Input protection: fuse, NTC, varistor, X and Y capacitors

A few parts right behind the inlet protect the supply from mains surges and the mains from the supply's noise. Repairs usually start here.

What does it do? The fuse prevents fire on a short, the NTC limits the big switch-on current, the varistor clamps lightning and sudden surges, and the X/Y capacitors and common-mode choke form the noise filter.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: Normal operation

Current passes through the fuse, the warmed-up (low-resistance) NTC and the filter to the bridge. The X and Y capacitors and the common-mode choke stop the supply's noise escaping into the mains.

1. Mains

230 V AC, ≈311 V peak.

On the scope

Multimeter≈230 V AC

🔍 How to spot it on a board

•A glass or ceramic fuse (sometimes in heat-shrink)
•A black or green disc NTC (e.g. 5D-15)
•A blue or yellow disc varistor (e.g. 14D471)
•A rectangular yellow/grey X2 capacitor and small blue Y capacitors
•A ring or square common-mode choke with two windings

🔧 How to measure it

•Fuse 0–1 Ω; if blackened, there's a big short behind it.
•NTC a few Ω when cold.
•Varistor OL on resistance; look for cracks or burns.
•Continuity on the choke windings.

⚠ Common faults

•Fuse blown: first measure the bridge and MOSFETs for shorts.
•NTC open: the device is completely dead, fuse intact.
•Varistor burst: there was a big surge; check the parts behind it too.

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