6.1 KiB
| tags | project | author | date | status | |||||
|---|---|---|---|---|---|---|---|---|---|
|
Project Drogheda | Helgen CTO | 2026-09-09 | Draft — first-pass estimate |
5 V source: on-board (V6 / U25) vs external Mean Well SD-15B-05 (V5)
Bottom line: the on-board 24→5 V block already captured in V6 costs ≈ US$2.2 / MX$37 per board in parts. The external Mean Well it replaces costs ≈ MX$330 per unit all-in as currently purchased. Net saving ≈ MX$290/unit (≈ MX$8,700 at 30 units, ≈ MX$29,000 at 100). The money is not the interesting part — the isolation loss and the 3 A → 3.5 A headroom are.
Prices: LCSC/JLCPCB live database (2026-09-09) · FX USD 1 = MXN 16.91.
1. What each version actually does
| V5 (as built) | V6 (as captured in the schematic) | |
|---|---|---|
| 24→5 V | External Mean Well SD-15B-05 → P2 (JST 2-pin) → VEH5V |
On-board U25 TPS54360B: VIN_PROT → VEH5V |
| Rating | 5 V / 3 A isolated (1.5 kVDC) | 5 V / 3.5 A, non-isolated, 60 V input |
| Entry chain | Q10 (IRFR5305) + D10 (SMBJ33A) → fan buck only |
F2 → Q23 → D32 (SMCJ33A) → C149/C150 → U25 → D34/L22 → C153/C154 |
| Extra parts on board | P2 connector |
15 fitted + 2 DNP (R124/R125) |
V6 keeps the fan buck (U2) and the 3V3_ETH buck (U12); U7 was deleted. Neither
ever made the main rail — this comparison is only about the VEH5V source.
2. On-board block — BOM cost per board
U25 TPS54360B · L22 SRP1265A-100M · D34 B560C · D32 SMCJ33A · C149/C150
2.2 µF/100 V · C151 100 µF/50 V · C152 100 nF · C153/C154 47 µF/16 V ·
C155/C156 · R120–R123 — plus the entry chain F2 · Q23 · D33 · R119.
| Line | 30 boards | 100 boards | 500 boards |
|---|---|---|---|
U25 TPS54360BDDAR (C524806) |
$0.487 | $0.428 | $0.419 |
L22 SRP1265A-100M 10 µH/10 A (C840531) |
$0.639 | $0.554 | $0.504 |
D34 B560C 5 A/60 V (C85100) |
$0.220 | $0.179 | $0.158 |
D32 SMCJ33A TVS (C699039) |
$0.114 | $0.091 | $0.079 |
C149/C150 2.2 µF/100 V 1210 ×2 † |
$0.180 | $0.180 | $0.160 |
C151 100 µF/50 V elco (C134502) |
$0.149 | $0.149 | $0.149 |
C153/C154 47 µF/16 V 1210 ×2 (C5440143) |
$0.185 | $0.185 | $0.127 |
C152, C155, C156, R120–R123 |
$0.022 | $0.022 | $0.022 |
| Buck subtotal | $1.996 | $1.788 | $1.618 |
Q23 IRFR5305 (C2624) |
$0.262 | $0.227 | $0.193 |
F2 2 A/1812 fuse †, D33 BZX84C15, R119 |
$0.144 | $0.144 | $0.144 |
| Entry subtotal | $0.406 | $0.371 | $0.337 |
| Parts / board | $2.40 | $2.16 | $1.96 |
| JLC extended-part setup (7 new uniques × $3, amortised) | $0.70 | $0.21 | $0.04 |
| SMT placement (~50 joints) | $0.10 | $0.09 | $0.09 |
| All-in / board | $3.20 · MX$54 | $2.46 · MX$42 | $2.09 · MX$35 |
† C149/C150 and F2 have no LCSC part pinned yet — estimated at $0.09 and $0.12.
Board outline stays 140×100, so the block costs zero PCB area money.
3. External Mean Well — cost per unit
| Item | MXN | Source |
|---|---|---|
| Mean Well SD-15B-05 | 300 | BOM-master.md §1 item 12 (as purchased locally) |
| — same part, direct import | 189–200 | Mouser $11.15 / Arrow $11.85 @100 |
| DC PSU bracket (ASA print) | 20 | BOM-master.md §1 item 3 |
P2 JST-XH + housing + crimps + wire |
~12 | est. |
| Total | ≈ 332 (≈ 221 if imported direct) |
Plus ~5 min/unit of mounting and wiring labour, and one more connector in the
vibration path — P2 is exactly the class of joint the RCA blamed.
4. Net
| Volume | External (local price) | On-board | Saving |
|---|---|---|---|
| per unit | MX$332 | MX$42 | MX$290 |
| 30 units | MX$9,960 | MX$1,260 | MX$8,700 |
| 100 units | MX$33,200 | MX$4,200 | MX$29,000 |
Against best-case direct import (MX$221) the saving is still ≈ MX$180/unit.
5. What actually decides this (not the money)
For on-board:
- 3 A → 3.5 A. The 10-Ago boot-loop was the SD-15B's 3 A ceiling. It also makes the
standing
RS1errata (3.12 A input limit sitting above a 3 A supply) go away by itself. - One fewer sub-assembly, one fewer connector, no import lead time on a PSU.
Against on-board:
- Isolation is lost. SD-15B-05 is 1.5 kVDC isolated; TPS54360B is not. HBOX ground becomes the cabinet 24 V return. The ignition already shares that return — worth a deliberate look at the ground topology before this is locked.
- The EMC burden moves onto our board. The copper plan already calls
U25's input loop the loudest on the PCB; that stops being Mean Well's problem. - No field-replaceable part. Today a dead PSU is a 10-minute swap; after this it is a board.
6. 🔴 Side finding — the MP1584s cost more than this whole question
Genuine MP1584EN-LF-Z (C15051) is $2.21 @100 on LCSC. V6 carries two (U2, U12)
= $4.41 / MX$75 per board, against $0.43 for the TPS54360B or $0.58 for the
AP64350 already qualified as U20.
Re-speccing those two rails onto a part already in the BOM saves ≈ $3.5 / MX$59 per board — more than the entire 5 V-source saving — and it also retires EMC findings 3 and 4, which named the MP1584s directly. Worth a separate decision.
7. Battery — LiPo vs Li-ion, first pass
| Energy | Price | MXN / Wh | |
|---|---|---|---|
| Li-ion Hyvel 18650 1S4P (current pack) | ~43 Wh | MX$204 | ≈ 4.7 |
| LiPo 2000 mAh 1S, SparkFun @ Sandorobotics | 7.4 Wh | MX$270 | ≈ 36 |
| LiPo 2000 mAh 1S, generic (indicative) | 7.4 Wh | ~MX$90–160 | ≈ 12–22 |
To reach today's 43 Wh with 2000 mAh pouches you need 6 in parallel: MX$540–1,620 against MX$204. Li-ion 18650 is 3–8× cheaper per Wh, and it is also the better cell for a box that runs hot — pouch cells swell, and heat is this product's standing failure mode.
LiPo buys thickness and form factor, nothing else. Treat the 2000 mAh cells as bench hardware. ⚠ The generic LiPo row is indicative — MercadoLibre/Amazon MX would not serve prices to an automated fetch; get one real quote before this line is used in a decision.