Step 10.13 — Measure the bed heater before you connect it¶
Step 13 of 80 · Chapter 10 — Wiring and Checkpoint #1
Parts:
- multimeter · tool
- the Bed L and Bed N extension leads, far ends still free · already on the bench
Do:
- Meter on Ω. Measure Bed L → Bed N at the free ends of the two extensions.
- Measure Bed L → the aluminium plate and Bed N → the aluminium plate, touching the M4×6 PE screw head.
Check
All three readings must land in these bands.
What you're looking at
You are measuring the heater pad before it can ever be energised. Across its own two leads it should read a few tens of ohms; to the aluminium plate it is bonded to, nothing at all. The photo identifies which lead is which.
Why 'tens of ohms', and how to compute your own target
LDO does not publish the pad's wattage on the Rev D BOM, so derive the number yourself: R = V² ÷ P, using the mains voltage the pad is wound for and the wattage printed on the pad's own label. Worked examples — not your value: 750 W at 120 V → 19.2 Ω; 750 W at 230 V → 70.5 Ω. Read your own label, do the division, and expect the meter to land within a few ohms of it.
| Measurement | Expected |
|---|---|
| Bed L → Bed N | a stable reading in the tens of ohms. Not 0 Ω. Not OL. |
| Bed L → plate / PE | OL on your meter's highest Ω range |
| Bed N → plate / PE | OL on your meter's highest Ω range |
A dead short means the pad is damaged and must not be connected. OL on L → N means an open pad or 125 °C thermal fuse — but first re-seat both leads in their WAGO levers and measure again, because an unseated core reads the same. Either way, do not go on until the number is right.
Source: LDO wiring guide § Wiring the bed heater · LDO Rev D 350 BOM · LDO wiring guide image bed_heater_wires_revd.jpg