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Registered Member #2909
Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145
i use tugstan switch they do great, but my big bank main switch is huge (it waigh over 50kg, 3phase 800A 400V power breacker) , but you can find tungstan contacks from old cuircuit breackers 20A
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Navrit Bal wrote ...
Please ignore my last post, I have been offered use of a 'HT' supply, which I gathered is a 'high tension' variable power supply by my school, although somewhat reluctantly because I'm not supposed to use anything over 50 Volts, for health and safety reasons of course. I absolutely can't blaim them for this but it can be quite annoying at times ...
Anyway, I'm starting off with 1 photoflash capacitor, the type I previously mentioned. I will make the test coil on the day, there will be something lying around I'll use the 3 SCRs in parallel as described for this but if it doesn't work, I'll have to critically dampen it... just for testing or make a quick V wedge aluminium switch, as that will be readily available.
Just as a heads up, you may need to parallel a few to get results at that voltage. One photoflash capacitor charged to 50V is only 0.15J (assuming 120uf), which might not even move your projectile. Either way I'm glad to see your nearing the build stage, keep us posted with the results and be safe!
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
Saz43 wrote ...
Just as a heads up, you may need to parallel a few to get results at that voltage. One photoflash capacitor charged to 50V is only 0.15J (assuming 120uf), which might not even move your projectile. Either way I'm glad to see your nearing the build stage, keep us posted with the results and be safe!
I'm using 330V 160uF, so that's 8.7Joules I believe, sorry for the misunderstanding
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Navrit Bal wrote ...
I'm using 330V 160uF, so that's 8.7Joules I believe, sorry for the misunderstanding
Is there any chance you can parallel a couple of these capacitors? I recommend at least 15 joules stored energy, minimum. Preferably more.
The thinking goes like this ... If you have 15J stored energy and do a good build, then after tune-up you might have 1-2% energy conversion efficiency. This yields about (15j)*(0.02) = 0.3J kinetic energy. How much velocity is that? Use the equation KE = 1/2 m v^2, and solve for velocity given your mass and energy. For sake of example, if we assume a very lightweight projectile of 0.1 grams, the math says you can get 2.4 m/s. On a good day. But before you tune and optimize things, it's unlikely the projectile will move very much, with only 15J stored energy.
So, connect a few capacitors together, if possible. Keep wires very short, solder connections, use heavy gauge wire, and have fun!
Cheers, Barry One day soon, YouTube, Twitter and Facebook will join together and be called YouTwitFace.
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I'll put at least 2 in parallel then, the reason why I'm starting with low energy levels is because my supervisor doesn't want me to start at the high energy levels. I'll use part of a paper-clip tomorrow, I'll use bits of paper-clips from 0.1 to 0.5 roughly, but I'll see what happens.
Also, since no one has answered this, wouldn't the best wire be made out of thick wire? I'm thinking this because it minimises insulation, resistance and inductance, are these properties not desirable? I'm taking the tungsten rods to school tomorrow, I'll devise something then...
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Navrit Bal wrote ...
I'll put at least 2 in parallel then, the reason why I'm starting with low energy levels is because my supervisor doesn't want me to start at the high energy levels. I'll use part of a paper-clip tomorrow, I'll use bits of paper-clips from 0.1 to 0.5 roughly, but I'll see what happens.
Also, since no one has answered this, wouldn't the best wire be made out of thick wire? I'm thinking this because it minimises insulation, resistance and inductance, are these properties not desirable? I'm taking the tungsten rods to school tomorrow, I'll devise something then...
You are correct to some extent.
Resistance IS bad, it leads to more energy being wasted as heat (Waste Heat = P = I^2*R). However if you resistance is TOO low, you might exceed the current rating of your switch and destroy it.
Inductance isn't necessarily bad. Obviously, if your inductance is too high, your current pulse will be wildly under-damped and you'll accomplish nothing. But if its to low, your current pulse will be too fast and your projectile might saturate from having all of the energy dumped into it too fast. You have to balance these factors in your design, and that's why Barry's simulators are so valuable.
You are certainly correct about insulation. The closer your coil turns are to the projectile, the more intense the magnetic field will be at the center. Insulation, although necessary, spaces out the turns and hampers efficiency. That's why most designers opt for magnetic wire, since it has such thin insulation.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Navrit Bal wrote ...
Is the ideal pulse time roughly 1-8 milliseconds? I've seen this suggestion somewhere, but I can't actually remember where...
You might have seen it on my Estimating Timing page, where it uses the projectile length and a guess at exit speed to make a swag (stupid wild-ass guess) at timing. It is based on a couple extremely crude simplifying assumptions, but it seems to work and the math is simple.
Run the numbers again with your situation to find out what you can expect.
Since the actual timing is determined by the LC time constant, all you have to do is build it toward the long end of the estimated time, and then tune up the coil by gradually removing turns. Fewer turns -> lower inductance -> shorter firing time.
Cheers, Barry I want to be the BEST DAD that an ungrateful brat has ever bitched about!
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Coilgun Questions
... 
I'll use the 3 SCRs in parallel as described for this but if it doesn't work, I'll have to critically dampen it... just for testing or make a quick V wedge aluminium switch, as that will be readily available.
