Ohm’s Law in practice

For an ohmic resistor at approximately constant physical conditions, voltage and current are proportional:

V = I × R

This relation is extremely useful, but nonlinear devices such as diodes need a different current–voltage model.

The real power comes from rearranging it. Whatever you don’t know, you can solve for:

• V = I × R — find the voltage
• I = V / R — find the current
• R = V / I — find the resistance you need

Across a 450 Ω resistor alone, a 9 V supply produces I = 9/450 = 0.02 A = 20 mA in the ideal resistor model. If an LED is added in series, subtract its forward voltage before calculating the resistor current, and choose a current within the LED and driver ratings.