The laptop power cable consists of two cables with a transformer in the middle. The transformer is making a tiny noise while plugged in. I think it's normal... is it normal? What is it exactly?


4 Answers 4


It's a transformer.

Not Optimus Prime, but a boring device that steps down mains voltage to the voltage your laptop requires.

A slight hum is normal.

  • +1 for the Optimus Prime reference. Yes, I'm just that shallow.
    – Chris_K
    Feb 4, 2010 at 19:54
  • Fantastic. Also, I am glad my laptop transformer is not like the one in the wikipedia picture... :p
    – Svish
    Feb 4, 2010 at 19:56
  • 4
    A transformer steps down mains voltage not current, they're not boring at all.
    – pavium
    Feb 4, 2010 at 20:02
  • Gah yeah, I wrote "steps up current" then thought it would be more sensible to think of it as going down to laptop, but only changed the "down" ... :|. (It's still correct to say it steps up current, yes? V=IR and all that. Physics lessons were a long time ago :) )
    – RJFalconer
    Feb 4, 2010 at 20:50
  • @Pavium - sorry mate, yes they are... ;)
    – CJM
    Nov 14, 2010 at 0:44

A hum is normal. If you hear a crackling or sparking noise, the transformer may be about to die, and may constitute a fire hazard.

The hum comes about because the transformer works by running the alternating wall current through some loops of wire to make magnetic fields, and running the fields through other loops of wire to reconstitute the electric fields at a different voltage..

A side effect of this is a set of periodically alternating mechanical stresses on the core of the transformer. Stresses mean motion, and alternating motion in the right band means audible vibration in the air.

  • 1
    "If you hear a crackling or sparking noise" >> I hear such noise when I move around the mouse or scroll. oO .. But not that sparking.. just like a high frequency noise. Should I worry?
    – Apache
    Feb 4, 2010 at 19:57
  • 3
    @Fujishiro: You have transformer noise that is correlated with mouse actions? This seems very unusual, and I have no advice to offer. Are you sure that mouse correlated noises are coming from the transformer and not from the laptop case? Feb 4, 2010 at 20:09
  • I had an PowerBook power brick die (with smoke and flames!) in this very manner. In that case, the sparking sound was unrelated to user actions, and it was only a matter of seconds between when they started and when it all went kablooey.
    – ephemient
    Nov 13, 2010 at 23:36

Laptop chargers these days might be switching-mode power supplies with a transformer which operates at a much higher frequency than the 50/60Hz mains, because transformers work better at higher frequencies, and can be made smaller and lighter.

Is the noise high or low pitched?


The thing in the middle of your laptop charging cord is known as a transformer or AC adapter.

The hum that it makes while plugged in to the wall outlet is called mains hum and is to be expected.

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