6 Simple AC Bridge Circuits Explained

An AC bridge is a circuit which can be used for measuring parameters like capacitance, resistance, Inductance using differential method, by comparing them with known values of similar components, positioned diagonally across a bridge circuit, and through an analogue meter placed at the center of the bridge.

Before we begin talking about the AC bridge circuits using capacitance and/or inductance, it may be important to understand several related terminologies. The capacitors and inductors that are ideally suited for the application are specified with reactance in an AC circuit, instead of resistance.

Important Parameters

Reactance represents the AC variant of "resistance," which is indicated in ohms. Impedance is one more relative term which is utilized to express the whole combined result of reactance and resistance in certain section of a circuit, and is indicated in ohms.

In the practical world capacitors and inductors both may consists of resistance and reactance, therefore while talking about them we imagine them also having an impedance and not just only reactance.

由于AC桥式电路腿可以包括电抗和/或电阻，因此我们将在阻抗和不受阻抗方面讨论这些桥的这些腿。

要精确，抵抗和阻抗可能是非常相同的。保持交流桥平衡的因素与直流桥完全相同，我们只插入阻抗（symbolized as Z）而不是阻力。

This provides us with extremely standard equations which can be used with any relevant circuit.

AC桥接良好地工作，以计算组件值，例如电容。如果我们在AC桥电路中有一个电容器，则可以利用其值中已知的桥中的组件的值来确定其值以及可能需要实现空读取的值的值。

1）测量小电容差异

如果我们想确定电容器电容的小差异，我们可以开始使用像图1所示的交流桥电路。如下1。考虑到电阻器的阻抗是严格的电阻并且电容器的阻抗实际上完全反应，桥的上部分支的阻抗根本不能与下部分支不同。

在这个电路中R1/C1 = R2/C2when the R3 is correctly adjusted for getting a zero or null reading on the voltmeter V.

This indicates that the second set of conditions needed for balancing (ZA = Z3 and Z4 = ZB) cannot be be fulfilled. The third conditions required for balance the AC circuit cannot be fulfilled either.

Having said that, we can see that the initial conditions (ZA = ZB and Z3 = Z4) could be fulfilled. The impedance ZA consists of R1 and a part of R3, while the impedance ZB comprises of R2 and a some part of R3. The impedance Z3 is attributed to the capacitor C1, while Z4 is the impedance that is used to represent C2.

如果C1仍然等于C2和任何R1d R2 difference is tweaked and corrected by R3, then we can achieve a well balanced bridge.

Here the impedance of active components, such as resistors and capacitors is determined by the frequency. However we find that the Fig. 1 is not dependent on frequency because the C1, C2 reactances could alter together whenever the frequency changes.

An additional variant of the above discussed AC bridge circuit can be seen in Fig. 2. Although this circuit is designed to work well, it isn't one of the favorite AC bridge versions due to the presence of the DC isolation across only one side of the meter (at point B).

在这个电路中R1 / R2 = C1 / C2when the variable resistor R3 is appropriately adjusted to get a zero or null reading on the meter V.

如果DC内容通过输入源，则可能导致效果大量不利。另外，它可能很复杂地偏置在点A时挂钩的仪表输入放大器，而不会在A的情况下修改输入信号电平。

2）仅使用电容器

It is possible to create an AC bridge circuit using only capacitors as demonstrated in Fig. 3 below.

在这个电路中，C1 / C2 = C2 / C4, when the variable resistor R1 is carefully adjusted until a zero or null reading is obtained on the meter V

Let's analyze the initial condition (ZA = ZB and Z3 = Z4). Given that the reactance of the capacitor pairs C1/C2 and C3/C4 can change in the same way in response to the changing frequency, the balanced state of the AC bridge is not dependent on the frequency.

