your professor was smart enough to know that dave was better than him by a ton

@@roseelectronics4582 What an odd thing to say.

@@roseelectronics4582 become his toilet paper then. You undermined that professor without knowing anything, so yeah that's what you're- a toilet paper

@@ItsBoyRed you absolutely hit the nail on the head there :D

@@roseelectronics4582 it ia always wiser to admire some bette than criticize

I am also sure that you’ve saved my oscilloscope.

I'm pretty sure this video is the reason why my Rigol MSO1104Z-SPlus is still alive as well... :-) VERY USEFUL (and money saving, probably).

I am also sure that you’ve saved mine! Thanks

+1

And I'm thankful to still be alive, had the probe in my mouth and on my head looking for brainwaves. Smh.

He didn't mention one point that frighteningly few experts seem to. By attaching your oscilloscope probe-ground to a device powered through an isolation transformer, you once again reference the device to mains earth. It defeats the isolation. It's even worse than using no transformer at all, since the isolation transformer also inherently defeats whatever GFCI/RCD that you might have on your home's wiring. While this method might be okay for lower voltages, using it for 100+ volts is a serious electrocution risk.

I'm certain Dave knows this, just like any expert, and I doubt he intentionally left it out. Instead, it just seems like something that doesn't come to their minds to warn us novices about.

@@gloomyblackfur399 would a differential probe negate that effect?

​@@Timothymukansi Yes, I would think so. That's probably the best way. But the cheapest way I could think to do it would be to A) always plug your device into an isolation transformer and, at the same time, B) plug your oscilloscope into a GFCI (or another isolation transformer) with a "cheater plug" to break the scope's connection to mains earth. The scope's BNC connectors can only become live in one of two ways--either through the device you are testing or through a short inside the oscilloscope's own wiring. Plugging your device into an isolation transformer would protect you against the former and plugging the scope into a GFCI (or another iso transformer) would protect against the latter. The cheater plug on the scope would prevent the issue of the scope "re-referencing" the device to mains earth through the alligator clip. So long as you do both A and B at the same time, with every device, I think you'd be safe from serious shocks (so long as you didn't grab two parts of the circuit at the same time with each hand). But the differential probe seems more foolproof--if really expensive 😕.

But seriously, do they teach anything useful in universities nowadays? I hated when I did my degree as a mature student. I've learned more during previous working years that many teachers knew about electrical principles. This is just a simple example of things you should learn and be aware

Universities, as they should, teach deep knowledge. It's up to you to learn the practical side. That's why you do holiday work, and gain further experience once leaving. They couldn't possibly cover it all in a few years at university.

No they don't. EE in school now is just math and theory. Even that, profs still can't teach shit but you have to learn everything by yourself.

Electronics are easy to teach and cheap for the university to afford the tools for, but they don't cause they don't give a fuck all they care about is their statics ( at least in here ) , even though I love this field I hated the study and after I graduated and got to work I learned a lot and understood what I have before.

@@flyinghuskey8466 Maybe in the US where universities are basically a scam to get 30-40k out of students per semester. All of this is taught in labs during your first year of electrical engineering in Quebec.

The main trap for beginners is the USB powered devices that are so popular...

@@erne75 can you elaborate for a total newb?

​@@gokblok its in the video lmao

That's a cheap hack and useful in most situmakations however you lose out on precision if you're logging data and not just looking at waveforms.

I second that. It leaves me wondering why Dave did not point that out. Not everyone has 300$ layin’ around to buy one of them isolation probes.

​@@padmanabhaprasannasimha5385 usually how much precision is lost using that setup?

​@@gasun1274 Depends on the scope but in general the higher the lowest values of the two probes, the more inaccurate the setup is. For example if your lowest signal is 10V over your mains reference and your other probe is at 11V, then most of the precision of your scope is lost between 0 and 10V. Even worst with 100V for one probe and 101V for the other probe. The difference is still 1V but here you loose 99%+ of the precision of your scope Hope that's clear enough!

A isolated tranformer to power up the load is safe, cheap and more simple. And older electrical circuits there's not grounding terminal, but in modern circuits almost all have the ground reference.

