Ohm's law (V = IR) is the foundation of electronics, but applying it in real PCB design involves practical considerations — trace resistance, tolerance, power ratings, and thermal effects that textbooks often ignore.
LED Current Limiting
The most common Ohm's law application: sizing a resistor for an LED. A red LED has a forward voltage of ~2V and needs 20 mA. From a 5V supply: R = (5 − 2) ÷ 0.02 = 150 Ω. Power: P = 0.02² × 150 = 0.06 W — a standard ¼ W resistor is fine.
For a white LED (3.3V forward) from 3.3V: there's no voltage headroom. You need a constant-current driver, not a simple resistor.
Voltage Dividers
Two resistors create a lower voltage: V_out = V_in × R2 ÷ (R1 + R2). Used for level shifting, sensor interfacing, and voltage measurement.
Practical considerations: choose resistor values that don't waste power but aren't so high that noise becomes a problem. 10 kΩ + 10 kΩ divides 5V to 2.5V while drawing only 0.25 mA. For ADC inputs, keep total resistance under 10 kΩ to avoid sampling errors.
PCB Trace Resistance
Copper traces have resistance too. A 1 oz copper trace (35 µm thick), 1 mm wide, 100 mm long: R ≈ 0.05 Ω. At 1A, that's 50 mV drop and 50 mW dissipation — usually fine.
But at 10A through a narrow trace: 0.5V drop and 5W of heat. The trace gets hot. Use the PCB trace width calculator (IPC-2221 standard): 10A needs about 2.5 mm wide on outer layers, or 5 mm on inner layers.
Try the Electric Resistance Converter
Convert between all electric resistance units instantly.