Capacitor specification, doesn't exist maybe (theoretical)

  • #1

Main Question or Discussion Point

Need to come up with specifications for a capacitor that can hold 20,000 or more amps
How many joules or watts or how much energy can this theoretical capacitor hold
how many farads, what would the current be and how many volts could it tolerate, how to increase volt tolerance. how to increase charge rate, decrease charge time?

how much surface area would be needed for the electrodes, thinking of hafnium as a electrode and thinking of teflon as a insulator since it has a dielectric strength of 60x10^6, but it melts around 650 degrees. is there something else with a high dielectric strength but higher melting point?

Any input for these questions is greatly appreciated
 

Answers and Replies

  • #2
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Amps is not a unit of capacitor capacity. Try it this way, what do you want to accomplish with this capacitor?

Are you working on a SF story?
 
  • #3
davenn
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Apart from @anorlunda response to storage capacity ( with which I agree)


how much surface area would be needed for the electrodes, thinking of hafnium as a electrode and thinking of teflon as a insulator since it has a dielectric strength of 60x10^6, but it melts around 650 degrees. is there something else with a high dielectric strength but higher melting point?
this statement of yours doesn't make sense

Why do you think the dielectric should be getting to 650 deg or more ??

Dave
 
  • #4
osilmag
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Capacitance is defined as $$ C = \frac {Q} {V} $$ which equals $$C = \frac {\epsilon A} {d} $$

The time constant is defined as $$ \tau = RC$$

20,000 amps is a lot of current, which is a lot of charge, so I'm guessing you might need a large area. Like an enormous capacitor.
 
  • #5
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As pointed out Caps do not hold current, they hold charge, so assuming you are asking about a Cap that can provide 20KA?

So HERE is a cap we use pretty frequently...

3900uF, 450V DC rating,,,, and ESR of .045 Ohms, so this can theoretically output ( at the terminals) about 8000 A ( starting at 400V ) --- but one time constant is 0.000195 Sec....and you are down 63% in V and thus in current.

So it "can" be done but you need to further define what you are looking to do - the Bus design that connects many caps in parallel needs to be included in the calulations - design. How long do you want the 20KA - etc.

Next - that 650 Deg temp spec, this cap above is electrolytic, will not survive this. A film cap may perform better but have lower capacitance ( less charge), almost 1/8th for the same size. ( This size can may be about 420uF/ 1100V) - still no where close to the 450 ( F ? C? ) spec...

Budget? These are both pretty high volume units with good $/performance, to dramatically get performance ( temp or otherwise) beyond these the cost goes up exponentially.
 
  • #6
jim hardy
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Need to come up with specifications for a capacitor that can hold 20,000 or more amps
How many joules or watts or how much energy can this theoretical capacitor hold
how many farads, what would the current be and how many volts could it tolerate, how to increase volt tolerance. how to increase charge rate, decrease charge time?
@DJ Jackson

There exist devices called "Thumpers".
They're used by electric companies for finding faults in underground cables.
Basically they charge a capacitor to high voltage,
then discharge it into a cable that has a short circuit.
The high current creates a small explosion where the short is.
A lineman walks along ground over the buried line and localizes the "Thump" with his feet as the Thumper makes a series of "Whump" sounds and shakes the ground.
That way the crew knows where to dig.

I have a Thumper capacitor.
It's 14 microfarads at 34 kv DC.
At that voltage it will hold energy = ½ CV2 = ½ X (14X10-6) X 34,0002 = 8092 Joules
One needs a wheelbarrow to carry it.
Its connections are large brass bolts so clearly it's optimized to deliver high pulses of current.
How much current it will deliver is figured by Ohm's law, Current = Volts/Ohms.

My point is twofold
1. You ought to get your basics squared away. Vocabulary is important. I dont know how to handle a question that conflicts with itself.
2. You can probably buy what you want off the shelf. But not at Home Depot.
http://www.sse.net.au/products/high-voltage-test-equipment/test-cable-fault-locator-thumper

High current surge capacitors have application in weapons. I won't go there.

Try it this way, what do you want to accomplish with this capacitor?
 
Last edited:
  • #7
sophiecentaur
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Why do you think the dielectric should be getting to 650 deg or more ??
Capacitors in power supplies can get hot due to so called Ripple Current and some dielectrics are significantly lossy at high frequencies. They can also roast when in the wrong place.
To know what would be suitable for the OP's application, the specified requirement needs to make sense, though.
 
  • #8
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While we knock ourselves out with more answers, the OP has not returned to PF since creating this thread so he hasn't seen any of the answers yet.

Thread closed.

@DJ Jackson , if you want to reopen this thread, click on my name and open a conversation.
 

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