Hi, I wanted to share how stupid I am, and that even when we know the theory and laws, if we're not familiarized with the real world stuff we may get surprised, or shall I say, shocked.

The story is actually short and stupid, but here it goes. I was taking apart a few minutes ago my old printer that stopped working few weeks ago because I wanted to get the 2 motors inside. I got them, and then I saw the transformer, the black box actually. I tried to open it just to see it, but it was damn difficult. Now an inner voice started to whisper inside my head: "hey, if it's difficult to open, call me crazy, but may be it's because they don't want you to open it". After a lot of pulling I finally opened it, started to observe it, and that's when I failed. I guess I touched and shorted the IC and I got a little shock. Not painful, but a little "scary". I'm seeing 2 capacitors rated 50V and 470uF, and a big one rated 400V 120uF.
I have two little white points in my finger, I guess a little burnt skin? They hurt a little.

I know capacitors can be charged and be like batteries, but I saw everything unplugged and I over trusted the circumstances. One thing is to know that a capacitor can get charged, and another one is to actually comprehend that a capacitor can get charged and shock you.

My first question is:
I thought capacitors only had one spec: the capacity, measured in farads. Why do they mark the voltage?
How dangerous are those capacitors?
What's the proper way to discharge them?

 

Capacitors have voltage ratings because they must be able to handle various voltages and the amount of material and type of construction influences this.

Higher voltage capacitors are also able to charge to higher voltages.

If something is completely disconnected from its power source, you can short the terminals of the cap using something with an insulated handle, like a screwdriver. If you want to be very careful, you can use a resistor, a "bleeder" which the circuit should have had. It should be relatively high current capacity >=1W and several kilohms.

Alternatively, you can buy a purpose made device for ~20-30 bucks.

 

The voltage rating is the maximum voltage that should be applied in any circuit it is used in.
They can give you a nasty jolt, very rarely would it kill you, unless exceptionally high voltage.
Often a resistor is placed across them to discharge at power off.
Max.

 

Basically, a capacitor consists of two parallel plates separated by an insulating layer which could be air, paper, plastic, etc. Hence it has a max voltage before flash-over occurs. Essentially all electrical and electronic components have a max voltage rating.

You can discharge a capacitor with anything that conducts electricity, even a screwdriver will do.
However, a screw driver is not recommended if the charge Q = C x V is huge.
The amount of energy stored in the capacitor goes up as the square of the voltage.

E = ½ C x V x V.

A 100Ω resistor will lower the discharge current resulting in something less than a big bang.

 

Welcome to the club.

Capacitors need a voltage rating just like any other component. You can use a 50V cap in place of a 10V cap but it might not physically fit. And a 50V cap is going to be more expensive.

I would say only dangerous if you have a heart condition. Though it is going to depend on the size of the cap.


I usually just short them to ground with a jumper wire

 

In the majority of cases any 'Jolt' would occur across one's hand and therefore not pose anything more than a burn at most.
An old apprenticeship trick was to crank a Megger across a large Cap and leave it on someones bench to pick up!.
Max.

 

Oh, I get it, so the voltage is there to indicate the maximum voltage it can handle. If I put it under 75V, that capacitor of 50V maximum could explode?

I'm reading 280V DC still in the large capacitor. Damn...

I still don't get one thing: the big capacitor only has 120uF 400V, that means it can store way less charge than the 470uF 50V. Nevertheless, it's way bigger. Which one is more dangerous?

 

Oh, I get it, so the voltage is there to indicate the maximum voltage it can handle. If I put it under 75V, that capacitor of 50V maximum could explode?

I'm reading 280V DC still in the large capacitor. Damn...

That works out to
½ x 120μ x 280 x 280 = 4.7J

That would hurt!

 

400V!!
50v is below the dangerous level.
Max.

 

I still don't get one thing: the big capacitor only has 120uF 400V, that means it can store way less charge than the 470uF 50V. Nevertheless, it's way bigger. Which one is more dangerous?

