Hybrid ac-current sensor based on the time modulation of an autonomous light source

We present an ac-current sensor based on the measure of the width of light pulses emitted by an autonomously powered light source. The working principle of the sensor is as follows: When the sinusoidal current to be measured overcomes a certain (arbitrarily fixed) threshold, the light source is switched on, and it remains so until the current falls below the threshold; at this point the light source is switched off until the following cycle begins to rise again the electrical current. Thus, the temporal width of the light pulses emitted by the source becomes a function of the ac-current amplitude. Since the width of a light pulse is a physical quantity immune to mechanical vibrations and other perturbing factors, it allows a reliable determination of the current amplitude from a remote site. The required threshold is provided by a self-contained simple electronics built using standard components. Unlike most optical current sensors reported in the literature, the proposed device is not based on the Faraday effect, and thus, it is cheap, robust and works without the need for precise alignment or additional optics. It could be potentially useful for industrial over-current protection purposes. Validation experiments are presented.