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Recipe

Power a board from something other than USB

Use when: a project needs a battery, an adapter or a bench supply instead of, or as well as, a USB cable

Goal: one source per rail, within the input's rated range, with every ground joined

Taught in Powering a board safely

Quick recipe

  1. Match the voltage to an input
  2. One source per rail
  3. Join the grounds
  4. Power every connected board together
  1. 1

    Find the input whose datasheet range contains your supply's voltage, including a fresh battery's highest value.

    Example

    A 9 V battery on an Uno R4 Minima: VIN or the jack, rated 6 to 24 V.

    Why: each input expects a range, and a voltage outside it skips or overloads the part meant to handle it.

  2. 2

    Unplug USB, or confirm in the board's documents that two inputs may be used together.

    • the documents say the inputs are isolated (diodes, a power switch) — both may stay connected.
    • they say "mutually exclusive", or say nothing — one at a time.

    Why: two sources on one rail fight, and the higher one back-feeds the lower.

  3. 3

    Every board and supply in the project shares one ground.

    Why: voltages only mean something measured from a shared zero (Chapter 2).

  4. 4

    Don't leave one board unpowered while another drives its pins.

    Why: a HIGH into an unpowered chip back-powers it through its input.

Common mistakes

  • Feeding a battery into the 5V pin "because it's about 5 V".
  • Leaving USB plugged in while testing with a bench supply.
  • Unplugging one of two connected boards and leaving the other running.