I've been reading about the fact that a bit of DC on the mains can cause toroidal transformers to hum more than normal. So I decided to try to measure it I've got no toroidal transformers I'm just interested to try to measure it. Any how I ran a 150 watt bulb via a half way rectifier and got around 480 millivolts DC from neutral to earth and around 10 millivolts live to earth and live to neutral I then reversed the diode but still the voltage was only from neutral to earth why would this be? Incidentally with the diode the other way round the earth was 480 millivolts POSITIVE to neutral. The way I measured it was with my best quality meter ( the true RMS 6000 count one) switched to DC Volts range. My other meters couldn't make sense of anything when connected to the test terminals on DC range with AC mains present. Why could I only measure the offset on the neutral side? Surely it shoul be present on both sides.
Some good answers below, with regard to the hum, I have never know a significant hum on a toroidial, that's mainly because hum comes form things vibrating, and toroidials are generally made out of solid ferrite material, so they don't vibrate much.
A lot of transformer hum, especially from low cost transformers comes from the metal laminations vibrating at a minute scale. If there are bolt heads you can tighten, do them up. If the transformer has not been dipped and the laminations are still accessable, then there is an easy way for major sound reduction....
Get a glass of warm water and One small drop of detergent, add Loads of salt stirring, and brush onto the laminates being VERY careful not to get any in the windings, tape over if you think you might.
The detergent will "thin" the water so it can get in between the laminates and the salt will oxidise and corrode the laminates. The corrosion will expand the laminates making them tighter, and stop the vibration. A few applications may be required.
I use to do this with cheap mains alarm clocks which often hummed like mad. I also used to work on a shop floor where we made custom transformers.The "coating" on laminates is actually an oxide type coating so there is no metal to metal connection, so a little bit of corrosion does no harm.
Some good answers below, with regard to the hum, I have never know a significant hum on a toroidial, that's mainly because hum comes form things vibrating, and toroidials are generally made out of solid ferrite material, so they don't vibrate much.
A lot of transformer hum, especially from low cost transformers comes from the metal laminations vibrating at a minute scale. If there are bolt heads you can tighten, do them up. If the transformer has not been dipped and the laminations are still accessable, then there is an easy way for major sound reduction....
Get a glass of warm water and One small drop of detergent, add Loads of salt stirring, and brush onto the laminates being VERY careful not to get any in the windings, tape over if you think you might.
The detergent will "thin" the water so it can get in between the laminates and the salt will oxidise and corrode the laminates. The corrosion will expand the laminates making them tighter, and stop the vibration. A few applications may be required.
I use to do this with cheap mains alarm clocks which often hummed like mad. I also used to work on a shop floor where we made custom transformers.The "coating" on laminates is actually an oxide type coating so there is no metal to metal connection, so a little bit of corrosion does no harm.
