Electronic Device And Electronic Circuit

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Thursday, February 04, 2010

Basic Levitation and Magnetic Levitation Circuit Project

Understanding Basic Levitation
In basic levitation (Fig 1) an object appears to float due to the invisible forces of magnets. A magnet creates a field that forms two opposing poles: North and South (Fig 2). Opposite poles attract each other while similar poles repel. For magnetic levitation, there is a fixed magnet and a smaller free moving permanent magnet, which is the object that will levitate. This object has two forces exerted on it: downward force from gravity and upward force from the fixed magnet.

Magnetic Levitation Schematic Circuit


Levitation Photodetector Circuit
This optodetector measures the position of the ball by the amount of light transmitted by the infrared LED. This is a linear signal across the small area of the detector -- it is not just "on" and "off".

Levitation Difference Amplifier Circuit
This circuit creates a control signal from the two optodetectors. It finds the difference between the two input voltages and amplifies it to get the ball's position. This stage is often called a comparator.

Levitation Output Amplifier Circuit
This circuit amplifies the control signal in preparation for the power output transistor. Why do we need this stage at all? Because we reduced the whole signal by one-ninth in the speed-plus-position circuit.


Perpetual top Levitation Toy
The point of the perpetual top levitation toy is simply that it continues spinning forever, and the challenge is to understand the driving mechanism. The top is made of plastic, and contains embedded in it a small permanent magnet, oriented perpendicular to the spin axis of the top. The base contains a transistor and a coil with two windings, the assembly being driven by a 9-volt power supply. A schematic of the electrical circuit is shown in the figure.
As one pole of the magnet (say the south pole) approaches the coil, a current in induced in winding A, in such a direction as to make the base of the transistor (an NPN) go positive. That makes the emitter-collector current flow through winding B, in the opposite direction to A. The current through B is larger than that of A (due to the amplification of the transistor), so by Lenz's law the magnet pole will be attracted to the coil.


Magnetic Levitation Circuit
This is a simple magnetic levitator circuit which suspends objects a set distance below an electromagnet. The physics behind it is to simply provide a magnetic force which equal and opposite to the gravitational force on the object. The two forces cancel and the object remains suspended. Practically this is done by a circuit which reduces electromagnet force when an object gets to close, and increases it when the object is out of range.


The following figure is a schematic diagram for the Electronics Board.

Operation of the Electronics Board circuitry may be of interest to some students who use the Levitation/Critical Temperature kit. Individual circuit functions may be understood and analyzed from the following explanations. Operational amplifiers (op amps) are characterized by two nearly ideal properties, which lead to a wide variety of applications. These properties are 1) high impedance between the non-inverting and inverting inputs and 2) high open loop gain. Usually, both of these may be assumed to be infinite. Infinite impedance means no current flows between inputs, and infinite gain means that negative feedback will drive input voltage difference to zero. A variety of circuits can be analyzed using these two properties and Ohm's law.

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