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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sat 03, 2018 4:23 am 
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Morcegao wrote:
Don't know if it's the BEST way, but some years ago, a co-worker showed me what must surely be the MOST FUN way. He would charge up this REALLY BIG industrial "power factor corrector" cap with the bench DC supply and then short it with a fine wire which would then vaporize with a bright flash and a loud BANG!

There's an interesting write up by Steve Hansen at the Bell Jar:
http://www.belljar.net

He has a nice booklet "Exploding Wires, Principles, Apparatus, and Experiments":
http://www.belljar.net/Exploding_Wires.pdf
Steve Hansen wrote:
The exploding wire phenomenon depends upon a very fast current rise, on the order of a few microseconds. Conn [1] described the process as comprised of six steps:
  • Heating of the wire by the heavy current passing through it.
  • Formation of a liquid column of metal replacing the wire.
  • Development of instability in the wire which forms unduloids (figures of rotation) and causes
    the appearance of striations in the metal vapor. (This is due to the mechanical and
    magnetic forces created by the discharge and heating.)
  • Upon the disruption of the arcs formed between the unduloids, a “dark interval” results during
    which no current flows through the wire; the voltage remains constant.
  • A sudden flash of light is next observed, the spectrum of which is continuous and independent
    of the material used. Thereafter emission and absorption lines are observed depending
    upon the materials used.
  • One or more sonic shock fronts are created during the same interval.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 04, 2018 5:18 am 
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Greetings to the Forum:

Since we are talking about discharging capacitors, here's a bit of a brain teaser for you:

Q=CV. This means that the the total charge Q (in coulombs) in a capacitance C in farads is equal to the voltage in volts times the capacitance.

Re-arranging with a bit of algebra, we get V=Q/C.

Now, it is also true that the ENERGY stored in a capacitor is: E=1/2 CV^2 where E is in joules, C is in farads and V is in volts.

So, let us imagine a couple of big capacitors... both are 1 farad (to make the math easy).

Now one of these capacitors has been charged to 10 volts. From Q=CV, we know that the charge stored in this capacitor is 10 coulombs. From E= 1/2CV^2, we know that the stored energy is 50 joules.

Now, let us assume that the other capacitor is completely discharged. We now connect it in parallel with the charged capacitor. Since both capacitors are identical 1 farad capacitors, connecting them in parallel results in a capacitance of 2 farads. Since we have 10 coulombs of charge total to share now between our two capacitors, we can plug that into our V=Q/C formula and we get a voltage across the two capacitors in parallel of 5 volts. Since the capacitance has doubled, the 10 coulombs is now divided by 2 farads instead of just 1.

However, if we apply the formula for energy, we have E = 1/2 CV^2 or 1/2 * (2 farads) * (5 volts) ^2. Simplifying, we have 1/2 of 2 *25 or 25 joules.

We had 50 joules to start with. We have simply changed the size of our storage tank. Yet now, we have only half of the energy we started with. Where did the energy go? :D

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 04, 2018 8:46 pm 
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Jthorusen wrote:
>...
We had 50 joules to start with. We have simply changed the size of our storage tank. Yet now, we have only half of the energy we started with. Where did the energy go? :D

Regards,



If I recall correctly from Physics 101, fully half the energy goes into electromagnetic radiation.


^^;;^^


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 04, 2018 9:22 pm 
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stevebyan wrote:
Morcegao wrote:
Don't know if it's the BEST way, but some years ago, a co-worker showed me what must surely be the MOST FUN way. He would charge up this REALLY BIG industrial "power factor corrector" cap with the bench DC supply and then short it with a fine wire which would then vaporize with a bright flash and a loud BANG!

There's an interesting write up by Steve Hansen at the Bell Jar:
http://www.belljar.net

He has a nice booklet "Exploding Wires, Principles, Apparatus, and Experiments":
http://www.belljar.net/Exploding_Wires.pdf


Interesting paper ... somewhat quantifies the energy of the BANG!... at least if you have a "feel" for what a Joule sounds like. From what I remember, it was about as loud as an M80. Anybody got an idea of how many Joules in a good M80?

I don't know how many Farads (or micros) those big old caps had, but I do recall that each one came off of a 1000 HP 4160V compressor and we would charge 'em up to around 1500V. DEFINITELY NOT when any of the bosses or potential snitches was around!

That paper talks a lot about safety ... if I remember right, we just used a couple of short fat welding cables and made the final connection by hand to to the cap (welding glove to protect in case of sparks when contacted). Guess we were somewhat lucky, but I spose the risks were part of the fun.


^^;;^^


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Mon 05, 2018 12:47 am 
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In high school electronics class back in the '60's our instructor liked to charge a filer cap on a surplus dynamotor and then pitch it to one of his students. That teacher was a real ass and I'm sure would be jailed today.

