ADR-016: Electronics & PSU Mounting Strategy

Status

Accepted

Context

The Amalgam requires decisions on where to mount the mainboard, host (Pi/SKIPR), and power supply unit (PSU). The placement affects:

Vibration Physics: - Electronics and stepper drivers are sensitive to vibration - High-torque Wade extruder and 3xZ motors generate significant vibration - Direct coupling vs isolation affects reliability and signal integrity

Serviceability: - Mainboard maintenance access - PSU replacement - Wire management for repairs

Thermal Management: - TMC2209 drivers generate heat - MOSFETs for heated bed can run hot - Convection airflow vs active cooling

Weight Distribution: - PSU weight adds to machine mass - Location affects center of gravity - Affects portability

With laminated plinth (ADR-011) and modular puck system (ADR-009), multiple mounting options exist.

Decision

We adopt a three-tier electronics mounting strategy with direct MDF coupling as Reference Spec.

Mounting Options

Option A: Direct to Laminated Plinth (Reference Spec) - Architecture: Mainboard and PSU bolted directly to laminated MDF plinth - Vibration Strategy: Direct coupling to vibration sink (MDF CLD kills frame vibration) - Weight: PSU adds to plinth mass (lowers center of gravity) - Thermal: Open bottom square, natural convection + optional quiet fan - Cost: $0 (uses MDF plinth from ADR-011) - Best For: Tier 3 Reference Spec builds

Option B: Printed “Brain Puck” Frame (Alternative) - Architecture: 3D-printed puck/frame holds mainboard and PSU - Vibration Strategy: Lightweight plastic, couples to workspace surface - Weight: Self-contained, portable - Thermal: Enclosed or honeycomb design, may require active cooling - Cost: Low (~$5-10 AUD in filament) - Best For: Tier 1-2 builds without plinth, portable maintenance

Option C: Hybrid - Printed Puck to MDF (Tier 3 Enhancement) - Architecture: Printed “Brain Puck” screwed into laminated MDF plinth - Vibration Strategy: MDF plinth provides damping, puck provides modularity - Weight: Puck + PSU contribute to plinth mass - Thermal: Open design with MDF backing for heat sink - Cost: Low (~$5-10 AUD in filament) - Best For: Tier 3 with modular electronics upgrades

Vibration Physics: Coupling vs Isolation

Direct Coupling to MDF (Reference): - Frame bolted directly to plinth (no rubber between) - MDF acts as massive vibration sink - Constrained Layer Damping (ADR-011) converts vibration to heat - Electronics experience minimal transmitted vibration - Recommendation: Direct coupling preferred for “Tractor” reliability

Bolting Through Rubber (Isolation): - Rubber pads between frame and MDF, bolts through to plinth - Creates tuned mass damper - Can reduce high-frequency transmission - Trade-off: Reduces coupling, may allow slight frame movement - Recommendation: Use rubber pads under plinth, not between frame and plinth

Isolation Under Plinth: - 4x rubber feet under MDF plinth corners - Isolates entire 20kg+ machine from desk - Prevents desk from becoming sounding board - Recommendation: Mandatory for all tiers with plinth

Electronics Placement Comparison

Feature Direct to MDF (Reference) Printed Brain Puck (Alternative)
Vibration Damping Superior (MDF mass) Poor (light plastic)
Maintenance Permanent (harder to remove) Excellent (modular)
Cost Minimal (screws) Low (filament)
Airflow Open, natural convection Restricted if enclosed
Portability Low (part of plinth) High (standalone)
Serviceability Moderate Excellent

PSU Mounting Strategy

Positioning: - Bottom Rear Corner: Standard placement, balances weight distribution - Bottom Front Corner: Easier access, but may affect Z-homing - On MDF Plinth: Recommended (adds mass, stable) - In Brain Puck: Only for puck-frame builds

Wiring Considerations: - High-Current Wires (heated bed): Use thick gauge (14-16 AWG) to prevent voltage drop - Low-Current Wires (signal): Keep away from high-voltage wiring to prevent EMI - Cable Chains: Required for X/Y axis loom to prevent snagging on moving bed - Wire Length: Keep as short as possible within safety margins

