One very useful skill for dealing with electronic systems is being able to identify capacitors and
safely discharge capacitors. You can see here I have a bunch of different type of capacitors.
So electrolytics, ceramic... the thing with all these capacitors right here in particular,
these five, is that none of these are dangerous. If they're fully charged it's not dangerous
to touch these, but the looks on these can be deceiving. This is especially true when it comes
to electrolytic capacitors, which are these guys here. For example if I showed you this capacitor
and this capacitor, which one of these two do you think is more dangerous?
Well if you guessed this capacitor, you're right. This capacitor right here is rated for 400 volts,
68 microfarad. This one is rated 35 volts, 10,000 micro farad. So even though this holds a lot more
charge, it doesn't hold the voltage required to electrically shock you, whereas 400 volts is
plenty enough to shock you. It probably wouldn't be a serious shock but it wouldn't be pleasant,
and it can get even more deceiving. For example, if I bring this one in.
Now, you might be thinking this one obviously looks dangerous, but this is actually rated
for just 25 volts. So if this was fully charged, and you touch these terminals,
you wouldn't even feel it. This one on the other hand, even though it's physically smaller,
this one is more dangerous. The sense that it's rated for 200 volts at 1000 microfarad.
So this one could give you a pretty substantial shock if it's fully charged.
Even more confusing, a capacitor like this one which is an x2 capacitor,
this one's rated for 275 volts yet it's physically smaller than even this one.
When you where you start getting into the really dangerous capacitors they start looking like this,
and you can see here I have the two terminals shorted out permanently. This is actually a
capacitor from a microwave oven and this capacitor is rated for two kilovolts.
So the truly dangerous thing about these is not only can it potentially shock you and cause
burn damage, or nerve damage, or something like that... if you touched
the terminal of this with one finger and the other terminal with the other finger,
well it's going to send 2100 volts through your chest, at that point, and potentially through
your heart. Now one feature of these very large capacitors, usually, is you can see this symbol
right here it has a capacitor and a resistor on top of it. What that means is that inside of
this can there's a capacitor, and then there's a resistor shorting out the two leads. So when
this is removed from power, it will start slowly discharging itself to make it relatively safe.
The reason I have it shorted out, though, and generally the reason you want to keep these
shorted out, is you never really want to rely on just one point of failure. So if that resistor
fails for any reason, this capacitor could potentially store quite a large amount of power
and you wouldn't be one any the wiser, but while these capacitors have safety mechanisms built in,
these ones don't. So there is no resistor inside of here. So if you charge this capacitor up,
it's going to maintain it's going to have some leakage, and it's going to
start discharging slowly over time. It will maintain a significant amount of charge
even when disconnected from power. So one thing to kind of help you understand how capacitors work
is to think of capacitors as sort of like electrical springs,
and like a spring normally when there's no load on it there's no real danger from it at all. If you
put a load on a spring, let's say you stretch out the spring, that would be the equivalent
of charging up this capacitor, for example.The issue with this is, think about what happens if
you take a spring that's been pulled out as far as it can and then let it go. Well it's going
to try and release all of its energy as fast as possible, and usually with pretty violent results.
The same is true with capacitors. For example, if you think of the capacitor like a spring,
if we were to take us like a screwdriver like this and short out the two terminals like this,
that would be the equivalent of taking our spring and just letting it go. So it's going to try and
release all of its energy all at once, and to demonstrate this, what i can do is i'll take this
35 volt 10 000 microfarad capacitor, and short out the two leads. So you can see what happens.
This is 35 volts so it's safe to touch even when fully charged, but it will still
have pretty explosive results if you try and discharge it. So let me charge up to 30 volts
All right now that's charged I can take these off.
Now take my screwdriver. Again, this is like taking the spring just letting it go.
So very quickly tried to release all of its energy onto my poor screwdriver here, which you can see
is a little bit blackened up. Now if you promise not to try this at home, I'll do this test again,
this time with a 400 volt capacitor, and what I'll do is I'll be charging it up to about 300 volts
using this transformer and rectifier. So I have my meter here also to keep track of the voltage.
So I'll turn the supply on real quick.
So about 250 volts on that capacitor, and this time, though, if you touched it with
your bare finger it would shock you. So I'm just going to use my screwdriver, keeping
a good bit of distance, yep. So you probably don't want to have that happen to your finger.
So you can discharge a capacitor like this. Like I said though it's pretty much like releasing a
spring under full tension. Not only can it, you know, obviously cause damage to
your tools like this here, but it could even cause damage to the capacitor itself.
This capacitor may not be very happy anymore that I did that.
So generally a better way to discharge capacitors is to use some type of resistor in place. So this
right here is a 10 watt 10 ohm resistor and this is a 100 ohm 5 watt resistor, and these
are ceramic resistors. So they can get pretty hot. You can also use larger resistors like this one,
although you have to keep in mind that discharging a capacitor this way is really
only safe when you're using it on one that is not physically able to shock you like this one here.
So I have my oscilloscope set up here and I'll show you the differences in voltage drop between
if I just do a direct short and if I hook up a resistor like this. Okay, so I'll go ahead and
charge up the capacitor up to 30 volts. Now I'll take my screwdriver and short out the two leads,
and you can see on the oscilloscope that the voltage dropped almost immediately. So
this time I'll do the same test but I'll use a resistor to discharge the capacitor instead of
just my poor screwdriver here. So I'll go ahead and charge the capacitor up,
and it's fully charged. So now I will apply the resistor to the leads there,
and you can see the voltage drop was much more steady this time.
Let's try it again just to emphasize it.
Now you can see the voltage drops off very very gradually, no sparks came out of this. So the
main takeaways from this video are to always pay attention to the labels on capacitors,
especially if you see large ones. The label really tells you what the capacitor will be
like like. For example, this one is not very dangerous, but this one is potentially dangerous
even though it's smaller, and when you're going to discharge your capacitor generally it's a
better idea to use a resistor. You want to make sure that when you do this, you're only doing
it on a capacitor that is safe to touch. If you're dealing with a larger capacitor like one of these,
I would suggest not trying to discharge this. I would suggest not even working
on the equipment if you don't feel comfortable with it just to avoid injury. Find someone who
is comfortable dealing with high voltage and knows how to handle large capacitors,
but most importantly, if you see a capacitor like this one, be extra careful because this
one will hurt you pretty badly if it is in some way defective or has not discharged properly.