What is a Resistor?

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses.

The main characteristics of a resistor are its resistance and the maximum power it can dissipate without overheating or failing. Other properties include temperature coefficient, noise, and inductance.

Types of Resistors

There are many different types of resistors designed for different applications. The most common types are:

• Carbon composition resistors
• Carbon film resistors
• Metal film resistors
• Wire-wound resistors
• Thick and thin film resistors
• Foil resistors
• Shunt resistors

In this article, we’ll focus on comparing carbon film and metal film resistors, two of the most widely used types today.

Carbon Film Resistors

What is a Carbon Film Resistor?

A carbon film resistor is a fixed form-factor resistor that uses a thin carbon film as the resistive material. It has a simple cylindrical geometry with a resistive coating painted or sprayed onto a ceramic rod. The thickness of the film determines the resistance value.

Carbon film is a widely used general purpose resistor type suitable for most applications not requiring tight tolerances or high-wattage power dissipation. They are inexpensive, reliable, and have reasonably low noise levels.

Characteristics of Carbon Film Resistors

Some key specifications of carbon film resistors include:

Characteristic	Typical Value
Resistance Range	1 ohm to 10 megohms
Tolerance	±5%, ±10%
Temperature Coefficient	±350ppm/°C to ±550ppm/°C
Maximum Power	0.125W to 2W
Voltage Rating	200V to 700V
Noise	-30dB to -40dB

Carbon film resistors have a number of advantages:

• Low cost
• Good stability with low noise
• Wide resistance range
• Adequate power handling
• Reliable with low failure rate
• Non-inductive
• Small size

Some disadvantages of carbon film resistors are:

• Relatively high temperature coefficient
• Moisture sensitivity
• Low precision
• Low maximum power rating

Applications

Carbon film resistors are suitable for general purpose use in many electronic circuits including:

• Voltage dividers
• Dropping resistors
• Gain setting
• Biasing
• Coupling
• Filtering
• Termination
• Pull up/down networks

They are widely used in consumer electronics, telecommunications equipment, instrumentation, computers and computer peripherals.

Metal Film Resistors

What is a Metal Film Resistor?

A metal film resistor is made by depositing a thin layer of metal alloy onto a ceramic substrate. Nichrome and tantalum nitride are commonly used metals. The film is patterned by a photolithographic process into a resistive element with metal caps applied to the ends for leads.

Metal film offers an upgrade over carbon film in terms of precision, temperature stability, and high frequency performance. The metal layer provides lower resistance with less parasitic inductance and capacitance.

Characteristics of Metal Film Resistors

Typical specifications for metal film resistors are:

Characteristic	Typical Value
Resistance Range	10 ohms to 10 megohms
Tolerance	±0.1%, ±0.5%, ±1%, ±2%, ±5%
Temperature Coefficient	±15ppm/°C to ±100ppm/°C
Maximum Power	0.05W to 2W
Voltage Rating	50V to 500V
Noise	-35dB to -50dB

Advantages of metal film resistors include:

• Excellent temperature stability
• Very low noise
• High precision and tight tolerances
• Good long-term stability
• Low voltage coefficient
• High maximum voltage ratings
• Moisture resistant
• Small size

The main disadvantages are:

• Higher cost than carbon film
• Low pulse handling capability
• Can be inductive at high frequencies

Applications

The precision and stability of metal film resistors make them ideal for applications such as:

• Precision voltage dividers
• Bridge circuits
• Filters
• Attenuators
• Instrumentation amplifiers
• Medical equipment
• Test and measurement
• Military and aerospace

They are commonly found in high-end audio equipment, precision power supplies, data acquisition systems, calibration standards, and other applications requiring accurate and stable resistance values.

How to Tell Carbon Film and Metal Film Resistors Apart

Carbon film and metal film resistors can look very similar, but there are some ways to distinguish them:

1. Markings: Carbon film resistors are usually marked with colored bands indicating their resistance value and tolerance according to a standard color code. Metal film resistors often have the resistance printed numerically.

2. Appearance: Carbon film resistors may have a rough, matte surface finish. Metal films are usually shiny and smooth. However, there is some variation between manufacturers.

3. Size: For the same power rating, metal films tend to be physically smaller than carbon due to their higher conductivity. Carbon composition resistors are even larger.

4. Leads: The end caps and leads of metal films are more substantial, often gold-plated for improved solderability. Carbon leads may be tinned.

5. Precision: If the marked tolerance is better than ±5%, it is likely a metal film. Carbon film resistors are rarely more precise than ±5%.

6. Measurement: Using an ohmmeter to measure resistance, metal films will be closer to their marked value. Carbon films are less predictable. Temperature changes have less effect on metal film resistors.

Carbon Film VS Metal Film Resistors Compared

The following table summarizes the key differences between carbon film and metal film resistors:

Property	Carbon Film	Metal Film
Material	Carbon	Metal alloy
Resistance Range	1 ohm – 10 megohms	10 ohms – 10 megohms
Typical Tolerances	±5%, ±10%	±0.1% – ±5%
Temperature Coefficient	±350ppm/°C – ±550ppm/°C	±15ppm/°C – ±100ppm/°C
Maximum Power	0.125W – 2W	0.05W – 2W
Voltage Rating	200V – 700V	50V – 500V
Noise	-30dB – -40dB	-35dB – -50dB
Cost	Low	Moderate to high
Applications	General purpose	Precision circuits

In summary, metal film resistors offer superior precision, temperature stability, and noise performance compared to carbon film. However, they are more expensive. Carbon film remains a popular choice for general purpose use where their characteristics are adequate.

The choice between carbon film and metal film depends on the specific requirements of the circuit. Factors to consider include:

• Required resistance tolerance
• Operating temperature range
• Applied voltage
• Power dissipation
• Frequency
• Budget

In non-critical applications, carbon film resistors will be suitable and more economical. For precise circuits that must maintain resistance under varying conditions, metal film is the better choice. Many circuits will use a mix of both types optimized for the particular role of each resistor.

Frequently Asked Questions

1. Can I replace a carbon film resistor with metal film?

In most cases, yes. Metal film resistors have better characteristics so substituting a carbon film with a same value metal film is usually an upgrade. However, check that the power rating and physical size are compatible.

2. Are carbon film resistors obsolete?

No, carbon film resistors are still widely used and manufactured. While metal film has superior properties, carbon film remains popular for its low cost and is still adequate for many general purpose applications.

3. Which resistor type is best for audio circuits?

Metal film resistors are preferred for audio because of their low noise and stability. They are commonly used in high-end equipment. However, carbon film is acceptable for less critical audio circuits such as simple amplifiers.

4. How does aging affect carbon film and metal film resistors?

Both carbon and metal film resistors change value as they age, but metal film is more stable. Carbon film can drift by several percent over time, while metal film typically changes less than 1% over many years. Aging is accelerated by high temperature and humidity.

5. Can I measure the temperature coefficient of a resistor?

Yes, by measuring the resistance at different known temperatures and calculating the change in resistance per degree. Temperature-controlled ovens or baths can be used to vary the resistor temperature. Specialized equipment like temperature coefficient testers are also available.

In conclusion, carbon film and metal film are two common resistor types with different characteristics. Metal film offers better precision and stability, while carbon film is a low-cost general purpose option. Understanding their properties allows selecting the best type for a given circuit.