Current sense resistors are very low value resistors used to measure the current flowing through it. Microhm Electronics MVR2321-4, MPR series, MMS series are most often used current sense resistor. The current through the resistor is represented by the voltage across the resistor, so by applying I = V/R as set down by the famous school teacher Georg Simon Ohm, the current is proportional to the voltage across the resistor.
 

 

This simple introduction to the topic is the beating heart of the rest of this current article. The topics covered here - resistor selection, high side or low side monitoring, and choosing a sense amplifier - are all based on this basic formula of electrical engineering.

 

Current sense monitoring helps to improve the efficiency of some systems and reduce losses. For example, many mobile phones have implemented current sensing to monitor and improve battery life while also improving reliability. If the current draw becomes too large, the phone can decide to throttle back the CPU frequency to reduce the load on the battery in order to extend battery life, while preventing the phone from overheating which increases reliability. There are even phone apps that can access the current sense reading circuit and make decisions to optimize the phone’s performance.

 

In addition to current sense monitoring using a resistor, two other less common methods are used. One uses a Hall Effect sensor to measure the flux field generated by a current. While this is non-intrusive and has the advantage of no insertion loss, it is somewhat expensive and requires a relatively large amount of PCB real estate. The other method, using a transformer to measure induced AC current, is also size and cost intensive; while also useful only for AC current.
 

 

There are three basic aspects of current sense monitoring using a resistor:

 

Choosing a low resistance precision sense resistor.

Choosing a sense amplifier chip. When sensing voltages across a resistance that is less than one ohm, small changes in voltage can have significant sense consequences. The sense amplifier amplifies that voltage change, making the insignificant, significant.

The location of the sense resistor. This refers to either sensing at the power supply, called High-Side Sensing, or at the ground connection, called Low-Side Sensing.

Precision current sensing applications are no longer made-from-scratch circuits; manufacturers have done all of the research and most of the work for modern designers.