We all need the source of these elusive and mysterious isolation transformers that everyone speaks of

@@rogerd4559 and disable an important safety feature... There's a reason why the bnc is earth referenced.

@@rogerd4559 no, buy a differential probe and stay alive

@@rogerd4559 Fine, but the isolation transformer should be used to isolate the device being tested, not the oscilloscope!

Yeah.... But even then, he got the symbology wrong!

Doc Brown ref. Lol

Those who have done the electric wiring job have saved your oscilloscope! (but not your life)... Be Happy ;)

It might be time to split!

Somerville, Massachusetts?

Is that one of those deals where you have to have the washing machine on and paused with a metal zip stuck in the mechanism before the toaster will work

dont most probes use an isolation capacitor in series?

@@rogerd4559 Huh? If you put a cap in series with the signal, that's just AC coupling. And it doesn't do anything about the ground connection of the alligator clip :)

Ouch.

That is kind of hilarious, as terrible as it must've been!

Step #1: Go change shorts. Step #2: Come back to figure out WTF just happened :)

@Janardan S yep

I think 10 times, look at the wires very carefully and then imagine the current flow before finally hooking up the clips of anything to any electrical circuit.

maybe you already figured this out but there are several options if you want to avoid this, one is a usb scope, and a laptop on battery, another is an isolated usb scope, there are also battery operated scopes now or you could use a differential probe.

I did exactly the same thing when connected a pc power supply instead the batteries that has also the dc minus connected to the mains earth just to make a simple test and it couldn't got any worst.The cpu was short circuited.

Yeah, it seems obvious to me; but then stupidity happens (to me, at least). Also, it seems to me that if there's an internal short somewhere in the DUT that leaks current to its chassis ground and you connect it to the scope's ground thru the probe's clip, then you might be in trouble. After all, what are GFI mains devices for? I have yet to draw the circuit in DaveCAD, so I'm not 100% certain.

...but as Dave said, in rare occasions the device's power pack can be positive-rail referenced. So you need to check this first.

Doesn’t work when you have to work with differential protocols, but it’s certainly a good general rule (after verifying where ground actually is)

Better: don't connect it at all unless you need it. If you don't get a signal from the tip alone, you need to reference the device under test to the oscilloscope. Think about that first, don't just clip on. (The thinking is what ultimately saves you.) The clip is also useful at high frequency and to remove noise. That's what it's actually designed for.

Paul F Are you saying that the positive rail of a laptop battery could be connected to mains Earth? I was considering powering my Arduino through my laptop connected to the wall and connecting the alligator clip of the probe to Arduino ground therefore Earth. Should I use a floating power supply instead to guarantee safety?

I’m no expert but I thought the ‘scope should remain grounded and the gear being tested should be isolated?

Interesting idea! -I believe he was using an analog scope in the video.- Edit: upon further inspection, it seems possible. His scope has the capability to invert CH2, and display the two signals added together! Also, fancy seeing you here! I enjoy your channel. :)

Yes you can use 2 channels. Usually the scope will have either an invert and add or subtract option for combining signals that will measure the voltage difference between the two probes like a balanced input. This is actually a very good way of making accurate measurements because you reject most if not all the common mode noise, ground loop problems, and no more shorting your ground reference, just leave your ground clip unhooked. However, you can end up using a lot of inputs this way. Your 4 channel scope has now turned into a 2 channel, which if you are reading a couple of inputs and need a reference clock to trigger off of, can be a bit of an issue.

Cause you'll lose a channel and if you are on 2 channel scope that's a lot.

That is basically a homemade differential probe

why not make a diagram showing how to do that?

While it is absolutely a good idea to explain this to all new electronics students, I would dearly hope that someone graduating with an Electrical Engineering degree would by that point understand the implications of mains referenced test equipment without someone needing to tell them.

@@stargazer7644 By the time I graduated I did, thanks to youtube videos unfortuneately, but no I don't think it's an obvious fact. The most common tool anyone in electronics uses is a multimeter and they come with differential probes. To someone in school there's no reason, if you're not explicitly told, to believe that an oscilliscope probe is any different. I think when you say that by the time an engineer graduates they should know that, you are missing the fact that by the time they graduate someone in one of their labs should have told them. If anything the oscilliscopes should be the reason people know about mains referenced test equipment, as you put it.

