Bridge Circuits: the basics

From Wheatstone bridges to bridge rectifiers, bridge circuits are a circuit format where two circuit branches are run in parallel with each other and a point on each of these is connected or bridged through another network.

Bridge Circuits Includes:
Bridge circuits: the basics Wheatstone bridge

A bridge circuit is a widely used form of electronic or electrical circuit format that is often used in measurement, or sometimes in control situations and circuits.

In essence a bridge circuit comprises two branch networks or legs which typically are connected in parallel, and some point in each of these legs is bridged in some form.

Wheatstone bridge - showing the two main arms: R1 + R4, and then R2 + R3 with a meter bridging the two arms — The Wheatstone bridge is one of the more widely known bridge circuits

The bridge was originally developed as a way of measuring various electrical parameters. Although they may seem rather archaic in some ways, this could not be further fromt he truth as they are widely used in many areas of electrical and electronics technology today.

Today, bridge circuits are used in many areas of electrical and electronic technology in a large variety of ways - often they are incorporated into circuits where the end user is unlikely to know that a bridge circuit is beng used -they are one of the quiet success stories.

Today bridge circuits are used in linear and non-linear circuits: they are used in many ways for applications including instrumentation and measurement, filtering, power conversion and more.

Key to understanding bridge circuits

Although the different types of bridge circuit may appear to have many differences, they are all very similar in their operation.

Typically they are drawn as a square rotated through 45° having their corners on horizontal and vertical lines because this gives a better clue to their operation.

In all cases the bridge circuits are based on the same principle: that is to compare the output of two potential dividers which share a common source.

The potential divider is normally created using just two electronic components which are typically resistors, although in the case of the various bridge circuits used, other components may also be introduced, often in series or paralel with the resistors.

The junction between the two resistors or network elements then has a defined proportion of the input voltage appearing across it.

The potential divider is a very useful method of reducing the voltage within a circuit with a minimum of electronic components, and it is also used as the basis for the various bridge circuits.

Basic potential divider or voltage divider circuit using two reistors as the electronic components — Basic voltage divider circuit using two resistors

Although the elements within the various sections of the bridges may include other components beyond the resistors, it is easiest to gain a view of their operation when just using resistors. Further analysis can be undertaken for the individual types.

It can be seen that the full input voltage labelled V_in appears across both of the resistors in series. As the same current passes through both resistors, assuming there is no load on the output, the output voltage will be a proportion of the input voltage.

The output voltage can be calculated from the formula given below.

V_{out} = V_{in} (\frac{R_{2}}{R_{1} + R_{2}})

Manipulating the equation, it can be seen that the ratio of the output over the input voltage is equal to the ratio of R to the total resistance, i.e. R₁ + R.

Bridge circuit types

There is a surprising number of bridge circuits that can be found and used: some are more popular and well known than others.

Wheatstone bridge: The Wheatstone bridge is the most widely known of the traditional bridge circuits. Consisting of four resistors, two in each arm, with meter bridging the centres of the two arms, the Wheatstone bridge is an essential circuit configuration for making many types of accurate measurement. It is often used in various forms of electronic measuring instrumentation to this day.
Wien bridge: This circuit was based on the Wheatstone bridge, but in one leg of the bridge, a series resistor capacitor combination was used and in the other section of the same arm a parallel capacitor and resistor was used. This gave a way of measuring a capacitor and resistor combination.
Maxwell bridge: A Maxwell bridge is another modification of a basic Wheatstone bridge which is used to measure an unknown inductance.
Heaviside bridge: This form of bridge circuit is used to discover the level of mutual inductance and it is a little more involved than some of the other types.
Bridge rectifier: Primarily used within power supplies, the bridge rectifier is a form of bridge circuit that uses four diodes to provide full wave rectification of an incoming alternating waveform. The AC waveform is applied to two opposing sides of the bridge and the full wave rectified signal is extracted from the other two.

Key to understanding bridge circuits

Although the different types of bridge circuit may appear to have many differences, they are all very similar in their operation.

Typically they are drawn as a square rotated through 45° having their corners on horizontal and vertical lines because this gives a better clue to their operation.

In all cases the bridge circuits are based on the same principle: that is to compare the output of two potential dividers which share a common source.

Bridge circuits from the Wheatstone bridge to the Wien bridge to the bridge rectifier and many more are surprisingly important in today's high tech electronics scene. While we may think they have have been consigned to the school physics laboratory to be forgotten, this is not so because they are so widely used in many areas of electrical and electronic technology today.