Resistor is passive component that oppose the flow of electric current and regulate voltage in circuits. According to Ohm's Law, V=IR, the voltage across a resistor is proportional to the current through it, and the power dissipated as heat is given by P=IV. It is tipically available in fixed, variable, and special types that respond to light or temperature. Resistor is commonly used to limit current, divide voltage, bias transistors, and protect electronic devices.
Types of Resistors
The chart below highlights the main types of resistors.
| Types | Definition | Resistors | Behavior | Applications |
|---|---|---|---|---|
| Linear (Fixed) | Resistance remains constant, obeys Ohm’s Law | Carbon Composite, Wire‑Wound, Thick Film (Fusible, Cermet, Metal Oxide, SMD), Thin Film (Carbon Film, Metal Film, SMD) | Provides stable resistance independent of voltage/current | Current limiting, voltage division, load balancing |
| Linear (Variable) | Resistance can be manually changed | Potentiometer, Rheostat, Trimpot | Resistance varies when user adjusts wiper/slider | Volume control, motor speed adjustment, calibration |
| Non‑Linear | Resistance changes with external conditions (temperature, light, voltage) | Thermistor (NTC/PTC), LDR, Varistor | Does not obey Ohm’s Law; resistance depends on environment | Temperature sensing, automatic lighting, surge protection |
Schematic Symbols of Resistors
Each type of resistor has its own schematic symbol. In ANSI (American National Standards Institute) diagrams it appears as a zig‑zag line, while in IEC (International Electrotechnical Commission) diagrams it is shown as a rectangle.
Resistor Labels
To identify a resistor's value, we look at color bands or printed labels. In a standard resistor color code system, the first two bands (for 4‑band) or first three bands (for 5‑band and 6‑band) represent the digits, the next band is the multiplier, the last band shows the tolerance, and in a 6‑band resistor the extra sixth band indicates the temperature coefficient. Decoding these colors is straightforward: each color from black to white corresponds to a digit, while gold and silver represent specific multipliers and tolerance levels, ranging from 5% to 20%.
Some resistors do not use color bands at all. Instead, they feature printed text code directly on the component, which specifies the resistance value in ohms, along with other technical ratings.
Series & Parallel Resistors in Circuit
Series‑parallel resistor circuits are required to achieve precise resistance values, distribute power safely, and provide design flexibility in real‑world electronics.
Series connection — Resistors are connected end‑to‑end, so the same current flows through each. The total resistance is the sum:
Req = R1 + R2 + R3 + R4 + ...
Parallel connection — Resistors are connected across the same two nodes, so the voltage across each is the same. The total resistance is found using reciprocals:
1/Req = 1/R1 + 1/R2 + 1/R3 + 1/R4 + ...
Power Ratings for Resistors
Always consider the power rating. This is the maximum wattage a resistor can handle without overheating or being damaged. Choosing the right rating depends on the component's size, material, and construction.
To calculate a resistor’s required power rating, multiply the voltage drop across it by the current flowing through it (P = V × I), or use the formulas P = I2 × R and P = V2 ÷ R. Always select a resistor with a power rating at least double the calculated value to prevent overheating.
Resistor Value Calculation for LED
Suppose we have a 12V DC power supply (VS) and want to power a 3W white LED with a forward voltage (Vf) of 3.2V–3.4V and a forward current (If) of 700mA. To calculate the current limiting resistor (RX), we need to follow these steps:
Step 1: Voltage Drop Across Resistor
VR = VS – Vf = 12V – 3.4V = 8.6V
Step 2: Required Resistance
RX = VR ÷ If = 8.6V ÷ 0.7A ≈ 12.3Ω
Step 3: Power Dissipation in Resistor
PR = VR × If = 8.6V × 0.7A ≈ 6W
Step 4: Safe Power Rating
PRating ≥ 2 × PR = 2 × 6W = 12W
Best practical choice: The exact calculated resistance value of 12.3 Ω is not available in the market. However, we can build a 12.3 Ω, 10 W resistor by combining standard resistors in series or parallel so that both the resistance value and power rating are achieved.
- For instance, connect a 10Ω (10W) resistor in series with a 2.2Ω (10W) resistor to obtain about 12.2Ω at 10W total.
- Or, connect a 22Ω (5W) resistor in parallel with a 27Ω (5W) resistor to get about 12.2Ω at 10W total.
Both combinations are close enough to 12.3Ω because resistor tolerances (±5% or ±10%) cover the small difference.
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