Series Resistor Calculator
Common Series Circuit Calculations
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 |
Quick Reference
Basic Formulas
- V = I × R (Voltage)
- I = V ÷ R (Current)
- R = V ÷ I (Resistance)
- P = V × I (Power)
Series Circuit Rules
- Rtotal = R1 + R2 + R3 + ...
- Itotal = I1 = I2 = I3 = ...
- Vtotal = V1 + V2 + V3 + ...
Power Formulas
- P = V × I
- P = I² × R
- P = V² ÷ R
Common Values
- LED current: 20mA typical
- Logic level: 3.3V/5V
- Power supply: 12V/24V
Conversion Factors
- 1mA = 0.001A
- 1kΩ = 1000Ω
- 1MΩ = 1000kΩ
Safety Margins
- Power: 2x typical
- Voltage: 1.5x peak
- Current: 1.25x max