Varistor (MOV)

A protection part that clamps overvoltage.

Schematic letter: RV / VDR
Unit / key rating: Clamping voltage (V)

Pinout

2 leads: No polarity; connected across the two mains lines.

Real photos

What is it for?

A varistor is almost an insulator at normal voltage. When the voltage passes a threshold (≈470 V for a 14D471) its resistance drops abruptly and it clamps the excess by shorting it.

It sits across the mains at a power supply's input and protects against lightning and surges. In a big surge it sacrifices itself and blows the fuse.

Things to watch out for

⚠ A burst or cracked varistor means the device saw a big surge: check the parts behind it too.
⚠ The first number in its code is the disc diameter (mm), the last three digits the voltage: 471 = 47 × 10 = 470 V.

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