The big one can store 120μ x 400 = 0.048 coulomb, E = ½ x 120μ x 400 x 400 = 9.6J

Small one, Q = 470μ x 50 = 0.0235 C, E = ½ x 470μ x 50 x 50 = 0.587J

It is quite obvious which one can give the bigger bang.

 

Incidentally, after you've discharged them, capacitors can charge themselves up again.
See dielectric absorption.
https://physics.stackexchange.com/q...ause-of-self-charging-of-unshorted-capacitors

 

Incidentally, after you've discharged them, capacitors can charge themselves up again.
See dielectric absorption.
https://physics.stackexchange.com/q...ause-of-self-charging-of-unshorted-capacitors

But usually only to a small fraction of the original voltage.

 

I thought capacitors only had one spec: the capacity, measured in farads. Why do they mark the voltage?

They have more than 2 parameters, but capacitance and working voltage are the most important.

How dangerous are those capacitors?

It depends on the type, capacitance, and voltage that they were charged to.

What's the proper way to discharge them?

Using a bleed resistor is generally considered the safest way. For large value electrolytic caps, you'll sometimes see the terminals shorted.

 

An old apprenticeship trick was to crank a Megger across a large Cap and leave it on someones bench to pick up!

When I was in school, a common prank was to charge up capacitors and put them in the parts drawer and wait for an unsuspecting victim to grab it.

 

Hi, I wanted to share how stupid I am, and that even when we know the theory and laws, if we're not familiarized with the real world stuff we may get surprised, or shall I say, shocked.

The story is actually short and stupid, but here it goes. I was taking apart a few minutes ago my old printer that stopped working few weeks ago because I wanted to get the 2 motors inside. I got them, and then I saw the transformer, the black box actually. I tried to open it just to see it, but it was god damn difficult. Now an inner voice started to whisper inside my head: "hey, if it's difficult to open, call me crazy, but may be it's because they don't want you to open it". After a lot of pulling I finally opened it, started to observe it, and that's when I failed. I guess I touched and shorted the IC and I got a little shock. Not painful, but a little "scary". I'm seeing 2 capacitors rated 50V and 470uF, and a big one rated 400V 120uF.
I have two little white points in my finger, I guess a little burnt skin? They hurt a little.

I know capacitors can be charged and be like batteries, but I saw everything unplugged and I over trusted the circumstances. One thing is to know that a capacitor can get charged, and another one is to actually comprehend that a capacitor can get charged and shock you.

My first question is:
I thought capacitors only had one spec: the capacity, measured in farads. Why do they mark the voltage?
How dangerous are those capacitors?
What's the proper way to discharge them?

most people just short them out with the nearest handy screwdriver. but it tends to pit (&weaken) the screwdriver and isn't great for the capacitor either.

You can use a power resistor with a few Ohms, if there's one laying about on the bench - I use an NTC thermistor mains inrush damper.

 

But usually only to a small fraction of the original voltage.

Of course, it all depends what voltage they started at. I knew we had discussed this recently and I found it:
https://forum.allaboutcircuits.com/...they-charge-on-their-own.157292/#post-1362134

 

A lightbulb works pretty well to discharge capacitors. When the filament is cold, it has very low resistance, but as the filament gets hot, it's resistance greatly increases, limiting current.

 

Yeah, I shorted the remaining 280V and there was a considerably loud spark, scary if you didn't expect it.

Talking about self charging capacitors... I guess once you short them for a long period, let's say 1 minute, they won't charge themselves again, right?
If so, that really goes against the law and God.

 

A lightbulb works pretty well to discharge capacitors. When the filament is cold, it has very low resistance, but as the filament gets hot, it's resistance greatly increases, limiting current.

The current is greatly limited anyway by the decaying charge. The instantaneous current at the moment of contact is high - but still not as bad as shorting it with a screwdriver.

 

most people just short them out with the nearest handy screwdriver.

.

I've blown a hole in a couple!
Max.