Principles of Current Sensing

When current flows through a resistor, there is a voltage or potential difference across the resistor according to Ohm's law. Dividing the voltage by the resistance gives the current flowing through the resistor. If the resistance is increased to obtain a large voltage, the resistor will overheat. To avoid this, a resistor with the smallest resistance possible should be used. When the voltage drop is small, the voltage detection range may be amplified by an amplifier circuit.

Applications of current sense resistors

Overcurrent sensing

Circuit operations may need to be stopped for safety when an overcurrent flows due to a malfunction or overload. If the current exceeds a set value, the voltage across the resistor can be judged as abnormal and the output interrupted.

Current control

A resistor can be used to detect the peak of sawtooth current flowing through the reactor of a DC-DC converter to use it for control. Since the phase and time of the current should be controlled precisely a three-phase motor, resistors can detect the current flowing in each phase and give feedback to the control circuit. This system is often used in electric power steering (EPS) and air conditioners.

Current management (Battery management)

Devices that operate on secondary batteries can sense current and the voltage applied to the battery in order to optimize circuit operation to increase battery life and the long-time operation of devices.

Purpose How it works Application
Overcurrent detection

Stops circuit operation for safety
when an overcurrent flows due to circuit malfunction.

  • Overcurrent protection of power supply circuits
  • Overdischarge/overcharge protection of secondary batteries
  • Overcurrent protection of motors
Current control

Senses current and gives feedback to a control circuit to control the value, duration, and phase of the current that flows through the circuit.

  • DC-DC converters
  •  Inverter power supplies
  • Current control of AC motors
Current management

Senses charging/discharging current of a secondary battery in real time to ascertain the battery life and optimally operate the power supply circuit.

  • Current management of devices that are powered by secondary batteries such as laptops and mobile devices
  • Current management of secondary batteries used in EVs/HEVs

Categories of current sense resistors

Generally, the resistance of current sense resistors should have a maximum voltage drop from 10 mV to 1 V when current flows. Therefore, a low resistance value of several ohms or less is used for current sensing. The type of resistor to be used depends on the current value to be detected.

To detect large current such as 10 A to 100 A, since very low resistance value of about 1 mΩ to 10 mΩ is required, and a metal plate type or metal foil type that is stable at low resistance value is most common.  The leaded metal film MF and MOSX series resistors are also used for overcurrent detection applications. A thick film type may be used to detect small currents with a larger resistance value of 100 mΩ to 1 Ω.

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Precautions for sensing large current

When sensing large currents, current sense resistors use very small resistances of 1 mΩ to 10 mΩ to reduce the power loss caused by the resistor. To detect the current accurately, the following precautions must be taken.

  • Layout of the voltage terminal pattern must be carefully designed
  • Inductance of the low ohm resistor must be low (when the time-based change of current dI/dt is large)

Voltage terminal pattern design

To accurately measure the voltage drop across a resistor, the layout of the voltage sensing patterns should be carefully designed on both sides of the resistor. It is recommended that the voltage sensing trace be designed from the center of the resistor’s electrode pads as shown in fig. (2). This is to avoid the influence from the voltage drop caused by the small resistance of the Cu foil pattern on the circuit board.  If the voltage sensing trace is designed from the side of the electrode pads as shown in fig. (1), the detected voltage drop will be influenced by both the resistance of low ohm resistor and the Cu foil pattern, which makes current sensing imprecise.

Voltage Terminal figure

Effect of inductance in low resistance resistors

Resistors are more likely to be affected by parasitic inductance when the resistance is low, with impedance increasing as the frequency gets higher. DC current is not affected by this, but AC currents such as a sawtooth wave that contains high frequency components will yield inaccurate detection results.

When AC current flows as shown in the figure below, the voltage drop of the resistor is the total sum of voltage drop due to resistance and voltage drop due to inductance. Thus, a low resistance resistor with minimum inductance should be used for large current sensing.

Four-terminal type current sense resistors

Current sense low resistance resistors are available in four-terminal types with voltage terminals for sensing voltage and current terminals for current flow. Shown below is an example of four-terminal type current sense resistors. Four-terminal types, which have voltage terminals, give better accuracy in current sensing with fewer sensing errors due to voltage terminal pattern design. However, two-terminal types can also accurately sense current if the voltage terminal patterns are properly designed. The disadvantage of four-terminal types is that the overall resistance value is larger than that of two-terminal types because of the structure, which creates resistance between the voltage terminal and the current terminal, in addition to the resistance for sensing current. Therefore, compared with two-terminal types, the power consumption and the heat generated by the resistor will be higher when the same current is applied. For resistance values that have to be below 10 mΩ in order to detect a large current, relatively more power is wasted due to the additional resistance between the voltage terminal and the current terminal. In addition, since the parasitic inductance is large due to the structure, it is not suitable for sensing current with a large current change (dI/dt).

Overcurrent detection  (example application)

Low resistance current sense resistors are sometimes used in series with the transistor source (or emitter) to detect overcurrent in the switching transistor. This resistor is required to be free of smoking or flame even if the transistor fails, by short circuit, causing a large current to flow into the resistor. Also, low inductance is required. For such applications, we recommend metal type low resistance resistors BPRSLTSLLR72LR

Click  here for the lineup of current sense resistors and power shunts.

Low Resistance /Current Sense Shunt Resistors

Power Rating and Resistance Distribution of Current Sense Shunt / Low Ohm Resistors

Current Sense Lineup