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Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
I've been designing a coilgun and now I have a decision to make:
I can either get one large capacitor or 10 smaller caps. The caps are regular filter caps. I can store the required amount of energy at required voltage in either setup, and both cost about as much.
Is there any (dis)advantage in using a single large cap or many smaller caps? I think if I put many smaller caps in parallel, I get lower ESR than using a single big cap.
Since size or mechanical complexity isn't really an issue I can go either way on this one, but I'd like to hear your opinnions on this.
Banned Registered Member #110
Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
Makes not difference is using a single stage. Multiple caps can be arranged to take on a shape, as in to fit inside a housing other than a simple rectangle.
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
The one big cap probably has a bunch of smaller caps inside... well at least some of them are that way. The many smaller caps allows you to change the configuration, higher voltage, higher capacitance, etc. So basically its more flexable. Requires a lot more wiring though...
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Many small capacitors are more likely to have a failure. So either could be an advantage depending on your preference.
Several small capacitors makes it possible for you to experiment with different voltages which is usually important unless you have made a lot of coilguns.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Many smaller caps in parallel generally have a lower ESR than one large cap (as you noted in the opening post). This is very important to coilguns; even a few milliohms does make a difference in the peak current.
It is very useful for experimentation purposes to have smaller caps to combine in different ways. Ignoring the total stored energy for a moment, the rise time and resonant frequency depends on the LC time constant. So it's helpful to try combining them for various total capacitance.
If multiple caps are so good, why aren't all coilguns built from thousands of them? Mechanical complexity and assembly is certainly a practical consideration that keeps it from becoming ridiculous. But more importantly the extra wiring leads to more self-inductance and of course resistance. We do need to keep wires as short as possible.
Cheers, Barry What would the Flying Spaghetti Monster do?
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
Thanks for all the replies, but I already bought the caps back in october.
I ended up going with the multiple cap approach, for the reasons pointed out here. Now if I can actually get to building the damn thing.
But, now, as we are still talking:
How would you arrange the caps for a dual stage accelerator? I have 400J worth of caps (10 caps, at 400V max), so what would be a good starting point for two stages (300J for the first, 100J for the second 200/200, or something else?)
I know I need shorter pulse for the second stage, so less energy for it sounds good, but I'm a bit of a coilgunning noob, so lets hear what the experts have to say!
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One big cap vs many smaller caps
