrically controllable switch31. The electrically controllable switch31may provide a single pole, single throw contact set having a first terminal23aconnected to a source of line voltage28and a second terminal23bconnected to the motor15to provide power thereto. The remaining terminal of the motor15is connected to line ground25. Accordingly, and as will be described in greater detail below, the low-voltage logic circuit19responding to signals from switches24controls the application of power to the motor15by providing a motor control signal21to the electrically controllable switch31.

During a standby mode, when the blender10is plugged in but the motor15is not running and none of the switches24is pressed, low voltage power is provided to the low-voltage logic circuit19through a first power conditioning circuit27. This first power conditioning circuit27has relatively low power dissipation (less than 0.03 W in one embodiment) and provides limited power to the low-voltage logic circuit19.

Generally the power provided to the low-voltage logic circuit19by the first power conditioning circuit27is insufficient for continuous activation of the electrically controllable switch31providing, for example, several milliamps of current flow in contrast to tens of milliamps required by the motor control signal21to activate the electrically controllable switch31. Nevertheless, the first power conditioning circuit27provides sufficient power from for the logic circuitry of low-power logic circuit19before activation of the electrically controllable switch31and, with energy storage, can provide a temporary activation of the electrically controllable switch31.

For this purpose, an energy storage element33is interposed between the first power conditioning circuit27to store energy during the standby mode to provide the low-voltage logic circuit19sufficient power reserves to temporarily activate the electrically controllable switch31in response to activation of either switch24aor switch24c.

Upon closure of the electrically controllable switch31, line voltage28is applied to the motor15and also to a second power conditioning circuit29. This second power conditioning circuit29also provides power to the low-voltage logic circuit19through the energy storage element33but has relatively high power dissipation (on the order of 3 W in one embodiment). Nevertheless the second power conditioning circuit29provides greater power to the low-voltage logic circuit19, power on the order of tens of milliamps and sufficient for continuous activation of the electrically controllable switch31. This second power conditioning circuit29, if operated continuously, would produce undesirable power drain and heating, but is operated only while the motor15is running and thus is acceptably limited. The second power conditioning circuit29may also benefit from a cooling airflow of a fan optionally associated with motor15. In this way, continuous power is provided to the low-voltage logic circuit19during motor operation without reliance on the energy storage element33. Because the energy storage element33must provide only sufficient energy storage for a brief period of time for closure of the electrically controllable switch31, the energy storage element33may be modestly sized.

Referring now toFIG. 3, line voltage28may be received by a first power conditioning circuit27comprised of a diode30and series connected resistors32. The line voltage28is received at an anode of a diode30to be rectified and have its voltage dropped by series connected resistors32. These resistors32are, in turn, connected to a 24 V power terminal34which also connects to the cathode of the zener diode36(having a 24 V breakdown voltage) and a capacitor38providing for energy storage. The remaining terminals of the zener diode36and capacitor38are grounded.

As noted, the diode30and resistors32provide a first power conditioning circuit27for standby power for the remainder of the circuit to be described. In this case, the resistors32provide a steady state current draw of less than one milliamp and thus a very low power dissipation both in the resistors32and in the zener diode36. During operation of the first power conditioning circuit, 24 V power is stored up over a period of time in capacitor38.

Once switch24ais pressed, a first NPN transistor40is turned on by means of 24 V power connected through the switch24athen through a resistor42to the base of the transistor40. The emitter of the transistor40is grounded. Noise suppression resistor44and capacitor46are placed in parallel between ground and the base of the transistor40to prevent false triggering from coupled of electrical noise.

The collector of the transistor40leads to a resistor48which in turn connects to a pull-up resistor50connected to the terminal34and to the base of a second transistor52. The second transistor52is a PNP transistor normally biased off by pull-up resistor50but turned on when transistor40pulls the base voltage down upon activation of switch24a. In the on state, current flows into the emitter of transistor52from the terminal34and out of its collector through a diode54and LED26to the coil56of a relay58which makes up the electrically controllable switch31. The signal through the diode54is the motor control signal21. A flyback diode59is placed across the terminals of the coil56, as is understood in the art, to suppress inductive electrical spikes. The voltage of the collector of transistor52is also connected to the junction between switch24aand resistor42to provide “latching” of the transistor40even when switch24ais released. Together the circuitry associated with transistors40and52makes up the low-voltage logic circuit19.

The relay58includes contacts60that are normally open and connect to line voltage28on one side and to a motor15of the blender10on the other side. The remaining terminal of the motor15returns through ground25which may be shared with line voltage ground. The contacts60that are directly connected to the motor15also connect to a secondary power conditioning circuit including rectifier diode64and resistor66. Resistor66has substantially lower resistance than resistors32, for example thirty times lower, to provide for higher current necessary to hold the relay coil56actuated after depletion of power from capacitor38at 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 diode36because the load provided by the coil56draws current away from the zener diode36. This secondary power conditioning circuit of diode64and resistor66dissipates substantially more energy than the first power conditioning circuit, on the order of 1 W, but only while the motor15is running.

When switch24bis pressed, it connects terminal34to the base of transistor52, turning off transistor52and transistor40and relay58and motor15. As a result, power from rectifier diodes64and resistor66is also turned off and power is again received primarily through rectifier diode30and resistors32.

Momentary operation of the motor15is obtained by pressing switch24cwhich connects terminal34to a diode68connecting through LED26to relay coil56.

Generally, it will be understood that the relay58may be replaced with a triac, for example, with the gate triac receiving the motor control signal21. The resistors32, for example, may be in a single integrated package for low-cost installation.