Leopoldo Rodríguez
d70dd806be
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01QCy5VHMVLkAmftYM6swWmx
142 lines
6.7 KiB
Markdown
142 lines
6.7 KiB
Markdown
---
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tags: [project-drogheda, hardware, bom, cost, power]
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project: Project Drogheda
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author: Helgen CTO
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date: 2026-09-09
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status: Draft — first-pass estimate
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---
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# 5 V source: on-board (V6 / U25) vs external Mean Well SD-15B-05 (V5)
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!!! warning "Designators from before 22-Oct (T43)"
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This document keeps the designators the board had on its date. Since 22-Oct (T43) the V5.6, V5.7 and v6 carry new ones, and several old names now belong to another part: on the V5.6/V5.7 the LTC4040 is U14 (was U6); on the v6, U6 is the modem and the LTC4040 is U17. To read it against today's board, use the [old → new table](https://drogheda.helgen.online/hardware/designadores-t43-22oct/#8-tablas-viejo-nuevo) (in Spanish); parts already gone by 22-Oct are not in it.
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> **Bottom line:** the on-board 24→5 V block already captured in V6 costs
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> **≈ US$2.2 / MX$37 per board** in parts. The external Mean Well it replaces costs
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> **≈ MX$330 per unit** all-in as currently purchased. **Net saving ≈ MX$290/unit**
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> (≈ MX$8,700 at 30 units, ≈ MX$29,000 at 100). The money is not the interesting part —
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> the isolation loss and the 3 A → 3.5 A headroom are.
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Prices: LCSC/JLCPCB live database (2026-09-09) · FX **USD 1 = MXN 16.91**.
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---
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## 1. What each version actually does
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| | **V5 (as built)** | **V6 (as captured in the schematic)** |
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|---|---|---|
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| 24→5 V | **External** Mean Well SD-15B-05 → `P2` (JST 2-pin) → `VEH5V` | **On-board** `U25` TPS54360B: `VIN_PROT` → `VEH5V` |
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| Rating | 5 V / **3 A** isolated (1.5 kVDC) | 5 V / **3.5 A**, non-isolated, 60 V input |
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| Entry chain | `Q10` (IRFR5305) + `D10` (SMBJ33A) → fan buck only | `F2 → Q23 → D32 (SMCJ33A) → C149/C150 → U25 → D34/L22 → C153/C154` |
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| Extra parts on board | `P2` connector | 15 fitted + 2 DNP (`R124`/`R125`) |
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V6 keeps the fan buck (`U2`) and the 3V3_ETH buck (`U12`); `U7` was deleted. Neither
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ever made the main rail — this comparison is only about the `VEH5V` source.
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---
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## 2. On-board block — BOM cost per board
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`U25` TPS54360B · `L22` SRP1265A-100M · `D34` B560C · `D32` SMCJ33A · `C149`/`C150`
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2.2 µF/100 V · `C151` 100 µF/50 V · `C152` 100 nF · `C153`/`C154` 47 µF/16 V ·
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`C155`/`C156` · `R120`–`R123` — plus the entry chain `F2` · `Q23` · `D33` · `R119`.
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| Line | 30 boards | 100 boards | 500 boards |
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|---|---:|---:|---:|
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| `U25` TPS54360BDDAR (C524806) | $0.487 | $0.428 | $0.419 |
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| `L22` SRP1265A-100M 10 µH/10 A (C840531) | $0.639 | $0.554 | $0.504 |
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| `D34` B560C 5 A/60 V (C85100) | $0.220 | $0.179 | $0.158 |
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| `D32` SMCJ33A TVS (C699039) | $0.114 | $0.091 | $0.079 |
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| `C149`/`C150` 2.2 µF/100 V 1210 ×2 † | $0.180 | $0.180 | $0.160 |
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| `C151` 100 µF/50 V elco (C134502) | $0.149 | $0.149 | $0.149 |
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| `C153`/`C154` 47 µF/16 V 1210 ×2 (C5440143) | $0.185 | $0.185 | $0.127 |
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| `C152`, `C155`, `C156`, `R120`–`R123` | $0.022 | $0.022 | $0.022 |
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| **Buck subtotal** | **$1.996** | **$1.788** | **$1.618** |
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| `Q23` IRFR5305 (C2624) | $0.262 | $0.227 | $0.193 |
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| `F2` 2 A/1812 fuse †, `D33` BZX84C15, `R119` | $0.144 | $0.144 | $0.144 |
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| **Entry subtotal** | **$0.406** | **$0.371** | **$0.337** |
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| **Parts / board** | **$2.40** | **$2.16** | **$1.96** |
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| JLC extended-part setup (7 new uniques × $3, amortised) | $0.70 | $0.21 | $0.04 |
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| SMT placement (~50 joints) | $0.10 | $0.09 | $0.09 |
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| **All-in / board** | **$3.20 · MX$54** | **$2.46 · MX$42** | **$2.09 · MX$35** |
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† `C149`/`C150` and `F2` have no LCSC part pinned yet — estimated at $0.09 and $0.12.
