1.Field of the Invention
The present invention relates to various kinds of electronic devices and appliances such as electronic wristwatches, portable communications devices, and domestic electrical appliances and, more particularly, to electronic devices fitted with power generators.
2. Description of the Prior Art
Heretofore, electronic devices fitted with power generators have been developed as electronic devices such as electronic wristwatches and the like. Various types are available as power generators. Especially, thermoelectric generators can be made compact and so they are being developed so as to be applied to various electronic devices and appliances.
An electronic wristwatch fitted with a thermoelectric generator is worn on an arm such that suction of heat by the thermoelectric generator is conducted from the arm. Heat is dissipated to the outside air. In this way, electric power is generated by making use of a temperature difference. Accordingly, when the electronic wristwatch is detached from the arm, no temperature difference is produced and so power generation may be stopped. If power generation is stopped for a long time, electric power stored in a storage device might be exhausted. Furthermore, when the outside air temperature and the body temperature on the arm become equal, power generation comes to a stop. In this case, there is the possibility that the electric power stored in the storage device is used up.
Since a thermoelectric generator produces only a small amount of electric power, if the storage device is exhausted, it is difficult to store a sufficient amount of electric power in the storage device is a short time after attaching the watch to an arm. For example, where a thermoelectric device has a temperature difference of about 2.degree. C., the electric power that can be generated is about 13.3 .mu.W, provided that the generated voltage is 0.4 V, the internal resistance is about 1500 .OMEGA., and the input impedance of a voltage step-up means for stepping up the generated voltage is 1500 .OMEGA. and that the loss of the voltage step-up means is neglected. The electric power consumed by the electronic wristwatch is approximately 1 to 2 .mu.W. Where a lithium-ion secondary battery having a diameter of 6 mm is used as a power storage device, energy of about 6.5 J can be stored. Accordingly, if the device is worn on an arm and should be fully charged, it takes a time of 135.8 hours even if consumption and loss in the load circuit are neglected.
Accordingly, if power generation of the thermoelectric generator is stopped for a long time as encountered when the electronic wristwatch is detached from the arm, the electric power storage device is exhausted. This brings the timepiece to a stop.
One proposed method for solving this problem is to reduce the frequency at which the second hand of a watch or clock is driven if the storage voltage drops, thus lowering the electric power consumed (see Patent Unexamined Publication No. 287080/1995). With this method, the electric power consumed can be decreased but cannot be reduced down to zero. Therefore, if power generation of the thermoelectric generator stops for a long time, there arises the problem that the storage device is exhausted.
If a thermoelectric generator is used in various electronic devices and appliances other than electronic wristwatches such as portable communications devices and domestic electric appliances, the same problem arises if no temperature difference occurs.
The present invention is intended to suppress decreases in the electric power stored in an electrical storage device of an electric device having a thermoelectric generator.