How to Measure Circuit Current Using a Shunt Resistor

In recent years, there has been an increasing demand for multifunctional and highly safe electronic circuits using current measurement technology. In this article, we will introduce a method of detecting current using a shunt resistor and actually operate this current detection circuit to see its detection effect.

Currently, there is an increasing demand for multifunctional, high-safety devices that require current measurement technology that utilizes newer electronic circuits suitable for their configuration.

For example, monitoring circuits for detecting overcurrent and abnormal circuit operation and safely stopping operation, functional circuits for battery charging and battery capacity detection, and equally important current monitoring circuits for motor control, so current monitoring technology is essential for modern circuit design.

Next we will introduce a method for detecting current and actually run the current detection circuit to see its effect.

Ultra-low resistance chip resistors for current detection (PMR)

Ultra-low resistance chip resistor for current detection / long side electrode (PML)

You may think that the current detection circuit is complicated, but in principle, it is just a simple circuit that uses "Ohm's Law", which is a very basic theoretical knowledge in the field of electronic circuits. A resistor for current detection is connected in series, and the voltage drop of the resistor is converted into a current value according to Ohm's law to achieve current detection.

A shunt resistor is an electronic component used to measure and detect current. The resistance value ranges from 100μΩ to several hundred mΩ. Ideally, you should use a shunt resistor with the lowest possible resistance, but in practice, you should choose the appropriate resistance value based on the amplification factor of the operational amplifier and the detection target range.

If a low-value resistor is used, the voltage drop will be small and the microcontroller will have difficulty detecting the voltage. Therefore, a high-precision operational amplifier with low input offset voltage should be used to detect the current.

A current sensing circuit using a shunt resistor and an operational amplifier is called a "current sense amplifier."

Also, the "shunt" in shunt resistor means "to avoid, to divert". Originally, it referred to connecting resistors in parallel to expand the measurement range of analog ammeters. Recently, chip resistors used for current detection are called shunt resistors. Although the usage has changed, it is common for the name to remain the same.

In principle, the current detection circuit using a shunt resistor is a simple circuit that only measures voltage. However, since the voltage drop across the shunt resistor is very small, it is necessary to create a circuit that can amplify the voltage with high accuracy. For this purpose, a differential amplifier circuit with an operational amplifier is used.

For the operational amplifier used for current detection, select a high-precision operational amplifier with low input offset voltage. Since the offset voltage causes measurement errors when detecting small voltage values, use a "high-precision operational amplifier" with as low an offset voltage as possible, or a "zero-drift amplifier" that can automatically adjust the input offset voltage.

We use a shunt resistor and an operational amplifier to make a current detection circuit and see how the circuit detects current. The current detection circuit is as follows:

The figure shows the current detection circuit to be made. The differential amplifier circuit detects the voltage of the shunt resistor, then amplifies it to a voltage signal of more than 15 times and outputs a 62mΩ chip resistor as a shunt resistor. The maximum measurable current value is determined by the power of the chip resistor. We are currently using a 1W resistor, so W = I2R, 1W ≒ 4A × 4A × 62mΩ, and the maximum current is finally calculated to be 4A.

ROHM Current Sense Chip Resistor LRT18 Series, 62mΩ 1W Chip Resistor

If the amplification factor of the current measurement circuit is too large, it will exceed the operating voltage of the operational amplifier, so the amplification factor needs to be adjusted according to the estimated maximum current value. The amplification factor we set this time is 15 times, so when the current flowing through the shunt resistor is the maximum value of 4A, the output voltage of the operational amplifier is 3V.

ROHM operational amplifier LMR1802G-LB. It is a sensor amplifier with low noise, low input offset voltage and low input bias current.

ROHM uses the industry's lowest noise operational amplifier.

EMARMOUR "LMR1802G-LB" operational amplifier has low input offset voltage 5uV (Typ) and is used in sensing devices.

An operational amplifier and shunt resistor mounted on a general-purpose board. Because these devices are mounted on a general-purpose board in an experimental environment, it is easy to solder them, but in actual circuit design, appropriate graphic design will be performed based on the shunt resistor data sheet.

Now that the current is sensed by the shunt resistor, let's explore how to sense the current using a simple circuit connected on a general purpose breadboard.

Connect a load to the completed current sensing circuit and observe the detected waveform. Connect a brushed DC motor to the load. If the current can be successfully sensed, you should be able to detect the current waveform when the motor coils are switched and the change in rotation after the load is applied.

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

The motor no-load current is 0.32 A. The effective value of the waveform output by the operational amplifier is 202 mV, and the calculated detection value is 0.3 A. The bandwidth of the oscilloscope and probe is 50 MHz.

As the motor runs, you can see the current change as the commutator switches. As the load increases, the motor speed decreases, and the change in current value detected by the shunt resistor is presented as a change in the voltage signal.

If you connect the current sense output of the op amp to a microcontroller board, such as an Arduino, you can monitor the motor current in real time, helping you detect abnormal conditions such as motor locking and coil shorts.

If the current can be detected, various functions can be added to the circuit, such as preventing overload of the motor body/drive circuit and detecting motor lock.

When current detection circuits are actually used to protect circuits, devices with low resistance and high current (not exceeding the maximum power of the shunt resistor) are selected.

We use general chip resistors, and among the existing high-performance shunt resistors, there are high-power models that can consume up to 5W power, and high-precision shunt resistors with ultra-low resistance values ​​of 0.1mΩ. You can choose the corresponding shunt resistor according to your needs.

This method of using a shunt resistor and an operational amplifier to sense current has been widely used because of its low cost, high accuracy, and ease of operation. However, sometimes adding a resistor can have an adverse effect on the circuit, so this method has a significant disadvantage: it cannot be used for high loads because the power loss in the shunt resistor is too large in this case.

In principle, the loss can be reduced by reducing the resistance of the shunt resistor, but in this case the voltage drop of the shunt resistor will also be reduced, and the detection voltage will become more difficult. Therefore, a trade-off must be made between choosing a shunt resistor with a smaller resistance value and achieving detection accuracy.

In particular, current detection circuits used to control brushless motors and DCDC converters and detect the remaining battery power require large currents and high detection accuracy. Therefore, we need to use not only small-value shunt resistors but also high-precision operational amplifiers.

When using shunt resistors for actual measurements, consider the maximum current of the load to be detected and the application scenario. This means you need to estimate the maximum current to be detected, the required accuracy, and the losses you can tolerate. It is also important to consider manufacturing costs when designing circuits and selecting components.

Please note that when using a current detection circuit to protect a circuit, since the current detection circuit itself is only used to detect current, you need to add protection and control functions. For example, you need to add a relay or a load switch to cut off the circuit. At the same time, you also need to determine the procedure or circuit for operation and under what circumstances to enable the protection function.

If a current sensing circuit is installed, the circuit and control will become more complicated and the threshold will be slightly increased, but this circuit is essential to improve the safety and functionality of the electronic kit.

The original text comes from the Internet, the author is unknown.

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