1. Field of the Invention
The present invention relates to a non-contact electric power transmission apparatus for transmitting electric power electromagnetically from a power supply unit to a load unit.
2. Description of the Related Art
In the present application, priority under 35 U.S.C. 119 is requested based on Japanese Patent Application No. 9-74,134, filed Mar. 26, 1997, entitled "Non-contact Electric Power Transmission Apparatus". The contents of that application are incorporated herein by reference.
A non-contact electric power transmission apparatus is disclosed in Japanese Unexamined Patent Publication (kokai) 6-311,658, entitled "Small Electric Appliances". The contents of that application are incorporated herein by reference. In this apparatus, referring to FIG. 14, when a load unit 2 having a power secondary winding L3 is properly positioned at a predetermined position with respect to a power supply unit 1 having a power primary winding L1 such that the power primary winding L1 and the power secondary winding L3 are electromagnetically connected each other, electric power is transmitted from the power primary winding L1 to the power secondary winding L3. The power supply unit 1 includes a diode D0 and a smoothing capacitor C0 so as to rectify and smooth the current from a commercial power supply AC. The power supply unit 1 further includes an LC resonant circuit and a transistor Q1 as a switching element. The LC resonant circuit includes an oscillation capacitor C1 and a power primary winding L1 for transmitting electric power to a power secondary winding L3 of the load unit 2. The series circuit of the LC resonant circuit, the transistor Q1, and a resistor R2 is connected in parallel to the smoothing capacitor C0. A series circuit of a resistor R1 and a capacitor C2 is also connected to the smoothing capacitor C0 in parallel. A base of the transistor Q1 and the connection point between the resistor R1 and the capacitor C2 are connected via a feedback coil L0 which is electromagnetically connected to the power primary winding L1.
The load unit 2 includes the power secondary winding L3. Electric power is induced in the power secondary winding L3 by electromagnetic induction between the power primary winding L1 and the power secondary winding L3. The power secondary winding L3 is connected to a secondary battery EB via a diode D5. The voltage induced in the power secondary winding L3 is rectified by the diode D5 and then charged in the secondary battery EB. The secondary battery EB is connected to a load M, for example, a direct-current motor via a switch SW1 which is operated by an operation unit (not shown). The power secondary winding L3 is connected to a signal primary winding L4 via a diode D6.
When the load unit 2 is not positioned at the predetermined position with respect to the power supply unit 1, voltage is intermittently supplied to the power primary winding L1. When the load unit 2 is positioned at the predetermined position with respect to the power supply unit 1, voltage is induced in the power secondary winding L3 by the voltage intermittently supplied to the power primary winding L1. The induced voltage is applied to the signal primary winding L4 and transmitted from the signal primary winding L4 to the signal secondary winding L2 by electromagnetic induction. When voltage is induced in the signal secondary winding L2, electric power is successively supplied to the power primary winding L1 and electric power can be transmitted from the power primary winding L1 to the power secondary winding L3.
In this apparatus, however, if the signal secondary winding L2 is positioned close to the power primary winding L1, since voltage is induced in the signal secondary winding L2 by the voltage supplied to the power primary winding L1, the problem arises in that it cannot be exactly determined whether the load unit 2 is positioned at the predetermined position with respect to the power supply unit 1. Accordingly, since the signal secondary winding L2 cannot be positioned close to the power primary winding L1, the signal primary winding L4 cannot be positioned close to the power secondary winding L3. Therefore, the problem arises in that the size of the apparatus cannot be reduced.