By Mark Stansberry
Updated Mar 24, 2022
Grasping voltage drops and resistors is essential for any electronics professional—engineers, technicians, and automotive mechanics alike. Although the concepts appear early in high‑school and introductory college courses, a solid foundation in basic algebra is required.
In a simple circuit composed of a single battery and one or more resistors, no resistor can exhibit a voltage drop that exceeds the supply voltage. This upper limit is the first rule you’ll always apply.
Series wiring connects each component end‑to‑end. For example, to build a series chain of two batteries and a resistor, link the positive terminal of Battery 1 to the negative of Battery 2, then attach Battery 2’s positive to one side of the resistor, and finally connect the other side of the resistor to Battery 1’s negative. That arrangement creates a single, unbroken path.
Parallel wiring ties corresponding terminals together. Connect the positives of both batteries, then the negatives. Adding a resistor in parallel simply means attaching one end to the shared positive bus and the other to the shared negative bus.
When components share a parallel arrangement, each one experiences the same voltage as the source. A 5‑V flashlight battery powering five parallel resistors will deliver 5 V across every resistor.
In a series chain, the individual voltage drops add up to the supply voltage. If a 5‑V battery feeds two equal resistors, each will drop 2.5 V (5 V ÷ 2). With unequal resistances, each drop scales with its resistance, yet the total still equals the supply.
Imagine a 10‑V battery across two series resistors: Resistor A = 4 Ω, Resistor B = 6 Ω. The voltage drops are 4 V and 6 V respectively—proportional to their resistances.
