Series vs Parallel Circuits

Two ways to wire two things#

Every circuit is either series, parallel, or a mix. Spotting which one you're looking at tells you instantly how voltage and current will behave.

• Series: components in a single line, one after another — one path for current.
• Parallel: components side by side across the same two points — multiple paths for current.

Series rules#

With one path, the same current flows through everything in the line:

1. Current is the same through every component.
2. Voltages add up — the supply voltage splits across the components.
3. Resistances add — R_total = R1 + R2 + R3 + ...

Example: two 3-ohm resistors in series make 6 ohms. On 12V that's I = 12 / 6 = 2A, and each resistor drops 6V. The catch with series: if one component opens, the single path is cut and everything stops.

Parallel rules#

With every branch tied to the same two points, they all see the same pressure:

1. Voltage is the same across every branch.
2. Currents add up — total current is the sum of the branch currents.
3. Total resistance drops below the smallest branch — 1/R_total = 1/R1 + 1/R2 + ...

Example: two 6-ohm resistors in parallel give 3 ohms — less than either one, because adding paths makes it easier for current to flow. A handy hook: the shared quantity is current in series, voltage in parallel.

How your robot uses both#

FRC power distribution is fundamentally parallel. The battery feeds the PDH (Power Distribution Hub) or PDP (Power Distribution Panel), and every motor controller, the roboRIO, and the radio hang off it as parallel branches. That's deliberate:

• Every device gets the full battery voltage (~12V), because parallel branches share voltage.
• Each branch draws the current it needs independently, because branch currents add at the battery.
• If one branch's breaker trips, the others keep running, because the paths are independent. A series design would drop the whole robot on a single fault.

The one place series shows up is the protection path: the battery, the 120A main breaker, and the PDH/PDP are wired in series, so the main breaker (and its switch) can kill all robot power at once. Series for the master cutoff, parallel for distribution — that's the pattern on every legal FRC robot.

Reading it on a real robot#

When you trace a harness, ask one question: does the current have one path or many? Components nose-to-tail are series (shared current, added voltage). Branches across the same two rails are parallel (shared voltage, added current). Once that's a reflex, a messy robot harness reads cleanly — battery and main breaker in series, feeding a big parallel bus of motor channels off the PDH.