Plasma-DMD/docs/design-review.md

Design review and cost estimate

Review date 2026-10-24. Covers the proposed module-only PSU (rev 0.2), compared with the routed rev 0.1 board, which used linear regulators copied from the Stern supply.

Verdict

The module-only design is a better fit for this project than rev 0.1:

• No linear losses, no transistors, no heatsinks.
• About 20 fewer custom PCB parts.
• A board roughly half the size.
• It also makes the −100 V / −112 V relationship more robust (see §3).

It is not ready to build yet. Four must-fix items (§8) block it: the unknown panel pinout, a weak point in the watchdog drive, TVS clamp levels that depend on the panel's ratings, and buying genuine modules.

Estimated cost: about €90–190, typically about €120 (§9).

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1. Scope and assumptions

• Panel: a 128×32 plasma DMD from a WPC game. Make, part number and pinouts are still unknown.
• Rails needed (PinWiki / Stern supply): about +62–65 V, −112 V, and −100 V. The −100 V rail is a fixed 12 V offset from −112 V. Plus +5 V logic, and +12 V if the panel uses it.
• Currents are unknown. The design is sized against the fuse limits. Real currents get measured at bring-up.
• Use: a hobby device in a closed enclosure. It is not a certified product.

2. Architecture (rev 0.2, module-only)

12 V brick (certified) ─F1 3A─┬─ TVS ─ Mini560 ──────────────── +5 V ─PTC─ TVS ─► ESP32, 74HCT245, panel logic
                              ├──────────────────────────────── +12 V ─JP1─► panel (if used)
                              └─ relay (watchdog) ─ +12V_HV ─┬─F2─ 45–390 V boost (65 V) ─F4─ RC ─ TVS ─► +65 V
                                                             └─F3─ iso 12→12 V ─ NCH6100HV (112 V, + to GND)
                                                                     ─F5─ RC ─ TVS ─► −112 V
                                                                     └─ B1212S stacked on −112 V ─ min-load ─► −100 V

3. Rail-by-rail review

Rail	Source	Regulation	Protection	Findings
+5 V	Mini560, fixed 5 V	Good (buck, fixed)	Add a PTC (about 1.5 A) and an SMBJ5.0A	Without the TVS, an HV-to-5V fault reaches the ESP32 and USB
+12 V	Brick, via F1	Brick	F1 3 A, P6KE15A	OK
+65 V	45–390 V boost board (8–32 V in, ≤0.2 A, 40 W)	Regulated, but uses burst mode at light load (jumpy ripple)	F4 100 mA, TVS clamp at about 72–78 V, RC filter, bleeder	Module is oversized but the most common one that reaches 65 V. Bench-test the ripple and the no-load voltage before connecting the panel.
−112 V	Isolated 12→12 V (about 6 W) + NCH6100HV at 112 V, floating (+ to GND)	NCH6100HV regulates	F5 100 mA, TVS clamp (P6KE130A class, check against the panel maximum), RC, bleeder	The 6 W isolated converter limits this rail to about 40–45 mA. Check against the measured current.
−100 V	B1212S-1W stacked: − on −112 V, + gives −100 V	Unregulated, about ±10% → offset 10.8–13.2 V	15 V zener across it, minimum-load resistor (about 10% load)	Advantage: the 12 V offset holds during start-up and shutdown too, because both rails move together. Rev 0.1 couldn't guarantee that. Consider a regulated 1–2 W isolated module if the offset tolerance matters.

Current direction checked: in the original circuit, Q6 sourced current into the −100 V node from GND, and the panel returns it at −112 V. The B1212S sources current from its + output into −100 V, and it returns to its − output at −112 V. Same direction, so the B1212S is never back-driven.

4. Safety review

Under IEC 62368-1 (AV/ICT equipment), anything above 120 V DC is class ES3, the highest-hazard class. This design has −112 V, and up to about 180 V between +65 V and −112 V.

#	Topic	Status
4.1	Mains isolation	OK. Provided by the certified brick (Class II).
4.2	HV and LV share one ground (same as the original pinball supplies)	Only the 3 mm spacing (basic insulation, about 2.5 mm needed at 250 V, pollution degree 2) protects the 5 V side. Needs: the TVS on +5 V (a fault blows F4/F5), and no USB connected while HV is on, or a USB isolator.
4.3	Exposed HV	The HV modules (including the floating isolated converter output and the NCH6100HV at −112 V) and the panel's own driver boards are live. Needs a tool-only enclosure with HV labels.
4.4	Fault current	Each HV rail has a 100 mA fast fuse, and the modules limit current. A TVS clamp plus fuse turns a runaway module into a blown fuse, not a damaged panel.
4.5	Stored charge	Bleeders discharge the rails to below 60 V within about 2 s.
4.6	Fail-safe HV enable	The relay is normally open and driven by the watchdog. But see 6.1: the drive is weak.
4.7	Live heatsinks	Gone in rev 0.2 (no linear regulators).
4.8	Counterfeit modules	The NCH6100HV designer warns that the fakes are poor quality and a fire hazard. Buy from an authorized seller.

5. Thermal and efficiency

	rev 0.1 (linear)	rev 0.2 (modules)
Regulator losses at 20 mA per rail	about 0.5 W (Q3) + 0.4 W (Q5) + up to 2 W (Q6/R13)	0 (only about 20 mW in the RC filters)
Worst case (fuse-limited)	about 10 W of heat, plus 3 heatsinks	Only module losses
Total heat, typical	about 4–6 W	about 1–3 W (boost modules at about 80–88% efficiency, isolated chain at about 70%)
Heatsinks / active cooling	3 heatsinks, no fan	None needed. Ventilation slots are enough.

Power budget, typical: about 3–5 W for the panel HV, about 1–2 W for panel logic and the ESP32, plus losses. That's about 8–12 W from the brick (≈ 1 A). The 36 W / 3 A brick has plenty of margin.

6. Control and firmware

1. The watchdog gate drive is marginal (must fix).
   • The charge pump runs from the ESP32's 3.3 V output. After two diode drops it gives about 2.1–2.3 V at the 2N7000 gate.
   • The 2N7000's turn-on threshold ranges from 0.8 to 3.0 V. Some parts will never switch the relay module (about 5–10 mA).
   • That fails safe (the HV stays off), but it isn't reliable.
   • Fix (no extra parts): feed the charge pump from a spare 74HCT245 channel, which swings 5 V. That gives about 3.8 V at the gate.
   • Alternative: replace the 2N7000 with a 2N3904/BC547 and a 10 kΩ base resistor.
2. The watchdog drop-out is about 1 s (C4 × R1). Using C4 = 1 µF gives about 0.1 s, which is less time with a stuck row on the panel.
3. GPIO16/17 are free on ESP32-WROOM modules, but are used for PSRAM on WROVER modules. Check which module your Wemos board has.
4. The firmware scans in a busy loop for now. Wi-Fi interrupts will cause jitter and flicker. Later, move the scanning to a high-priority task, or to I2S/DMA output.
5. The panel signal polarities (enable, latch, row data) are still unverified.

7. PCB and mechanics

• rev 0.1 (170 × 125 mm, routed, DRC clean) is obsolete if rev 0.2 is adopted.
• rev 0.2 estimate: about 100 × 80 mm.
  • On the board: terminal blocks, 5 fuses, TVS clamps, RC filters, bleeders, the watchdog, the 74HCT245, the B1212S (SIP-4) and the panel connector.
  • Off the board: the Mini560, the boost modules, the isolated converter and the relay module.
• Keep the 3 mm HV/LV spacing, the DRC rules and the routing scripts. They carry over.
• Mount the modules on standoffs inside the enclosure, keep the HV wires short, and give each module its own fuse.

