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dryan

Capacitor

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Well, I was taking apart this computer monitor for fun. I took the back cover off, detached the screen from the front frame, and started popping the wires out of the connector headers. I got two or three off, then turned the monitor to reach some other ones. I looked down to see a huge capacitor, the size of a can of food. 400V 390uF. :eek:

 

From taking apart disposable cameras, I've found its not too pleasant to touch the leads of high voltage capacitors. So, out of curiosity, what would this one do to me? And how can I discharge it safely (assuming its charged now), a MOhm resistor?

 

Thanks!

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If the capacitor has a full charge on it, you could receive a pretty severe shock from contact with the two terminals. Equipment with capacitors are required by Underwriter Laboratories to have permanent and automatic means to discharge capacitors to a residual voltage of under 50-volts nominal within one minute after disconnecton from the power source. Still, it only requires 15-mA to kill a person, so don't take any chances.

 

All you need to do to completely discharge the capacitor is to short the two leads. A simple loop of insulated wire is all that is needed.

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Just make sure there's a fairly high resistance otherwise you're going to melt the wire, especially with a capacitor that big.

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Still' date=' it only requires 15-mA to kill a person, so don't take any chances.

[/quote']

 

But 15 mA where? Across the heart, presumably. So unless you are going to discharge the capacitor by grapping one terminal with your hand and touching the other to your foot, it's not likely to be lethal.

 

As Dave said, insulated wire isn't enough for a big capacitor - a power resistor with high R is what should be used, so it discharges slowly.

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But 15 mA where? Across the heart' date=' presumably. So unless you are going to discharge the capacitor by grapping one terminal with your hand and touching the other to your foot, it's not likely to be lethal.

[/quote']

 

Electrocution is a bit quirky. I doubt discharging the cap arm-to-arm would even do much (BUT DON’T DO IT!). There have been people who survived arm-to-arm shocks from pole pigs and survived with no permanent injuries.

 

There has also been sombody who died from a AA (1.5v).

 

And of course younger people tend to be able to withstand much larger shocks too. I wouldn’t let that cap scare you off. Just get into the habit of searching for capacitors when ever you open something up, and discharging them with a screwdriver.

 

Edit: I just deleted half my post because now that I think about it may not have been good advice.

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There has also been sombody who died from a AA (1.5v).

Did he eat it? ;)

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Just make sure there's a fairly high resistance otherwise you're going to melt the wire, especially with a capacitor that big.

 

High resistance??? :confused: Won't that mean lots of electrical energy would be changed into heat, therefore melting the wire?

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If the resistance is too low (a wire would have almost zero resistance), then the electricity will flow too quickly, and the energy will be released in a single spark.

High resistance will let the energy travel a bit more slowly.

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Well, won't 0 resistance mean that electricity won't be changed to heat? I would think that a higer resistance 'wastes' electricity, and what it is wasted to is heat! So, more resistance -> more heat -> LOTS of heat -> wire melting!

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It's like shorting out a battery. If you have a motor or a LED or something that gives resistance, you won't short it out. But if you just connect the leads with a wire, then it's going to heat up.

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shorted with a 0.1ohm wire you get:

 

V=IR

voltage=Current x Resistance

400=I0.1

I=4,000

 

4000 amps flowing through the wire, even so it will be a very short pulse and a screw driver wouldnt be effected by it much. Just make sure the side of the cap with the grooves isnt facing you. And dont get freaked out and stab yourself or somthing.

 

Did he eat[/b'] it? ;)

 

Eh I dont think you want to know what he did with it...

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V=IR

voltage=Current x Resistance

400=I0.01

I=4000

 

ummm... it will be 40,000 amps, not 4,000

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ummm... it will be 40,000 amps, not 4,000

oops! fixed. It should have said .1 like it said at the top.

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hehe! :)

 

Isn't 4000 amps LOTS of current? Then why don't electric companies use that current instead if its not going to be changed to heat?

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Well higher voltages also create higher current. Thats why the power lines run at 6kv-100kv and thus need a pole big to step the voltage down to 120.

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It probably will create heat (maybe sparks), which is why he advises me to watch where I aim my screwdriver...

Thats why I'll be using at least a 100 ohm resister, which gives a more managable 4 amps.

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Your all nuts! Short the leads with a short peice of any small gauge wire or a few quick taps with a screwdriver is all you need to do. If the unit was denergized for any period over a few minutes, the internal circuitry has already dischraged most of the eneregy from the cap. All that will be left is some residual charge. 4,000 or 40,000 amperes at 1/100 of a second isn't going to melt a copper conductor anyway.

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Well' date=' I was taking apart this computer monitor for fun.

[/quote']

 

Did you try to drive the flyback?

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V=IR

 

More importantly' date=' for purposes of whether you melt the wire, etc. P = I[sup']2[/sup]R, so if you drop the current by 10, you drop the power by 100.

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Your all nuts! Short the leads with a short peice of any small gauge wire or a few quick taps with a screwdriver is all you need to do. If the unit was denergized for any period over a few minutes, the internal circuitry has already dischraged most of the eneregy from the cap. All that will be left is some residual charge. 4,000 or 40,000 amperes at 1/100 of a second isn't going to melt a copper conductor anyway.

 

You don't want to assume that, and you obviously haven't been around any really big, industrial-sized capacitors, or heard any "war stories" from people who have. It's the people who assume that the big ones can be treated like the little ones that get into trouble. You don't take chances that the capacitor might be mislabelled or anything, follow proper safety protocol, and you don't get hurt.

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Eh I dont think you want to know what he did with it...

 

By that I assume his problems were all behind him, if you take my meaning.

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By that I assume his problems were all behind[/i'] him, if you take my meaning.

 

His problem was behind him, but the guy who did the autopsy sure took care of that.

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