How to measure If there I’d DC on the mains

[Pelliott321]

I would like to get back to my original question
How to measure DC on the mains.
Tom had one answer. Is there another way with out using a Scope that has fft

[DaveR]

Hmmm. That looks familiar.

[DaveR]

I would like to get back to my original question
How to measure DC on the mains.
Tom had one answer. Is there another way with out using a Scope that has fft

I did this once, I don't remember exactly what I did, but it seems to me if you us a bridge rectifier and two equal sized big caps to rectify the mains to plus and minus DC, you should be able to put a resistor divider between + and - and measure the middle point. It should be zero volts. You want to do this to the mains, not through a transformer. Make sure your caps are rated for the voltage.

[Pelliott321]

you just described the DC blocker circuit sans resistor

DC-Blocker-2.3-print-300x175.png (20.27 KiB) Viewed 10878 times

[Pelliott321]

here is the link
https://www.atlhifi.com/shop/fully-assembled-devices/dc-blocker-trap-filter-assembled-in-case/

[DaveR]

Not exactly. I was talking about a standard power supply like this one:

powersupply.png (21.62 KiB) Viewed 10865 times

The AC mains to to 2 and 3 on the bridge rectifier, the resistors go across the two capacitors. Leave the junction of C1 and C3 floating - don't tie it to ground. Measure the voltage at the junction of the two resistors/capacitors to AC ground. You'd want resistors that are matched as closely as possible to get the most accurate result. Keep in mind that the AC will rectify to 1.4 times the line voltage. I predict the result will be zero volts DC.

[FerdinandII]

Do you really need the caps? Seems like they might introduce error since they would need to be matched.
Can you just use a dmm to measure two half wave rectified outputs, one positive and one negative?

Not exactly. I was talking about a standard power supply like this one:

powersupply.png

The AC mains to to 2 and 3 on the bridge rectifier, the resistors go across the two capacitors. Leave the junction of C1 and C3 floating - don't tie it to ground. Measure the voltage at the junction of the two resistors/capacitors to AC ground. You'd want resistors that are matched as closely as possible to get the most accurate result. Keep in mind that the AC will rectify to 1.4 times the line voltage. I predict the result will be zero volts DC.

[ratbagp]

I've been pondering this and maybe there is another solution. Of course, it may just be a brain fart.

First, attach the AC to a suitable inductor and place a compass on top of the inductor. If my thinking is correct, the compass needle should not move if the AC is perfectly balanced. If there is DC on the line then the compass needle should move. If you reverse the AC connections, the compass needle should move in the reverse direction.

ray

[dberning]

The power mains is supplied from a transformer, so it better be free of dc. Now where I could see that you could end up with a small amount of dc, and it would be quite small, is if you have something in your house that has a half-wave rectifier. This would inject dc. Equipment with half-wave rectification is very low power generally, and might include something like a clock radio from pre 1960 such as the 5-tube ac-dc radios with series tube filaments. On the other hand, you would have numerous loads in your house that would short dc out such as motors, transformers, etc.

[Pelliott321]

After doing lots of internet searching on other boards it looks like this is another small problem that really does not matter much. There are a few commercial products out there, plus a bunch of stuff on eBay. The subject does come up in discussions about noise and other possible problems with AC mains.
I’m just trying to help out a friend and my own curiosity, but you would think there should be an easy way to verify that there is no DC on the mains