How to Measure Circuit Current with a Shunt Resistor

Published by DevicePlus Editorial Team at March 15, 2021

Measure the current to run the circuit safely

There is an increasing need for multi-functional devices and safety improvements that utilize current measurements for devices equipped with newer electronic circuits.
For example, a monitoring circuit for detecting overcurrent and the abnormal operation of a circuit and stopping it safely, a function for battery charging and battery capacity measurement, and current monitoring are also essential for motor control, so current monitoring has become an indispensable technology in modern circuit design.
We will introduce a method for detecting current and actually run the current detection circuit to see how it behaves.
The basics of current detection circuits and shunt resistance

Ultra-Low Resistance Chip Resistor for Current Detection (PMR)

Ultra-Low Resistance Chip Resistor for Current Detection / Long Side Electrode (PML

You may think that current detection circuits are complicated, but the principle itself is a simple circuit that utilizes “Ohm’s law”, which can be said to be the basis of electronic circuits. A resistor for current detection is inserted in a series, and the voltage drop of the resistor is converted into a current value using Ohm’s law to perform current detection.
The resistor used for current detection is called the “shunt resistor”.
A shunt resistor is an electronic component used to measure and detect current. Resistance values range from 100μΩ to several 100mΩ, with the most commonly used resistance value being several mΩ to several hundred mΩ. Ideally, you should use a shunt resistor with the lowest possible resistance, but in reality, you should select it according to the amplification factor of the operational amplifier and the current value to be detected.
Especially with low resistances, the amount of voltage drop is small, and it is difficult for the microcontroller to detect the voltage, so use a high-precision operational amplifier with a small input offset voltage to detect the current.
A current detection method that uses a shunt resistor and an operational amplifier is called a “current sense amplifier.”

By the way, the shunt of the shunt resistor means “Shunt: avoid / drive away.” Originally, it meant the resistor was inserted in parallel to expand the measurement range of the analog ammeter. Recently, the chip resistor itself for current detection has been called a shunt resistor. Even if the usage changes with time, it’s common for the name to remain the same.

Connecting shunt resistors to differential amplifier circuits

In principal, a current detection circuit using a shunt resistor is a simple circuit that only measures voltage. However, since the voltage drop of the shunt resistor is small, it’s necessary to make a circuit that can amplify the voltage with a high level of accuracy. Therefore, a differential amplifier circuit that uses an operational amplifier is used.

For the operational amplifiers used for current detection, use a high-precision operational amplifier with a low input offset voltage. Since the offset voltage causes measurement errors when detecting small voltages, use a “high-precision operational amplifier” with the lowest possible offset voltage, or a “zero drift amplifier” that automatically adjusts the input offset voltage.

Detect the current value of the circuit with the current detection circuit

Let’s actually make a current detection circuit using a shunt resistor and an operational amplifier and check how the current is detected. The current detection circuit is as follows.

This is the current detection circuit to be manufactured. The differential amplifier circuit detects the voltage of the shunt resistor, then amplifies it to a voltage signal that is 15 times higher and outputs it.

The 62mΩ chip resistor is used for the shunt resistor. The maximum current value that can be measured is determined by the power consumption of the chip resistor. We are currently using a 1W product, so it becomes W = I2R, 1W ≒ 4A × 4A × 62mΩ, and it’s calculated that the current can flow up to 4A.

ROHM current detection chip resistor LRT18 series, 62mΩ 1W chip resistor

If the amplification degree for measuring the current is too large, it will exceed the operating voltage of the operational amplifier, so adjust the amplification degree with the assumption of the maximum current. Since the amplification degree is set to 15 times this time, 3V is output from the operational amplifier when the maximum shunt resistance current of 4A flows.

ROHM operational amplifier LMR1802G-LB. A sensor amplifier featuring low noise, a low input offset voltage, and a low input bias current.

ROHM uses the industry’s highest low noise operational amplifier,　EMARMOUR “LMR1802G-LB”, as the operational amplifier. This operational amplifier has a small input offset voltage of 5uV (Typ), and it’s used for sensing devices.

Operational amplifier and shunt resistor mounted on a universal board. Since it’s experimentally mounted on a universal board, it’s simple to solder, but in the actual circuit design, an appropriate pattern design is performed based on the shunt resistance data sheet.

Since the current is detected by the shunt resistor, let’s explore how to detect currents using a simple circuit wired on the universal board.

Measure the voltage with an oscilloscope and observe the movement of the current

Connect the load to the completed current detection circuit and observe the measured waveform. Connect a DC brush motor to the load. If the current can be detected easily, it should be possible to measure the current waveform at which the motor coil switches, as well as the state of change when a load is applied to the rotation.

The shunt resistor is connected in series with the motor and power supply. The motor operates at 5V.

The no-load current of the motor is 0.32A. The effective value of the waveform output from the operational amplifier is 202mV, which is detected as 0.3A in the calculation. Both the oscilloscope and probe bandwidths are 50MHz.

When you rotate the motor, you can see that the current changes according to the switching of the commutator. When the load is increased to stop the rotation, the change in the current value detected by the shunt resistor also changes as a voltage signal.

If you connect the current detection output of the operational amplifier to a microcontroller board, such as an Arduino, you can detect the current of the motor in real time, enabling you to detect abnormalities such as motor lock and coil layer short.
If the current is detected, various functions such as the protection of the motor body / drive circuit from motor overload, as well as the detection of motor lock, can be added.

Achieving higher accuracy and greater current detection by changing the shunt resistance

When you actually use the current detection circuit as circuit protection, select a product with a low resistance and a large current that does not exceed the maximum power of the shunt resistor.
We are using a general-purpose chip resistor, and as for the high-performance shunt resistors, high-power types that can handle high power up to 5W and highly accurate shunt resistors with ultra-low resistances of 0.1mΩ are also available. You can choose from a variety of shunt resistors to suit your needs.

Reference link: Chip resistor for current detection (shunt resistor) | ROHM
https://www.rohm.co.jp/products/resistors/current-detection-resistors/information

Summary

This method of detecting current using a shunt resistor and an operational amplifier is widely used method because it’s low-cost, highly accurate, and easy to use. However, if a resistor is added and the circuit is adversely affected, it also has a distinct disadvantage: it cannot be used for high loads where the power loss of the shunt resistor is large.
In principle, the loss can be reduced by lowering the resistance value of the shunt resistor, since the voltage drop of the shunt resistor also decreases and it becomes more difficult to detect minute voltages, so there is a trade-off between the small shunt resistor and the detection accuracy.
In particular, current detection, which is used for controlling brush-less motors and DCDC converters and detecting the remaining amount of battery, requires both large currents and high-detection accuracy. Consequently, not only is a low shunt resistance required, but a high-precision operational amplifier is required as well.
When actually detecting currents using a shunt resistor, consider the maximum current and the application of the load you want to detect. This means that you need to assume the maximum current and predict how much accuracy and loss can be tolerated. Essentially, you’ll need to think about cost when proceeding with circuit design and component selection.

You should remember when using current detection for protection that since the current detection circuit itself only detects current, it’s necessary to add functions for protection and control. For example, it’s necessary to add a relay or a load switch so that it can be cut off, while you’ll also need to decide on the program or circuit for operating it, and under what conditions the protective operations are performed.
If a current detection circuit is installed, the circuit and control will become complicated and the threshold will rise a little, but it is an indispensable circuit for improving safety and stepping up electronics kits.