Wall Warts, Mains Ground, Oscilloscope - Page 1
Forum section: electronics / beginners

author: jobog
author_rank: Regular Contributor
timestamp: February 26, 2015, 03:06:43 am
content: I'm will be using the breadboard shown in the picture and powering it from a wall wart with 2 input prongs, no ground prong. I have also just purchased my first DSO scope and will be using it for projects. Since the wall wart only has the 2 input prongs can I assume it is isolated from the mains ground, I know the neutral line goes back to ground in the main panel? Can I safely probe around on the power supply without fear of damaging the scope? If this is true then does this also apply to any device that only plugs in to the hot and neutral? I should add that I'm in the US and plugging into 120V. I have watched Dave's video on scopes and mains ground but I'm not totally clear on all of it. Will be watching it again to try to clarify things. Since Australia operates on 230V all electrical appliances there must use 3 prong connectors, 2 hots and a neutral, and does this make a difference in the danger of connecting the scope to mains ground. I know in the US when operating off of 220V we have 3 wire devices that use 2 hots and a neutral and 4 wire devices that use 2 hots, neutral, and ground. Don't want to blow up my new scope but I'm really confused with this whole mains ground issue.

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2015, 03:20:57 am
content: A few answers in this recent thread:https://www.eevblog.com/forum/beginners/safe-to-use-an-unearthed-oscilloscope/

author: RJFreeman
author_rank: Regular Contributor
timestamp: February 26, 2015, 04:42:02 am
content: Quote from: jobog on February 26, 2015, 03:06:43 amYes that is correct. but the plug pack (wall wart) should* have an isolation transformer inside, with the secondary (low voltage side) isolated from mainsQuoteActive and Neutral should be isolated from the low voltage side so are not relevant to the low voltage side.Quoteyes, in this instance. as the low voltage output from the plug pack is isolated from mainsQuoteNot quite, while we do run three prong plugs, one prong is Earth, one is Neutral, and the third is active - it's just that out active is 230V rather than 120VQuoteThat would be because your base supply is 120V, which would be between one phase and Neutral and your 220V supply would be between two different phases (our equivalent would be three phase which we would refer to as 415V) this gets slightly more complex to explain and I would need to draw pictures for that. but two or three phase power is not really relevant to your concern since you should be isolated from the mains by your Wall wart*Should? older style (heavier) Wall warts contained transformers that did fully isolate mains and the low voltage side (although in theory there could be some oddball out there like an auto transformer, but this would be extremely unlikely) - more modern lightweight Wall-warts contain a switch mode power supply (which again contains a transformer, just a much smaller one) and while any UL etc certified ones of these SHOULD be safe, cheaper ones off Ebay have been called into question (see also Iphone chargers etc.) and I have also had issues about many of the 'safe and certified' ones that I have had cause to dismember as well.To be clear maybe a couple of pictures or model number etc. of the wall wart you are planning to use?