365kJ Capacitor Bank

sepulker

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

Cliff_J

Are you certain that its not microfarads instead of milifarads?

sepulker

Yeah, they definately say 1800MF (and not 1800UF)

Averagesupernova

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.

willib

Nate i have to ask , how large are they physically??
dimentions in inches?

sepulker

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?

Averagesupernova

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.

sepulker

Because i'm on vacation and I don't have them with me haha.

But you answered my question about the markings. Now im 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?

Manchot

Yeah, the largest capacitor I've ever seen sold is 5000 Farads, and it was quite expensive (since it was meant for industrial use).

willib

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

exequor

you have some nice projects on your website sepulker; I like the night vision project in particular.

faust9

He has two caps (In parallel I presume) not one.

sepulker

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

willib

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 doesnt 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...92/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..

willib

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..

sepulker

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, thats 9000UF at 800V (i think thats 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.