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 some clever cheap current detection circuits
1. Transistor current detection circuit
If the transistor is simply turned on and off to detect the current, the transistor should be turned on by about 0.7V. When the current is relatively small, a large series of resistors are needed, 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 base resistance of the triode. 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 made 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, two TL431 are used to realize current feedback. The precision 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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