EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - Page 1
Forum section: electronics / general

author: .frank.
author_rank: Newbie
timestamp: February 26, 2016, 06:22:02 pm
content: Watched the video what...5 times? I still don't get the concept so here is a scenario with pictures.Couple of facts before I lay this out:I have a pic16f690 microchip, and I am simply probing the clock out frequency on pin 3I can use a 9v battery or a walwart power supply (no 3rd ground pin) on the breadboard to power itI live in the USA where the neutral and ground pins of my power outlets are bonded together (Also verified with ohm meter)Here's the rub...If I use a 9v battery on the breadboard, then this circuit becomes completely isolated UNTIL I connect the ground clip of the scope probe to any ground rail of the breadboard - WHY? Because the scope has a 3rd earth pin for the 120v 60hz wall power, WHICH IS BONDED TO THE NEUTRAL.What I have discovered is that the second I use the ground probe from the scope on ANYTHING it will always take an isolated circuit and make it...well not isolated.It seems to me no different than in the video watching Dave connect up a 5v usb port to power a board and thus de-isolating it.I need some help in understand what's going on and why I haven't blown anything up yet - as per the video if I hook up the breadboard to earth ground I should be shorting things out. How is this not happening?I have included 3 pictures to help...In all 3 pictures I am using a 2 pin wall power transformer - again the power supply itself IS isolated (checked) but the minute I plug it in, the neutral will automatically go to ground anyway.Picture 1 - probe ground to nothingPicture 2 - probe ground to scope groundPicture 3 - probe ground to breadboard ground

author: rrinker
author_rank: Super Contributor
timestamp: February 26, 2016, 07:26:50 pm
content: If you are powering from a battery, you are only "un-isolating" the circuit on one side - the ground. There's no path to send line voltage into the scope input. Now if using USB, or a non-isolated power supply, you MIGHT be OK or you might not - I know I've run into plenty of outlets where some bozo mixed up the hot and neutral, or if two different outlets are being used and they are on separate circuits, there is a chance for there to be a voltage across the grounds. Or if the power supply does not have a polarized plug and it is connected backwards - you have scope ground connected to the hot side of the circuit under test and the scope probe to the neutral side. Oops. There are lots of things you can get away with without damaging anything, but should there be an unexpected condition, poof. If you never do the potentially fatal (to the equipment) things, then you will never have a nasty surprise.

author: bitseeker
author_rank: Super Contributor
timestamp: February 26, 2016, 07:52:36 pm
content: Connecting the grounds together isn't the explosive setup. Here's an example where things can go boom.1. Your circuit/device is plugged into a wall outlet and is ground referenced (i.e., 0V is at the same potential as the ground of the outlet).2. You want to see the voltage change across a component in the circuit/device. Let's take a resistor somewhere in the middle of the circuit (i.e., neither side of it is at 0V), for example.3. The lower-potential side of the resistor is at, say, 18V at a particular moment in time, relative to ground.4. The higher-potential side of the resistor is at, say, 24V at the same moment in time, relative to ground.5. In the hopes of seeing the 6V across the resistor, you want to connect the ground wire of the scope probe to the low-potential side of the resistor and the probe to the high-potential side.6. BAM! The 18V at that point of the circuit has been shorted to ground through the scope.On the other hand, if the circuit/device was floating/isolated from ground, then,1. The low-potential side of that resistor is not 18V above ground since it has no ground reference.2. Connecting the ground of the scope to that point of the circuit then makes it 0V, relative to ground. No short circuit occurs because the only ground is still the one provided by the scope's ground wire.3. Then, connecting the probe of the scope to the high-potential side of the resistor shows 6V, because the low side is at 0V (due to the scope's ground being connected to it) and what used to be the circuit's 0V before connecting the scope is now at -18V relative to the scope's ground.Thus, the problem is not that an isolated circuit is grounded by the scope's ground wire. The problem occurs when the circuit is already grounded and the scope's ground wire is placed anywhere in the circuit except where it's 0V.That was a bit long. Hopefully, it made sense and I didn't derp the details. Corrections are, of course, welcome.