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Board outline stays 140×100, so **the block costs zero PCB area money**.
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---
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## 3. External Mean Well — cost per unit
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| Item | MXN | Source |
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|---|---:|---|
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| Mean Well SD-15B-05 | **300** | `BOM-master.md` §1 item 12 (as purchased locally) |
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| — same part, direct import | *189–200* | Mouser $11.15 / Arrow $11.85 @100 |
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| DC PSU bracket (ASA print) | 20 | `BOM-master.md` §1 item 3 |
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| `P2` JST-XH + housing + crimps + wire | ~12 | est. |
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| **Total** | **≈ 332** *(≈ 221 if imported direct)* | |
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Plus ~5 min/unit of mounting and wiring labour, and one more connector in the
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vibration path — `P2` is exactly the class of joint the RCA blamed.
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---
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## 4. Net
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| Volume | External (local price) | On-board | **Saving** |
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|---|---:|---:|---:|
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| per unit | MX$332 | MX$42 | **MX$290** |
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| 30 units | MX$9,960 | MX$1,260 | **MX$8,700** |
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| 100 units | MX$33,200 | MX$4,200 | **MX$29,000** |
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Against best-case direct import (MX$221) the saving is still **≈ MX$180/unit**.
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---
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## 5. What actually decides this (not the money)
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**For on-board:**
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- **3 A → 3.5 A.** The 10-Ago boot-loop was the SD-15B's 3 A ceiling. It also makes the
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standing `RS1` errata (3.12 A input limit sitting *above* a 3 A supply) go away by itself.
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- One fewer sub-assembly, one fewer connector, no import lead time on a PSU.
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**Against on-board:**
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- **Isolation is lost.** SD-15B-05 is 1.5 kVDC isolated; TPS54360B is not. HBOX ground
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becomes the cabinet 24 V return. The ignition already shares that return — worth a
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deliberate look at the ground topology before this is locked.
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- The EMC burden moves onto our board. The copper plan already calls `U25`'s input loop
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the loudest on the PCB; that stops being Mean Well's problem.
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- No field-replaceable part. Today a dead PSU is a 10-minute swap; after this it is a board.
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---
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## 6. 🔴 Side finding — the MP1584s cost more than this whole question
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Genuine `MP1584EN-LF-Z` (C15051) is **$2.21 @100** on LCSC. V6 carries two (`U2`, `U12`)
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= **$4.41 / MX$75 per board**, against **$0.43** for the TPS54360B or **$0.58** for the
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AP64350 already qualified as `U20`.
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Re-speccing those two rails onto a part already in the BOM saves **≈ $3.5 / MX$59 per
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board** — more than the entire 5 V-source saving — and it also retires EMC findings 3 and 4,
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which named the MP1584s directly. Worth a separate decision.
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---
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## 7. Battery — LiPo vs Li-ion, first pass
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| | Energy | Price | **MXN / Wh** |
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|---|---:|---:|---:|
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| **Li-ion** Hyvel 18650 1S4P (current pack) | ~43 Wh | MX$204 | **≈ 4.7** |
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| **LiPo** 2000 mAh 1S, SparkFun @ Sandorobotics | 7.4 Wh | MX$270 | **≈ 36** |
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| **LiPo** 2000 mAh 1S, generic (indicative) | 7.4 Wh | ~MX$90–160 | **≈ 12–22** |
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To reach today's 43 Wh with 2000 mAh pouches you need **6 in parallel**:
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MX$540–1,620 against MX$204. Li-ion 18650 is **3–8× cheaper per Wh**, and it is also the
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better cell for a box that runs hot — pouch cells swell, and heat is this product's
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standing failure mode.
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LiPo buys thickness and form factor, nothing else. Treat the 2000 mAh cells as bench
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hardware. ⚠ The generic LiPo row is indicative — MercadoLibre/Amazon MX would not serve
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prices to an automated fetch; get one real quote before this line is used in a decision.
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