Powering an SO-101 Arm: 12 V vs 7.4 V Servos, Which Adapter, and How Much Current

An SO-ARM101 pair needs two different power supplies: 12 V for the follower's STS3215-C018 servos and 7.4 V for the leader's 7.4 V servos. Mixing them up is the fastest way to destroy a leader arm. Here is how the power side should be built.

Voltage per arm

Arm Servos Rated voltage Adapter
Follower 6 × STS3215-C018 (30 kg·cm) 12 V (6–12.6 V) 12 V 5 A, DC 5.5 × 2.1 mm
Leader 6 × STS3215 7.4 V (C001 / C044 / C046) 7.4 V (5–8.4 V) 7.4 V 3 A, DC 5.5 × 2.1 mm

How much current

  • One STS3215 draws about 0.9 A rated and 2–3 A briefly at stall.
  • A follower lifting a 200 g object with an extended arm peaks at 4–5 A on the shoulder and elbow together. A 12 V 5 A adapter is the minimum; 12 V 6–8 A gives headroom for STS3250 shoulders.
  • A leader moved by hand draws under 1.5 A; a 7.4 V 3 A adapter is plenty.

Where the power goes in

Into the servo adapter board, not into USB. The Waveshare Bus Servo Adapter (A) and the BusLinker V3.0 both take a 5.5 × 2.1 mm barrel jack (or screw terminals) and pass the voltage straight to the servo bus. Their USB-C port carries data only.

Wiring rules that prevent problems

  1. Label the two adapters at the plug: a 12 V plug in the leader's jack is the classic mistake.
  2. Use centre-positive adapters; both boards expect it.
  3. Keep servo power and computer on the same power strip to avoid ground offsets that show up as bus noise.
  4. Switch power off before plugging or unplugging servo cables; hot-plugging a 5264 connector can spike the bus.
  5. Never power servos from a laptop USB hub through the adapter; it cannot supply the current and will brown out.

Batteries

A 3S LiPo (11.1–12.6 V) runs a follower and a 2S LiPo (7.4–8.4 V) runs a leader, which is how the LeKiwi mobile base is powered. For a desk arm, mains adapters are simpler and do not need charging or a low-voltage cutoff.

FAQ

Can I run the leader from 12 V?

No. The leader's 7.4 V STS3215 models are rated to 8.4 V; 12 V destroys them. Use a 7.4 V or 8.4 V adapter, or a 2S LiPo.

Can I run the follower from 7.4 V?

The 12 V C018 servos will run at 7.4 V with much less torque; the arm sags under load. Use 12 V.

Can both arms share one adapter?

Only if they take the same voltage. A leader and follower need one 7.4 V and one 12 V supply.

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