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.
Measuring a fraction of a volt of DC in the presence of hundreds of volts of AC is no mean feat - ideally you low pass filter.before the meter to get the AC component down.
Hello, Is the diode on the Neutral side of the lamp? When a diode conducts there is a small voltage drop across it. In the case of a silicon diode about 0.6V (varies with temperature). I think you are just measuring this voltage drop that's why it reverses when you reverse the diode. When to measure L-N you will get the same reading with or without the diode in circuit unless there is a lot of upstream impedance. Its asking too much to expect even a good meter to find mv of DC in 230V+ of noisy ac.
You would need some form of active low pass filter with a very low cut off frequency. Google will produce some designs.
The simplest low pass filter is basically is a series R and a C to ground, such that the RC time constant is long compared ot the period of the mains, and then the cap charges to the mean DC value, with a ripple that is attenuated. Clearly the resistor and capacitor need to be suitable for the votages (both AC and DC ) that you expect.
If your DC meter is a digital one that has say a megohm input impedance, then you can afford perhaps 100K in series, as this will push the DC reading off by ~10%
If we take that 100K as an example the capacitor value needed to reduce 720V p-p (==230V RMS) to say 1`% of its original value (7.2Vp-p or 2.3V RMS) would be 1000j ohms, or at 50Hz, a touch over 3uF. (the reactance of the capacitor Xc is 1/(2pi FC) )
You would get greater suppression but more phase shift splitting the 100k in half into a pair of 47K, and using a paiir of 1uf capacitors to create an R-C-R-C network, to get two lots of 47K against 3k (15 squared is more than 100....)
Take care how you do this - if some how you end up with rectified mains on the caps, the energy stored may be heart stopping.
As a useful rule of thumb about when to start to worry, in the lab we treat anything where (CV2)/2 is more than about 1 joule as potentially deadly, and require resistors for automatic discharge. Above 10 joules we require multiple automatic discharge paths, so several things go wrong before we reach a dangerous situation.
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.
The answer is, most likely, the resistance of the Neutral between the "point of common coupling" between N and PE.
... and that is the reason that Neutral is a Live conductor!
To explain further, a half-wave rectified DC signal as both AC and DC components. The AC components "cancel" in your multimeter, so you are left with the DC component. Other meter types will react differently depending on how they "average" the voltage ... that is the only difference between a "true rms" and "normal" multimeter, the way they "average" the voltage (or current, or power).
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.
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.
