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Power Budget

Current draw calculations based on component datasheets. All servos are dumb PWM.

Apparatus (24V System)
PSU 24V, 480W (20A max)
ESP32 Powered separately via 5V USB adapter
Component Voltage Max Current (each) Qty Total Current Source
Stepper motors (via TMC2209) 24V 2A RMS (driver limited) 4 ~8A TMC2209 spec
24V 80mm fans 24V ~0.15A 4 ~0.6A Typical 80mm fan
Total ~8.6A 43% of 20A PSU

GEO-DUDe (12V System)
PSU 12V, 600W (50A max)
AC input 120V mains through 3-wire 15A slip ring

Servo Specifications

Two independent identical arms, 5 servos per arm, 10 servos total.

Servo Voltage Stall Current (each) Qty (total, 1 per arm) Total Stall Power Source
Base (HOOYIJ 150kg) 12V 8.0A 2 16.0A 12V bus direct
Shoulder (ANNIMOS 150kg) 12V 8.0A 2 ~16.0A 12V bus direct
Elbow (ANNIMOS 80kg) 7.4V 5.0A 2 10.0A Buck conv 1 (7.4V)
Wrist rotate (RDS3218 20kg) 5V 1.6A 2 3.2A Buck conv 3 (5V)
Wrist pan (RDS3218 20kg) 5V 1.6A 2 3.2A Buck conv 3 (5V)

Other Loads

Component Voltage Max Current Power Source
Raspberry Pi 5V 2.5A Buck conv 2 (5V)
PCA9685 PWM driver 5V ~0.05A Buck conv 2 (5V)
12V fan 12V 0.15A 12V bus direct

Current by Rail

Rail Components Total Max Current Capacity Status
12V direct 4x 150kg servos (2 base + 2 shoulder, one per arm) + fan ~32A stall 50A PSU (30A fused) OK
7.4V buck 1 2x 80kg elbow servos (one per arm) 10A 20A converter (8A fused at 12V in) OK
5V buck 2 Pi + PCA9685 (always on) ~2.6A 20A converter (3A fused at 12V in) OK
5V buck 3 4x RDS3218 wrist (two per arm) (after toggle switch) ~6.4A stall 20A converter (8A fused at 12V in) OK

Buck Converter Allocation

Converter: 20A 300W buck - Input 6-40V, Output 1.25-36V adj, 20A max / 15A continuous

Buck # Output V Load Max Current Headroom Notes
1 7.4V 2x elbow servos (one per arm) 10A stall 5-10A spare After toggle switch
2 5V Pi + PCA9685 ~2.6A 12-17A spare Before toggle switch (always on)
3 5V 4x RDS3218 wrist (two per arm) ~6.4A stall 9-14A spare After toggle switch
4 - Spare - -

Realistic Operating Current vs Stall

The "kg" rating on hobby servos is the stall torque - the absolute maximum when the motor is locked and drawing peak current. During normal operation, servos draw a fraction of this.

Assumptions:

  • The arms are manipulating a lightweight subscale satellite model, not heavy payloads
  • Servos operate at 20-40% of stall torque during typical motion (holding against gravity, positioning)
  • Current scales roughly linearly with torque for DC motors
  • All servos will never simultaneously stall - that would mean every joint on both arms is jammed at once
  • Brief acceleration spikes may hit 50-60% of stall but are transient (milliseconds)

GEO-DUDe Realistic Current Draw

Servo Stall Current Realistic Operating (per servo) Qty (total) Realistic Total Notes
Base (150kg) 8.0A 2-3A (~30% load) 2 4-6A Holding arm weight against gravity
Shoulder (150kg) 8.0A 2-3A (~30% load) 2 4-6A Highest static load (arm cantilevered)
Elbow (80kg) 5.0A 1.5-2A (~35% load) 2 3-4A @ 7.4V Lighter distal load
Wrist RDS3218 (20kg) 1.6A 0.5-0.8A (~40% load) 4 2-3.2A @ 5V Fine positioning, light torque
Pi + PCA9685 - 2.6A 1 2.6A @ 5V Constant
Fan - 0.15A 1 0.15A @ 12V Constant

