Amalgam: Power Supply Configurations

⚠️ Outdated Tier System: This document uses the old Tier 0/1/2/3 framework and explores multi-PSU configurations. Current canonical decisions are in: - ADR-012: Mainboard & Host Architecture (electronics configuration) - ADR-016: Electronics & PSU Mounting Strategy - docs/reference/MASTER_PARTS_LIST.md (current PSU specs per configuration)

With the simplified two-donor model (ADR-025), most builds use scavenged PSUs; refer to current BOM for accurate specs.

Overview

Amalgam requires power for seven stepper motors (1×X, 2×Y, 3×Z, 1×E), a hotend, heated bed, control boards, and cooling fans. A single scavenged PSU from a donor printer (typically 150W–250W) is often insufficient. This document covers safe configurations for single and dual PSU setups.

Power Budget Analysis

Component Power Requirements

Component	Power Draw	Notes
Control Boards (×2)	~10W	Logic, drivers idle
Cooling Fans	~10W	Hotend, part cooling, electronics
Stepper Motors (×7)	~70W	Peak during rapid moves
Hotend (E3D V6)	40W	40W heater cartridge
Heated Bed (235×235)	150W–220W	Largest single load
Total	~280W–350W	At full operation

Typical Scavenged PSU Ratings

Donor Printer	Voltage	Wattage	Sufficient?
Anet A8	12V	240W	❌ No (12V system)
Ender 3 (original)	24V	270W	⚠️ Marginal
Ender 3 Pro/V2	24V	350W	✅ Yes (barely)
Prusa MK2	12V	240W	❌ No (12V system)
Prusa MK3	24V	240W	⚠️ Marginal
CR-10	12V/24V	360W–500W	✅ Yes (check voltage)

Conclusion: Most single scavenged PSUs are marginal or insufficient. Dual PSU configuration is recommended.

⚠️ CRITICAL SAFETY WARNINGS

The Voltage Trap (12V vs 24V)

This is the most dangerous mistake in scavenger builds.

If your donor printer is older (Prusa MK2, Anet A8, early CR-10), it likely uses a 12V system.

The Danger: Connecting a 12V component to 24V doesn’t double the power—it quadruples it.

Power = Voltage² / Resistance

12V bed at 12V: P = 144/R = 150W (normal)
12V bed at 24V: P = 576/R = 600W (FIRE HAZARD)

What Will Happen: - Connectors will melt - MOSFETs will explode - Heating traces may crack and arc - Fire is likely

Before Connecting Anything: 1. Check labels on heated bed (underside) 2. Check capacitor voltage ratings on boards 3. If it says “12V” → Do NOT connect to 24V PSU

Capacitor Voltage Ratings

Scavenged control boards have electrolytic capacitors (cylindrical components).

The Check: Look at the voltage rating printed on the capacitor. - If it says “16V” → Will explode on 24V - If it says “25V” or “35V” → Safe for 24V

If capacitors are underrated: Replace them or use that board only with a 12V PSU.

Configuration 1: Single PSU (Simple)

When This Works

PSU is 350W+ at 24V
Heated bed is 24V compatible
All components match PSU voltage

Wiring Diagram

┌─────────────────────────────────────────────────┐
│                 MAINS AC INPUT                   │
│            [Fused IEC Inlet + Switch]            │
└─────────────────────┬───────────────────────────┘
                      │
                      ▼
              ┌───────────────┐
              │    PSU 24V    │
              │    350W+      │
              └───┬───────┬───┘
                  │       │
               (+24V)   (GND)
                  │       │
       ┌──────────┴───────┴──────────┐
       │                             │
       ▼                             ▼
┌─────────────┐              ┌─────────────┐
│   Board 1   │──── USB ────│   Board 2   │
│ (X,Y1,Y2,E) │              │ (Z1,Z2,Z3)  │
└─────────────┘              └─────────────┘
       │                             │
       ▼                             ▼
   [Hotend]                     [Heatbed]
   [Fans]                       [Z-Motors]

Limitations

Single point of failure
May brownout during bed heating + rapid moves
Limited upgrade path

Configuration 2: Dual PSU (Recommended)

Why Dual PSU?

Load Distribution: Each PSU handles a manageable load
Isolation: Electrical noise from bed heater separated from motion system
Redundancy: If one PSU is marginal, the other compensates
Scavenger Reality: Two 200W PSUs are easier to find than one 400W

The Three Golden Rules

Rule 1: Common DC Ground (MANDATORY)

Both control boards must share the same reference for “zero volts.”

The Problem: Without common ground, the “0V” on Board A might be 0.5V different from Board B. This causes: - USB communication errors - Data corruption - Potential damage to Raspberry Pi USB ports

The Fix: Run a thick wire (14–16 AWG) between the Negative (−V or COM) terminals of both PSUs.

PSU 1 [-V] ════════════════════ PSU 2 [-V]
              (Common Ground)

Rule 2: Never Parallel the Positives (CRITICAL)

NEVER connect the +V terminals of two PSUs together.

