Why does a charger make a humming sound only at a low load?

Main answer · Wecent Editorial Team

Short answer: A faint hum at light load is common and usually not a fault. At low load many chargers switch in short bursts to save energy, and the repetition rate can fall into the range we can hear. Parts such as the transformer and capacitors can vibrate at that rate. It is a concern only if the hum is loud, changes suddenly or comes with heat, smell or a damaged charger.

What causes the hum

Efficient chargers do not run continuously when the load is small. They send bursts of energy and rest between them. If the burst rate falls into the audible range, the magnetic parts and some capacitors can vibrate and make a faint hum or buzz.

Why only at low load

At high load, the charger switches continuously at a much higher, inaudible frequency. At low load, the controller changes to a burst mode to reduce standby losses, and the pattern can produce sound.

Typical sources

• Transformer. Windings or the core vibrate slightly.
• Ceramic capacitors. Some types change shape with voltage and can sing.
• Loose parts. An unsecured part rattles.
• Poor varnish or gluing. Windings move.

What is acceptable

A faint sound in a quiet room, only when the charger is idle or lightly loaded, is normal for many designs. A loud buzz or a sound that wakes you is a design problem.

When to be careful

1. The sound is loud or rattling.
2. It changes suddenly or gets worse.
3. The charger is hot or smells.
4. The case is cracked or loose.
5. There are sparks or flickering lights.

If any apply, unplug it and stop using it.

What users can do

• Try a different socket or a different device on the charger.
• Place the charger on a soft or damped surface, if it is not covered.
• Replace it if the noise bothers you in a bedroom.

What brands can do

• Specify a noise limit at low load, measured in a quiet room.
• Choose controllers and capacitors with low acoustic noise.
• Use glue or varnish to secure transformer windings.
• Test at several load levels.

Quick summary

A light hum at low load is common. Loud, changing or hot units are not normal.

Where this answer says to confirm with Wecent, the detail is not published on the product page and has to come from the datasheet or the factory. General facts come from the regulators, standards bodies or device makers named in the text.