Dumb oscope question - Page 1
Forum section: electronics / beginners

author: DanielSpokane
author_rank: Regular Contributor
timestamp: May 22, 2017, 06:03:11 am
content: Hi, I just bought an Oscilloscope to use for trouble shooting tube amps. I bought a Siglent SDS1102X which I love so far. My question is this: After buying the oscope I read several warnings about dumb things you can do that will destroy your scope. One of them saying you should never measure your house line voltage. I'm trying to understand this. Is the reason you shouldn't measure the line voltage because your scope and your house share the same ground and so if you accidentally touch your scope ground to the line hot it ground your hot through the scope? If that is the case... it seems like if you used a volt meter ahead of time and identified the hot wire and made sure to put the ground to ground and probe to hot it should be fine? Right?... am I thinking about this wrong?

author: cvriv
author_rank: Frequent Contributor
timestamp: May 22, 2017, 06:46:18 am
content: If it says not to do it... You probably shouldn't do it. You probably shouldnt do not because you might accidentally probe the hot with your ground, but maybe because of the possibility of a high amount of current that could surge through the scope? Thats my guess.

author: ataradov
author_rank: Super Contributor
timestamp: May 22, 2017, 06:50:23 am
content: There is no practical reason to measure mains directly. You risk damaging the scope, and gain absolutely nothing, so what's the point?

author: jonovid
author_rank: Super Contributor
timestamp: May 22, 2017, 06:54:10 am
content: Dave did a video on this

author: tggzzz
author_rank: Super Contributor
timestamp: May 22, 2017, 06:58:28 am
content: See the safety references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/If you don't understand them, then you might destroy equipment and/or people.Make very sure understand when/why to use HV differential probes.

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2017, 08:02:19 am
content: There maybe times that you have to measure high voltage circuits or high energy circuits, combine both like you have with mains and it needs specialist gear to protect yourself and your equipment.Good advice is to only connect to HV and high energy circuits with Differential and/or 100:1 probes.With ordinary scope use the reference lead on the probe is at mains ground potential and therefore extreme care must be used as to where this signal reference connection is made in order not to create a simple ground loop with can damage the scope and/or probe.With the knowledge the probe reference lead is at mains ground potential can also be used to some small advantage with low frequency signals and when just looking for some indication of approximate voltage or just is there power in my circuit/DUT........how ?Just remove the reference lead from the probe and rely on the measurement asrelative to mains ground, that's the scope chassis just where the probe lead connects to anyway.I've seen repair techs rip the reference leads off their probes knowing there's a return path via mains ground and when some better signal fidelity is required connect a independent scope grounding lead to the DUT. This has the advantage of zero risk of inadvertent shorting of the clip on the probe to any part of the DUT while chasing test points all over an unprotected and powered PCB.If chasing the best signal fidelity of course more specialized techniques must be used.There's never a dumb question about safety, keep asking until you are crystal clear on what you need to know.Think about what you are doing and stay safe out there.

author: Shock
author_rank: Super Contributor
timestamp: May 22, 2017, 08:15:14 am
content: SDS 1000X/SDS1000X+ SeriesMeasurement Categories:The oscilloscopes can make measurements in measurement category I.WARNING This oscilloscope canonly be used for measurements within its specified measurement categories.Category Definitions:Measurement category I is for measurements performed on circuits not directly connected to MAINS.Examples are measurements on circuits not derived from MAINS, and specially protected (internal) MAINS derived circuits. In the latter case, transient stresses are variable; for that reason, the transient withstand capability of the equipment is made known to the user.

author: DanielSpokane
author_rank: Regular Contributor
timestamp: May 22, 2017, 08:47:27 pm
content: Thanks for the responses.Regarding my situation. I don't necessarily need to measure line voltage using my oscilloscope. Mainly I'm just curious and like to feel I understand what is going on and why this is a safety concern.As far as what I will be using the scope for, I will need to measure various points in a tube amplifier (using the scope for tube amp trouble shooting and design). A couple of you have mentioned transformer isolated circuits. I'm trying to figure out what that means. Does that mean if I take a tube amp and plug it in without the ground plug, the amp is isolated from the mains (ie the hot and the neutral are isolated via the power transform and the ground is not hooked up)? ... and thus I could theoretically use the scope on any point in the amp (provided the voltages are below the required max for the scope and/or I use a 1x10 probe)Seems like that could be dangerous too since the amp is then not grounded. But for a few tests maybe ok?... assuming I test to make sure the grounded components aren't carrying any potential with respect to ground. Thoughts?

