When faced with the exam question or bench inquiry, which of the following are two classes of resistors, the fundamental and universally accepted answer is Fixed Resistors and Variable Resistors. While secondary classifications exist—such as linear versus non-linear (thermistors, varistors) or low-power versus high-power—the physical and functional division of the entire resistor family rests on whether the ohmic value is immutable by design or adjustable by the user.
Understanding this primary division is only the starting point. On the workbench, knowing that a component is a 'fixed resistor' tells you very little about how it will behave in a high-frequency audio path, a high-voltage snubber network, or a precision current-sense circuit. This guide breaks down the material science, marking systems, substitution logic, and failure modes of both classes so you can select, read, and troubleshoot them with confidence.
The Two Primary Classes: Fixed vs. Variable Specifications
Fixed resistors are manufactured to provide a single, specific resistance value. They dominate printed circuit boards (PCBs), serving as pull-ups, current limiters, voltage dividers, and feedback network components. Variable resistors—encompassing potentiometers, rheostats, and trimmers—feature a mechanical wiper that slides across a resistive element, allowing real-time adjustment of resistance or voltage division.
The table below maps the most common sub-types across both classes, detailing the exact parameters you need to check against a datasheet before specifying a part.
| Class | Sub-Type / Construction | Typical Tolerance | Tempco (TCR) | Current Noise | Primary Use Case |
|---|---|---|---|---|---|
| Fixed | Carbon Film | ±5% | 200 to 1000 ppm/°C | High (-10 dB) | General purpose, legacy repairs, non-critical pull-ups |
| Fixed | Metal Film (e.g., Vishay MRS25) | ±1% to ±0.1% | ±15 to ±50 ppm/°C | Very Low (-30 dB) | Precision analog, audio signal paths, feedback loops |
| Fixed | Metal Oxide Film | ±2% to ±5% | ±250 ppm/°C | Low | High-temperature environments, high-voltage snubbers |
| Fixed | Wirewound (e.g., Ohmite 160) | ±1% to ±5% | ±20 to ±90 ppm/°C | N/A (Inductive) | High power dissipation, braking, current sensing (if bifilar) |
| Variable | Carbon Track Potentiometer | ±20% | N/A (High Drift) | High (Wiper Noise) | Audio volume controls (Log taper), user interfaces |
| Variable | Cermet Trimmer (e.g., Bourns 3296W) | ±10% | ±100 ppm/°C | Moderate | One-time factory calibration, bias setting on PCBs |
| Variable | Conductive Plastic Pot | ±5% to ±10% | ±500 ppm/°C | Very Low | High-cycle joysticks, studio audio faders, servos |
Decoding Physical Markings and Color Codes
Knowing what the markings mean is a mandatory skill for both through-hole and surface-mount device (SMD) workflows. While variable resistors usually have their nominal value and taper printed directly on the casing (e.g., B10K), fixed resistors rely on standardized color bands or numeric SMD codes.
Through-Hole Color Bands
Modern precision metal film resistors use a 5-band system. The first three bands represent significant digits, the fourth is the multiplier, and the fifth is tolerance. A 6th band, if present, indicates the temperature coefficient (Tempco).
- Example: Brown (1) - Black (0) - Black (0) - Red (x100) - Brown (±1%).
- Calculation: 100 x 100 = 10,000 Ω (10 kΩ) at ±1% tolerance.
For standard 4-band carbon or metal film resistors, the first two bands are significant digits. All About Circuits provides an excellent interactive breakdown of the IEC 60062 color code standard if you need to memorize the multiplier colors.
SMD Resistor Codes
Surface mount resistors use printed numeric codes due to their microscopic size. There are three dominant standards:
- 3-Digit Code (5% tolerance): First two digits are significant, third is the multiplier (power of 10).
103= 10 x 10³ = 10,000 Ω (10 kΩ). - 4-Digit Code (1% tolerance): First three digits are significant, fourth is the multiplier.
4702= 470 x 10² = 47,000 Ω (47 kΩ). - EIA-96 Code (1% precision): Uses two numbers and a letter. The numbers correspond to a lookup table (e.g.,
01= 100,68= 499), and the letter is the multiplier (A=1,B=10,C=100).01C= 100 x 100 = 10 kΩ.
Variable Resistor Taper Markings
Variable resistors include a letter prefix indicating the taper—how the resistance changes relative to the shaft rotation. Be careful: regional standards differ.
- Asian/Global Standard:
B= Linear,A= Logarithmic (Audio),W= Reverse Log. - US/Vintage Standard:
A= Linear,C= Logarithmic. (Always verify with a multimeter if the origin is unknown).
