feat(pcb): per-console power distribution + monitoring CAN node (PDU) — INA180 telemetry

[mottihoresh]

Body consolidated 2026-07-08 — the original exploratory scope (below-the-fold history in comments) converged over 6 comments into a built board. This body now reflects current state; the comment thread holds the decision history.

Status — hardware done, awaiting boards

Per-console Power Distribution + Monitoring CAN node (PDU). PCB routed, DRC 0/0, ERC-clean → PR #217 (pcb/pdu-power-202 → main). Bare boards ordered (JLCPCB, in transit); LCSC components pending (batched with the Right-Nav order). Firmware split → #216. Rev1 = instrumentation-first (measure real cockpit load, let data drive final fuse/shunt sizing).

Role / architecture

• ~3 PDUs, one per console (Left · Right · Main Instrument). Each injects + monitors + fuses its console's power locally.
• Sits PSU → PDU → console J_BUS. GND + CAN stay continuous across all PDUs; +12V/+5V are segmentable (a heavy zone dead-ends its OUT power pins → next zone gets a fresh injected feed).
• Built on the PanelGroup base (STM32F103C8 + SN65HVD230 CAN + AMS1117) + design-block library (feat(pcb): OpenSkyhawk Design Block library (reusable power/CAN/MCU/harness circuits) #201). Own NODE_ID.

Locked design (supersedes the original body — see DESIGN-NOTES.md for full BOM/rationale)

Function	Part	Note
Current sense ×2	INA180A2 (SOT-23-5, gain 50) + 0.1%/25 ppm dividers → STM32 ADC	analog high-side; NOT INA219/INA226 (relaxed accuracy → cheaper, inspectable SOP; INA219 = documented fallback if absolute-V ever needed; INA3221 QFN-banned)
Shunt ×2	CSRF2512 10 mΩ 2512 (C346481)	Kelvin (4-terminal) via pad routing; footprint swappable
Voltage sense	0.1%/25 ppm dividers, both sides of each fuse	rail-V + fuse-open detect
Temp	NTC B3450 ±1% + STM32 internal sensor	TMP117 dropped (WSON-banned)
Fuse ×2	12V 5 A · 5V 2 A MINI blade + holder	source-side, off-board consumable; GND never fused
PSU input	Mini-Fit Jr 6-pin (2×+12V / 1×+5V / 3×GND)	J_PSU_IN
Bus ×2	Mini-Fit Jr 8-pin J_BUS_IN/OUT	connector split dropped — CAN/power split is a Y-harness into the fabbed 8-pin J_BUS
Input TVS	D1 SMBJ12A (12V) · D2 SMBJ6.0A (5V, LCSC C5331096)	per the #214 input-rail standard

• Copper: large pours (12V 6–10 mm / 5V 5–8 mm / GND max, via-stitched) — sized for Kelvin accuracy + thermal margin, not current (a console is ~2–3 A). Fuse-aware neck rule: ≥2 mm per-console, ≥5 mm whole-bus.
• Servo power (new standard): 12V rail → local buck → servos, never logic 5V (+ bulk cap + polyfuse). Servos become a 12V load (captured by the 12V INA180); keeps 5V monitoring clean.
• Fault bitmap (over CAN): 12V/5V UV · 12V/5V OV · overcurrent · fuse-open · sensor-fail · CAN-fail · temp warn · temp critical.

Remaining before this closes

• Merge PR feat(pcb): PDU power-distribution + monitoring board (#202) #217 (PCB) + PR docs: docs-drift cross-check → GitHub Projects; power per-console fusing #212 (power docs, unmerged).
• Order SMBJ6.0A (C5331096) for the 5V position (12V stays SMBJ12A); confirm the 12V bulk cap ≥ 25V.
• KiCad touch: swap D2 schematic symbol SMBJ5.0A → SMBJ6.0A (value/MPN/LCSC only, footprint unchanged).
• Receive boards + components → hardware bench verify.
• Docs to land: servo-power standard → hardware-standards.md; 12V load-budget refresh (power.md LED-only table → servos-on-12V ~6–8 A); commit the PDU pour widths.