Thermal Management: - PSU Ventilation: Ensure airflow, don’t enclose - TMC Drivers: Can overheat at high currents, add slow quiet fan if needed - MOSFETs: May need heatsinks or active cooling for high-current beds

Wiring Loom Design

Voltage Drop Mitigation: - Heated bed draws highest current (10-15A) - Use 14 AWG or 16 AWG wire for bed power - Keep bed power wires short and direct - Monitor for overheating wires (should not feel hot)

EMI Protection: - Keep endstop and probe wires away from motor/bed power wires - Use twisted pairs for signal wires - Shield long signal cables - Separate high-current and low-current wiring in cable chains

Cable Chain Routing: - X/Y axis loom from mainboard to gantry - Separate: motors (power), endstops (signal), probe (signal) - Use cable chain or umbilical (bundle held by filament) - Prevent snagging on moving bed components

Consequences

Benefits

  • Tiered Flexibility: Direct MDF (Reference), Printed Puck (Alternative), Hybrid (Best of both)
  • Vibration Optimization: Direct coupling to CLD plinth provides industrial-grade damping
  • Modularity: Printed puck enables easy mainboard upgrades without unbolting frame
  • Serviceability: Multiple options from permanent (Reference) to modular (Puck)
  • Thermal Management: Open designs provide natural convection, fan options available

Trade-offs

  • Direct to MDF: Permanent, harder to remove for mainboard replacement
  • Printed Puck: Poor vibration damping (lightweight plastic)
  • Hybrid: Adds filament cost, but provides modularity + damping

What This Enables

  • Tier 3 Reference: Direct MDF mounting for maximum damping
  • Tier 1-2 Alternative: Printed puck builds without plinth
  • All Tiers: Proper wire management (voltage drop, EMI protection)
  • Future Upgrades: Modular puck allows mainboard swaps

What This Replaces

  • Mainboard mounting to M12 frame (transmits vibration)
  • PSU mounting to moving gantry (adds mass to moving parts)
  • Uncontrolled wire runs (voltage drop, EMI issues)
  • Enclosed electronics boxes (poor thermal management)

BOM Implications (Generic)

Tier 3 Reference: Direct to MDF Plinth

  • Parts needed:
    • MDF plinth (already in ADR-011)
    • 4x Rubber feet for plinth (vibration isolation)
    • Mounting screws for mainboard and PSU
    • Optional: Slow quiet 120mm fan for electronics cooling
  • Cost implication: Very Low (~$10-15 AUD for feet + screws)
  • Vibration Damping: Superior (MDF mass)
  • Maintenance: Moderate (permanently mounted)
  • Thermal Management: Open, natural convection

Tier 1-2 Alternative: Printed Brain Puck

  • Parts needed:
    • 3D-printed “Brain Puck” frame/enclosure
    • Mounting hardware (standoffs, screws)
    • Optional: 4x Rubber feet for puck
  • Cost implication: Very Low (~$5-10 AUD in filament)
  • Vibration Damping: Poor (lightweight plastic)
  • Maintenance: Excellent (modular, portable)
  • Thermal Management: May require active cooling fan

Tier 3 Enhancement: Hybrid (Puck to MDF)

  • Parts needed:
    • 3D-printed “Brain Puck” with mounting holes
    • Wood screws to secure puck to MDF
    • MDF plinth (already in ADR-011)
    • 4x Rubber feet for plinth
  • Cost implication: Very Low (~$5-10 AUD in filament)
  • Vibration Damping: Superior (MDF mass)
  • Maintenance: Excellent (puck can be unscrewed for upgrades)
  • Thermal Management: Open with MDF backing (heat sink)

Universal Requirements (All Options)

  • PSU: 12V or 24V (sufficient current: 20A for 12V, 12A for 24V)
  • Wiring:
    • 14-16 AWG for heated bed
    • 18-20 AWG for hotend/fans
    • 22-24 AWG for signals
    • Shielded cables for long signal runs
  • Cable Management: Cable chains or umbilicals for X/Y loom
  • EMI Protection: Separate high-current and low-current wiring