Job Selection and Safe Substitution Rules
Selecting the right class and material for the job prevents noise injection, thermal runaway, and catastrophic failure. When your parts bin is missing the exact BOM component, you must substitute safely based on electrical stress, not just ohmic value.
Which Type for Which Job?
- Audio Signal Paths & Preamplifiers: Use Metal Film (Fixed) or Conductive Plastic (Variable). Carbon composition and carbon track pots generate high current noise (measured in nV/√Hz) which will manifest as a noticeable hiss in high-gain stages.
- High-Voltage Snubbers & Bleeders: Use Metal Oxide or Thick Film. Metal film resistors can suffer from internal micro-arcing across their laser-cut helical tracks under high voltage spikes. Metal oxide handles surge energy much better.
- Current Sensing (Shunts): Use Four-Terminal Wirewound or Metal Foil. You need an extremely low Tempco (under ±15 ppm/°C) so the resistance doesn't drift as the shunt self-heats from I²R losses.
- User Volume Controls: Use Logarithmic (Audio Taper) Carbon or Plastic Pots. Human hearing is logarithmic; a linear pot will cause 80% of the perceived volume change to happen in the first 20% of the knob rotation.
Never substitute a standard metal film or carbon film resistor for a fusible resistor (often marked with a single black band or specific flameproof coating). Fusible resistors are designed to fail open safely and predictably under short-circuit conditions, acting as both a resistor and a fuse. Substituting a standard part can cause the resistor to catch fire or explode when a downstream component shorts.
Safe Substitution Matrix
If you are out of stock, follow these hierarchy rules:
- Wattage: You can always sub a higher wattage (e.g., 1/2W for 1/4W) if physical PCB clearance allows. Never sub lower.
- Tolerance: You can sub a tighter tolerance (1% for 5%). Never sub a looser tolerance in a timing circuit (like a 555 timer RC network) or a precision voltage divider.
- Material: Metal film can safely replace carbon film in 95% of general-purpose circuits. Carbon film should never replace metal film in precision analog circuits.
Bench Failure Modes and Visual Symptoms
Resistors are generally reliable, but they are not immortal. When troubleshooting a dead board, recognizing the visual and electrical symptoms of resistor failure can save hours of tracing. According to reliability data from component manufacturers like Vishay's Resistor Design Guides, failure mechanisms are heavily dependent on the construction class.
Fixed Resistor Failures
- Carbon Composition (Moisture Absorption): Common in vintage audio gear and old CRT monitors. The phenolic binder absorbs ambient moisture over decades, causing the resistance to drop significantly (sometimes by 20-50%). Visual Symptom: The outer wax or epoxy coating may show micro-cracking, or the leads may show green copper oxidation creeping under the body. Fix: Replace all carbon comps in the signal path with metal film; baking them temporarily reverses the drift, but it will return.
- Metal Film (Overvoltage/Overpower): Fails open. The thin metal layer vaporizes at the laser-cut helical spiral. Visual Symptom: Often looks perfectly normal to the naked eye, but a 10x jeweler's loupe will reveal a tiny scorch mark or a microscopic fracture across the body. Multimeter reads 'OL' (infinite).
- Wirewound (Thermal Cycling Fatigue): Fails open at the end-cap weld. Repeated heating and cooling causes the weld between the resistive wire and the metal end-cap to fracture. Visual Symptom: Body is intact, no burn marks, but resistance is infinite. Sometimes, pressing on the lead with a probe temporarily re-establishes contact.
Variable Resistor Failures
- Carbon Track Pots (Wiper Wear & Oxidation): The physical wiper wears away the carbon track, or environmental sulfur oxidizes the surface. Visual/Electrical Symptom: 'Scratchy' audio output, dead spots where the signal drops out completely, or massive DC offset spikes in an amplifier. You can measure this by putting a DMM on the wiper and sweeping the shaft; the resistance should change smoothly, not jump erratically.
- Cermet Trimmers (Wiper Lift): If adjusted with excessive downward force from a metal screwdriver, the wiper can lift off the cermet track. Visual Symptom: The adjustment slot may show metal gouging from a steel screwdriver (always use ceramic or plastic alignment tools). Electrically, it will read open or erratic.
By understanding that the two primary classes of resistors—fixed and variable—branch out into highly specific material chemistries, you move beyond simple continuity testing into true component-level engineering. Whether you are decoding an EIA-96 SMD code, selecting a low-noise metal film for a microphone preamp, or diagnosing a moisture-laden carbon comp in a 1970s receiver, the physical construction dictates the electrical reality.
