Basic Guide To Resistor Selection

Step 3 - Resistor Technology

Although resistors may appear identical on schematics, the underlying technology used to manufacture them varies massively. The technology used by a resistor determines far more than just resistance value; it influences noise, stability, temperature behaviour, long-term drift, environmental robustness, and suitability for specific applications. In circuit diagrams, treating all resistors as equivalent is simple and convenient, in reality, it is incorrect and expensive in the long run.

Carbon Composition

One of the oldest resistor technologies is the carbon composition resistor, and are almost always through-hole parts using a mixture of carbon powder and binder as the resistive element. The manufacturing is incredibly simple, extremely cheap, and has been in use for decades (likely more than a century now). Carbon composition resistors are mechanically robust and can tolerate brief overloads, which historically made them popular in older power and valve-based equipment.

However, their granular structure causes several serious problems; they exhibit high noise, poor stability, and have very wide tolerances, often around 20%. In fact, at low currents and voltages, their electrical behaviour can become inconsistent due to the discontinuous conductive paths within the material. Such resistors are now obsolete, being replaced with carbon film, thick film, and thin film technologies, but their legacy is one worth remembering.

Carbon Film

Carbon film resistors address some of these shortcomings found in carbon composition resistors. Instead of a bulk carbon mixture, a thin carbon film is deposited onto a ceramic substrate and laser trimmed to value. This manufacturing method significantly improves tolerance, consistency, and overall predictability. Despite carbon film resistors being many decades old, they are still inexpensive to produce, commonly available, and extremely popular in prototyping and small volume production. That said, they still remain relatively noisy, are sensitive to temperature changes, and can drift over time. Thus, they are suitable for non-critical biasing and signal paths, but where precision or low-noise applications are concerned, using them is not advisable.

Metal Film

Metal film resistors improve on carbon film technology in almost every single way. These resistors use a metal-based resistive film deposited on a ceramic core, again, trimmed to value using a laser. The resulting design has much tighter tolerances, often down to 0.1%, improved temperature coefficients, lower noise, and far better long-term stability. For analogue signal chains, reference networks, and measurement circuits, metal film resistors are usually the go-to technology. While they cost more than carbon film parts, the performance improvement gained makes them well worth their price, especially in sensitive analog applications.

Thick Film (SMT)

When it comes to surface mount technologies, the most common resistor types are thick film and thin film. Thick film resistors dominate the market due to their low cost, versatility, and availablity in a wide range of values and power ratings. They are manufactured by screen-printing a granular cermet paste (yes, that's the correct spelling) onto a ceramic substrate, followed by firing and laser trimming. Thick film resistors are excellent for bulk applications, digital circuits, and power-related roles where absolute precision is not critical. However, they suffer from higher noise, poorer temperature coefficients, and non-linear effects. Some thick film resistors even exhibit resistance changes in response to strong electric fields, a feature that is rarely desired.

Thin Film (SMT)

Thin film resistors take a slightly different approach to their thick film counterparts. A very uniform resistive layer is deposited using vacuum-based techniques (such as chemical vapour deposition), and then trimmed via a laser with high precision. This process is far more complex and expensive compared to thick film manufacturing techniques, but the performance gains are well worth the challenges, especially in high-end applications. Thin film resistors offer excellent tolerance, low noise, stable temperature behaviour, and minimal drift over time. For sensitive analogue circuits, instrumentation, precision references, and high-resolution measurement systems, thin film resistors are usually the correct choice.

In practice, resistor technology should be selected based on circuit sensitivity rather than habit. For logic pull-ups and general biasing, thick film or carbon film parts are usually fine. For anything involving accuracy, stability, or low noise, metal film and thin film technologies quickly justify their cost. The schematic symbol may be simple, but the physics underneath it is not.