Metal film resistor
A metal film resistor is a type of fixed resistor that uses a thin metal film as the resistive element. They are constructed by depositing a thin layer of metal alloy (typically nickel-chromium) onto a ceramic substrate.
Metal film resistors are the industry standard for precision applications, offering superior stability, accuracy, and low noise characteristics compared to carbon film resistors. They are easily identifiable by their body color, which is typically blue or light blue.
Construction
The manufacturing process involves high-precision vacuum deposition techniques.
- Deposition: A thin film of metal alloy, usually Nickel-Chromium (NiCr), is deposited onto a high-grade ceramic rod (substrate) using vacuum sputtering or evaporation techniques. The thickness of this film determines the initial resistance range.
- Trimming: A laser cuts a helical groove into the metal layer. This process is highly controlled to achieve precise resistance values with tight tolerances.
- Termination: Metal end caps with connecting leads are pressed onto the rod.
- Coating: The resistor is coated with several layers of protective epoxy. Unlike carbon film resistors (which are usually beige/tan), metal film resistors are typically coated in blue.
Characteristics
Metal film resistors are prized for their high performance in critical areas:
- Tolerance: They offer high precision, with standard tolerances of ±1% (F-class), ±0.5%, or even ±0.1%.
- Temperature Coefficient (TCR): They have excellent temperature stability. The resistance changes very little with temperature, typically ranging from ±50 to ±100 ppm/°C.
- Noise: They generate very low current noise (1/f noise) because the metal structure is more homogeneous than the carbon granules in carbon film resistors.
- High Frequency Performance: They exhibit low inductance and capacitance, making them suitable for high-frequency and radio frequency (RF) applications.
Applications
Due to their precision and low noise, metal film resistors are essential in:
- Audio Equipment: Used in pre-amplifiers, mixers, and synthesizers to maintain a high signal-to-noise ratio (SNR).
- Instrumentation: Critical for multimeters, oscilloscopes, and medical devices where measurement accuracy is paramount.
- Active Filters: Used in filter circuits where stable resistance values are required to maintain specific frequency cutoffs.
Comparison with Carbon Film
| Feature | Metal Film | Carbon Film |
|---|---|---|
| Resistive Material | Nickel-Chromium Alloy | Carbon Film |
| Body Color | Blue | Beige / Tan |
| Tolerance | High (±1% or better) | Moderate (±5%) |
| Noise | Low | Moderate to High |
| Cost | Moderate | Low |
| Primary Use | Precision / Audio / RF | General Purpose |
See also
- Resistor
- Thin-film technology
- Electronic color code
- Signal-to-noise ratio