Plasma-DMD/docs/safety.md

Safety

About 175 V sits between the +65 V and −112 V rails. Treat every HV node as live until you have measured it.

Design measures

1. Only certified mains equipment. The 12 V brick is the only mains-connected part. Buy a certified brand (e.g. Mean Well GST36A12) from a normal retailer, not AliExpress.

2. Hardware HV watchdog. The relay on the HV modules' 12 V feed is driven through a charge pump:

   GPIO21 ──┤├──── A ──►|── B ───┬───────┬──── 2N7000 gate      2N7000 drain ─► relay IN (active-low)
          100nF    │   1N4148    │       │                      2N7000 source ─► GND
                   ┴ 1N4148     ═ 10µF  ▯ 100k
                   ▲ (anode GND) │       │
                  GND           GND     GND
   

   Node A gets a diode to GND (cathode at A), and a diode from A to B (anode at A). This rectifies the toggling signal into a DC voltage on B. A constant high or low level gives no charge.

   The firmware toggles GPIO21 once per scanned row (about 2 kHz). If it crashes, or the pin sticks high or low, or the ESP32 resets, the cap discharges and the relay drops within about 1 s. A stuck row can't burn the panel. (Check the polarity of your relay module's input. Some are active-low.)

3. Fail-safe defaults. The relay is normally open. Put a 10 kΩ pull-down on the watchdog path. Pull the panel ENABLE line to its off state at the 74HCT245 input, so a booting ESP32 can't light anything.

4. Sequencing. 5 V comes up, then the ESP32 boots and starts scanning, and only then does the watchdog close the relay.

5. Fusing.

   Where	Fuse
   12 V input	3 A
   Each HV module input	1 A slow-blow
   Each raw HV rail	100 mA fast

6. Discharge. 330 kΩ bleeders on raw and regulated rails. HV-present LEDs are a hint only; always measure before you touch anything.

7. Layout.

   • Put all HV parts on one side, with ≥ 3 mm clearance between HV tracks.
   • Use wire and caps rated for ≥ 300 V/500 V.
   • Use a keyed connector to the panel.
   • Close it in a plastic enclosure with no exposed HV.
   • Tie all grounds at one point.

8. Semiconductors from real distributors (Mouser, Farnell, Reichelt). Fake HV transistors are common on AliExpress.

Bring-up procedure

1. Build the 5 V and 12 V side only. Check the voltages.
2. Connect one HV module at a time, with no load. Set NCH6100HV #1 to 90 V and #2 to 130 V. Lock the pots with nail varnish.
3. Connect the regulator section, still without the panel. Check for about +65 V, −112 V and −100 V and that the bleeders discharge them.
4. Test the watchdog: stop toggling GPIO21, and the HV must drop within about 1 s.
5. Map the panel's power connector against the measured rails. Get a second check (photo/label).
6. First power-up with the panel: 1 kΩ in series with each HV rail and a running test pattern. Measure the current per rail, then remove the series resistors.