Author Topic: Mains Plug attenuator  (Read 702 times)

0 Members and 1 Guest are viewing this topic.

Offline sahko123Topic starter

  • Frequent Contributor
  • **
  • Posts: 324
  • Country: ie
Mains Plug attenuator
« on: March 21, 2020, 05:10:25 pm »
I'm having trouble with interference on my mains and wanted to create a plug that simplified probing the mains and made it a good bit safer. My idea was to use high value resistors (1M to 10M) to simply attenuate the mains to reduce the voltage by a known amount is there anything to consider safety wise or traps that I might fall into?
Asking for a friend
 

Offline bob91343

  • Super Contributor
  • ***
  • Posts: 2675
  • Country: us
Re: Mains Plug attenuator
« Reply #1 on: March 21, 2020, 08:42:59 pm »
You are asking for serious trouble here.  Any test equipment connected to mains will create a dangerous situation.  You should be using a differential probe but those are expensive, especially if you want safety with over 100 Volts common mode.
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 10174
  • Country: gb
Re: Mains Plug attenuator
« Reply #2 on: March 21, 2020, 08:43:30 pm »
Nope, sorry. That will inevitably end up with you connecting the ground clip to mains neutral and causing a safety issue.

The other problem is that inserting a straight 1 - 10M resistor would form a low pass filter with the input capacitance of the scope probe, causing you to lose the high frequency noise that you're looking for.

The 'proper' way to do it would be use a high voltage differential probe, but you're looking at significant cost. However I think you could probably get away with a small (as low wattage as possible) toroidal mains transformer, say, 24V output. This would give you the important mains isolation that is needed.

One of the downsides of toroidal transformers, compared to ordinary ones is that they tend to couple more high frequency noise. This seems like exactly what you want.
Best Regards, Chris
 

Offline sahko123Topic starter

  • Frequent Contributor
  • **
  • Posts: 324
  • Country: ie
Re: Mains Plug attenuator
« Reply #3 on: March 21, 2020, 09:34:18 pm »
i more or less just wanted to know what the mains voltage is at multiple points in my house using a handheld multimeter and divide the voltage down by a known amount
Asking for a friend
 

Offline Prehistoricman

  • Regular Contributor
  • *
  • Posts: 216
  • Country: gb
Re: Mains Plug attenuator
« Reply #4 on: March 21, 2020, 09:59:17 pm »
If you did want to scope it, would the following work?
I don't see how it could be a safety risk (either to the operator or equipment).

It's basically a potential divider on both wires to earth. Should have low common-mode voltage and isn't able to provide enough current to hurt anyone.

Offline sahko123Topic starter

  • Frequent Contributor
  • **
  • Posts: 324
  • Country: ie
Re: Mains Plug attenuator
« Reply #5 on: March 21, 2020, 10:07:21 pm »
yeah that's almost exactly what i've done just a 10m and 100k potential divider (live through 10m into 100k to earth same with neutral). all this is contained in a UK plug
Asking for a friend
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf