The Stuttung bridge is a circuit that is very suitable for precision measurement of resistance. Although it has been invented for nearly 150 years, it is still favored by electronic engineers due to its extremely high test accuracy.

What is the working principle of the electric bridge?

Look at this relatively simple way of understanding. In this drawing, there are two branches: R1 and R2 form one, and R3 and R4 form the other. According to the resistance value on the drawing, the voltage value between R1 and R3 will be equal, and the voltage value between R2 and R4 will also be equal. This is because the ratio of the resistance value between R1 and R2 is equal to the ratio of the resistance value between R3 and R4, so their voltages are also equal. This makes the potentials of point A and point C the same. If you connect A and C with a wire, there is no charge flowing between these two points. At this time you can say that the bridge is balanced.

Now we assume that the value of resistor R4 is unknown (and set it as Rx), and connect an ammeter in series at points A and C in the previous circuit. If the current representation number is 0, we can immediately get the value of the resistance Rx-because as mentioned above, when there is no current flowing between the two AC points, the voltage at the two AC points must be the same, so R1 and R2 can be obtained The ratio of the resistance values between R3 and R4 is also equal. Then the resistance value of Rx can be calculated. Rx=(R2 x R3)/R1.

But if the number of G is not zero, what should we do? At this time, the voltages of AC two points are definitely not equal. We can replace R2 with a variable resistor and repeat the test. In the experiment, turn on the power. And began to adjust the variable resistance. Note that in the previous picture, point B and point D are actually the same point in the circuit, so we can draw them together. In the process of adjusting the resistance, you will find that the indication of the ammeter has changed, and after adjusting to the absence of the indication of the ammeter, it means that the state has reached the same state as in the first case. At this point, you can follow the calculation formula in the first case to substitute the resistance value of the variable resistor and calculate the resistance value of Rx.

What exactly can an electric bridge do?

In fact, by calculating the voltage difference between the two AC points in the schematic, you can directly calculate the resistance of an unknown resistance through the resistance of three known resistances. There are two main advantages of this calculation: you can use this Ways to measure the resistance of a resistance that is difficult to reach, such as the resistance of a temperature measuring resistor buried in concrete. On the other hand, the accuracy of such measurement is also higher than that of the meter when directly measuring resistance. When measuring the resistance of a small resistance, we no longer need to amplify it, but with the help of the other three high-precision resistances, we can directly calculate the high-precision resistance value directly through the difference of the voltage. .

A more classic application is to measure small changes in stress. The manifestation of all stress changes reflected on the stress plate is that when the stress plate is bent, its resistance changes very slightly. This kind of stress plate is mainly buried deep inside the building or bridge to detect the change of internal force.

When measuring resistance, we also use a bridge circuit to convert the change in resistance value into a change in voltage value and send it to the amplifier. By measuring the voltage value, the resistance value can be obtained. This measurement method is very common and effective.

Understand the principle of the electric bridge, next time you design a temperature-sensitive or photosensitive circuit, do you understand what method should be used to measure the tiny resistance?