Capacitors and their ratings, impact. / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Capacitors and their ratings, impact.

1 2 3 next

Move Thread

LAN_403

Angry_Llama

Sun Aug 31 2008, 02:40AM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

First, let me say "Hi"- I'm new here and I'm really enjoy building high-voltage based electronics. Right now, though, I'm building a coilgun. So far, I have figured out the majority of the construction, but get completely lost in two sections, one of which I hope you can help with now, and the other when I encounter it again.
In the construction of my coilgun, I will be using a large amount of salvaged capacitors. For this device, what is the more important rating, voltage or capacitance? I have two 63V 10000 ÂµF capacitors, and one 16V 10000 ÂµF capacitors, amongst the piles of lower voltage and capacitance. Which should I use? Should I give up and just order better ones?

Backyard Skunkworks

Sun Aug 31 2008, 03:30AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

The 63v cap will provide enough power to get a nail going pretty fast if your coil has low enough resistance. Generally it's a good idea to have atleast 150V for coilguns, but lower voltages can work if the reistance is low enough.

Try to shoot for under half an ohm in the coil and the 63V cap will make a decent first gun.

Also remember to put in a crowbar diode that opposes the direction of current flow. It will save your capacitor from an early death.

Mechanical triggering might work at this voltage with sufficient inductance, otherwise opt for some SCRs rated around 400A peak.

Barry

Sun Aug 31 2008, 04:05AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Welcome to 4hv! It turns out the important factor is the amount of potential energy it stores, which is a combination of both capacitance and voltage:

PE = 1/2 * C * V^2

where C is capacitance in farads, and V is voltage.

Did I read your note correctly that you have some 10,000 ÂµF caps where some are rated at 16v and some at 63v? Using these numbers for an example:

PE = (0.5) (0.010 F) (63 v)^2 = 19.845 joules

PE = (0.5) (0.010 F) (16 v)^2 = 1.26 joules

It would be tempting to connect all your caps in parallel, so the capacitance adds up, but then the voltage rating is limited to the lowest rating in the bunch. Since they're in parallel they would all see the same voltage.

Cheers, Barry
Haikus are easy
but sometimes they don't make sense
refrigerator

Angry_Llama

Sun Aug 31 2008, 05:46AM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Thanks Barry! I can always get behind the whole physical aspect of things, it's the easy part for me :) You read right that there were 10,000 microfarads at 63 Volts and 16 volts. Also dealing with that, do i understand correctly that if I were to wire them in series, I would gain voltage but halve capacitance?
Backyard Skunkworks, I'm not sure what you mean by trigger. I have a momentary SPST switch that I was using for that, is that a bad idea? from what you're saying, it seems that wouldn't end well.
Finally, in a related part, this is more of a desktop test then a handheld sort of item, I wouldn't want to waste materials on a case for a prototype. I'm using an old marks-a-lot marker as my barrel, how long of a coil should I make? When I go to wind more layers, do i wind on top going back to the start of the lower coil as in the opposite direction of the lower layer, or do I go back to the beginning and go in the same direction as the lower? I apologize for the multitude of questions, but some things I can
t find explained clearly (or at all). Once I get a working secondary or tertiary prototype, I think I'll compile a repository of knowledge on the subject...
EDIT: I noticed from the formula- voltage is actually more important than capacitance, especially since capacitors usually don't exceed the microfarad...
Informed EDIT: Backyard Skunkworks, the switch I was using is not even 5 Amps, so, to answer my own question, Bad idea. Why exactly do I need a trigger switch, why can't I use two pieces of copper?

Barry

Sun Aug 31 2008, 01:45PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Whew, a lot of questions, but you have to start somewhere. Btw it would be worthwhile to edit your posts for organization, putting related topics together so it's easier to answer.

Yes, when various cans have the same capacitance rating, the only thing that matters is the voltage rating. The stored energy goes up as the square of the voltage so you can see why 100v - 300v capacitors are so attractive.

