Resistive bridge circuits remain the dominant pressure-sensing technology. This article examines the operating principles of Wheatstone bridge configurations in pressure sensors, compares thin-film, thick-film, and metal foil technologies, and addresses key resistor design considerations.
There are many pressure-sensing technologies, including capacitive bridges, inductive-based solutions, piezoelectric and optical methods, but none combine the simplicity, manufacturability, and broad applicability of resistive bridges, which explains their continued dominance in pressure sensor design.
Resistor bridges are widely used in pressure sensors because they efficiently convert small strain-induced resistance changes into measurable voltage signals.
Pressure Sensor Applications
Pressure sensors are used in a wide range of industrial and military applications. They often operate in harsh environmental conditions where high temperatures, high moisture levels and contaminants are present. Some common applications include:
- Fluid and gas level monitoring.
- Hydraulic systems.
- Alarm systems.
- Automotive (MAP sensors, brake pressure, tire monitoring).
- Industrial process control.
- Medical devices (e.g., disposable pressure transducers).
- Aerospace applications.
Resistor Bridges Principle of Operation
When pressure acts on a membrane or substrate, the deformation changes the cross-section of the conductor tracks. This, in turn, causes a shift in the electrical resistance (measured in milliOhms). It is this change in electrical resistance that a pressure sensor records.
By wiring four resistors in a Wheatstone bridge, opposite arms increase/decrease resistance in complementary ways. This unbalances the bridge, producing a differential output voltage proportional to the applied strain or pressure. The Wheatstone bridge configuration provides differential measurement, which boosts sensitivity while cancelling common-mode errors.
Ideally, output voltage should be directly proportional to applied strain, but nonlinearity can arise from diaphragm deflection mechanics or resistor placement. Careful diaphragm geometry and bridge design deliver the linear response, essential for accurate measurements across the full range.
Resistor Bridge Technology Comparison
Thick-film resistors are corrosion-resistant and robust. They are generally used in more chemically aggressive environments than thin film. Thick-film is also significantly cheaper and has greater power handling capability than the thin-film equivalent.
Metal foil strain gauges remain the benchmark for stability and accuracy; however, they are relatively bulky and costly. Thin-film resistors on steel or silicon offer excellent precision and stability in high-temperature environments. They can withstand very high pressures (1000’s of Bar) and have better long-term stability than thick film. Unfortunately, the thin film deposition process is complex and thin film resistors are more expensive than thick film.
In contrast, thick-film resistors on ceramic substrates provide a robust, versatile, and cost-effective solution. They can be customised to match the demands of specific applications and, although they don’t offer extreme precision, they are often the best overall choice for general pressure sensing applications.
Thick Film Resistor Bridges Design Considerations
Thick film pressure sensors use four resistors in a Wheatstone bridge configuration. The resistors are screen printed onto the back of a ceramic membrane diaphragm and then hermetically joined to a main ceramic body. Connection is made from the membrane through the sensor body to the connection points on the surface.
The sensitivity of thick film resistor network bridges is related to ceramic thickness. Very thin, lapped ceramic delivers very sensitive sensors, while thicker ceramic allows for higher pressure ranges.
When designing with thick-film resistor bridges, the main challenges are temperature drift, humidity sensitivity, noise, and long-term stability. Success depends on careful resistor material selection and protective coatings.
To address temperature drift, thick film resistor manufacturers choose resistor pastes with low TCR and good long-term stability and maintain tight control of the screen-printing and firing process. Glass, epoxy, or hermetic packaging is essential in high-humidity or chemical environments. In some high-performance applications, external compensation is required to address offsets driven by poor resistor tolerance matching or noise.
It is important to select the right technology for the application by comparing functionality with cost. A range of standard sensors is available, but if they do not fit the demands of the application, a manufacturer of specialist thick film resistors can help.