I wanted to design a custom LED light bulb, so I bought a couple and tore them down in the hope to reverse engineer them.
Most of them implement a circuit with a chip shown as below, with the circuit the green area implemented on the PCB, with 1 additional 20 Ohm resistor in heathrink, shown in red, making the connection between the bulb base and the PCB.
Is this a current limiting resistor? How come there is no fuse and no varistor? It's a bit surprising because even bulbs from reputable brands (like OSRAM) don't implement fuses or varistors. Shall I add one on my custom circuit? Would it be before or after the resitor?