What Is a Current Sense Resistor?
Current sense resistors are used to convert the current in a circuit into a voltage that can be easily measured. They are a low-cost, high-performance way to measure current in many applications and can be customized to meet specific needs of the current sense resistor manufacturer.
A current-sensing resistor is similar to a standard chip resistor except that it is engineered for extremely low resistance values and power loss. It converts current into a small, proportional voltage by following Ohm's law. In addition, it features very low voltage drops and is typically positioned in-line on the conduction path between the power source and the load.
There are a variety of different types of current-sensing resistors, some with low TCR (temperature coefficient of resistance) and some with higher TCR. The TCR of the resistor can change with temperature, so it is important to consider the application's operating conditions when selecting the right type for a given application.
The temperature rise that occurs in a resistor containing dissimilar metals can cause a thermal EMF, which will affect the sense voltage of the resistor. This EMF will then impact the accuracy of the current measurement.
Another consideration is the heat dissipation capacity of the resistor. This can vary depending on the application and may require the resistor to be mounted on a heatsink.
Moreover, the transient currents that can occur in motor drives or other circuits need to be accommodated when designing for high-current sensing. This requires careful planning, larger surface areas, and greater ruggedity.
Temperature-Coefficient-of-Resistance Shunts
One of the most popular options for high-current sensing is a bulk metal shunt element. These self-supporting, non-conductive pieces of resistance metal alloys provide excellent performance for power dissipation and thermal characteristics.
They are available with power ratings from a few milliwatts to tens of watts and are offered in several resistance values, including 0.5 ohms. They also offer a wide range of package sizes.
The resistor must be carefully designed to prevent voltage spikes from forming at its connection points, which can lead to poor SNR. A ballast resistor of 1000 times the value of the sense resistor should be placed in series with the circuit to minimize these spikes.
Other considerations that should be addressed when designing for a current sense resistor are temperature coefficients and tolerances. These are critical to the performance of the resistor and should be determined in advance of manufacture.
A standard chip resistor has a TCR of about +-500 ppm/degC, while low-temperature sense resistors can be manufactured with TCRs as low as 0.05%/degC. These low-temperature shunts are ideal for applications with large temperature changes, such as in power supplies and battery chargers.
The use of a current-sensing resistor is an excellent solution for making precise current measurements in many applications and can be a great way to save power and improve system efficiency. But it is important to recognize the tradeoffs that come with using a current-sensing resistor and to consult the application engineers at resistor vendors or experts in high-current sensing for help with these design challenges. Get a general overview of the topic here: https://en.wikipedia.org/wiki/List_of_resistors.