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2016, 07:53:42 pm
content: @ .frankYour confusion arises from the fact most call the probereferencelead the ground lead.This so often done to remind the inexperienced that with most scopes the reference lead is bonded to mains earth through the scope chassis.But with any signal of interest to be observed correctly on a scope,that signal must have a reference connectionwhich when the DUT is mains powered must be mains ground in order not to damage your scope, eg. EEVblog #279.When wishing to examine a DUT one first identifies the POI, then you must determine a safereferencepoint, be it mains or battery powered.

author: bitseeker
author_rank: Super Contributor
timestamp: February 26, 2016, 08:00:38 pm
content: Quote from: tautech on February 26, 2016, 07:53:42 pmYes, guilty of that terminology usage as well. Thanks for the reminder, tautech.

author: rrinker
author_rank: Super Contributor
timestamp: February 26, 2016, 08:20:15 pm
content: Likewise - the Brits definitely have us beat in this respect by using two different terms. Common, ground, and common ground seem to be used interchangeably in the US and it leads to this confusion. I will sometimes find myself explaining circuit common to someone but referring to it as ground and then catch myself - unlike I did here. It doesn't help when the usage comes across in commercial product manuals. For example, the DCC system I use for my model trains. When you hook multiple booster units to the main command station, you need to have a robust circuit common between the devices for them to all function properly, however the manufacturer has labeled the terminal provided for this purpose as "ground" and refers to it as such in the manual. It is not intended to be connected directly to earth ground, and definitely not for each device - if spread across a large room you are just asking for ground loop issues. Some people have recommended a single point connection to earth ground via a 10 meg resistor as an ESD discharge path. How much it actually helps in this particular case is questionable, since the circuit design is such that the part you are must likely to touch, the rails, have their power generated by a circuit that generates a square wave signal centered around the common (ground) terminal reference. There are other brands of compatible equipment that generate the square wave totally above the reference, with the signal reference for the square wave (which in these applications transmits the power as well as encode the control signal) at half the peak. In the former, you can measure voltage between one rail and the 'ground' terminal and then the other rail and 'ground' and add them together to get the peak to peak voltage. On the latter, from one rail to the common you get nothing, and from the other rail to common you get the peak.(and I know I've once again interleaved the terms which probably does nothing to alleviate confusion - the thing is, *I* know which I mean, but that doesn't help the other guy. We (collective) need to use common when we mean common reference point and ground only when we mean an earth ground. Fat chance...)

author: Fungus
author_rank: Super Contributor
timestamp: February 26, 2016, 09:02:08 pm
content: Quote from: .frank. on February 26, 2016, 06:22:02 pmCorrect.Quote from: .frank. on February 26, 2016, 06:22:02 pmCorrect.Quote from: .frank. on February 26, 2016, 06:22:02 pmIt's not the "de-isolating" that's harmful.What's harmful is connecting your oscilloscope ground clip to a part of your circuit that isn't at ground level. You can do it on an isolated circuit, not on a non-isolated circuit.

author: Brumby
author_rank: Unknown
timestamp: February 26, 2016, 10:16:49 pm
content: Two points cover this:Quote from: Fungus on February 26, 2016, 09:02:08 pmandQuote

author: iXod
author_rank: Frequent Contributor
timestamp: March 02, 2016, 03:08:19 pm
content: Early experience with Tek 465 in EE lab in college. I was a lab tech (maintaining high-atmosphere particle sensor laser system). Needed to probe mains for some reason and thought "Why not?". Well, after the hearing came back, I see that the probe's ground lead was destroyed and mains breaker tripped. Plugged in another probe and '465 kept on truckin'.465: amazing gear.

author: krby
author_rank: Contributor
timestamp: December 25, 2019, 09:04:23 pm
content: Just got an oscillosope for Christmas (Siglent SDS-1202X-E) and I just finished watching #279 about how not to blow it up. I want to make sure I understand how to check for where it could be safe to put the grounding clip. I'm really just confirming my reasoning.- I'm in the US (120V 60Hz), my scope is earth grounded (confirmed by using a trusted DMM to check resistance between the BNC outer sleeve and the ground of wire of an outlet).- Right now, I'm using the stock single ended probes that came with the scope, in 10x mode.So, I've identified a spot I want to attach the ground clip on the circuit. I believe this spot is at the same potential as earth ground, that's why I'm choosing it. To verify, could I take my DMM and measure voltage across that spot and an ground plug on the same AC circuit where the DMM is plugged in (ideally as close as possible to the same plug)?My reasoning is that is this measures 0mV on the DMM, then no current will flow thru the probes ground clip. Do I have that right? Or is there something I'm still not understanding?

author: typoknig
author_rank: Regular Contributor
timestamp: March 24, 2020, 12:35:06 am
content: At this point in the video, Dave shows that the BNC connectors on the scope are shorted together. This being the case, why don't scopes just have a dedicated ground lead like a DMM? Seems like it would be much more convenient than the short ground lead at the of most probes.