Resistors
A resistor is the simplest and most common electronic component. Its job is to oppose the flow of current — think of it as a deliberate bottleneck in a water pipe. By controlling how much current can flow, resistors set operating points, limit current through LEDs and transistors, divide voltages, and form filters. You will find resistors in every circuit in your radio from the microphone amplifier to the transmit chain.
How Resistors Work
Every material has some resistance to the flow of electrons. Resistors are manufactured to precise resistance values using materials — carbon film, metal film, or wire — that have a predictable, stable resistance.
Resistance is measured in ohms (Ω). One ohm is the resistance through which a potential difference of one volt causes a current of one ampere. This relationship is Ohm's Law: V = I × R. A 1 kΩ resistor with 5 V across it carries 5 mA of current; a 10 kΩ resistor under the same voltage carries only 0.5 mA.
Resistors are passive components — they do not amplify or generate signals. They only dissipate energy as heat. That dissipation is the resistor doing its job: absorbing electrical energy to control the circuit.
Types of Resistors
Common resistor types. Carbon film resistors (left) are the most common in general circuits. Metal film resistors offer better precision and stability. Wirewound resistors handle high power. SMD chip resistors are used in modern surface-mount PCBs.
View LargerCarbon Film
A thin carbon film is deposited on a ceramic rod and a spiral groove cut to set the resistance value. Carbon film resistors are inexpensive and available in the full standard value range. Tolerance is typically ±5%. They are found in virtually every general-purpose circuit.
Metal Film
A metal alloy film replaces carbon, giving tighter tolerances (±1% or better) and lower temperature coefficient — the resistance changes less with temperature. Metal film resistors are preferred in audio circuits, measurement bridges, and anywhere stable, precise values matter. They are identifiable by their blue or green body (as opposed to the tan/beige of carbon film).
Wirewound
Resistance wire is wound on a ceramic former. Wirewound resistors handle high power (from 1 W to hundreds of watts) and maintain their value under high temperature. Their disadvantage is significant inductance at RF frequencies, making them unsuitable for RF applications but ideal for power supplies and dummy loads at audio/low frequency.
Surface Mount (SMD)
Chip resistors are tiny rectangular components soldered directly to PCB pads. They are marked with a three- or four-digit code (e.g., 472 = 47 × 10² = 4700 Ω). All modern transceivers use SMD resistors internally.
Reading the Color Code
Through-hole resistors use colored bands to mark their value. Hold the resistor so the bands are closest to the left end, or so the tolerance band (gold, silver, or brown) is on the right.
Resistor color code chart. Each color maps to a digit (0–9), a multiplier, and a tolerance. The mnemonic "BB ROY of Great Britain had a Very Good Wife" gives the order: Black Brown Red Orange Yellow Green Blue Violet Gray White.
View Larger| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1,000 | — |
| Yellow | 4 | ×10,000 | — |
| Green | 5 | ×100,000 | ±0.5% |
| Blue | 6 | ×1,000,000 | ±0.25% |
| Violet | 7 | ×10,000,000 | ±0.1% |
| Gray | 8 | — | ±0.05% |
| White | 9 | — | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
4-band resistors (the most common) use bands 1 and 2 as digits, band 3 as the multiplier, and band 4 as the tolerance. Example: Yellow–Violet–Red–Gold = 4, 7, ×100, ±5% = 4700 Ω (4.7 kΩ) ±5%.
5-band resistors (precision types, usually metal film) use bands 1, 2, and 3 as digits, band 4 as the multiplier, and band 5 as the tolerance. Example: Red–Red–Black–Brown–Brown = 2, 2, 0, ×10, ±1% = 2200 Ω (2.2 kΩ) ±1%.
4-Band Resistor Color Code Decoder
Select the color of each band from left to right. Hold the resistor so the gold or silver tolerance band is on the right.
5-Band Resistor Color Code Decoder
Used on precision (1% tolerance) metal film resistors. Three digit bands, one multiplier band, one tolerance band.
⚖ Experiment: Verify Resistor Values with a Multimeter
Practice reading the color code, predict the resistance, then measure it to see how close the actual value is to the marked value.
- Assorted resistors (10–20 mixed values, from a parts kit or pulled from old electronics)
- Multimeter set to resistance (Ω) mode
- Pen and paper
- Pick up a resistor and read the color bands from left to right (gold or silver band on the right).
- Use the color code table to calculate the marked value and write it down.
- Set your multimeter to the resistance range appropriate for that value (auto-ranging meters choose for you).
- Touch the probes to both leads of the resistor and read the display.
- Compare your measured value to the marked value. Calculate the percentage error: error% = |measured − marked| / marked × 100.
