Current sense resistors are very low value resistors used to measure the current flowing through it. 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. Microhm Electronics mainly focus on the manufacture of low ohm current sense resistors, NMS series, MMS series, MPR series for example.

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