Having just spent a day or two struggling to find the cause of an occasional buzz in a toroidal mains transformer, which turned out to be DC on my mains supply, I'm curious to know just how much DC there typically is on people's mains. I don't know if my supply is unusually troubled, or actually rather clean (and the transformer is too sensitive), or somewhere in the middle.
Has anyone made any measurements ? Would people be interested in making some, so we can establish a norm ?
The values I had were, I thought, tiny. But they made all the difference to this transformer. Given that they are tiny though it does take just a little effort to measure them. IT ALSO INVOLVES MAKING DIRECT CONNECTIONS TO THE MAINS, SO IF YOU'RE NOT CERTAIN THAT YOU CAN DO THIS SAFELY THEN PLEASE DON'T TRY IT !
The setup I used was a 220k resistor, which I was sure would handle the full mains voltage and the power (a quarter of a watt or so) very comfortably, in series with a 22uF non-polarised capacitor (in my case polypropylene film) which I decided should be rated for more than 10V (there will be several volts of AC across it, as well as whatever DC is on the mains). I connected the spare lead from the capacitor to the neutral pin an IEC plug (the one with the pins) and the spare lead from the resistor to the IEC's live pin. I hooked a DMM set to DC volts across the capacitor. I then plugged an IEC mains lead trailing socket) onto the plug and, last of all, I connected the 13A plug at the other end of the mains lead to a 13A outlet in the wall.
I've watched the DC voltage reading on the DMM all day. It's tended to read between 10mV and 25mV. But yesterday evening it was reading 65-75mV and that was sufficient to make the transformer buzz. I was staggered. So staggered that I made up a synthetic supply with a 1:1 isolating transformer and an adjustable DC source in series so I could turn the DC up and down at will. Sure enough, at 40mV DC the transformer was quiet and at 60mV DC it was buzzing.
So is my 75mV unusually high ? If anyone has measured the DC component on their mains then I'd be very interested to hear how large it was and how much it varied.
Mods - I'm not sure if this is the right place for this thread. But there are passing comments on the problem of mains DC making toroids buzz here
https://www.vintage-radio.net/forum/...d.php?p=673158, without any measurements though.
Cheers,
GJ