Learning Library · Circuit basics · 2

Series vs. parallel circuits

The same three resistors behave completely differently depending on how they are wired. In series they share one current. In parallel they share one voltage. Flip the switch and compare.

Series: one path, one current

Current has only one route, so the same current flows through every resistor. The source voltage is split between them in proportion to their resistance: the largest resistor takes the largest share. The individual voltages always add back up to the source voltage. That is Kirchhoff's voltage law, and the reason a string of resistors works as a voltage divider.

Parallel: many paths, one voltage

Every branch sits directly across the source, so every resistor sees the full voltage. Current divides between the branches, and the smallest resistor carries the most. The branch currents add up to the total. That is Kirchhoff's current law. Adding a branch always lowers the equivalent resistance, which is why plugging more loads into one circuit draws more current.

Try this

• In series, set R1 to 10 kΩ and the others to 100 Ω. R1 takes almost all of the voltage.
• Switch to parallel with the same values. Now R1 carries the least current and the 100 Ω branches do the work.
• Set all three to 1 kΩ. In series, R_eq is 3 kΩ. In parallel, it is 333 Ω, a third of one resistor.

Why this matters when you measure

A voltmeter is always connected in parallel with what it measures, so it adds a branch to the circuit. A typical DMM has a 10 MΩ input resistance. Across a 1 kΩ resistor that extra branch changes almost nothing. Across a 10 MΩ resistor it halves the equivalent resistance and the reading drops noticeably. An ammeter goes in series, so it adds a small resistance, called its burden, to the one path the current has.

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