Where to put oscilloscope ground clip...newbie question - Page 1
Forum section: electronics / beginners

author: panzerfaust
author_rank: Newbie
timestamp: June 21, 2017, 09:39:13 am
content: Hi everyone, my first question here,When probing around with crt tv's and computer monitor circuits i have always used differential measurements using up both the channels. I'm mostly looking at the transistors that is pulsing the flybackSo my question is ..in sweden crt tv:s do only have live and neutral 0-230v, and no ground wire , so there is no obvious grounding point for the probe, is it still possible in some way to measure safely using one channel only and using ground clip?I have not blown any scope yet, but have blown several transistors when trying to probe between collector -emitter and base-emitterwith one channel and ground clip

author: MosherIV
author_rank: Super Contributor
timestamp: June 21, 2017, 10:37:23 am
content: HiWelcome to the forum.Old fashioned CRT TV are really dangerous! I do not know about the later ones but early and mid generation ones actually had the metal chassis at live mains voltage.So to safely probe around in a TV, you normally needed to run either the TV or your scope through an isolation transformer.Check out Dave's vlog :

author: rstofer
author_rank: Super Contributor
timestamp: June 21, 2017, 05:33:30 pm
content: Do you have a schematic?Probing indiscriminately without the benefit of a schematic will never end well. The use of an isolation transformer will help but you need to identify circuit ground and, without a schematic, you're just guessing.Once I thought I had found a 'ground' point, I might measure it to earth ground on the scope with a DMM.Just guessing but I imagine the neutral conductor is grounded at the panel or transformer. Knowing that doesn't really help because you don't know how the electronic ground is created.ETA: Is your scope grounded to earth? Are any of your outlets 3 wire (hot, neutral, ground)?

author: Electro Detective
author_rank: Super Contributor
timestamp: June 21, 2017, 06:23:35 pm
content: I would play it safe and stay with the differential method with the ground clips REMOVED!assuming low voltages being probed,and especially if the spending budget for better safer methods is less than $00.00Otherwise you need a proper fully 'mains isolated' transformer to power the TV, end of storyDave's video above is amust watchand Mr. 12voltvids explains it straight up too

author: Vtile
author_rank: Super Contributor
timestamp: June 21, 2017, 11:01:29 pm
content: Neutral is usually (always) considered as ground connected. The so called ground pin ( yellow-green) is safety ground and later invention.The non-grounded old style two wire system is fully safe (as safe as safetygrounded system without RCD) in a log cabin without grounded surfaces (floating environment) or objects (obviously, this doesn't happen in repair situation as there is both potentials in reach of repair persons body).Your device do have potential ground connection through neutral.PS. Welcome to the board. Hows the midsommar in west.

author: Electro Detective
author_rank: Super Contributor
timestamp: June 22, 2017, 01:07:29 am
content: Yep, you can do a lot with dry wood, rubber mats, safety eye-wear, and gloves,if waiting on the log man-cabin to be finishedOr wait till the kids grow up and jack their cubby houseHey guys, does it actually STRESS the power supplies of CROs and DSOs if you defeat the earth/ground pin ?will rectifiers fragment,capacitors burst forth to oblivion,transformers unwind themselves into a 1960s bad hair day at Woodstock... ?or is it half baked voodoo BS to scare off clueless prodding fools ?example: running the oscilloscope from a full floating isolation transformeror sine wave UPS in back up mode isolated from the mains (and concrete floor!)240 volts in, and isolated (and fault current limited) 240 out to power the oscilloscope.How much snafu factor can there be?

author: MosherIV
author_rank: Super Contributor
timestamp: June 22, 2017, 08:31:11 am
content: This is a forum for beginners.The last thing we want to do in a beginners forum is to teach them dangerous practices especially if they do not know what they are doing.