Why: No two PSUs output exactly the same voltage. - PSU 1 might output 24.1V - PSU 2 might output 23.9V

If connected, they will “fight”—the higher voltage PSU tries to charge the lower one. This causes: - Excessive current flow between PSUs - Overheating - PSU failure - Potential fire

The Fix: Keep positive circuits completely isolated. - Board A gets +V ONLY from PSU 1 - Board B gets +V ONLY from PSU 2

Rule 3: Single AC Switch (SAFETY)

Both PSUs must turn on and off together.

The Risk: If one PSU is on and the other is off while the printer tries to move: - “Back-powering” through stepper motors - Control board damage - Unpredictable behavior

The Fix: Wire both PSUs to a single fused AC inlet and power switch.

Dual PSU Wiring Diagram

┌─────────────────────────────────────────────────────────────┐
│                      MAINS AC INPUT                          │
│               [Fused IEC Inlet + Switch]                     │
│                    [Emergency Stop]                          │
└─────────────────────────┬───────────────────────────────────┘
                          │
            ┌─────────────┴─────────────┐
            │     Terminal Block        │
            │   (Live/Neutral Split)    │
            └─────┬───────────────┬─────┘
                  │               │
                  ▼               ▼
          ┌───────────┐   ┌───────────┐
          │   PSU 1   │   │   PSU 2   │
          │   24V     │   │   24V     │
          └─┬───────┬─┘   └─┬───────┬─┘
            │       │       │       │
         (+24V)  (GND)   (+24V)  (GND)
            │       │       │       │
            │       └───────┴───────┤  ← COMMON GROUND
            │           ║           │
            │           ║           │
    ┌───────┴───┐       ║       ┌───┴───────┐
    │  Board 1  │       ║       │  Board 2  │
    │(X,Y1,Y2,E)│       ║       │(Z1,Z2,Z3) │
    └─────┬─────┘       ║       └─────┬─────┘
          │             ║             │
          ▼             ║             ▼
      [Hotend]          ║         [Heatbed]
      [Fans]            ║         [Z-Motors]
                        ║
              (Common Ground Bus)

    ⚠️ +24V lines NEVER connected between PSUs

Recommended Load Distribution

PSU	Board	Components	Rationale
PSU 1	Board 1	X, Y1, Y2, Extruder, Hotend, Fans	“Fast” motion, lower power
PSU 2	Board 2	Z1, Z2, Z3, Heatbed	“Slow” motion, high power

Why This Split: - Heatbed electrical noise isolated from X/Y signals - High-current bed circuit on dedicated PSU - Z-motors move slowly, less sensitive to noise

Configuration 3: Mixed Voltage (12V + 24V)

When This Is Necessary

One donor is 12V (older printer)
One donor is 24V (newer printer)
You have a 12V heated bed

The External MOSFET Solution

Use the 12V PSU exclusively for the heated bed, controlled by an external MOSFET triggered by the 24V board.

┌─────────────────────────────────────────────────────────────┐
│                      MAINS AC INPUT                          │
│               [Fused IEC Inlet + Switch]                     │
└─────────────────────────┬───────────────────────────────────┘
                          │
            ┌─────────────┴─────────────┐
            │                           │
            ▼                           ▼
    ┌───────────────┐           ┌───────────────┐
    │    PSU 1      │           │    PSU 2      │
    │    24V        │           │    12V        │
    └───┬───────┬───┘           └───┬───────┬───┘
        │       │                   │       │
     (+24V)  (GND)               (+12V)  (GND)
        │       │                   │       │
        │       └───────────────────┴───────┤  ← COMMON GROUND
        │                   ║               │
        ▼                   ║               ▼
┌─────────────┐             ║       ┌───────────────┐
│   Board 1   │─────────────╫──────→│ External      │
│  (All MCU)  │  Signal     ║       │ MOSFET        │
└─────────────┘  (3.3V)     ║       └───────┬───────┘
        │                   ║               │
        ▼                   ║               ▼
    [Hotend]                ║         [12V Heatbed]
    [Steppers]              ║
    [Fans]                  ║
                            ║
                  (Common Ground Bus)

External MOSFET Notes

Cost: ~$5–10 (HA210N06 module or similar)
Triggered by low-current signal from main board
Switches high-current 12V to bed
Must share common ground with 24V system

⚠️ Do NOT Use Software PWM Limiting

Some guides suggest using max_power: 0.25 in Klipper to run a 12V bed on 24V.

Why This Is Dangerous: - If Klipper crashes with the pin HIGH, bed goes to 400% power - No hardware protection - Fire risk

Always use hardware voltage matching, not software limiting.

Configuration 4: Server PSU Scavenge

The “Free Power” Option

Server PSUs from old HP, Dell, or IBM rack servers are often available free from e-waste centers.

Common Types: - HP “Common Slot” PSUs (750W–1200W) - Dell PowerEdge PSUs (500W–1100W)

Server PSU Characteristics

Feature	Value	Notes
Voltage	12V DC	Single rail, high current
Power	500W–1200W	Massive headroom
Efficiency	80%+ (Gold/Platinum)	Low heat, quiet
Cost	Free–$20	E-waste goldmine

The 12V Limitation

Server PSUs output 12V, but Amalgam prefers 24V.