Split off / related

• Firmware → feat(firmware): PDU monitoring node — telemetry firmware + new CAN telemetry packet class #216 (telemetry node + new CAN telemetry packet class).
• Enclosure — tabled (mount undecided).
• Related: feat(pcb): OpenSkyhawk Design Block library (reusable power/CAN/MCU/harness circuits) #201 (design blocks) · feat(pcb): bus power distribution + power spec (base boards) #203 (power spec/PSU, merged) · docs: docs-drift cross-check → GitHub Projects; power per-console fusing #212 (power docs) · feat(pcb): PDU power-distribution + monitoring board (#202) #217 (PDU PCB) · docs(hardware): input-rail TVS fleet standard (SMBJ12A/SMBJ5.0A) #218 (TVS docs) · feat(pcb): central PSU power-entry / distribution board (24-pin ATX -> 3x console feeds) #215 (optional central PSU entry board) · feat(hardware): standardize input-rail TVS protection across CAN-node boards #214 (input-rail TVS standard, closed).

[mottihoresh]

PSU selected + power spec documented (PR #203)

Source PSU: GOLDEN FIELD NX650 — 650W, 80+ Gold, fully modular, DC-DC, ~$60. Over-provisioned ~13× (cockpit ≈ 50W); chosen for clean modular cabling + expansion headroom. DC-DC topology → no minimum-load resistor needed.

Constraints found (feed the injection + monitoring design)

• Single +12V rail = 54A, and consumer ATX has NO per-pin / per-connector protection — only rail-level OCP (~60A) + dead-short SCP. A 15A fault through a 5A connector burns it long before OCP trips. → Per-lead source-fusing is mandatory — the fuse is the per-connector protection (12V ~5A / 5V ~3–4A, GND unfused). Strengthens the Fusing section above.
• +5V is only on the IDE/SATA cables — PCIe / CPU / 12V-2x6 are +12V-only. The NX650 ships 2× IDE cables (3×SATA + 1×Molex each) → only 2 independent feeds that carry 5V. Impacts multi-zone injection: a 3rd 5V zone needs a 3rd IDE/SATA cable or a custom 6-pin→Mini-Fit lead. Use the Molex ends (~11A) for heavier feeds, SATA (~few A) for light; 12V-only zones (backlight) can use the PCIe cable.
• All ports share the same internal rails (single-rail DC-DC) — the limit is the cable/connector, not a separate rail. Sum of all leads ≪ 54A / 18A.
• Modular cables are brand-specific — ATX 3.1 standardizes only the device-side connectors (SATA/Molex/PCIe/24-pin), not the PSU-side modular sockets. Never reuse another PSU's modular cables.
• Turn-on: jumper PS_ON# → GND (no motherboard).

Soft-power tie-in for this node

The monitoring MCU could run on +5VSB (always-on, 2.5A) and own PS_ON# control — giving a soft power button + AC-present sense + per-zone soft-enable, alongside the INA219 telemetry. Folds power sequencing into the same node.

Documented

docs/_source/base-boards.md → new Source fusing & per-console injection + PSU / source supply sections (PR #203). The segmentable pass-through (jumper / DNP) design-in is reflected there so future boards stay re-injectable with no redesign.

[mottihoresh]

PDU design review — refined scope for the power node

Converged design after external review + load re-analysis. The node is now framed as a PDU (Power Distribution Unit) — a smart per-console power injection node, not a per-device distributor.

Architecture

• ~3 PDUs, one per console: Left Console · Right Console · Main Instrument Panel. Each injects + monitors + protects its console's power locally. (= the per-console injection plan, made concrete.)
• Sits between PSU → console J_BUS (feeds the base-board bus pass-through in base-boards.md). GND + CAN stay continuous across all PDUs.

