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.
I think that there is a rule or regulation that limits the DC current that may be drawn from AC mains.
Most valve radio sets obtained the HT supply by half wave rectification of the mains. Some TV sets used a half wave rectifier to run the valve heaters.
With the decline in the use of valve equipment, it was thought that this problem was going to get better. Not so ! many cheap imported domestic appliances draw significant DC current from AC mains.
Electric blankets, full mains for high heat and half wave for low heat. Rice cookers ditto.
Travel kettles and hair dryers, full mains in 100/120 volt mode, and half wave in 220/240 volt mode.