​@@kevincrowe7832 "Thanks to youtube videos"? Someone with an EE degree should be able to DESIGN this type of test equipment. If someone doesn't even understand how mains powered equipment is safety grounded, how in the world could they design it?

Ha ha...

A great case for using an isolated transformer!

but earth pin you mesured but there is no connection between main and earth right? So how can you actually make a short circuit? Considering also that there is a transofrmer in the power supply which isolate the rest of the circuits, ok there is ground but still I don't see any point that the circuit could be shorted. Maybe I am missing something?

@@grzesiek1x You're missing that if there's a fault, connecting a mains referenced circuit to an earthed GND can make a dead short. It seems that the comment I originally replied to eight years ago is gone.

@@nrdesign1991 hmmm 🤔 but if there is a fault in the transformer I could be shocked even without oscilloscope (I mean probing with the scope it wouldn't change the situation because I could be shocked even connecting a diode) ? Another thing is that my USB port and also headphones port have 1 M Ohm of resistance between main's ground and them... I ask because I don't follow emotions in electronics but facts and science is important to me.

@@grzesiek1x This isn't about shocking you, but killing your equipment with a short.

ha ha me too

Watch them at 0.5 speed :)

+Paulo Constantino hahaha oh man now that's stuck in my head

lmao, I keep imagining him saying all kinds of stuff. Like on his introduction, to me he says "I'm your host, David Fucking Jones."

I think we pretty much all do that. Lol

just change ".. i'll short live" .. to: " ...i'll short life ..."

Isolate something - either the device under test or the test instrument. If the device under test is the whole electrical grid, you've got one obvious choice...

only if you connect your probe's earth lead to live wire in your wall socket.

Afaik ground is connected to neutral somewhere in the grid. So you might get away with just leaving the earth clip dangling loose. I never tried it though, you need quite some faith in your science to put your 500$+ scope at potential risk. I would at least put a transformer between it with like 5v output or so.

Simply Electronics find a 240 outlet with a scope that is capable of that voltage rating and probes that meet or surpass 240vac

I apologize for asking this simple question (to you guys) but I really need the answer. Crocellian really brought into focus my concern. Most of our houses in New England have three wires brought into their homes. one is 120 vac, one is 120 vac but phased shifted 180 degrees to permit 220 VAC to form and finally a "ground". The problem that I am having is understanding what happens with a 120 VAC circuit. One wire IS hooked to the ground. Caling that wire "neutral" seems erroneous. Having a separate ground plug (the round one) seems superfluous. AND it seems like I exposing ANY circuity powered by AC to earth ground exposing myself to the problem David talks about. So what do i do to attempt to create isolation? I can't really see a way unless I put an isolation transformer between the device I want to troubleshoot and the AC power source. Please help me.

william burling the ground wire is used as a safety ground. Referencing the chassis to one conductor, which is named Neutral, provides safety so if the line shorts to the chassis it will open a breaker. Otherwise it could be possible for the wash machine, for instance, have one conductor short to its chassis, and maybe the clothes dryer have its chassis shorted to the other conductor. Motors in each could became damaged and get a wire shorted to the chassis. This could electrocute a person touching each machine. So purposely referencing the chassis to the same conductor creates a safe environment.

@@KissAnalog I think what he's getting at is 'Neutral is bonded to mains earth back at the main breaker panel of a house. So, at least for American homes, seeing only two wires (hot and neutral) spade going into an AC adapter doesn't necessarily mean you are isolated. I learned that really quick about 30 years ago...don't ask how!

