So although this isn't exactly related a higher percent to BMET,
this is still I think a pretty important safety video to make because in the labs usually there
are oscilloscopes and multimeters, and they can both be used for lots of different purposes,
even not necessarily just electronics. So you can use them for a bunch of different applications,
but a very important rule is- there's some common misconceptions about oscilloscopes
and multimeters. Some of those can be rather dangerous. So the very first one I put out
there is that oscilloscopes are absolutely not multimeters even though they can operate
in similar functions. They both measure voltage oscilloscopes, measure voltage over time. They
internally operate very differently. Your average multimeter is going to not only be isolated,
it's going to have its own battery system. So it's an isolated power supply inside of those,
but also the leads for this, the positive and negative leads, have a very high resistance
and impedance between them. So when you take your probes for your multimeter,
whatever you connect them to, even if there's a very high potential between them,
there's not very much current that's going float through these probes.
That's good, but it kind of gives you the impression that you can just poke these
anywhere and it'll probably fine. For multimeters that's basically true. You can stick these probes
arbitrarily into different potentials, and it won't really matter because these are,
for one, a floating supply and there's not going to be any power going through this.
Oscilloscopes on the other hand, are quite a bit different. You have the probe itself,
which this is what connects to your circuit, or the test point. What you have here is your
grounding strap. I might look at this and say okay this is similar to your multimeter
in that this is your positive lead and this is your negative lead. It's sort of true for
the positive one. It is absolutely not true for the negative one, for the negative lead. This
is not a negative lead or a negative probe. What this is, in fact, is a common lead back
to earth ground, and it's actually a very very low impedance, usually around a couple ohms.
To better illustrate that, I can show you an actual example of how this works.
Okay so here I have a multimeter, and my oscilloscope. To demonstrate the differences,
I will be using mostly the multimeter to just show you different aspects of the oscilloscope.
This is a four port oscilloscope Rigol. You may not have one like this. It may have two ports,
it doesn't really matter. The concept is the same unless you have a certain type of oscilloscope
isolated inputs, but most of the ones like the ones in the BMET labs are not going to
have isolated inputs. They're all going to be with common together and what I mean by that...
so I switched my multimeter over to continuity mode. So what I can do now is check to see what
has a direct connection between each other. Now you can see on here these BNC connectors,
that's what these are called, are the parts where the probes actually plug in to your oscilloscope.
An important fact to know is that these BNC, the outer parts of these BNC's are common.
So that means that effectively these two outer shields are connected directly to each other,
and that's the same for all of these. If I took this off and probed from this one to this one,
that's a dead short. Now what that means in terms of your oscilloscope probes,
I here I've got two probes right here and these are both set to 1x. So about 1 million
ohms of impedance between each of these, but these alligator clips connect directly to
the outer shields of the oscilloscope, which means that in effect both of these alligator
clips are connected to each other. So what that means overall is that it presents two different
problems. The first problem is if you have two of these hooked up to your to your circuit,
and you plug this one into five volts, and you plug this one into ground. What will
happen is electricity is going to flow through this, through the oscilloscope,
out to this, and cause a short circuit. For low voltages and low currents that may not
be a huge deal because most likely what will happen is your circuit will just stop working.
It may damage the circuit a little bit in some way, but the more dangerous aspect of this,
is that both of these, and all of these shields on the oscilloscope are common to main earth ground.
So if I take my probe here, and I stick this in to ground... you probably shouldn't try this at home
unless you know what you're doing. Stick that lead on the ground. It probably doesn't have a great
connection though. I'll fix that and I connect this. So that illustrates that this is connected
directly to the outlets ground port. So let's look at an example where this can be a problem. This
is a really condensed version of what Dave did over at EEVblog. So if you want to see the full
explanation for this, head over there and look for his video on how to blow up your oscilloscope.
This is sort of a very condensed version of that. So an idea of where this presents a problem...
Let's say you have your circuit over here, and let's say that your circuit is connected
just to the outlet for the wall. So you have live, you have your neutral,
I should have done this the other way... but whatever, and let's make ground green. Same
for your oscilloscope you have the hot, you have the neutral, and then you also have the ground.
