Using a current sense resistor is a cost-effective way to measure current. This type of resistor is used in electronics and can be used for power management, motor control, and battery charging. However, there are some concerns to be aware of when using this type of resistor.
First, it is important to determine how much current the system is capable of delivering. This is important in choosing the value of the current sense resistors. The larger the resistor, the more accurate the measurement. For example, a 0.1O resistor will provide 100mV of voltage drop when the system has one Amp of current. A higher value will improve the signal-to-noise ratio of the circuit and increase the resolution. A higher value will also increase the power rating of the resistor, so it will be able to survive a higher current.
Another issue to consider is the temperature coefficient of resistance. This is a measure of the resistance of the sense resistor to the ambient temperature. A low TCR reduces the temperature dependence of the current measurement, which improves accuracy. Most standard chip resistors have TCRs of +-500 ppm/degC. However, a special-construction sense resistor may have TCRs as low as +-100 ppm/degC.
Finally, it is important to make sure that the resistor is not being routed in parallel with high-current bearing traces. This can cause the resistor to self-heat. This will cause the resistor to drift from its nominal value, and can also cause the current measurement to be inaccurate.
When designing the interface between the sense resistor and the load, it is important to capture small voltage changes. This will improve the accuracy of the reading and reduce the impact of wasted power. It is also important to keep the voltage across the resistor low to minimize the heating of the resistor. A low sensed voltage can be used to determine the location of short circuits, but it can also limit access to the load.
The size of the resistor can also impact the thermal condition of the resistor. Larger resistors improve the accuracy and resolution of the measurement, but can also increase the power dissipation of the circuit. A general rule of thumb is to multiply the power rating by two to get a resistor that will dissipate enough power to handle the maximum current.
A large number of resistors are available in the milliohm range, including standard resistors and ultralow-resistance resistors. Some vendors of ultralow-resistance resistors include Ohmite, Caddock Electronics, and TT Electronics.
While it may be tempting to choose a larger resistor to increase the dynamic range of the measurement, it is important to keep in mind the impact that larger resistors can have on PC-board layout and thermal conditions. In addition, the thermal resistance of the sensing element package should be considered. Having a larger resistor can also impact the signal/noise ratio of the circuit.
There are many options for sizing a current sense resistor, and each has its unique considerations. Ultimately, a sense resistor's power rating and thermal resistance will determine how it will perform in your application. You can get more enlightened on this topic by reading here: https://en.wikipedia.org/wiki/Current_sensing.
