Introduce three clever cheap current detection circuits
In power supplies and other equipment, current detection or feedback is usually required. Current detection usually uses a series-sampling resistor to amplify the voltage across the resistor. If you want to improve the detection accuracy, you will often use a more expensive instrumentation amplifier. The op amp offset voltage is relatively large.
Here are a few clever cheap current detection circuits.
1, triode current detection circuit 
If the transistor is simply turned on and off to detect the current, the transistor should be turned on to about 0.7V. When the current is small, the resistor needs to be used in large series, and a large feedback voltage is wasted. As shown in the above figure, it can be used relatively small. The resistance can be detected by consuming a small voltage, and the sensitivity of the detection can be adjusted by adjusting the resistance of the triode base. This circuit can be used in places such as chargers that need to display the presence or absence of charging current.
2, high sensitivity current detection circuit
This circuit uses two diodes for current sampling. The sensitivity is very high. The current can be used in a wide dynamic range. It is suitable for high-power or high-voltage applications. The disadvantage is that the voltage needs to be damaged by about 1.4V.
3, TL431 current feedback circuit
The TL431 is inexpensive and widely used in the feedback loop of the switching power supply, but its FB voltage is 2.5V. It is a large sampling resistor used for current feedback and wastes voltage. In the figure, the current feedback is realized by two TL431s, and the precise current feedback can be realized with a relatively small sampling resistor. If there is a voltage feedback network, the voltage feedback circuit of U3 is connected.
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