These feed back into your breaker box. This can be just break box, and hot goes to your breakers.
Let's just say they're both on the same circuit, and your neutral... let's go back to your neutral
busbar. The grounds go to your ground busbar in your breaker panel. Now, an important thing to
keep in mind is that in most domestic situations the neutral is connected directly to the ground.
So if you were to take a power strip, for example, with the power off, and plug it
into an outlet, and then probe between... do a resistance check between neutral and ground,
there will be a very low resistance because in the breaker panel these are connected to
each other. So effectively the neutral and ground are at the same potential. So
you've got your circuit and you got you oscilloscope. Both of them were working
fine. You have your probe and you're probing around the circuit trying different things,
and it may work fine for a while, and let's say you make a mistake. Let's say you forget
which one is it which. You're not familiar with the circuit, or don't know if it's isolated,
so you just say, "oh I'm just going to probe on this probe, the neutral". You know, you may
not know it's connected neutral, but that's what you end up connecting it to. Either through some
circuit path or something it eventually ends up back in neutral, and let's say instead of taking
the ground lead and putting it to ground, for example, let's say you accidentally put it to
the hundred twenty volt. So your alligator clip is now on the 120 volt live input.
This is where the problem starts happening because if you had a multimeter, this would be
fine. With your multimeter you can probe basically anywhere. You can say I'm going to probe neutral
and hot, and that will be fine. Nothing's going really to happen unless you accidentally set
this to the current mode, and have the probe in the current slot. That would be a direct short,
but otherwise if you're just doing voltage there's basically no current
that's going to flow through this multimeter and you're safe. You're not safe with this,
though. What will happen is electricity will come in through your breaker, through the hotline,
to your outlet. It will go from your outlet, to your circuit, to the 120 volt input. Now what will
happen is because this alligator clip has such a low resistance, a low impedance, the electricity
will go through the ground clip, through the probe for your oscilloscope, into your oscilloscope,
through the outlet from the oscilloscope, back through to the panel, and back to the neutral,
which is then effectively common to each other because that's where it goes out to
the transformer. What you have effectively done is connected the neutral and hot line directly
to each other through this path. So, what happens then? Well now this poor little oscilloscope probe
is going to try and pass the mains current, full mains voltage through this dinky little cable.
What will most likely happen, this probe will probably blow up. It'll vaporize. It may spark,
it may blow apart. Bad things will happen. It may damage the oscilloscope. It's not as likely
because this stuff is beefier cable, and usually rated for pretty high currents, whereas this
little probe is not. So if you're probing with this in your hands, for example, that can be bad,
because if you're for some reason holding the probe like this, this thing will get super hot or
blow up in your hand, and that's bad. You don't want that to happen. So this circumstance... I
mean the main takeaway from this, is if you don't know what you're doing with the oscilloscopes...
First of all, do not use these on anything that goes into the wall when you're starting
out unless you're absolutely sure how it works, and you're sure that you're not plugging these
ground probes into something that can potentially cause it to blow up. The second is to always plug
these into the same potential. So if you have any little bit of difference between these two,
current will flow through them, and that's bad. It can either cause a circuit to start messing up,
or it can cause damage to your circuit, or even damage to your oscilloscope probe,
or the oscilloscope itself. What can save you from this is a GFCI ground fault connection interrupt,
I believe, I always forget the acronym for this. Anyway, it's an outlet that's designed so that
if there is a difference between the live and neutral in the terms of current going in and
out of the live and neutral, it will break open the connection. This is what would happen here.
If you have the mains voltage coming in through here, into the oscilloscope, down into ground,
your GFCI is going to detect that imbalance and break open the circuit very quickly. Saving your
oscilloscope, saving your hand, and saving your probe. This will not protect you against things...
like if you have these two probes connected to different points in your circuit, these will still
carry current. It's not going to detect that. What that will detect is an accidental connection
between live and ground. So anyway, very very high overview, very quick rundown of why oscilloscopes
and multimeters are different, and why they can be dangerous. As I said, more information you
can find on EEVblog. Just Google or YouTube, "how not to blow up your oscilloscope", is the name of
the video, and he goes into a much more detail of the situation and explanation of how this works.