Metal Oxide Film Resistor

The metal oxide film resistor is widely used these days being superior in performance to the carbon composition and carbon film resistors.

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As the name indicates, metal oxide film resistors axial leaded resistors that are made from a thin film of metal oxide that has been deposited into a ceramic or glass rod.

Today, the metal oxide film resistor is used in more niche applications. The general performance of the metal film resistor is better and they are a bit cheaper.

What is a metal oxide film resistor

As the name implies the metal oxide film resistor uses metal oxide as the resistive material within the resistor - the most common are tin oxide and tun chloride, although other oxides can be used.The metal oxide resistor is typically manufactured by taking a ceramic or even a glass rod, which is heated then heated in a vapour to deposit a thin layer of the metal. This is then oxidised or chlorine added to give the resistive film.

This process creates a resistive layer that is chemically bonded to the substrate, making it incredibly tough and stable

Once the resistive layer is established, the value is established by cutting a helix in the layer using a laser. This makes the conductive path longer and narrower, considerably increasing he resistance.

Why use metal oxide film resistors?

Metal film resistors are far more common, so this begs the question about why metal oxide film resistors should be used

High Temperature Stability: They can operate at much higher temperatures than metal film or carbon film resistors. While a metal film might start to drift or fail at high heat, metal oxide offers much beter performance under these conditions.
Surge & Overload Endurance: They are much better at handling momentary surges of power. If a circuit has a current "spike" or surge during startup, a metal oxide resistor is better able to tolerate this.
Flameproof Coating: Most metal oxide resistors are coated with a silicone-based, non-flammable material. If they do fail due to extreme overload, they usually do so without catching fire or emitting thick smoke which can damage other parts of the circuit.
Long-term Reliability & Stability: Because the material is already an oxide (it has already "reacted" with oxygen), it doesn't degrade or oxidize further over time as easily as pure metal films might.

The downsides for metal oxide film resistors is that they are more expensive and they tend not to come in the very close tolerances that are available for metal film resistors.

Metal oxide film resistor performance

Typical performance figures for metal oxide film resistors are given below as a guide to the performance

Metal Oxide Film Resistor Performance Guide
Parameter	TypicalPerformance
Typical tolerance availability	±1%, ±2%, ±5%, ±10%,
Value range	<1Ω - ~10MΩ
Load life (% change over 1000h)	2
Max noise (µV/V)	0.2
Temperature coefficient (ppm/°C)	~±300
Voltage coefficient (%/V)	0.0005
Max resistor temperature (°C)	300
Package styles	Available as leaded parts, very rarely as SMD

Metal film vs metal oxide film resistors

Comparison of Metal Film and Metal Oxide Film Resistors
Feature	Metal Film	Metal oxide film
Tolerance	Very high (down to 0.1%) Although 1% & 2% are typical	Lower (usually 1% to 5%)
Noise	Very low (great for audio)	Moderate (hissing in sensitive circuits)
Power rating ranges	Low to Medium	Medium to High
Best for	Precision / Audio / Computing / General use	Power Supplies / Industrial / Protection