Overview
A servo's current depends on its load. A micro servo moving freely may take 100–250 mA, but at the moment it starts or when it stalls against something it can briefly draw 500 mA to over 1 A. Standard-size servos can draw 2 A or more. The Uno on USB has 500 mA in total for itself and everything attached.
Signs of a power problem
- The board resets or the sketch restarts when the servo moves.
- The USB connection drops, or the computer warns about a USB device drawing too much power.
- The servo jitters, moves weakly, or stops partway.
Wiring a separate supply
- Use a regulated 5–6 V source that can supply at least 1 A per small servo: a 5 V 2 A adapter, a hobby BEC, or a 4-cell NiMH pack (4.8 V).
- Connect the supply's + to the servo's red wire only. Never connect it to the Uno's 5V pin.
- Connect the supply's − to the servo's brown wire and to an Uno GND pin. Without this common ground the signal has no reference and the servo will not respond, or will twitch randomly.
- Keep the signal wire on the Uno pin as before.
- Add a 470–1000 µF electrolytic capacitor across the servo supply (+ to +) to absorb current spikes.
What not to do
- Do not use a 9 V battery: the voltage is too high for most servos and the battery cannot deliver the current.
- Do not feed 4×AA alkaline cells (up to 6.4 V fresh) to servos rated 4.8–6 V without checking the datasheet.
- Do not power servos from the Uno's 3.3V pin.
Code
No code changes are needed: the sketches in Servo basics and the workshop projects work unchanged. Only the power wiring differs.