We use diversion resistors and computing amplifiers to make a current detection circuit and check how the circuit detects the current. The current detection circuit is as follows:

The picture shows the current detection circuit you want to make. Differential amplification circuit detect the voltage of the diversion resistance, and then enlarged the voltage signal of more than 15 times and output the 62MΩ patch resistance as the shunt resistance. The measured maximum current value is determined by the power of the patch resistance. We are currently using 1W resistance, so W = I2R, 1W ≒ 4A × 4A × 62MΩ, and finally calculated that the maximum current is 4A.

ROHM current detection patch resistance LRRRRRRRRRRR18 series, 62MΩ 1W patch resistance

If the amplification multiple of the measuring current circuit is too large, it will exceed the working voltage of the computing amplifier, so you need to adjust the amplification multiple according to the estimated maximum current value. The amplification multiple we set this time is 15 times, so when the current of the excessive overcurrent resistance is 4A, the output voltage of the operation 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 operations amplifier.

EMARMOUR "LMR1802G-LB" operational amplifier has a low input offset voltage 5uV (TYP) and used for sensing equipment.

The computing amplifier and shunt resistance installed on the general board. Because these devices are installed on the universal board in the experimental environment, it is easy to welded, but in the actual circuit design, appropriate graphics design will be carried out according to the diversion resistance technology specification book.

Since the current is detected by a diversion resistor, let's explore how to use a simple circuit connected to the general board to detect the current.

shunt resistor current measurement

Use an oscilloscope to measure the voltage value and observe the current flow direction

Connect the load to the complete current detection circuit and observe the detection waveform. Connect a DC brush motor to the load. If the current can be successfully detected, the current waveform of the current when switching in the motor coil should be detected and the change of rotation after applying the load.

Dial resistance is connected in series with motor and power. The working voltage of the motor is 5V.

The air load current of the motor is 0.32A. The effective value of the waveform output of the operation amplifier is 202mv, and the detection value is 0.3A. The bandwidth of the oscilloscope and probe is 50MHz.

When the motor is running, you can see that the current changes with the switching of the directioner. As the load increases, the speed of the motor decreases, and the changing current value detected by the shunt resistance will be presented in the form of voltage signal changes.

If the current detection output of the computing amplifier is connected to the microcontroller board, such as Arduino, you can detect the electrical current in real time to help you find abnormal conditions such as motor lock and coil short circuit.

If the current can be detected, a variety of functions can be added to the circuit, such as avoiding the electrical subject/drive circuit overload, and detecting motor locks.

To achieve higher accuracy and a larger range of current detection by changing the shunt resistance

When the actual current detection circuit is used to protect the circuit, a device with low resistance and large current (not exceeding the maximum power of the diversion resistance) will be selected.

We use universal patch resistance, and in the existing high -performance diversion resistance, there are high -power type resistors that can consume up to 5W power values, and high -precision diversion resistors with 0.1MΩ ultra -low resistance value. You can choose the corresponding diversion resistance according to your needs.

Summarize

This method of using diversion resistors and computing amplifiers to detect current has been widely used because the method is low, high accuracy and easy to operate. However, adding a resistance will adversely affect the circuit, so there is a significant disadvantage of this method: it cannot be used for high loads because the diversion resistance power loss is too large in this case.

In terms of principle, the loss can be reduced by reducing the resistance value of the diversion resistance, but in this case, the pressure drop of the shunt resistance will also be reduced, and the detection voltage will become more difficult. Therefore Between detection accuracy.

In particular, the current detection circuit for controlling a brushless motor and DCDC converter, and the detection of the remaining power of the battery requires large current and high detection accuracy. Therefore, we not only need to use small resistance diversion resistors, but also use high -precision operational amplifiers.

When using the diversion resistance for actual measurement, consider the maximum current and application scenarios based on the detected load. In other words, you need to estimate the maximum current you want to detect, the accuracy you need, and the loss that can be afforded. There is also a very important point. When designing circuit design and component selection, you also need to consider manufacturing costs.

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

If the current detection circuit is installed, the circuit and control will become more complicated, and the threshold will be slightly increased, but the circuit is essential for improving the safety and functionality of the electronic kit.