author: tautech
author_rank: Super Contributor
timestamp: May 22, 2017, 08:59:49 pm
content: Regardless of probe ratings for anything more than ~150V I use 100:1 probes, my equipment and personal safety is paramount.If your probe reference lead is ONLY connected to the equivalent of mains neutral which is of course also mains GND you will be fine, remember to check with a DMM that the DUT chassis and/or DUT circuit GND are in fact at the same potential.Isolating transformers without a carry trough earth/GND are the best bet as then the DUT then has zero mains GND reference/tie.

author: danadak
author_rank: Super Contributor
timestamp: May 22, 2017, 09:24:27 pm
content: Might be useful -http://www.newark.com/pdfs/techarticles/tektronix/FFM.pdfhttp://www.tek.com/document/application-note/three-facets-floating-measurement-solutionsRegards, Dana.

author: edavid
author_rank: Super Contributor
timestamp: May 22, 2017, 09:34:37 pm
content: Quote from: DanielSpokane on May 22, 2017, 06:03:11 amYes, exactly.Quote from: DanielSpokane on May 22, 2017, 08:47:27 pmThey are alluding to certain notorious old devices that have no power transformer, such as "hot chassis" TV sets and "All American Five" radios.Since all tube audio amps do have power transformers, it's not relevant to you.

author: jbb
author_rank: Super Contributor
timestamp: May 22, 2017, 10:00:20 pm
content: Quote from: DanielSpokane on May 22, 2017, 08:47:27 pmIn general, removing/disconnecting a ground pin is a risky plan. There are some situations where it can help a lot with testing, but it's not something I'd do lightly. When you have gained some experience in electrical systems design you can make your own evaluation.I suggest the following thought experiment:In the beginning, the chassis was groundedMaybe some internal circuitry - on the isolated side of the transformer - was grounded. Maybe the DC 0V (aka Ground).You disconnect (or 'lift') the chassis ground from the power plugAs this point in time you're probably OKYou want to measure something, so you connect your 'scope ground to DC 0V and measure.No problems yetYou want to measure something else - maybe the voltage across the output transformer primary.You connect your scope probe to DC+ (could be well over 100V depending on amplifier).The '0V' rail is now forced down (because DC+ is connected to mains earth via your scope), so amplifier chassis is now live to negative(DC+) volts.You rest your hand on the chassis while poking around with the 'scope probe and get a shock.When we consider the above scenario, we see that it takes 3 steps to shock yourself: steps 3, 8 and 10. Steps 3 and 8 were 'necessary' for your measurement plan. Step 10 was a mistake. Maybe you're tired. Maybe someone distracted you. Maybe you dropped the probe and went to recover it. What steps 3 and 8 did was put you one mistake away from danger.And now I look like a safety nutter / the fun police... When you receive a significant electrical shock, several things can happen. You can suffer electrical burns at the point of contact (painful, not so bad). You can fall off your chair (or ladder for extra fun times) and injure yourself. You can suffer an immediate cardiac event (e.g. ventricular fibrillation), collapse and possibly die. Good thing you've got a buddy there keeping an eye on you, right?You can even damage part of your heart and/or set up an irregular rhythm, feel OK, and then suffer a cardiac event when you go to sleep for the night, and your partner wakes up next to a dead body. So if you do receive a significant shock (static discharge onto a door handle doesn't count) you should probably go see your doctor or local emergency department to get checked out. It will likely take at least a couple of hours, or all bloody day depending on your triage level. (Pro tip: tell them you had an electric shock as soon as you get in because a) it's important and b) it'll increase your triage priority level.) It may cost a lot of money (depending on public health care, insurance or lack thereof).So that's why I wouldn't float the ground.

author: jbb
author_rank: Super Contributor
timestamp: May 22, 2017, 10:01:54 pm
content: Oh, also...The original question was by no means dumb. It's good to not just blindly follow rules / common knowledge but also try to understand them. Sometimes the rules are wrong. Sometimes special circumstances apply.