Rails monitored: 12V + 5V

3.3V is generated locally on each board (AMS1117) → no central 3V3 monitoring.

Current monitoring — INA226 (supersedes the earlier INA219 note)

One INA226 per rail (12V, 5V). Chosen over INA219 for an instrumentation board:

• 16-bit ADC (vs 12-bit); input offset ~±10µV vs ~±100µV → ~10× better low-current accuracy (idle current is the whole point of Rev1).
• Alert pin → hardware over/under-current IRQ to the STM32 (not polling) → serves fault detection.
• Programmable averaging (1–1024) → clean readings of PWM LED + servo loads. 36V headroom.
• Cost delta trivial at our volume (6 units × ~$1 ≈ $6). Bare VSSOP-10 on the PCB, not a module.
• (Alt for a simpler later rev: INA260 = integrated 2mΩ shunt, no Kelvin — but fixed shunt, coarse at low current. Rev1 = INA226 + external shunt.)

Power path + shunt

Power In -> Fuse -> Shunt -> Console Output

• Fuse protects everything downstream.
• Shunt: 10mΩ, 2W, Kelvin (4-terminal) → ±81.92mV / 10mΩ = ~8.19A full-scale, 0.25mA/LSB. Design the footprint so a different shunt value can be swapped later.
• Kelvin sense: INA226 sense lines connect directly to the shunt Kelvin pads via thin dedicated traces — never share high-current copper.
• Fuse-open detection needs voltage sense on BOTH sides of the fuse (INA226 alone reads "0 current" — can't tell fuse-open from no-load).

Temperature — TMP117

One digital temp sensor near (not under) the 12V shunt. Overheat detection + thermal-design validation for Rev1.

Fault detection (firmware, over CAN as a bitmap)

12V/5V UV · 12V/5V OV · overcurrent · fuse-open · sensor failure · CAN failure · temp warning · temp critical.

Copper — 2-layer, 1oz

Large pours (not traces): 12V ~6–10mm, 5V ~5–8mm, GND max + heavily via-stitched top↔bottom. Note the pours are sized for low-R (Kelvin accuracy) + thermal margin at the aggregation point, not current (a console is only ~2–3A). No narrow necks at connector / fuse / shunt / output.

Servo power — 12V→buck, NEVER logic 5V (new standard)

5–6 MG90 servos/console ≈ 2–3A running (up to ~6A stall) if put on 5V → brownout + I²C glitches + blows the 2-feed 5V budget. So:

12V console rail -> local buck (5/6V) -> servos   (+ local bulk cap + polyfuse)

Same power on 12V = ~1–1.5A (buck ~halves rail current) + isolated from logic. Servos become just a 12V load → captured by the 12V INA226; 5V monitoring stays clean. No dedicated servo PDU/output needed.

Revised per-console load budget

Rail	Load	~Current/console
12V	LEDs ~1A + servo-buck ~1–1.5A	~2–3A
5V	logic + OLED only (servos moved off)	~1A

Corrected LED budget: 500 LEDs → 100× 5-series strings → ~3–4A total cockpit-wide (not 20A). Keeping servos off 5V also resolves the "+5V only on 2 IDE/SATA feeds" constraint.

Rev1 = instrumentation platform

Primary purpose is measurement, not optimization. Collect idle / startup / lighting / peak / normal current → let real data drive final fuse sizes, whether more/specialized PDUs are justified, shunt values, and rail separation. Revisit the 3-zone architecture after a few months of data.

PDU outputs (Rev1)

• 12V — monitored + fused
• 5V — monitored + fused
• No dedicated servo output (handled locally via buck).

To do

• Servo-power standard → add to hardware-standards.md (12V→buck, local cap + polyfuse, off-logic-5V).
• PDU built on the PanelGroup base + design-block library (feat(pcb): OpenSkyhawk Design Block library (reusable power/CAN/MCU/harness circuits) #201); assign a NODE_ID.