As electronic circuits get smaller and become less power hungry, small amounts of energy in our surroundings can be used to power them up. For bio-implantable and wearable devices it is possible to extract energy from body movements. Until now, three main transducers have been introduced for energy scavenging purposes, i.e. piezoelectric, electromagnetic and electrostatic. In electrostatic method a circuit utilizes capacitance changes of a variable capacitor to convert kinetic energy to electrical energy.Here we present a method to simulate this kind of generators in PSPICE and use the PSPICE simulations to simulate a popular circuit in an electrostatic generator. It will be shown that this circuit suffers from charge depletion. In order to solve this problem, we propose two different techniques. The effectiveness of proposed methods is proved by simulation and using these methods, we estimate the amount of energy that can be extracted from body movements (as large as 300mW). Practical issues in using these techniques are discussed.

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