Re: [femm] Total current in a high energy disharge.

[Dave Squires <djsquires@xxxxxxxx>]

If you want to calculate a ballpark number use the basics and
don't muddled up with fancy LCR analysis.
You need R to find the time constant.
Then you need the voltage you will charge to.
Then tau = RC gives you the time of discharge.
You know C. If R is very small for a short miminum discharge time.
If you are going to use an SCR then use the on-resistance specification.

Assume 0.01 ohm short circuit just for kicks.
RC = 0.007*0.01 = 0.00007 second = 7e-5 = 70us

Now use the basic equation i = Cdv/dt
Assume dv/dt = 400v/70us and solve for i.

i = 7e-3*400/7e-5 = 1e2*400 = 40,000 amps!!!
over a 70us window approximately.  Get the picture!
Yes that is real! Forty thousand amps!!
You will have a blinding flash plasma 
explosion if you use the short circuit approach with a big bussbar.
Not recommended if you want to survive the experience unhurt.

So increase the resistance to slow it down and decrease the current
peak.  An SCR will have higher resistance than a metal to metal
short. However, you can set the current to a safe level by putting
in a series resistance to limit the RC time constant to change the dv/dt
in the above equation to get the desired current....highly recommended.

Work it backwards to get what you want.  Be careful!
If you can't do the math backwards then abandon the attempt
and understand what you are doing first.
What you have is a big, dangerous capacitor.  Watch out!
Keep it shorted at all times when not using it so it won't accumulate
charge and knock you flat when you pick it up the next time.

Good luck,
Dave Squires
 

David Meeker wrote:

 

fucian@xxxxxxx wrote:

Hi, Im new to this stuff and cant download right now.Can anyone tell
me how to find amperage when inductance and capacitance and volts are
know?Let me tell you what Ive got.I have a flat solenoid of 7uH and a
7000uF 450 volt cap.I need to know how much current the solenoid will
pull and how fast the current rise is so I can get a proper SCR for
it.Can anyone help me?Oh yea, this is a non-repetitive
application.Just a charge up...Fire type.

Hmm--It's strange for me to be the voice of reason, but if you can't analyze an LCR circuit, you have no business messing around with one at this sort of power level.  Well, even if you can analyze an LCR circuit, you have no business messing around with one at this sort of power level unless you have been trained by a professional.  It's easy to get yourself injured or dead if you don't know what you're doing (e.g. electrocution, exploding capacitors, "splash", etc).

"Don't try this at home, kids!"

Dave.

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