The transformer is the boring part. Yes, I rewound the secondary — a few turns of fat cable, hundreds of amps at a volt or two. Every spot welder build does that.

What caught my eye was the control board on its way to the bin. A Samsung microwave mainboard is a little marvel of cost-engineered safety design:

• a zero-cross detector (PC817 opto straight off mains L)
• a relay arming circuit that deserves its own paragraph
• the display, encoder-friendly button inputs, a piezo, relays
• a Toshiba TMP89FM42 — an 8-bit TLCS-870/C1 with 32 KB flash

The arming circuit is the part I fell in love with. The power relay isn't just switched by an MCU pin. Its driver chain is powered from a 1 µF capacitor, and that capacitor is charged through a PNP transistor that is AC-coupled to the MCU pin — a 4.7 nF cap in series with the base. A stuck-high or stuck-low pin does nothing. Only a continuously toggling pin keeps the chain alive. If the firmware crashes, the relay physically cannot stay closed. That's the kind of hardware you want between a microcontroller and a transformer that can weld steel.

Plan: erase the original firmware, write my own controller — weld pulse timing via the display and encoder, welds synchronized to mains zero crossings, all the safety behavior preserved.

Spoiler: the chip had other plans.