A mains-powered board¶
This lesson builds a zero-cross detector: a board that plugs into 230 V mains and gives a microcontroller a pulse each time the mains voltage crosses zero, plus 5 V to run it. Along the way you will see the checks WholeBoard applies to anything that touches mains.
Danger
Mains can kill. WholeBoard's checks are an aid, not a certification. Do not build or power a mains board unless you know how to do it safely, have the design reviewed, and put it in an enclosure that keeps fingers out.
1. Ask for it¶
A zero-cross detector for 230 V mains: screw terminals in, a fuse, a varistor, an HLK-PM01 for 5 V and an optocoupler giving a zero-cross pulse. A 3-pin header out with 5 V, ZC and GND.
2. What the designer declares¶
Because the board touches mains, the design has a mains section naming the nets that carry the full mains
voltage:
"mains": {"nets": ["AC_L", "AC_N", "AC_LF"], "volts": 230}
AC_LF is the live after the fuse. isolated defaults to true: the low-voltage side (the header) is meant to be
safe to touch, so it must be separated from the mains side by reinforced insulation.
If the designer forgets this section but uses nets like AC_L together with mains parts (a fuse holder, a
varistor, an AC-DC module), the build is refused until it is added.
3. The design check¶
Before anything is drawn, WholeBoard checks:
- A fuse on the mains input (
F1, the 5×20 mm holder). - An isolation barrier: something the mains side cannot reach through. Here there are two: the HLK-PM01
supply (
PS1) and the optocoupler (U1). Everything reachable from the mains nets through other parts (the zero-cross resistorsR1andR2, the optocoupler's LED) is the mains side.
4. Spacing¶
At 230 V, overvoltage category II:
| Between | Rule | Distance |
|---|---|---|
| Mains nets (live and neutral, before and after the fuse) | IPC-2221 B2, outer layers | 2.5 mm |
| The mains side and everything else | IEC 62368-1 reinforced clearance | 3.0 mm |
| The mains side and everything else, along the surface | Reinforced creepage | 5.0 mm |
These go into the board's net classes and .kicad_dru, so TraceMaker routes to them and KiCad's DRC checks them.
The Mains requirements page has the figures for other voltages.
5. The slot¶
The LTV-814S optocoupler is in an SO-4 package. Its input pins (mains side) and output pins (low-voltage side) are about 4.5 mm apart along the board: short of the 5 mm creepage. WholeBoard cuts a 1 mm milled slot under the part, between its two rows of pins, and the log says so. Creepage is measured along the surface, so the path now goes round the slot: 6.4 mm.
6. After DRC: the mains check¶
After the copper pour and DRC, WholeBoard measures every pair of items across the barrier itself (pads with their real shapes, tracks, vias, zones), including the path round the slot. The Board checks card lists each pair with its clearance and creepage against the requirement. Any failure stops the build and goes back to the designer.
7. Before you order¶
- Check the fuse rating and the varistor's voltage against your mains.
- Keep the board in an enclosure; the enclosure step can add the openings for the terminal block.
- Have someone qualified review it.
See Mains isolation for how the mains side is found and measured.