Implementation Notes

Direct to MDF Mounting (Reference)

Mainboard Mounting: - Use M3 or M4 standoffs (height ~10mm) - Secure to MDF with wood screws - Ensure flat mounting surface (no warping) - Leave clearance for connectors on all sides

PSU Mounting: - Bottom rear corner of plinth - Use 4x M4 or M5 bolts through PSU mounting holes - Secure to MDF with large fender washers - Ensure ventilation holes are not blocked - Leave space for wiring access

Wiring Layout:

Bottom Square Layout:
┌─────────────────────────────────────┐
│          [MDF Plinth]             │
│  ┌─────┐    ┌──────────────┐    │
│  │PSU  │    │  Mainboard    │    │
│  └─────┘    │  (SKIPR/Manta)│    │
│              └──────────────┘    │
│         [Cable Chain to Gantry]    │
└─────────────────────────────────────┘

Printed Brain Puck Design

Puck Frame Specifications: - Fits within bottom M12 square - Holds mainboard + PSU (if compact) - Honeycomb or perforated sides for airflow - Mounting holes for MDF attachment (if hybrid) - Cable management loops for wire routing

Mounting to Puck: - Mainboard on standoffs (10mm height) - PSU either on puck or next to puck - Leave access for connectors and fan mounting - Wire management channels to prevent tangling

Hybrid Puck to MDF: - Puck screwed into MDF with 4x wood screws - Puck provides modularity, MDF provides damping - Remove puck for mainboard upgrades without unbolting frame

Wire Management

Voltage Drop Calculation:

V_drop = I × R × L
I = Current (heated bed ~12A)
R = Resistance per meter (14 AWG: 0.0083 Ω/m)
L = Length in meters (keep <2m if possible)

Example: 12A × 0.0083 × 1.5m = 0.15V drop (acceptable)

EMI Protection: - Separate power and signal wires by at least 20mm - Use twisted pairs for signal wires - Shield long signal cables (connect shield to ground at one end) - Keep motor wires away from endstop/probe wires

Cable Chain Routing: - Bundle X/Y axis wires (motors + endstops + probe) - Use cable chain from mainboard to X/Y gantry - Prevent snagging on moving bed or Z-rods - Leave slack for full travel range

Thermal Management

TMC Driver Cooling: - Check driver temperature with Klipper: DUMP_TMC STEPPER=stepper_x - Add slow quiet 120mm fan if drivers > 60°C - Point airflow across heatsinks, not directly at components

PSU Ventilation: - Ensure ventilation holes are not obstructed - Keep at least 10mm clearance around PSU - If enclosed, add exhaust fan for hot air removal

MOSFET Cooling: - Check heated bed MOSFET temperature during heating cycle - Add small heatsink or directed airflow if > 60°C

Safety Considerations

  • PSU Mounting: Secure with proper bolts, don’t rely on gravity
  • Wire Gauge: Use appropriate gauge for current (voltage drop = fire risk)
  • Ventilation: Don’t enclose PSU without exhaust fan (overheating = fire risk)
  • Water Protection: If using laminated plinth, seal MDF edges to prevent moisture absorption
  • Strain Relief: Use cable glands or zip ties to prevent wire fatigue

References

  • docs/reference/ai-conversations/electronics-mounting.md: Complete mounting discussion
  • docs/adr/011-laminated-plinth.md: Laminated plinth baseboard foundation
  • docs/adr/009-puck-system.md: Modular puck mounting system
  • docs/adr/012-mainboard-host-architecture.md: Mainboard selection
  • Klipper Wiring Guide: Wiring
  • AWG Wire Gauge Chart: Wire Size Calculator

Evolution Notes

This ADR establishes direct MDF coupling as Reference Spec with printed puck as alternative. The decision framework prioritizes: - Vibration damping (MDF mass over portability) - Serviceability (modular pucks for easy upgrades) - Thermal management (open designs, natural convection) - Wire management (voltage drop, EMI protection)

Future mounting systems will be evaluated against: vibration damping performance, serviceability, thermal efficiency, and modularity for mainboard upgrades.