Bridge

The concept of a bridge: an instrument that measures various quantities (such as resistance, capacitance, inductance, etc.) by comparison. The simplest is a circuit consisting of four branches. Each branch is called the "arm" of the bridge.

Basic interpretation

A quadrilateral measuring circuit composed of components such as resistors, capacitors, and inductors is called a bridge. People often refer to the four sides as bridge arms. As the measuring circuit, a power supply is connected to one diagonal end of the quadrilateral, and the other diagonal line is connected to the zero instrument at both ends. Adjust the parameter values ​​of some components on the bridge arm so that the voltage across the zero-pointing instrument is zero, and the bridge is balanced. Using the bridge balance equation Z1Z3=Z2Z4, the parameters of the device under test (such as resistance, inductance and capacitance) can be obtained from the values ​​of the known components in the bridge arm.

Principle introduction

Assume that the four resistors are fixed. When s is closed, if "R3*R2=R1*R4" is satisfied, that is, the product of the diagonal resistances is equal, then Uad is equal to 0, that is, there is no voltage between ad. Using this principle, when one of the four quantities on either side of the equation is unknown, if the values ​​of the remaining three resistors are adjusted so that the equation holds, then the unknown can be obtained by formula. But in fact, as long as there is an adjustable variable resistor on each side of the equation, then one of the other two resistors is a fixed value, and the remaining one is inevitably available. With this principle, a resistance measuring box can be made. This principle uses the concept of "bridge" or the concept of "balanced bridge".

Common bridge

Commonly used are the Wheatstone Bridge and the Kelvin Bridge.

Wheatstone Bridge

Wheatstone bridge test resistance

The bridge is an electromagnetic basic measuring instrument that uses a comparative method to measure physical quantities. There are many types of bridges. The resistance for measuring medium resistance (10 to 10^6 ohms) should be measured with a Wheatstone single-arm bridge; For higher resistance values, high-resistance bridges or megohmmeters are generally used. To measure resistances with small resistance values, double-arm bridges (Kelvin bridges) are generally used. The bridge has high accuracy and good stability, so it is widely used in electromagnetic measurement, automatic adjustment and automatic control. The Wheatstone single-arm bridge is the most basic DC single-arm bridge. DC single arm bridge.

Kelvin Bridge

The Kelvin Bridge is also known as the "armed bridge." Measure 10^(-6)~10^2 ohm low resistance DC bridge. FIG R is the resistance across the line (including the wiring resistance between the Rx, Rs two resistors, the contact resistance and internal wiring resistance), the resistance Rs of the standard. When adjusting R1, R2, R3, R4 and Rs in the bridge so that no current (i.e., sensitivity is not a pointer deflection galvanometer G), the following conditions are true equilibrium: R1R2 = R3R4 and Rx = R1R2Rs. Rx and R independent calculation formula, thereby reducing the contact resistance and the wiring resistance error generated.