Current Through Toggle Switch (Realistic)

Everything after the toggle switch, referred to 12V input:

Branch Realistic @ native V Referred to 12V input Stall @ 12V
Base (12V direct, 2 servos) 4-6A 4-6A 16A
Shoulder (12V direct, 2 servos) 4-6A 4-6A 16A
Elbow (7.4V buck, 2 servos) 3-4A @ 7.4V ~2.5A (efficiency ~80%) ~6.2A
Wrist (5V buck, 4 servos) 2-3.2A @ 5V ~1.7A (efficiency ~80%) ~2.7A
Fan 0.15A 0.15A 0.15A
Total through toggle switch ~12-16A ~41A

Conclusion: A 40A toggle switch provides 2.5x margin on realistic load.

Wire Gauge Sizing (Revised)

Wire gauge must be rated for the fuse rating (not the load), since the fuse is what limits current in a fault. Only 16, 18, and 22 AWG wire is available.

Segment Old Gauge Old Fuse Revised Gauge Revised Fuse Justification
PSU to bus trunk 8 AWG 40A 2x 16 AWG parallel 30A 2x 16 AWG gives ~24A capacity, 30A fuse protects
Bus to toggle switch 8 AWG - 2x 16 AWG parallel - Matches trunk
Toggle switch to fuse boards 8 AWG - 16 AWG - After toggle, splits to individual branches
GND trunk 8 AWG - 2x 16 AWG parallel - Matches trunk
Base/shoulder servo branch 14 AWG 20A 16 AWG 8A per servo Per-servo fusing on arm fuse board
Buck 1 input 16 AWG 8A 18 AWG 8A
Buck 2 input 18 AWG 3A 18 AWG 3A No change needed
Buck 3 input 16 AWG 8A 18 AWG 8A
Buck outputs to fuse boards 16 AWG - 18 AWG -
Fan 22 AWG 1A 22 AWG 1A No change needed
Signal / low current 22 AWG - 22 AWG - No change needed

About Servo Wire Gauge

Hobby servos come with thin built-in leads (typically 22-26 AWG). This is fine because:

  • Each lead only carries current for one servo
  • Wire runs are short (15-30cm from distribution point to servo)
  • Even the 150kg servos at 8A stall use ~20 AWG factory leads - the wire is rated for thermal load over short bursts
  • The fuse upstream protects the servo's own wiring in a fault condition

The wire gauges in the diagrams are for the distribution runs (PSU to bus, bus to toggle switch, toggle switch to fuse boards). From the fuse board to each servo, use the servo's own factory leads.

Fuse Sizing

Fuse Branch Realistic Draw Peak/Stall Rating Wire Gauge Notes
AC inline Mains hot before slip ring ~3A @120V ~5A 6A slow-blow Mains cable
Main DC 12V bus after PSU ~16A ~41A stall 30A 2x 16 AWG parallel
Base servo (each, x2) Per base servo, one per arm ~3A 8A stall 8A 16 AWG Per-servo on arm fuse board
Shoulder servo (each, x2) Per shoulder servo, one per arm ~3A 8A stall 8A 16 AWG Per-servo on arm fuse board
Elbow servo (each, x2) Per elbow servo, one per arm ~1.8A 5A stall 5A 18 AWG Per-servo on arm fuse board
Wrist rotate (each, x2) Per wrist rotate, one per arm ~0.7A 1.6A stall 3A 18 AWG Per-servo on arm fuse board
Wrist pan (each, x2) Per wrist pan, one per arm ~0.7A 1.6A stall 3A 18 AWG Per-servo on arm fuse board
Buck 1 input Elbow servos at 12V in ~2.5A ~6.2A 8A 18 AWG
Buck 2 input Pi + PCA9685 at 12V in ~1.1A ~1.1A 3A 18 AWG Before toggle switch (always on)
Buck 3 input Wrist servos at 12V in ~1.7A ~2.7A 8A 18 AWG
Fan line 12V fan 0.15A 0.15A 1A 22 AWG
Apparatus main 24V bus ~8.6A ~8.6A 12A 14 AWG Steppers are driver-limited