Careful about connecting different capacitors in series. If they have different ratings then it becomes difficult (even unsafe) to charge and discharge them. More about it here.

Switches? I say try your SPST and see what happens. It will work at least once.

Just don't count on it lasting a long time. This usage tends to arc and pit and even weld the contacts. The 5A rating is for continuous duty and you can exceed that in pulse duty as in coilguns. You can exceed it by 100x or more but it obviously shortens its lifespan.

Almost any kind of switch will work, the bigger the better. Even copper plates will work. Too bad they don't sell mercury-wetted contact wall switches any longer. Mechanical switches have the advantage of lower contact resistance than SCRs and silicon devices. But in multi-stage coilguns a mechanical switch can't be used in subsequent stages.

Winding? As long as all the turns go on in the same direction, it is not critical how the coil is wound. If you start winding with clockwise turns then they must all be clockwise or they'll cancel each other's effects.

A random wound coil will work pretty well. A neat tidy coil can work a little better. There are some small gains to be had with neat tidy coils; using less wire results in lowering both resistance and inductance. So wind one layer from left to right, and continue immediately stacking the next layer from right to left, to minimize the amount of wire. By the way, keep all the other wires very short in the main high-current path, for the same reason.

Firing tube? With only a few dozen joules of potential energy in your caps, it will work better with a smaller diameter. It will be more impressive to shoot small nails; bolts and such won't get much speed. A marks-a-lot tube may be 5 - 10mm across, and you need is a tube is a tube just barely bigger than a nail. Try straws and ballpoint pen tubes.

Cheers, Barry
I'm diagonally parked in a parallel universe

Angry_Llama

Sun Aug 31 2008, 01:52PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Thanks Barry, that's pretty much the majority of my questions, save one final one: Because the resistivity (and thus its resistance) of a wire decreases in lower temperatures, wouldn't a coilgun be inherently more or less efficient depending on the time of year and climate of the user? Or is the change due to temperature differences negligible? I was never expected to know the differences temperature brought into the equation, it was always a simple standard, hence my question.

youngcoilgunner

Sun Aug 31 2008, 03:41PM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
Location:
Posts: 45

the effects of climate would almost be negligible. however, If your talking about the difference between -20 degrees, and 120 degrees, there are some noticeable resistance changes

Angry_Llama

Sun Aug 31 2008, 09:42PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

Thanks again, folks. Right now, it seems like I'm going to go with the 2 10000 microfarad, 63V caps for my desk prototype, and use an old bic casing as the barrel. As for the coil, I'll try to make it 25mm in length and about 7 layers.(I could use help with the numbers for this part, it's the only one i'm not sure about.) For a trigger, I'll sacrifice this SPST switch, as it's useless to me anyways, and I believe that, since I already have it, using the flash circuit from a camera instead of making my own DC-DC converter is best... Anything I'm missing?
EDIT: Well, that was fast. I found out the capacitors were completely worthless, they are not working at all. Multimeter confirms this. Now, I need suggestions for capacitors, where to buy, capacitance/voltage, and amount to get...

Backyard Skunkworks

Mon Sept 01 2008, 06:22AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

First off, is the charging plarity right on them? Second, did you fire them WITHOUT a protection diode? (sometimes one shot could do it, although not likely.

For other sources of capacitors, I'd reccomend a photoprocessing store. They will give away used disposable camera casings each with a 330V 80-160uF cap, between 4.5 and 9J. Just remember the protection diode! (Thats a diode that faces the oppisite way of the main current flow, to kill the inductive kickback).

Angry_Llama

Mon Sept 01 2008, 11:15PM

Registered Member #1669 Joined: Sun Aug 31 2008, 02:33AM
Location: 127.0.0.1
Posts: 13

They weren't even wired in yet! They were salvaged from an old stereo Amp, and I am assuming that this was why it wasn't working. I'm going to go to my local CVS, as I can't think of any dedicated photoprocessing stores in my area (any of the ones I once knew have gone out of business with the advent of the digital age.)

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob