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).

---

## 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](https://www.ebay.de/itm/296810265488), [neonixie-l (designer)](https://groups.google.com/g/neonixie-l/c/e8nm73UTEyk/m/e-skGAfqBAAJ), [Elecrow NCH8200HV](https://www.elecrow.com/nch8200hv-nixie-dc-dc-high-voltage-boost-power-converter.html)
- 45–390 V boost module: [AR5388 datasheet](https://uelectronics.com/wp-content/uploads/2026/07/AR5388-Convertidor-Boost-8-32V-a-45-390V-5A-Datasheet.pdf), [Sigmanortec](https://sigmanortec.ro/en/voltage-booster-module-zvs-dc-dc-45-390v-adjustable)
- B1212S-1W: [DigiKey B1212S-1WR3](https://digikey.cn/en/products/detail/mornsun-america-llc/B1212S-1WR3/13968708), [Robu](https://st.robu.in/product/b1212s-1wr3-dc-power-supply-module-1d1212-dc-dc-isolation-module-12v-to-12v1w-isolation-module)
- Mornsun 6 W isolated: [smbom URA2412YMD-6WR3](https://www.smbom.com/product/ura2412ymd-6wr3-3774098), [YMD-6WR3 datasheet](https://cdn.shopify.com/s/files/1/1180/9692/files/URA_YMD-6WR3.pdf)
- Mean Well GST36: [Farnell GST36E12](https://ie.farnell.com/mean-well/gst36e12-p1j/adaptor-ac-dc-12v-3a/dp/2815899), [RS GST36B12](https://de.rs-online.com/web/p/steckernetzteile/1176146)
- Rail voltages: [PinWiki Williams WPC](https://www.pinwiki.com/wiki/index.php?title=Williams_WPC)