8. Findings

ID	Priority	Finding	Action
M1	Must	The panel make, power and data pinouts are unknown. J2 follows the Stern CN2 order.	Photograph the label and connectors and map every pin. Wrong wiring can destroy the panel.
M2	Must	The watchdog gate drive is marginal (6.1).	Drive the charge pump from the 74HCT245 (5 V), or use an NPN transistor.
M3	Must	The TVS clamp levels depend on the panel's maximum voltages.	Choose them once the panel is identified. Clamp above the setpoint, below the panel maximum.
M4	Must	Counterfeit NCH6100HV modules are a fire hazard.	Buy from an authorized seller.
S1	Should	+5 V has no fuse or clamp.	Add a PTC of about 1.5 A and an SMBJ5.0A.
S2	Should	Shared ground with USB.	No USB while HV is on, or use a USB isolator.
S3	Should	Exposed HV on the modules and the panel.	Tool-only, vented enclosure with HV labels.
S4	Should	The B1212S is unregulated.	Fit a minimum-load resistor and a 15 V zener, or use a regulated isolated module.
S5	Should	Module quality varies (burst mode, no-load overshoot).	Bench-test each module on a dummy load before connecting the panel.
S6	Should	The 6 W isolated converter limits −112 V to about 40–45 mA.	Measure the panel current. If it's higher, use a 10 W converter.
N1	Nice	1 s watchdog drop-out.	C4 = 1 µF for about 0.1 s.
N2	Nice	GPIO16/17 on WROVER modules.	Check the module type.
N3	Nice	Scan jitter with Wi-Fi.	Scan from a high-priority task or I2S/DMA.
N4	Nice	RC filter values are a guess.	Tune after measuring with a scope.
N5	Nice	Extra margin on the HV side.	Conformal coating.

9. Cost estimate (rev 0.2)

Prices are rough, as of October 2026, in euros including VAT, for a single unit. AliExpress items vary.

Modules and power

Item	Qty	Estimate
12 V 3 A brick, certified (Mean Well GST36E12 / GST36B12)	1	€17–25
Mini560 5 V buck	1	€1–2
45–390 V adjustable boost module (+65 V)	1	€6–12
NCH6100HV, genuine (−112 V)	1	€8–20
Isolated DC-DC 12→12 V, 6 W, regulated (Mornsun YMD-6WR3 class)	1	€7–10
B1212S-1W isolated (−100 V offset)	1	€1–3 (€4–6 for a regulated type)
Relay module, 5 V, opto	1	€1–2
Wemos D1 mini ESP32	1	€4–8
Subtotal		€45–84

Board parts (Mouser / Farnell / TME / LCSC)

Item	Estimate
5 × 5×20 mm fuse holder + fuses (3 A T, 2 × 1 A T, 2 × 100 mA F) + spares	€5–8
TVS: P6KE15A, SMBJ5.0A, HV clamps × 2, 15 V zener	€2–3
PTC 1.5 A	€0.5
RC filters: 47 Ω 1 W + 22 µF/250 V, × 2–3	€3–4
Bleeders, minimum load, 2 LEDs	€1–2
Watchdog parts (diodes, caps, transistor, resistors)	€1
74HCT245 + socket + decoupling	€1–2
Terminal blocks × 7–8 (clone vs Phoenix)	€4–10
Molex KK-396 8-pin header + housing + crimps	€2–4
Header, jumper, standoffs, screws	€2
Subtotal	€22–37

Build

Item	Estimate
PCB, 5 pcs, 2-layer, about 100 × 80 mm (JLCPCB/PCBWay) + shipping	€10–25
Vented plastic enclosure	€8–15
Wire, ferrules, heat-shrink, 14-pin IDC ribbon for the panel data	€5–10
Shipping / small-order fees (distributors, AliExpress)	€0–20
Subtotal	€23–70

Total

	Low	Typical	High
rev 0.2, module-only	≈ €90	≈ €120	≈ €190
rev 0.1, linear (for comparison)	+€15–25 for transistors, zeners, 3 heatsinks, 5 W resistor, plus about €5–15 for the larger PCB
Original route (replacement DMD PSU board)	€50–170 for the board, plus an AC transformer and mains wiring

10. Next steps

1. Identify the panel (M1). That unblocks M3 and the J2 pinout.
2. Order the modules (genuine NCH6100HV, M4). Bench-test each one on a dummy load (S5).
3. Update the schematic, PCB, BOM and docs to rev 0.2, including M2, S1 and S4.
4. First power-up: series resistors and current measurements (S6). Then tune the RC filters (N4).

Sources

• NCH6100HV price and counterfeit warning: eBay listing, neonixie-l (designer), Elecrow NCH8200HV
• 45–390 V boost module: AR5388 datasheet, Sigmanortec
• B1212S-1W: DigiKey B1212S-1WR3, Robu
• Mornsun 6 W isolated: smbom URA2412YMD-6WR3, YMD-6WR3 datasheet
• Mean Well GST36: Farnell GST36E12, RS GST36B12
• Rail voltages: PinWiki Williams WPC