Current Detection Circuit Design Tips

Use tricks to use tricks. Today I will teach you some clever tricks on current detection circuits. You must know that current detection or feedback is usually required in power supplies and other equipment. Current detection usually uses a series sampling resistor to amplify the voltage on the resistor through an amplifier. If you want to improve the detection accuracy, a more expensive instrument amplifier is often used. I think that 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 we simply use the transistor on and off to detect the current, the transistor will turn on at about 0.7V. When the current is relatively small, we need to use a larger resistor in series, and at the same time waste a larger feedback voltage. As shown in the figure above, you can use a smaller The resistance of the transistor consumes very little voltage to detect the current I. The sensitivity of the detection can be adjusted by adjusting the base resistance of the transistor. This circuit can be used in chargers and other places where it is necessary to display whether there is charging current.

2 High-sensitivity current detection circuit

This circuit uses two diodes for current sampling. The sensitivity is very high, and the current can achieve a large 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

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