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Dangerous Capacitor

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mark_3094

New Member
At what size is a capacitor dangerous?

If (in theory) I charged a 4700µF cap, then touched the leads, how bad would the shock be?

Thanks
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Probably no shock at all, it's the VOLTAGE that gives you a shock, and a 4700uF is mostly pretty low voltage.

As for what 'voltage' is dangerous, it's a very widely debated point - 110V mains in various parts of the world doesn't prevent death by electrocution, and it doesn't seem any safer than the 230V mains elsewhere.

I seem to remember reading years back that some areas of the USA (farms or some such) even used 60V mains, and that people had been killed by that - I would think you would have to work really hard to get killed by 60V.
 

Sceadwian

Banned
Mark, it's not just the farad rating that makes a cap dangerous, it's the voltage too. Your 4700u cap question has no reference without a voltage. The energy in joules a capacitor has available is 1/2 CV^2 where C is the capacitance in Farads and V is the voltage. Capacitors can discharge their energy VERY fast so they become exponentially more dangerous as the voltage increases. There is no hard cutoff point for lethality or danger as it's all dependent on how it's discharged into a person.
 
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Sceadwian

Banned
220u at 400 volts can kill if you touch one wrong.
 
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stevez

Active Member
When you say dangerous that could mean potential to cause harm. A small spark in an explosive situation (near gasoline for example) could be dangerous. Let's assume your concern is electrocution. We'll further assume that electrocution means your heart could stop.

My memory may be faulty but I thought that 20 milliamps was the threshold of current that if passing thru or near the heart could cause it to fail. I may be incorrect in the value but let's just assume I am correct.

The flow of current depends on the load resistance and the voltage available. A quick google revealed that the resistance of the human body might be 100,000 ohms for contact with dry skin dropping to 300 ohms with good, intimate contact (most skin, exposed flesh, conductor imbedded in skin). Again, these values may be off but we'll use the info until we have something better.

With a threshold of 20 milliamps and a resistance of 100,000 ohms we'd need 2,000 volts of potential. If the resistance is 300 ohms then 6 volts potential is sufficient.

What I didn't do here is account for the limited time that the potential exists as the capacitor discharges.

Again, the data may not be quite right but I hoped to illustrate that the answer to your question is quite dependent on conditions. Stick the ends of the capacitor leads thru your skin and it could turn deadly. You might not want to do that but you could do it by accident.
 

Pommie

Well-Known Member
Most Helpful Member
If you charge your 4700µF capacitor to 400 volts then it would contain ½ x 0.0047 x 400² which is 370 Joules. This is the same energy as a defibrillator and could easily stop your heart.

Mike.
 

Boncuk

New Member
If you charge your 4700µF capacitor to 400 volts then it would contain ½ x 0.0047 x 400² which is 370 Joules. This is the same energy as a defibrillator and could easily stop your heart.

Mike.
Only an electric chair works more reliably.

Boncuk
 
You said, "dangerous". Danger has infinite forms. Ancillary damage could occur from the "startle response", or involuntary muscle contraction. You could jerk back, letting fly the screwdriver in your hand and impaling the cat... or your mother-in-law. (Hmmm...That might be worth a jolt!) Seriously, I've had caps explode and embed themselves in my ceiling tiles. (Think "eyes" here.) If you're thinking of playing a joke on someone, just make REAL sure you can outrun her boyfriend!! (Voice of experience!)

p.s.--If you don't survive your experiments, can I have your stuff?
 

Hero999

Banned
It's not just voltage but current and energy.

DC is generally regarded to be much safer than AC, 60VAC will give you quite a shock and 60VDC will only shock you if you have damp skin.

Capacitors are always charged with DC voltages and there is a limited amount of energy stored in the capacitor. Obviously the larger the capacitor, the higher the voltage, the greater the risk of a lethal shock becomes.

A really small capacitor charged to a high voltage will not kill you, for example a 100pF capacitor charged to 10kV will only give you a mild static shock. A much larger capacitor charged to a lower voltage is much more likely to kill you: a 1000:mu:F capacitor charged to 350V will cause far more harm than that tiny 100pF capacitor even though the voltage is much higher.

I read somewhere that capacitors are only really dangerous if they're charged to more than 120V: at this voltage you will receive a shock but it's unlikely to kill you. I don't know what the lethal capacitance is, it's probably more about energy than anything else.
 

mark_3094

New Member
OK, well basically I was wondering if I could charge up my 4700uF cap, get someone to touch the two leads (bit of a prank you see) and get a bit of a shock.

I don't want to cause any harm to anyone though. That's the main concern I had.

I have had all the warnings about discharging capacitors before working on CRT monitors flash through my head...


If I charged it to 12v, would that cause a small zap without hurting anyone? Would 24v be better or too much?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
OK, well basically I was wondering if I could charge up my 4700uF cap, get someone to touch the two leads (bit of a prank you see) and get a bit of a shock.

I don't want to cause any harm to anyone though. That's the main concern I had.

I have had all the warnings about discharging capacitors before working on CRT monitors flash through my head...


If I charged it to 12v, would that cause a small zap without hurting anyone? Would 24v be better or too much?
Neither would be felt at all by the vast majority of people, under the vast majority of conditions - much too small a voltage.
 

colin55

Well-Known Member
You will find that if the voltage is AC, it will produce a much greater effect on the "target."

I have produced a card, that when opened, it shocks the user. If the voltage is DC, it has almost no effect, even at 150v. But an AC voltage at 120v will make them drop the card.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I have produced a card, that when opened, it shocks the user. If the voltage is DC, it has almost no effect, even at 150v. But an AC voltage at 120v will make them drop the card.
You've obviously never played the fun game of charging a 100uF to 350V and throwing it to the young apprentice! :p
 

Pommie

Well-Known Member
Most Helpful Member
You will find that if the voltage is AC, it will produce a much greater effect on the "target."

I have produced a card, that when opened, it shocks the user. If the voltage is DC, it has almost no effect, even at 150v. But an AC voltage at 120v will make them drop the card.
Your design is faulty, 150V DC will kill.

Mike.
 

Pommie

Well-Known Member
Most Helpful Member
You've obviously never played the fun game of charging a 100uF to 350V and throwing it to the young apprentice! :p
Played that game and it is fun. However, I wouldn't try it with a 4700uF cap.

Mike.
Hmmm, I haven't tried that on my sons know all mates. Could be fun.
 

Hero999

Banned
Your design is faulty, 150V DC will kill.

Mike.
Not very likely, you've got more chance of saying ouch, that hurt.
 

Pommie

Well-Known Member
Most Helpful Member
Not very likely, you've got more chance of saying ouch, that hurt.
150VDC will kill you. You are confusing low amperage supplies with true 150VDC. It's a bit like stating that the 20kV from a spark plug won't kill you - of course it won't because when you conduct it to ground the voltage is greatly reduced. An 11kV overhead line will turn you to carbon.

If my memory serves me correctly, anything over 48V is considered lethal in the UK. On farms I believe it is 25V!! That wet mud sure does conduct well. (bit of artistic (argumentative) license there, it's actually because bigger animals conduct better and so can be killed by lower voltages.)

Mike.
 
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