Metal oxide film resistor
A metal oxide film resistor is a type of fixed resistor that uses a resistive element made from a metal oxide, typically tin oxide (SnO₂), deposited on a ceramic rod.
These resistors are renowned for their ability to withstand high temperatures, high power loads, and high voltage surges. They bridge the gap between general-purpose carbon film resistors and high-power wirewound resistors, making them ideal for medium-power applications where stability under heat is crucial.
Construction
The manufacturing process differs slightly from other film resistors due to the chemical reaction involved.
- Deposition: A ceramic rod is heated to a very high temperature. A solution of metal chloride (such as tin chloride) is sprayed onto the hot rod. The heat causes a chemical reaction (hydrolysis), forming a durable film of metal oxide (SnO₂) on the ceramic surface. Antimony oxide is often added to adjust the resistance value.
- Trimming: Similar to other film resistors, a helical groove is cut into the oxide layer using a laser or grinding wheel to calibrate the resistance to the precise value.
- Coating: Because these resistors are designed to run hot, they are coated with a flame-retardant silicone or non-flammable paint. The body color is often grey, light pink, or sometimes light blue.
Characteristics
Metal oxide resistors are chosen for their robustness rather than extreme precision.
- High Temperature Stability: They can operate at much higher temperatures (up to 200°C or more) than carbon or metal film resistors without failing or drifting significantly.
- Pulse and Surge Handling: They have excellent surge current capability and overload endurance, outperforming metal film resistors in rugged environments.
- Tolerance: Typically ±5%, though ±1% versions are available.
- Noise: They generate less noise than carbon composition resistors but are generally noisier than metal film resistors.
- Reliability: The oxide film is chemically very stable and resistant to humidity and oxidation.
Applications
Due to their thermal durability and power handling (typically 1W to 5W), they are widely used in:
- Power Supplies: Used as bleeder resistors, inrush current limiters, and voltage dividers in high-voltage circuits.
- Audio Amplifiers: Frequently used in the power output stage (e.g., emitter resistors for power transistors) where high currents and heat are present.
- Industrial Electronics: Used in environments requiring high reliability under thermal stress.
Comparison with Wirewound Resistors
Metal oxide resistors often compete with wirewound resistors in the 1W–5W range.
| Feature | Metal Oxide Film | Wirewound |
|---|---|---|
| Inductance | Low (Non-inductive) | High (Inductive) |
| High Frequency | Good | Poor |
| Pulse Handling | Good | Excellent |
| Cost | Low | Moderate to High |
| Size | Compact | Bulky |
See also
- Resistor
- Tin dioxide
- Wirewound resistor
- Electrical breakdown