对于第3条条件（za = zb = z3 = z4）同样是真的。另一种替代方案可以是C1和C3是相同的，并且C2和C4是相同的。通过满足这些条件，并且输入频率微调以跨点A和B获得相同的相移，可以在桥接电路中实现PEREFCT平衡。

然而,上述条件阻止了桥from being independent to the input frequency. Regardless of what conditions we use for getting the AC bridge circuit properly balanced, if the value of any one of the capacitors is changed will cause the generation of an AC voltage output between points A and B.

由于点A和B都与输入交流电源保持良好，因此将该高阻抗点连接到任何优选的差分放大器，以进一步处理结果，因此相当容易。

3）使用电感和电容在一起

Inductors and capacitors both could be applied in an AC bridge circuit, as indicated in Fig. 4.

In this network,L1/R2 = R1/C1，当R3完全调整以使仪表V上的零或零点。

So far, the phase shift symbols of active components did not demand consideration. However, inductors and capacitors both can give rise to phase shift. Capacitors possess a positive phase shift, and inductors can generate negative phase shift. Therefore, in case an inductor is connected in one leg of the of the AC bridge, a capacitor must be connected at the other opposite leg (unless you may want to work with an additional inductor hooked up at the adjacent leg of the AC bridge). These elements will possess equal and opposite phase shift only when they are positioned diagonally across from one another in the AC bridge diagram.

However , the phase shift needs to be identical for the left-hand and the right-hand portions of the circuit together. This may be only achievable at frequency levels where the reactances of the active components are identical. Therefore, the L1 and C1 reactances in Fig. 6 should be identical to one another at the operating frequency.

如果实现了附加条件，可以平衡所示的桥。该规则是，如果L1电抗变得等于R1电阻，并且C1电抗变得与工作频率的R2电阻相同。这意味着R1应该与R2相同。

We can evaluate the reactances of the capacitive and inductive components in an AC bridge circuit by replacing R1 and R2 with variable resistance, or potetiometers. After this we can measure the resistance, once proper balancing of the pots are accomplished.

4) Using Identical Inductances and Identical Capacitances

更容易更容易的方法是在交流桥腿中插入相同的电感和相同的电容，如下图所示。如图5所示，通过在同一桥内的另一条腿上具有电感和电容来平衡桥接电路。

在这个电路中l1 / c1 = l2 / c2when R1 is carefully tweaked to get a zero or null reading on the meter V

In this AC bridge circuit, the phase shift across the two left and right sections are the same for a particular frequency. A little modification in either the capacitance or inductance within one of the bridge legs will cause the generation of an AC output at points A and B. This bridge circuit could be balanced by fine-tuning C2 instead of a adjusting the resistor for the balancing.

5) Using Only Inductors

An AC bridge circuit using only inductors can be seen in Fig. 6 below.

在这种配置中，L1 / L3 = L2 / L4when R2 carefully adjusted to get a zero or null reading on the meter. R1 is used for fine tuning and ensure that the meter reading comes perfectly to the zero mark

By adding a sensitive voltmeter in series with a variable resistor, it becomes possible to calibrate to any desired output range within bridge's tolerable voltage range. This bridge can perform very nicely for detecting the inductance value of any type of inductor that may be introduced in the bridge.

6）在另一个桥内

There are also AC bridge circuits that include one bridge inside another bridge. Such types of circuit can be used for determining the combined results of more than a couple of versions of the circuit, as indicated in the Fig. 7 below.

近仪表附近的桥接网络类似于全波整流器，让我们了解DC表的灵敏度，以显示交流电压的变化。

If necessary it may be possible to include more than one element to the specific legs of the bridge.

虽然这可能使系统数学非常复杂地，但余额所需的因素可能不会改变，意味着在密切关注和调整相移的同时保持节点的相同电压差。关于相移的理解，究竟需要在交流桥电路中彼此相互补充的组件非常重要，如果您打算将电路利用到您最大的优势。通过AC桥接电路的基本原理良好，可以解决许多未来的电子设计并发症。