Transformer Hum - Page 1
Forum section: electronics / general

author: arkufahl
author_rank: Newbie
timestamp: January 14, 2023, 04:09:52 pm
content: I've purchased and used step down transformers (120VAC to 24VDC, for example) and they hum. I've read-up on it and understand why... but why do the transformers I see in phone chargers (for example) not hum? Does it have to do with one being better quality than the other?

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 14, 2023, 04:13:11 pm
content: Modern phone chargers use switching power supplies. Much smaller transformers on much higher frequencies. They too "hum" but you can't hear it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 14, 2023, 05:24:12 pm
content: I think there are two parts to the answer.First the wall wart transformers are much smaller, designed for smaller currents. Even if the same percentage of power is lost to magnetostriction there is less mechanical motion. And that motion is less efficiently coupled to the air. So the hum is there, just below audible threshold.Second, there are very real differences in construction in various transformers. Some of these might be associated with "quality", but others are purely related to the design goals of the transformer. Some transformers are potted with tar to reduce the physical motion. The wall warts have a plastic housing for safety and appearance reasons. This housing attenuates any noise generated by the transformer housing. In any case I own several step down transformers similar to the types you describe that produce no audible hum.Another example which is instructive is the ballast which was used in older fluorescent lights. These were typically filled with tar (and sometimes other fillers) for sound suppression reasons. Over time the tar frequently melts and leaks out and the ballast begins to hum.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 14, 2023, 09:46:17 pm
content: Just placing a guess here, but larger physical cavities for sonic effects to occur in resonate at lower frequencies. Might it be that transformers in phone chargers are small enough to make the hum ferquencies inaudible, while those in things like laptop and computer monitor power supplies are larger so will resonate at lower (audible) frequencies? I certainly hear the hum from monitor power supplies when they are on standby and the room is otherwise silent.

author: Brumby
author_rank: Unknown
timestamp: January 15, 2023, 05:41:45 am
content: Switchmode units - the sort that are small and light - have active electronics and tend to operate at frequencies that are above human hearing range. You see often them in phone chargers and laptop supplies as well as modern TVs, but are used in a wide range of applications.Simple transformers are the ones that you can hear hum.I feel the volume of this hum is dependent on two factors:1. The current flowing through the windings. The greater the current, the greater the magnetic field and, thus the greater the forces acting on anything conductive within that field.2. The physical size of the transformer. The bigger the unit, the more surface area there is to couple to the surrounding air. Bolt it to a chassis and that coupling increases significantly.The fact that you will often find big transformers handling larger currents, means they will more likely to hum more audibly than small ones.This hum can be a useful diagnostic tool. If you hear hum, then you know it has AC power running through it. Sometimes you might have to use a screwdriver as a stethoscope to hear it. Also - once you have heard a few transformers operating - you will get to have a good idea of when a transformer is operating normally and when it is struggling. This can alert you to a problem before the magic smoke has made an appearance.