Building a 365kJ Capacitor Bank for Experiments | Abiscus.com

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Discussion Overview

The discussion revolves around the construction of a capacitor bank for experiments involving high currents. Participants explore the calculations for energy storage, safety considerations, and various configurations for increasing voltage, including the use of Cockroft-Walton voltage multipliers. The conversation includes technical details, safety concerns, and practical applications of the capacitor bank.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning
  • Experimental/applied

Main Points Raised

  • Nate calculates the energy storage of his capacitors as 364.5kJ, based on the formula (1/2)CV², but later acknowledges uncertainty about the capacitance unit being microfarads instead of millifarads.
  • Some participants question the validity of Nate's calculations, suggesting that the capacitors are likely rated in microfarads, which would significantly reduce the energy calculation to 182.25 J.
  • There is a discussion about the physical size of the capacitors, with Nate describing them as approximately 8 inches tall and 4 inches in diameter.
  • Participants suggest measuring charge/discharge times as a way to verify capacitance without needing to charge to full voltage.
  • Nate inquires about the differences in charging time and energy between using a Cockroft-Walton voltage doubler and stacking capacitors in series.
  • One participant mentions the current limitations of Cockroft-Walton configurations compared to stacking capacitors for higher power output.
  • There is a mention of high voltage capacitors found in microwave ovens and their energy storage capabilities.
  • A participant discusses potential capacitor purchases and configurations for achieving higher energy outputs, including the use of various switching methods like spark gaps and thyristors.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the correct capacitance unit and the implications for energy calculations. There is no consensus on the best method for increasing voltage or the safety measures required for the capacitor bank. Multiple competing views on capacitor configurations and their effects on performance remain unresolved.

Contextual Notes

Limitations include the potential misinterpretation of capacitor ratings and the dependence on specific configurations for voltage and current outputs. The discussion does not resolve the mathematical discrepancies or the safety implications of the proposed designs.

sepulker
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I am pursuing a degree in Electrical Engineering with a minor in Physics, and I am almost done with my 4th out of 5 years ar Northeastern University (go SOX!) in Boston. I have a solid grasp of most theories related to the field.

However, exploiting my knowledge upon a physical device for the first time is different from using a pencil and a piece of paper. I am sure anyone will agree that there is no shame in double-checking yourself and questioning others with more experience.

I plan to begin experimenting with very high currents, and to do this I would like to build a capacitor bank. By luck I already have two Mallory 1800MF 450V capable electrolytic capacitors. Obviously, the energy I could store in these guys seems to be: (1/2)*(1.8)*(2)*(450)^2 which leads to 364.5kJ!

This seems like an awful lot of power, and also seems fansastically dangerous!

I plan on charging the bank with a rectified transformer (probably a microwave transformer since they are free from the junk yard) under a voltage divider for 450V. I will add safety devices like a complete 1/4inch plastic housing, bleeder resistors, and a voltage monitor, plus a huge red light that warns me not to go near the thing.

A few questions (since I have never made something like this before)

1. Is 364.5kJ correct?
2. What are the not-so-obvious dangers?
3. What can I use to discharge it safely?
4. Anything else that I should know/research before attempting this?
5. Once it's working, what devices can I use it for?
6. How would a Cockroft Walton voltage mult. config. affect energy and such?


If you would like to check out my website, it contains a lot of projects I have completed in the past:



www.abiscus.com


Thank you for your time!
-Nate
 
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Are you certain that its not microfarads instead of milifarads?
 
Yeah, they definitely say 1800MF (and not 1800UF)
 
That doesn't mean that it isn't microfarads. I have seen caps printed that way. Physically they would need to be HUGE in order to have a capacity that large at a high voltage.
 
Nate i have to ask , how large are they physically??
dimentions in inches?
 
They are probably about 8 inches tall with a diameter of about 4 inches (which would make a lot more sense for a smaller capacitance)

So industry standards aren't as concrete as I thought?... some companies use an "M" and some use a "U" for microfarads?
 
If you still aren't sure why not just charge it up and measure the charge/discharge time? You don't have to charge it up to full voltage.
 
Because I'm on vacation and I don't have them with me haha.

But you answered my question about the markings. Now I am almost 100% sure theyre microfarads. Thanks

SIDE QUESTION:
I don't want to spend time doing the analysis...

What are the differences in charging time, energy, etc of Cockroft-Walton voltage doubling configurations compared to other methods?
 
Yeah, the largest capacitor I've ever seen sold is 5000 Farads, and it was quite expensive (since it was meant for industrial use).
 
  • #10
You also made a mistake with the formula 1/2CV^2..
unless i am mistaken your new Joule calculation is .5(0.0018)(450)^2 = 182.25 J
 
  • #11
you have some nice projects on your website sepulker; I like the night vision project in particular.
 
  • #12
willib said:
You also made a mistake with the formula 1/2CV^2..
unless i am mistaken your new Joule calculation is .5(0.0018)(450)^2 = 182.25 J

He has two caps (In parallel I presume) not one.
 
  • #13
Right, I was assuming two.

I have another question:
I want to increase the maximum voltage I can put into the bank. Assuming all capacitors are the same model... is it healthier to:

1. Stack capacitors in series
2. Use a voltage doubler like the Cockroft-Walton

It would be interesting to see the pros and cons
 
  • #14
sepulker said:
Right, I was assuming two.

I have another question:
I want to increase the maximum voltage I can put into the bank. Assuming all capacitors are the same model... is it healthier to:

1. Stack capacitors in series
2. Use a voltage doubler like the Cockroft-Walton

It would be interesting to see the pros and cons
in my opinion , for raw power , stack them , besides the current output of the Cockroft-Walton and its variations is low, on the order of 100mA , although they do talk about one that looks like it could produce more current , it has sort of a complex triggering scheme because it doesn't use P channel FETS because at the time it was written they were more expensive than they are now.. http://murray.newcastle.edu.au/users/students/1999/c9221792/doubler.htm your doubler circuit
how much voltage are you looking to make?
Power fets are only rated for around one hundred volts or so , so stacking them would be your only choice..
 
Last edited by a moderator:
  • #15
did you know that there are high voltage capacitors in microwave ovens , on the order of 2100V ..
@ 1.1uF that comes to 2.4255 Joules..
 
  • #16
On Ebay, I found Nippon Chemi-Con capacitors rated at 400V 3600UF. The guy has 60 available. So let's see... each one can hold 288J each. If I buy 10 and split the stack, that's 9000UF at 800V (i think that's right?) at around 2.9kJ. I can stack it more for a better approach to a spark gap... actually...

What would be better for switching the current? A spark gap? Thyristor SCR, Thyratron? (Maybe I don't have to stack them)

If I assume my load is a coil and it has a impedance low enough to allow a fast pulse, which should be enough energy to crush a soda can, right?

This would be perfect for my demonstration.
 

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