Options:

Use 12V System: If all your components are 12V compatible
Series Two PSUs: Connect two 12V PSUs in series for 24V
DC-DC Boost: Use a boost converter (less efficient)

Series Connection for 24V

┌─────────────┐      ┌─────────────┐
│  Server     │      │  Server     │
│  PSU 1      │      │  PSU 2      │
│  12V        │      │  12V        │
└──┬──────┬───┘      └──┬──────┬───┘
   │      │             │      │
(+12V)  (GND)        (+12V)  (GND)
   │      │             │      │
   │      └─────────────┘      │
   │            │              │
   │        (JOINED)           │
   │      This becomes         │
   │      the "middle"         │
   │                           │
   ▼                           ▼
(+24V)                       (GND)
   │                           │
   └─────── TO PRINTER ────────┘

Critical: The “joined” point (PSU1 GND to PSU2 +12V) must be isolated from chassis ground.

USB Back-Powering Prevention

The Problem

When control boards are connected to the Klipper host (Raspberry Pi) via USB, the USB cable can try to power the board through the 5V pin.

The Risk: - If PSUs are off but Pi is on, board draws power through USB - USB traces are rated for ~500mA, boards may try to draw more - Can fry the Pi’s USB port or the board’s voltage regulator

The Fix: 5V Pin Tape Trick

Look inside the USB-A connector (the end that plugs into Pi)
Identify the 5V pin (usually leftmost or rightmost, red wire)
Place a small piece of electrical tape over just that pin
Data pins (D+ and D−) remain connected

USB-A Connector Pin Layout:
┌─────────────────────┐
│  [5V] [D-] [D+] [GND]  │
│   ▲                    │
│   │                    │
│  TAPE THIS PIN         │
└─────────────────────────┘

Alternative: Some boards have a jumper to disable USB power input—check your board’s documentation.

Wiring Best Practices

Ferrules Are Mandatory

Scavenged wires are often: - Brittle from age - Frayed at the ends - Prone to loose strands

The Risk: A loose strand can bridge terminals, causing shorts, arcing, or fire.

The Fix: Use crimped ferrules on every wire going into a screw terminal.

Ferrule crimping kit: ~$15–25
Prevents strand separation
Creates secure, repeatable connections

Wire Gauge Guidelines

Circuit	Minimum Gauge	Notes
Heated Bed	14 AWG	High current, longest run
Hotend	18 AWG	Moderate current
Stepper Motors	22 AWG	Low current
Logic/Signals	24 AWG	Very low current
Common Ground (Dual PSU)	14–16 AWG	Must handle fault current

Earth/Ground Safety

Mains Earth (Green/Yellow) must be connected to: - Metal frame of both PSUs - Metal frame of printer (if any) - Heated bed frame (if metal)

This protects you if a wire shorts to the frame.

Klipper Multi-MCU Configuration

Basic Dual-Board Setup

In printer.cfg:

[mcu]
serial: /dev/serial/by-id/usb-board_1_xxxxx

[mcu aux]
serial: /dev/serial/by-id/usb-board_2_xxxxx

Pin Addressing

For the primary MCU:

[stepper_x]
step_pin: PB13

For the secondary MCU:

[stepper_z]
step_pin: aux:PB13

Timing Synchronization

Klipper automatically synchronizes timing between MCUs. The common ground connection is essential for this to work reliably.

Troubleshooting

Symptom: USB Disconnects Randomly

Likely Cause: Common ground not connected or poor connection

Fix: - Verify thick wire between PSU negatives - Check for corrosion on terminals - Use star-ground topology if multiple grounds

Symptom: Heated Bed Heats Slowly

Likely Cause: PSU voltage sag under load

Fix: - Check PSU wattage vs. actual load - Use dedicated PSU for bed - Check wire gauge (undersized wires drop voltage)

Symptom: Steppers Skip or Lose Position

Likely Cause: Voltage brownout during bed heating

Fix: - Separate bed onto different PSU - Reduce stepper current slightly - Add bulk capacitor near stepper drivers

Symptom: Board Won’t Power On

Likely Cause: - 12V board connected to 24V (check for damage) - USB back-powering with low-current source

Fix: - Verify voltage compatibility - Apply 5V tape trick - Check for blown fuses or burned components

Summary: Recommended Configuration

For Amalgam “Tier 0” scavenger build:

Setup	Recommendation
PSU Count	Two (from donor printers)
Voltage	24V preferred, mixed 12V/24V acceptable
PSU 1	Board 1: X, Y1, Y2, Extruder, Hotend, Fans
PSU 2	Board 2: Z1, Z2, Z3, Heated Bed
Common Ground	14–16 AWG between PSU negatives
AC Side	Single fused inlet, single switch
Safety	Ferrules, earth bonding, 5V tape trick

The Golden Rules: 1. ✅ Common DC Ground (tie negatives together) 2. ❌ Never parallel positives 3. ✅ Single AC switch for all PSUs 4. ✅ Verify voltage compatibility before connecting

References

Klipper Multi-MCU Documentation
Meanwell PSU Specifications
RepRap Wiki: Power Supply
3D Printing Community Safety Guidelines