Apart from the piezo generator all the following are based on the same simple design: magnets are rotated (or translated) near to coils of wire (Faraday Induction). There is no coil switching so these are AC generators. On magnet movement, voltages are induced (generated) into the coils of wire. (Note: the magnets need to be the very strong - Rare Earth Neodymium type). As long as the magnetic field developed by the magnets does not diminish too much throughout the volume enclosed by the coils the peak voltage generated by these simple devices is given by:
V = A x N x dB/dt
where V = the peak voltage (V)
A = the cross sectional area of the coils (m²)
N = the number of turns
dB/dt = rate of change of magnetic field strength (T/s)
Note: as these are AC generators the average voltage (root mean square) will be less that the peak voltage calculated above
(especially in the case of the shake-a-gen, which produces quick pulses).
As these generators have no moving contacts they are very simple, rugged and will work over a very wide range of temperatures. They can be made completely water proof and will work when completely immersed in water (as long as the live electrical connections are suitably enclosed). As they have no ferro-magnetic components (except the magnets) they are less efficent than other designs but are much easier to turn.
STORING THE ENERGY FROM THE GENERATORS - rectifier and storage device
(a simple circuit to make a DC voltage from the generators and so be able to store the energy.
THE CREATIVE SCIENCE CENTRE
Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ
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