Several clever current detection circuits

　　Current detection or feedback is usually required in power supplies and other equipment. Current detection usually uses a series sampling resistor and an amplifier to amplify the voltage on the resistor. If the detection accuracy is to be improved, a more expensive instrument amplifier is often used because the offset voltage of ordinary op amps is relatively large.

　　Here are some clever and cheap current detection circuits.

　　1 Transistor current detection circuit

 

　　If the current is simply detected by turning on and off the transistor, the transistor needs to be turned on at about 0.7V. When the current is relatively small, a relatively large resistor needs to be connected in series, and a large feedback voltage is wasted. As shown in the method in the figure above, a relatively small resistor can be used to detect the current I by consuming very little voltage. The sensitivity of the detection can be adjusted by adjusting the transistor base resistance. This circuit can be used in chargers and other places where it is necessary to display whether there is charging current.

　　3 High-sensitivity current detection circuit

 

　　This circuit uses two diodes for current sampling, which has very high sensitivity and can achieve a large dynamic range of current. It is more 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

 

　　TL431 is cheap and widely used in the feedback loop of switching power supply, but its FB voltage is 2.5V. When it is used directly for current feedback, a large sampling resistor is required, which wastes voltage. In the figure, two TL431s are used to realize current feedback, and a relatively small sampling resistor can be used to realize precise current feedback. If there is a voltage feedback network, the voltage feedback circuit of U3 can be connected in parallel.