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Mon 05, 2018 3:04 am 
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If you want an even bigger flash / bang, take a toilet paper core cardboard cylinder, stuff a wad of the paper in each end, run the fine wire down the middle, and fill the core with a couple of ounces of aluminum powder (the kind they sell for mixing in paint ... AKA Aluminum Bronze). Just make sure you are not looking directly at it when you touch it off ... flash burn of the retina.

DAMN! The stuff we used to do ... sorta amazing we're (mostly) still around to tell the tales!


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Mon 05, 2018 3:16 am 
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I used to make noise with this cap bank:
Attachment:
capbank.jpg
capbank.jpg [ 43.05 KiB | Viewed 331 times ]


330,000 mfd at 50 volts. The power resistor was to limit the charging current from a large power supply. The 150W bulb was for discharging. With some practice, it could sound like a large firecracker. Once, I set the grass on fire. Since then I have amassed enough newer caps to make 2 farads at 50V, but they are PC mount, which will require some careful wiring.

Bob

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Wed 07, 2018 12:04 pm 
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Greetings to the Forum:

Only Morcegao has had a go at my brain-teaser. Are there no other brave souls out there? :?:

Regards,

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sat 17, 2018 5:40 pm 
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Jthorusen wrote:
Greetings to the Forum:

Only Morcegao has had a go at my brain-teaser. Are there no other brave souls out there? :?:

Regards,

OK, I'll give this one a shot...

The most plausible explanations I've heard regarding the capacitor paradox:

1.) It does nothing more than illustrate the limitations of an idealized circuit model. In this circuit, the charged capacitor would discharge to 5V, while supplying infinite current to the discharged capacitor, which would, in turn, charge up to 5V in infinitesimal time.

2.) The flow of current through a conductor must create a magnetic field (Michael Faraday nods his head in agreement). The creation of the field during the charging time requires energy, which comes from the energy initially stored in the capacitor. Of course, the idealized circuit with zero inductance for the conductor means that calculating the magnetic field strength is mathematically impossible in this circuit.

Adding inductances and resistances into the idealized circuit allows one to calculate the amount of energy dissipated as both electromagnetic radiation and heat. IIRC, most of the energy lost in a real circuit in this scenario is usually heat.


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 18, 2018 3:05 am 
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Greetings to the Forum!

ALL of the answers to my brain-teaser were correct.... all two of them!

Congratulations to lexrageorge and Morcegao who were the only ones to take a stab at it.

For a low resistance connection (large hunk of wire) the energy is radiated away as EM radiation (see spark transmitter). For a connection made with a resistance, most of the energy is dissipated as heat in the resistor. Were one to make the connection with an inductance, the circuit would "ring" at its resonant frequency, again radiating away most of the energy in the form of radio waves.... with the balance lost to heating effects. (Note that the large hunk of wire mentioned above is a special case of the general case of inductance.... the wire just has very low but not zero inductance.)

And there you have it.... sneaky physics.

Regards,

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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 18, 2018 11:15 am 
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The home made capacitor reformer here, actually for catching faulty newer ones, switches to a 10K resistor for discharging. Also there is a cheep & very versatile Chinese tester for identifying transistors inductance, capacitance, resistance, ESR....... Feed that a charged cap & you will destroy it, a couple of Radio Club members here have tested it by doing that, & it does. :oops:

Marc


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 18, 2018 4:22 pm 
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smeezekitty wrote:
I don't like direct shorting the cap. Besides the risk of damage, it scares me! I don't like *pops*I suggest using a resistor. If the circuit is drawing current, the caps will discharge in only a few minutes. It's best to test before touching no matter what.
There is more: Other than certain, not all, photo-flash and specifically designed capacitors, none should be discharged by direct short circuit. High internal currents and magnetic effects can possibly open connections, break the continuity of the metalization or in some instances, open a protective link ruining the capacitor. In terms of metalized and so called "soggy foil" capacitor designs, areas of metalizing will become isolated and reduce the capacity. Sometimes slightly, those small isolated areas will randomly connect and create a noise spikes. In sensitive, high impedance circuits the spike can cause un-desired operation of a device. I have confirmed this in the development of photoflash devices.
There is the hazard of spraying metal, particularly copper.
Google search: [Copper expansion in arc flash] wrote:
The tremendous temperatures of the arc cause the explosive expansion of both the surrounding air and the metal in the arc path. For example, copper expands by a factor of 67,000 times when it turns from a solid to a vapor. The danger associated with this expansion is one of high pressures, sound, and shrapnel.
Arc Flash and Blast Hazards per NFPA 70E

YMMV

Chas


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 Post subject: Re: The best way to discharge a high voltage capacitor
PostPosted: Mar Sun 18, 2018 11:22 pm 
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The Flash Gun & Electric Fence capacitor are Pulse Grade. The caps in the electric fence (one 20uF @900V) charge & discharge with a pulse every second. One cap has failed in one of them since 1983 and they run 24/7. Rarely being switched off. That may be for high fire danger, or the firewood crushing the fence its on.

Marc


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