- Repeat for at least ten resistors. Note which ones fall within their stated tolerance band.
Most resistors will measure within their stated tolerance of the marked value — usually within ±5% for gold-band parts. A ±1% brown-band resistor should be very close indeed. Occasional outliers that measure significantly out of tolerance are failed or damaged parts. This exercise builds confidence reading color codes and using your multimeter for component verification.
Series and Parallel Resistors
Resistors can be combined to create values not available as a single standard part, or to share current and power across multiple components.
Resistors in Series
When resistors are connected end-to-end in series, the same current flows through all of them and the total resistance is simply the sum of the individual values:
Two 1 kΩ resistors in series give 2 kΩ. The voltage across each resistor is proportional to its fraction of the total resistance — this is the voltage divider principle, which appears constantly in bias networks and reference circuits.
Resistors in Series
Enter up to four resistor values. Leave unused fields blank. All values in ohms (Ω).
Resistors in Parallel
When resistors share the same two nodes — both ends connected together — they are in parallel. The voltage across all of them is identical, and the total resistance is always less than the smallest individual resistor. The formula is:
For two equal resistors in parallel, the result is simply half the individual value. Two 10 kΩ resistors in parallel give 5 kΩ. Parallel combinations are used to increase current handling and to obtain non-standard values.
Resistors in Parallel
Enter up to four resistor values. Leave unused fields blank. All values in ohms (Ω).
Power Rating
Every resistor has a maximum power it can dissipate without overheating. Power is calculated from Ohm's Law: P = V² / R = I² × R = V × I. The most common power ratings are ¼ W and ½ W for signal-level circuits. High-power applications such as dummy loads and bleeder resistors use 1 W, 2 W, 5 W, or higher ratings.
Ham Radio Applications
Resistors appear in virtually every section of a radio.
Current Limiting
LED indicators, transistor base circuits, and vacuum tube cathode resistors all rely on series resistors to control current. Without them, excess current flows and destroys the component.
Bias Networks
Two resistors in a voltage divider set the bias voltage for a transistor amplifier stage, establishing the operating point (quiescent current) for linear operation. The ratio R1:R2 determines the bias voltage; the individual values determine the stiffness of the divider.
Attenuators
Pi and T attenuator pads use precision resistors to reduce signal levels by a known number of decibels while maintaining the characteristic impedance (usually 50 Ω) of the system. They are essential in RF testing and in level-matching between equipment.
Dummy Loads
A dummy load is a non-radiating 50 Ω load for testing transmitters. High-power dummy loads use multiple wirewound or carbon composition resistors in parallel to share the power, or a single large metal-oxide resistor rated for the full transmit power.
Noise Sources
A resistor generates thermal noise — small random voltage fluctuations caused by electron thermal motion. This noise sets the fundamental receive sensitivity limit of any amplifier. Understanding resistor noise is essential for designing low-noise preamplifiers and for appreciating why receiver noise figure matters in weak-signal work.
Frequently Asked Questions
How do I identify the first band if I cannot see a gold or silver band?
Most resistors have the tolerance band (gold or silver) spaced slightly farther from the body end than the other bands. Look for the wider gap — that end is the right, and you read from the left. If the spacing is unclear, you can measure the resistance and compare it to E12 or E24 standard values to determine the color code orientation.
What is a preferred value (E12, E24) series?
Resistors are not manufactured in every possible value — they come in standard series. The E12 series has 12 values per decade (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2) spaced so that any target value is within ±10% of the nearest preferred value. The E24 series doubles that to 24 values per decade, suitable for ±5% tolerance. Precision E96 and E192 series cover ±1% and ±0.5% work.
Does the orientation of a resistor matter when installing it?
No — resistors are non-polarized components. Current flows equally in either direction and the resistance is the same regardless of which lead is connected to the positive side of the circuit. The only exception is some specialist components such as thermistors and varistors that may have directional characteristics.
Why do parallel resistors always give a lower value than any individual resistor?
Adding another path for current reduces the total opposition. Think of lanes on a motorway — more lanes, less congestion. The combined resistance is always less than the smallest branch because the total current is always greater than the current through any single branch, and by Ohm's Law a higher current at the same voltage means lower resistance.
Can I use wirewound resistors in RF circuits?
Generally no. Wirewound resistors behave as inductors at radio frequencies because the resistance wire is wound in a coil. This inductance causes their impedance to rise with frequency, making them unsuitable for RF circuits. Non-inductive wirewound resistors exist (using a bifilar winding technique) and can be used at moderate RF frequencies, but standard wirewound resistors should be confined to DC and audio applications.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.