# 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.