Basic Bridge Circuits

A bridge circuit splits an input voltage into two paths, compares the voltages across different branches, and provides an output. These circuits are widely valued for their accuracy and versatility in comparing, balancing, or measuring electrical properties. These circuits find applications in sensing, calibration, and even filtering. They consist of a combination of four resistive, capacitive, or inductive elements arranged in a quadrilateral (bridge-like) structure depending upon measuring application. Bridge circuits may be drawn in schematic form in an H-shape or in a diamond shape, although the diamond configuration is more common.

The voltage source powering the bridge circuit is called the excitation source. This source may be DC or AC depending on the application of the bridge circuit. Bridge circuits, first invented in the early 19th century by Samuel Hunter Christie, have been pivotal in technological advancements, from measuring unknown electrical resistances to enabling advanced sensors today. Bridge circuits are used extensively in both the laboratory and industry, these circuits stand testament to the fundamental principles of electrical engineering. Let's dive deep into understanding bridge circuits. See, how the bridges are implemented in a circuit schematics with other components.

Types of Basic Bridge Circuits:

Bridge circuits have remained at the forefront of electrical measurement techniques for generations. Among the plethora, five distinct bridge circuits have been universally acknowledged for their precision and versatility. Let's delve deeper into each of these. Here's an overview of some Top 10 types of bridge circuits:

1. Wheatstone Bridge:

This bridge is fundamentally a diamond configuration of four resistors and used to measure unknown resistances. When balanced, the ratio of two adjacent resistors equals the ratio of the other two, allowing for the determination of an unknown resistance. In this way if three resistance values are known, a fourth unknown resistance value can be obtained. It finds extensive use in labs for accurate resistance measurements and in industries where sensors change resistance based on some physical parameters.

It has applications in strain gauges for mechanical measurements and resistance thermometers for temperature measurement. For example: A strain gauge mounted on a mechanical structure detects deformation by changing its resistance. The Wheatstone Bridge converts this resistance change into a measurable voltage.  

2. Kelvin Bridge:

Same as wheatstone bridge, a Kelvin bridge circuit is used to measure unknown electrical resistors, under 1 Ohm. It is particularly intended to measure resistors that are assembled as four-terminal resistors. It has high-accuracy resistance measurement, particularly for low resistances. While measuring very low resistance in wires or contacts, the Kelvin Bridge eliminates errors from lead resistances.

3. Maxwell Bridge:

It is used for measurement of unknown inductance. It is given by James C. Maxwell, this bridge specializes in evaluating unknown inductances using known capacitances and resistances. The bridge is balanced by varying resistance and capacitance until the detector shows no deflection. It's particularly adept at measuring inductors with medium quality factors, ranging between 1 and 10.

4. Hay Bridge:

Hay's bridge is an enhanced version of Maxwell’s bridge, specifically designed for more accurate inductance measurements, especially in cases where the inductor's quality factor is high. This bridge is optimal for high-quality factor inductors, typically those exceeding a factor of 10.

5. Schering Bridge:

It is used for measurement of unknown capacitance and dielectric loss. The Schering bridge is vital for high-frequency applications. In a common configuration, a known capacitor and a resistor are connected in series in one arm, while an unknown capacitor and its parallel resistor are placed in another. This arrangement is widely used in capacitor manufacturing for quality checks and in laboratories for precise capacitance measurements.

6. AC Bridges:

Used for measuring complex quantities like inductance, capacitance, and impedance. They operate with alternating current and are essential for high-frequency applications.

For example:  A Wien bridge circuit is used for accurate measurement of capacitance in terms of frequency and resistance. It is also used for measuring audio frequencies. It balances when the impedance ratio in one leg matches the other leg's ratio.

7. Diode Bridge:

A diode bridge circuit is used to provide the same polarity of output for each polarity of the input. A diode bridge circuit, also known as a bridge rectifier, is a configuration of four diodes arranged in a bridge topology. It is commonly used to convert alternating current (AC) into direct current (DC) in power supply. There are three types of diode circuit which implies AC to DC conversion, half wave, full wave and bridge rectifier.

8. H Bridge:

A H-bridge circuit is used to allow DC motors in robots to move forward and backward by enabling a voltage across a load. It is basically a circuit used in motor driver applications where a controlling signal is used to drive the motor clockwise and counterclockwise.

9. Fontana Bridge:

A Fontana bridge circuit is used to implement an ample frequency band voltage to the current converter. The Fontana Bridge Circuit is a specialized electrical circuit used for precise measurements of capacitance and resistance in various configurations. It operates on the principle of balancing impedances in a bridge setup, making it effective for accurate component evaluation in laboratory and industrial applications.

It is commonly employed in testing and calibrating capacitors, offering high sensitivity and reliability. The circuit’s design allows it to handle both high-precision measurements and low-loss capacitor testing. Its versatility makes it a valuable tool in advanced electronics testing.

10. Carey Foster Bridge:

A Carey Foster bridge circuit is used to measure tiny differences between two large resistances to calculate low resistances. It is particularly useful for determining the unknown resistance of wires or components by comparing them with a known standard resistor. The circuit achieves accuracy by balancing the bridge through a null method, eliminating errors from wire resistances in the setup.

Collectively, these bridge circuits underline the importance of precise measurements in electrical engineering, each serving a distinct purpose to cater to a myriad of applications.

Why Are Bridge Circuits Important?

Bridge circuits offer exceptional accuracy by nullifying external influences when balanced. Their advantages include:  

1. Precision in Measurement: Essential for laboratory and industrial applications.  

2. Dynamic Range: Can handle low and high electrical properties.  

3. Flexibility: By adjusting components, bridge circuits can measure different parameters.  

4. Simplicity: Despite their effectiveness, the design is straightforward and easily adaptable.

Applications of Bridge Circuits

Bridge circuits have a vast range of applications in electronics, including:  

• Sensing and Instrumentation: Used in strain gauges, thermistors, and piezoelectric sensors.  
• Electrical Testing: Capacitance and resistance calibration.  
• Signal Conditioning: Filtering and signal comparison.  
• AC/DC Conversions: Used in rectifiers for converting alternating to direct current.  
• Inductor and Capacitor Testing: Bridges like Maxwell and Schering are widely used in laboratories.  

Conclusion

Bridge circuits form the backbone of precision measurements and signal processing in electronics. From the Wheatstone Bridge for resistance measurement to the Schering Bridge for capacitance testing, their diversity and utility are unparalleled. Understanding the fundamentals of bridge circuits empowers engineers and technicians to implement accurate and reliable systems across various fields. Whether you're a student, a hobbyist, or a professional, mastering bridge circuits is an essential step in exploring the world of electronics.