Bridge Circuits Explained: Measuring, Rectifying, and Driving Loads

Bridge circuits are not one interchangeable family. A measurement bridge compares electrical quantities, a diode bridge rectifies an input, and an H-bridge controls the voltage applied across a load. Recognizing that difference is the first design decision: a balanced sensor interface and a switching motor stage need very different components, PCB layouts, and tests.

Begin With the Required Output

A measurement bridge produces a differential signal related to a resistance or impedance relationship. A rectifier produces a consistent output polarity from an alternating input. A motor bridge selects switching states to apply either polarity across its load. These functions can coexist on one board, but their output nodes must not be treated alike.

*Figure 1: Measurement, rectification, and motor drive are distinct bridge functions. These are conceptual applications, not wiring diagrams.*

For example, connecting a microvolt-level sensor return through the same copper neck as a motor current return can introduce an error even when both schematics are individually correct. Function classification therefore belongs before component selection and placement.

A Wheatstone Bridge With Unambiguous Nodes

The Wheatstone bridge compares two resistor dividers. Its output is the difference between their midpoint voltages, so an equal midpoint voltage produces zero differential output. Resistive sensors deliberately move away from this balanced state when the measured quantity changes. Analog Devices explains the basic bridge relationship.

Define the connections before using a formula:

Apply excitation voltage VEX between the upper supply node and the lower node, defined as 0 V.
Put R1 above midpoint A and R2 below A in the left branch.
Put R3 above midpoint B and R4 below B in the right branch.
Measure VOUT = VA − VB with a high-input-impedance differential receiver.

For ideal resistors and negligible receiver loading:

VA = VEX × R2 / (R1 + R2)

VB = VEX × R4 / (R3 + R4)

The bridge balances when R1 × R4 = R2 × R3. The resistor labels are local to this explanation; another schematic may number its arms differently.

Worked Example: A Small Resistance Increase

Assume a 3.3 V excitation and four nominal 1,000 Ω resistors. Initially, A and B both sit at 1.65 V. Now increase only R4 to 1,001 Ω while keeping the other three unchanged:

VB = 3.3 × 1001 / 2001 ≈ 1.650825 V

VOUT = 1.650000 − 1.650825 ≈ −0.825 mV

This is an illustrative calculation, not a sensor specification or measured result. Reversing the receiver inputs changes the sign. Its engineering lesson is that a sub-millivolt useful signal can ride on a common-mode level near half the excitation voltage. The receiver must accommodate both quantities, including offsets and overloads.

Measurement Accuracy Requires More Than Balance

A balanced bridge does not cancel every disturbance. Resistance matching, unequal temperature changes, input offset, noise, and wiring resistance can still affect the result. Increasing excitation raises sensitivity, but also increases resistor dissipation; sensor limits and self-heating must be checked.

A ratiometric interface derives its ADC reference from the bridge excitation. Ideally, proportional excitation changes then affect the measured signal and reference together. This is a specific cancellation mechanism, not immunity to all noise. Analog Devices' excitation and measurement guidance also distinguishes wiring errors from excitation errors.

For low-resistance measurements, separate current and voltage-sense connections prevent the force-lead voltage drop from entering the sensed quantity. Keithley's four-wire measurement explanation describes this Kelvin principle. A Kelvin connection is not automatically a Kelvin double-bridge circuit; the terminology describes related but different arrangements.

Bridges can also contain reactive components. A Wien network is frequency-selective and can form part of an oscillator. Its behavior depends on capacitance, resistance, and frequency; the resistor-only balance relationship above cannot simply be reused. Establish whether the circuit is comparing a measured quantity or providing feedback before selecting its analysis method.

A Diode Bridge Rectifies; It Does Not Measure Balance

A conventional single-phase bridge rectifier uses four diodes, with a pair carrying current on each conducting half-cycle. The load sees the same polarity for either input polarity. Bridge rectification is a form of full-wave rectification, not an alternative category to it; Vishay identifies that function in its single-phase bridge rectifier documentation.

Without filtering, the result remains pulsating. A reservoir capacitor changes charging current into pulses, so diode, capacitor, copper, and connector selection must consider that current waveform. Check reverse-voltage rating, surge conditions, dissipation, and the actual load. On the PCB, keep charging-current paths away from sensitive sensing returns. A rectifier alone provides no galvanic isolation.

An H-Bridge Selects Load Polarity

An H-bridge uses two switching legs with the load between their midpoints. Selecting opposite diagonal switches reverses the applied load voltage; pulse-width modulation can control its average behavior. A motor's instantaneous current also depends on inductance, back EMF, and the selected recirculation state. TI's H-bridge control note describes operating states and current-sensing trade-offs.

Turning on the upper and lower devices of one leg together can short the supply. Driver timing, protection, and suitable dead time address this shoot-through risk. Inductive current must also have a controlled path during switching. Keep local supply decoupling close to the power stage and review switching loops separately from current-sense routes.

Frequently Asked Questions

Does every bridge require a zero-voltage balance adjustment?

No. A measurement bridge may use a null condition or measure departure from balance. Diode bridges and motor H-bridges perform power-conversion functions and do not use Wheatstone balance adjustment.

Can a bridge sensor output connect directly to an ADC?

Sometimes, if the ADC's differential range, common-mode limits, input loading, noise, reference arrangement, and protection meet the sensor requirements. Resolution alone does not establish compatibility.