Ohm's Law & Power

Every embedded system is, underneath the firmware, an electrical circuit. Ohm's law is the first relationship that lets you reason about one quantitatively.

Ohm's law

For an ideal resistor, voltage and current are directly proportional:

V = I × R

where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Rearranged, this also gives you I = V / R and R = V / I — three forms of the same relationship, used constantly when sizing components.

Worked example: an LED with a forward voltage of 2.1 V is driven from a 5 V supply and should be limited to 15 mA. The series resistor absorbs the remaining voltage:

R = (V_supply − V_LED) / I = (5 − 2.1) / 0.015 ≈ 193 Ω

You'd pick the nearest standard value above this, typically 220 Ω, to stay safely under the current limit.

Power

Power dissipated by a resistive element follows directly from Ohm's law:

P = V × I = I² × R = V² / R

This is why resistor power rating matters as much as resistance value — a 1/8 W resistor asked to dissipate 0.5 W will overheat and fail, regardless of how correct the resistance value is.

Ohm's law describes a single component. The next sub-lesson, Kirchhoff's Laws & Network Analysis, covers how to analyze a full network of components.