@@williamburling3229 Our system is called "TN-C-S grounding, single phase, with grounded center tap." In the house wiring and appliances, neutral is an electrical system wire. It is allowed to carry current in normal operation. It isn't touch-safe and it must be insulated. Code calls it the "grounded conductor" (groundED). *The neutral wire can develop several volts of AC* difference along its length during normal operation, simply because it has non-zero resistance. This means one neutral can have several volts of potential relative to nearby other neutrals or the grounding wires. It's not touch-safe or equivalent to the grounding wire. Another reason why they're not equivalent is that a GFCI outlet uses a small common-mode test voltage L&N vs G to probe for N-G continuity. This detects (and trips) if a neutral wire has rubbed or pinched against a grounded part. Despite the non-equivalence, some old work (WWII in particular) was allowed to run a common to some 240/120 appliances. This is simply less safe than a separate grounding conductor. The grounding wire is touch-safe. It can be connected in strange ways - if two appliances share plumbing or a mechanical connection, or through a metal building frame, their grounding wires will not be insulated from each other. It doesn't normally carry current. (It will carry a small amount of current due to parasitic capacitance, static electricity, etc.) Code calls this the "equipment grounding conductor" (groundING) The distribution system is different. It uses a multiply grounded power conductor. Utility electricians usually call it the "ground" wire, but I think "common" is more clear. It's touch-safe IF you only touch it at one point. It's bolted directly to utility poles without an insulator. Safety is ensured by balancing the distribution circuit and making this common wire surprisingly large on the poles. (It's at least as thick as the hots even though it is multiply grounded and carries less current.) These conflicting systems are reconciled by connecting all three of those conductor types together at one and only one point, which code calls the "service equipment." This one point is touch-safe: it touches the utility's distribution circuit at one point. The grounding conductors are at the same potential as long as they don't carry significant current. The neutral does carry significant current, so it's not touch-safe. Grounding conductors from a neighbor are generally within a few volts. Still touch-safe (except when swimming pools are involved) but if you run a wire long enough you can find a dangerous disagreement in "ground" potential. Regular earth electrodes helps a little with this problem, but they are more of a lightning-protection measure. Mostly it's not a problem because people don't run long conductors unless they are a telco or electric utility. Many old electronic devices had a "hot chassis" - connected to hot or to neutral. Neutral through a polarized plug is a little safer than fully hot. All those devices need an insulated case for safety, and they can't be probed unless they're powered through an isolation transformer. New electronics are either grounded-chassis or floating-chassis. Any low-voltage power system inside the device should be galvanically isolated from both the hot and the neutral. The low voltage system will be referenced to the chassis and might be floating or grounded relative to the environment. Modern AV equipment is often isolated from the AC input and grounded through its connections - the plug is polarized only because correct polarity reduces hum. (Quite a lot of cheap crap doesn't obey those isolation rules.) Grounded-chassis can be probed, but the reference clip can only go to a signal ground. The clip is not required for low frequency, low precision. Floating-chassis can be probed. It's a good idea to only reference clip to signal ground, but you can often get away with differential measurements. I think it's still a bad idea. If an isolation transformer is set up with a floating secondary, you can do the same thing. An isolation transformer with a floating secondary is a code violation; you're not allowed to use it as permanent wiring. In all cases the secondary-side groundING conductor must be connected to the secondary groundED (neutral) tap. That makes a floating secondary. A grounded secondary also connects them to the primary groundING conductor. Figure 2 is a good schematic of what's required for a permanent-service transformer in the US. electrical-engineering-portal.com/purpose-of-shielded-isolation-transformer

Jordan Rodrigues. Hey thanks, I’ve actually read all of that! Bunch of good info. It does get confusing for guys who work on old tube equipment. Whether or not to float scopes is usually the subject of debate. Many have done it for decades and are still alive to tell you why they do it. This is what I’ve been told.... They give up safety for lower noise. Old tube amps that are hot chassis are noisy because the caps and resistors don’t have a earth ground to drain into. So floating their scopes keeps the noise down because the amp circuit isn’t trying to drain through the scope ground. I don’t know if that’s the absolute truth of why they float scopes, but that’s what I’ve been told. I’d rather use a isolated transformer on the old tube amps and keep my scope grounded. All I’ve ever done is work on modern tube amps with chassis earth ground, so I have never worried about blowing up anything in my hand while my other hand is in my back pocket.

I just did that innocent mistake :)

How did you determine the true ground of the DUT?

When using multiple probes, the safest thing is to unclip (and set aside) all but one of the clip-on alligator wires.

...... No one who wants to learn looks stupid.

@@robbieaussievic You're right, I actually ended up getting that job.

