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capacitor bank questions. Answered

hello all, this'd be the first post i've ever made, and may be in the wrong place.......

edit- i guess it was in the wrong place, since no one replied.... so i'll try here....

i'm tryin to make a cap bank from 30+ flash cameras (possible? possible outputs? how do i calculate that?)  and i have some question/problems.

is it possible to take a bank like the above and use as a spot welder?   if so, how would i trigger the spark without the trigger becoming the spark gap? 

another possible use i have for it is a rail gun...... theoretically, if counter evidence isn't provided by all of you much more tech inclined ibblers

a second concern is that the caps come in at least three different micro farad ratings. would one explode if it's rating is lower than the higher ones?  (the ratings are between 80ish to 150 ish (don't remember))

i rather need the weilder for a possible steampunk project that i just bought a bunch of metal junk for....(knowing me though, i'll never get around to doing it....)


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10 years ago

You don't really provide enough information. Capacitors are rated in units of "farads" (or more usually, microfarads, nanofarads, or picofarads), which tells you how much total charge they can store.

If you have multiple capacitors, you can connect them in series, or in parallel (like resistors). The rules for addition are:

  • In series: 1/Ctot = 1/C1 + 1/C2 + 1/C3 + ...
  • In parallel: Ctot = C1 + C2 + C3 + ...

Notice that the different Ci do not have to be equal. Just do the arithmetic to get the total. If you want to maximize the capacitance, you should wire them in parallel (proving that is your homework problem).

For a spot welder, you want as much current as you can get. Current is just charge/time. The charge you get stored in a capacitor bank is Q = CV, where V is the voltage you used when you charged it up. Time in this kind of simple circuit is t = RC (technically, that is the "time constant" for the circuit, the time for the capacitor to discharge to 1/e = 37% of its full charge).

So if you know the resistance, and you know the total capacitance, you can work out just how much current you're going to get: I = Q/RC = CV/RC = V/R. Hey, that looks just like Ohm's law. How convenient :-)

Capacitors usually have a voltage rating, which is just the maximum voltage you can use when charging, equivalent to the total charge they can store before they break down. For a spot welder, as I said, you want to maximize current, which means maximizing the charging voltage. I don't know what camera flash units are rated for in voltage.

Reply 10 years ago

wow. you really know what your talking about. but. i thought a farad was a measure of current? ============================================

every flash camera capacitor i've worked with is always at 330 volts, and all of them are powered by one measly AA battery. i've still taken chunks out of drill bits with the smallest of them, so i'll be useing causion.....

from what you said, i'm reading ' it doesn't matter if you hook up a 160uf with an 80uf' right?

the micro farads all seem to either be 80, 100 120, or 160, with most being 120 or 80. also, i have quite a few that don't follow the same labeling conventions, perhaps you could help? in pic, from left. 80uf 160uf 120uf unknown, labeled 918
(the strange burnt look on the right was mearly and effect created when i removed my hideous bare gut from the pic. belive me, you don't want to see that.)

what information was it that i was missing, so i can fill in?

any ideas on the trigger mechanism?


10 years ago

wow, you're a popular guy, you know that? everyone's lining up to ignore you....
(ignore me, i'm just entertaining myself pending a reply(.....i didn't mean ignore the thread! come on now! lol))