Series Resistor Calculator

1. LED Current Limiting

Calculate LED series resistor values:

12V LED resistor: R = (12V - Vf) / If
5V LED resistor: R = (5V - Vf) / If
3.3V LED resistor: R = (3.3V - Vf) / If

2. Voltage Division

Common voltage divider ratios:

12V to 5V: R1 = 1.4kΩ, R2 = 1kΩ
5V to 3.3V: R1 = 510Ω, R2 = 1kΩ
9V to 5V: R1 = 820Ω, R2 = 1kΩ

3. Ohm's Law Applications

Common calculations using Ohm's law:

Find voltage: V = I × R (e.g., 0.02A × 100Ω = 2V)
Find current: I = V ÷ R (e.g., 12V ÷ 600Ω = 0.02A)
Find resistance: R = V ÷ I (e.g., 5V ÷ 0.02A = 250Ω)
Calculate power: P = V × I (e.g., 12V × 0.015A = 0.18W)

4. Series vs Parallel Comparison

Property	Series Circuit	Parallel Circuit
Total Resistance	Sum of all resistances	Less than smallest resistance
Current	Same through all resistors	Divides between paths
Voltage	Divides between resistors	Same across all resistors

5. Common Circuit Examples

LED Circuit (12V)

For a typical red LED (Vf = 2V, If = 20mA):

Supply voltage = 12V
Voltage drop = 12V - 2V = 10V
Required resistance = 10V ÷ 0.02A = 500Ω

Voltage Divider (5V Reference)

To create a 5V reference from 12V:

R1 = 1.4kΩ (top resistor)
R2 = 1kΩ (bottom resistor)
Output voltage = 12V × (1kΩ ÷ 2.4kΩ) = 5V

6. Design Considerations

Power Rating Selection

Choose resistors with adequate power rating:

P = V² ÷ R (power calculation)
Use 2x safety margin for reliability
Consider temperature derating

Tolerance Selection

Common tolerance values and applications:

±1% for precision voltage dividers
±5% for general purpose
±10% for non-critical applications

7. Troubleshooting Guide

Common issues and solutions:

Excessive heating: Check power rating and current
Incorrect voltage division: Verify resistance values
Circuit instability: Check for loose connections
Component failure: Inspect for physical damage

8. Resistor Color Codes

Common color codes for through-hole resistors:

Color	Value	Example
Brown Black Red	1kΩ	1k resistor color code
Brown Black Orange	10kΩ	10k resistor color code
Brown Black Green	1MΩ	1M resistor value

9. Common Applications

LED Applications

LED resistor for 5V: R = (5V - 2V) / 0.02A = 150Ω
LED resistor for 12V: R = (12V - 2V) / 0.02A = 500Ω
LED resistor for 3.3V: R = (3.3V - 2V) / 0.02A = 65Ω

Current Sensing

Low-side current sensing
High-side current measurement
Motor current monitoring

Protection Circuits

Input protection resistors
Pull-up/pull-down resistors
Current limiting for semiconductors

10. Advanced Calculations

Temperature Effects

Consider temperature coefficient (TC):

ΔR = R × TC × ΔT
Typical TC: ±100 ppm/°C
Operating range: -55°C to +125°C

Power Derating

Power derating above 70°C:

Derate 1% per °C above 70°C
Zero power at 175°C
Use larger safety margin at high temps

Common Series Circuit Calculations