A current sensor is an electrical device that measures the flow of electrical current along a specific electrical line. It accomplishes this by taking a precise measurement of the drop in voltage at a resistor placed in the path of the current. This allows the current sensor to generate an estimate of the level of current flowing through the line. The output of the current sensor is given either as a voltage reading or as a continuing current which is roughly proportional to the level of current moving along the tested path.
Traditionally used to protect circuits from damage and general current reporting, current sensors are now used in a much broader range of applications. These include performance monitoring and enhancement; protecting against overcurrents, which can overload a line and place it at risk; and battery operated circuits, such as those with battery rechargers. Rechargeable batteries can especially benefit from current sensing, because without an accurate measurement of current, they can easily become overloaded or otherwise useless.
A current sensor works by attaching onto an active electrical line. Once attached to the line, the sensor reads the flow of electricity going through the line as it travels through a resistor. A resistor is a device placed in the path of a current which impedes the flow of electricity, ordinarily used to control and subdue the amount of current moving through a line. Voltage drops correspondingly as it passes across a resistor, depending on the overall level of resistance; by measuring the proportion of this decrease, the sensor can calculate the overall current of the line.
There are two general approaches to current sensing; low side sensing and high side sensing. Low side sensing connects the sensor between the original load and the grounding connection. This type of sensing provides an inexpensive method of measuring the current, but has a downside of adding extra resistance to the grounding path of the line. High side sensing, on the other hand, connects the sensor between the supply and the load. It typically requires a more expensive sensor — as a higher level of accuracy is required — but does not disrupt the grounding current like current sensing through low side connections.