Series and parallel

There are only two ways to join parts in a circuit. In series they sit in one line, like beads on a thread. In parallel each part has its own path, like lanes on a road.

Series

• The same current flows through every part.
• The voltage is shared between the parts.
• Resistances add up.
• If one part fails, everything stops.

R total = R1 + R2

Parallel

• Every part gets the full voltage.
• The current is shared between the paths.
• The total resistance gets smaller.
• If one path fails, the others keep working.

R total = (R1 × R2) ÷ (R1 + R2)

Two 1 kΩ resistors make 2 kΩ in series, but only 500 Ω in parallel.

Batteries follow the same idea

Two 3.7 V cells in series give 7.4 V at the same capacity. In parallel they stay at 3.7 V but last twice as long. Only join cells of the same type, age and charge.

LEDs on a 5 V supply

Two red LEDs in series use about 4 V, which leaves 1 V for the resistor. At 15 mA: 1 ÷ 0.015 = 67 Ω, so use 68 Ω. In parallel, give each LED its own 220 Ω resistor. Never share one resistor between parallel LEDs, because one LED can take most of the current and burn out.

Try the battery calculator in Tools to see how series and parallel cells change voltage and run time.