You will still charge the grounds on the scope and charge them with potential to zap you or short if the potential is not same. The ground is there to protect you not the equipment.

I have a scope that is powered on USB and I can power it via battery just for such. The battery isolation keeps the boom potential lower still. 😊

+Paul Wayper Normally a probe's leads have their ground tied together inside the scope. So if you have one ground lead connected, all probes are now referenced to whatever the ground lead is connected to.

Electrically yes, electronically NO. Try doing that with mixed audio or high frequency signals and you can EASILY get into a world of bother with incorrectly shielded signals, bad return paths and/or earth loops. The main thing as in ALL electronic and electrical measurement is to properly understand WHAT you are doing, WHY you are doing it and HOW to do it correctly! SOMETIMES it may be necessary to use MULTIPLE different scopes. As an example, if you have two scope leads in an RF environment and only ONE signal ground connection back to the scope how can you be certain that currents flowing down the common shield are not interfering with currents flowing down the SAME shield from the second signal source? Or producing a skewed voltage reading due to tiny increases in voltage drop down the shielding? They are BOTH shielded but there is only ONE common signal return path. There has to be a return path but SHOULD it be down ONE shield only? If BOTH shields are connected at the scope (they almost always will be if only via chassis ground) are you creating an RF loop antenna with the two shielded cables providing a complete loop which can massively increase common mode noise? It's NOT as straightforward to get this stuff right in every circumstance as people may think! One reason why GOOD differential scope probes cost so much - it's not just the design but the precision of manufacturing for what will be a VERY SENSITIVE piece of measuring equipment.

Rule of RF: Every wire is an antenna. If you want your probe leads to NOT be an RF radiator, connect the ground ... that's why probes use coax!

Only for low frequency work.

Yes, for which you need to use differential-probe or 2-channels with channel1 - channel2 math operation on, most digital have this (old analog have + and invert, channle1 + invert( channel 2).

Hello, Can i put the alligator to the building ground and go with the probe measuring the 400v phases? (one at time of course)

+Bob Bowie 0.1A is still enough to damage circuitry. But it would certainly be safer, yes.

+Bob Bowie The proper solution would be for scopes to follow the principles of almost all other tools and test equipment, and be built to 'Class 2' double insulated standards, where an earth is not required. The evidence is overwhelming that Class 2 equipment has a far better safety record than Class 1 (single insulated and earthed) equipment. How often do you see drills with an earth these days? For some reason the scope manufacturers have always dug their heels in at this suggestion, and they are about the last of the Mohicans to still be building to Class 1 standards.

It just comes down to money. Remember the old CRT television sets with the _live_ chassis? The fix was.........to go to LCD.

+Phantasmotronogun One other addition to this would be good-- a switch for floating each probe's ground.

+ReactanceIsFutile Tektronix did produce an add-on to do that, only problem was that if the voltage to ground exceeded 50v it automatically reconnected the ground. It actually created a worst-case scenario because an ohmmeter check would suggest an earth-free probe, but the probe shield would actually short-out if it came in contact with high voltage. It could also kill the operator by providing an (unexpected) shock path to ground. Basic issue is that old-school electricians place an excessive reliance on earthing. Even in the consumer environment it is now being recognised that earthed metal is as much of a hazard as it is a safety device, because it can act as the return path for a shock from another voltage source. This is especially so on the testbench. The best arrangement is to have no exposed metal which could become live.

OpenGL4ever Twisted pair Ethernet has isolating magnetics

Jonathan Schneider I am not sure about that isolating magnetics thing what you said, but what i know is, that it is not allowed to use twisted pair ethernet to connect two buildings. For such things optical fiber is a must. And i would assume there is a reason for that.

The concerns are lightning not earth loops which are of no concern _whatsoever_ with isolated twisted pair. But never ever use PoE ports

Jonathan Schneider The source is lightning in most cases but the real concerns are a different voltage potential.

Twisted pair Ethernet (does any other version of Ethernet interface/cabling still being built ?) uses differential receiver/transmitter pairs which of course mean there is no ground reference required/used. Only 2 of the 4 pairs of wires are used for Data. PoE of course is different.