Thick film resistors deliver resistance values from a few Ohms to GigaOhms whilst handling significant power loads in a compact form factor. They deliver stable performance characteristics at a reasonable cost.
Thick film resistors are the preferred choice across numerous applications. However, they do have limitations, so it is important to understand when to specify thick film resistors and when an alternative technology may be a better choice.
Thick Film Resistor Construction
The thick film resistor manufacturing process involves screen-printing resistive paste onto a ceramic substrate. This process is cost-effective and delivers rugged devices with high power-handling capabilities.
The resistance is adjusted by altering the material formulation, thickness, geometry or firing process. Hence, thick film resistors have a wide range of resistance values.
Advantages of Thick Film Resistors
The advantages of thick film resistors over other resistor technologies are as follows:
- High power-to-size ratio
- High resistance values
- High power-handling capability
- Stability at high temperatures
- Cost-effectiveness
- Suitability for harsh environments (vibration, humidity, etc.)
Thick film technology has some limitations. These include a higher temperature coefficient of resistance (TCR) when compared to thin film or metal foil resistors. However, thin film and metal foil lack the wide resistance range and power-handling capabilities of thick film resistors.
Thick film resistors have moderate tolerances (±1% to ±5%) and exhibit higher noise than other resistor technologies. A specialist thick film resistor manufacturer can minimise these limitations to an extent.
Thick Film Resistor Technical Specifications
Key thick film resistor performance characteristics are shown below. These are typical figures based on standard resistor devices offered by major resistor manufacturers. In most cases, custom resistor manufacturers can push the performance envelope for specialist applications. However, there are often trade-offs to consider, as optimising one parameter can negatively affect others.
Resistance range: 0.01 ohm to 10’s GOhm
Power ratings: 0.07 W to 2 kW
Temperature coefficients: ±25 ppm/°C to ±700 ppm/°C
Voltage ratings: to 75 kV (for single elements)
Tolerance options: (±1% to ±20%)
Frequency: Up to 1 MHz
It is important to be wary at the extremes. For example, although it is possible to manufacture high value thick film resistors in the GOhm range, they are sensitive to external environmental factors and nearby noise fields and tolerances are orders of magnitude higher than those specified above. Temperature co-efficients are generally in the ±100 ppm/°C range
When to Specify Thick Film Resistors
Thick film resistor selection depends on specific performance requirements and operating conditions. Thick film technology excels in the following applications:
- High Power Handling: Power supplies, LED drivers and motor drivers
- High Resistance: Leakage detection and high-impedance circuits
- Harsh Environmental Conditions: Automotive, industrial and military electronics
- High Voltage Applications: X-ray machines, CRT monitors, high-voltage power supplies and surge protection
- Where there are space constraints
- Where cost is a concern
hick film resistor components provide a combination of electrical, thermal and mechanical properties, making them suitable for many applications. However, certain scenarios may favour alternative resistor technologies. These include:
- High precision applications
- Applications requiring very high stability
- Low noise applications
- Extreme power applications
- Extremely high-frequency circuits (thick film resistors are non-inductive)
Design Considerations When Using Thick Film Resistors
When specifying any resistor technology (including thick film) for demanding applications, it is important to consider the following:
Power Rating: Ensure the resistor can handle the required power dissipation without overheating or failure. In high-temperature environments, it may be necessary to derate the resistor.
Temperature Coefficient of Resistance (TCR): Assess the stability of resistance with temperature changes (see above).
Resistance Tolerance:Choose an appropriate tolerance level for the application.
Pulse and Surge Handling: Evaluate the resistor’s capability to withstand high-energy pulses or surges.
Environmental Conditions:Consider exposure to humidity, vibration, shock and corrosive environments. Also, ensure the resistor meets relevant standards.
Long-Term Stability: Assess drift in resistance value over time, particularly in high-temperature environments.
Packaging and Mounting:Select the right resistor footprint for the application.
Compliance with Standards:Verify that the resistor meets appropriate industry standards.
Thick film resistors offer an effective balance of performance and cost. Whilst they may not match the precision of thin film or metal foil alternatives, the combination of power-handling capability, wide resistance range and robust construction makes them suitable for demanding applications.
Before implementing thick film resistors, consider operating conditions, including temperature profiles, power derating, surge requirements and environmental factors. Where standard resistors do not match the application requirements, custom resistor manufacturers can often offer solutions.