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Barry

Thu Oct 28 2010, 04:14PM

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

GhostNull wrote ...
Yep, that is called critical dampening:

only problem is that it causes some energy to be lost and makes the pulse longer.

Critical damping design causes lots of energy to be lost. To be precise, the peak current is guaranteed to be 36.8% of the maximum possible or less. Been there, done that, tried it in my Mark 4 coilgun. It's easier to build but has poor performance.

Myke wrote ...

I think that the contacts won't weld as long as you close the gap fast enough (shouldn't be a problem at a low voltage) and that you keep them closed...

Myke is exactly right. The reason it works is that, due to the coil's inductance, the current will start at zero at switch closure, and build up to its maximum over time. Remember V = L di/dt or look at the RLC simulator. As long as the switch is fully closed before the current gets too high, then no energy is lost in the contacts. Therefore it has no contact arcing or wear. It does take some care in mechanical design to ensure sufficient surface area and to minimize contact bounce.

Now all I have to do is get off my butt and finish building my Mark 5 to prove it, lol. Actually, I have seen this working first-hand already with the 7kJ coilgun by Discovery Channel.

Cheers, Barry
Words of Wisdom: "Never go to bed with anyone crazier than you"

Navrit Bal

Fri Oct 29 2010, 01:58PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I might initially add resistance to critically dampen it just to get it working, even though it will be pretty poor. After I get it working I will at least attempt to get a non critically dampened circuit working.

Barry, would you say that a mouse trap might be a good place to start for quick mechanical closing? Or should I use what you have on your Mark 5 coilgun, the V shape trap, or something similar?

Also, could you explain this :
"At a certain critical angle (around 8 degrees for copper) the wedge will "stick" or lock in place, and needs no other mechanism to maintain contact."

Barry

Fri Oct 29 2010, 03:34PM

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

Navrit Bal wrote ...

Would you say that a mouse trap might be a good place to start for quick mechanical closing? Or should I use what you have on your Mark 5 coilgun, the V shape trap, or something similar?

Also, could you explain this :
"At a certain critical angle (around 8 degrees for copper) the wedge will "stick" or lock in place, and needs no other mechanism to maintain contact."

A mouse trap is quick, alrighty, but contact bounce needs to be studied. Build a prototype and put an oscilloscope on it to measure contact bounce. Ironically, the faster the contact moves, the more bounce it may have. But really, you have to build it to see.

Most relays and other contactors with flat mating surfaces can have contact bounce of 10 milliseconds or more. In typical small coilguns where the entire firing time might be between 4 and 8 milliseconds, you can see it would be a problem.

In my Mark V coilgun, I measured contact bounce to be usually 0 but sometimes 2 msec. To give myself timing leeway on using higher power, I chose a massively heavy projectile so that the firing times can be much longer than usual. I don't expect high speeds (but I do hope for high kinetic energy, lol).

The "critical angle" concept arose in the 50s as part of high-power switching using mechanical contactors. The idea is to eliminate contact bounce by mating wedges instead of flat surfaces. A wider angle would allow the moving portion to bounce and possibly fall out. A narrower angle would capture the moving part so tightly that it would be difficult to remove again. But the 8-degree angle for copper lets it stick in place, and yet be easy to pull out to open the circuit.

Cheers, Barry

Navrit Bal

Fri Oct 29 2010, 07:38PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Since it's only 4 pounds, I think I might get the trap... I briefly thought about simply using salt-water with one wire already in there, then dropping the other in, but quickly realised that massive electrolysis would probably occur, correct me if I'm wrong

Wouldn't it be best to fire vertically, to slow down the acceleration. Also, I would also prefer higher kinetic energies and efficiency.
Do you know what the critical angle for brass is, or how to find it? I'm asking because I have a random rectangle of brass lying about.

ScotchTapeLord

Fri Oct 29 2010, 08:15PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

The water is a neat idea but even when saturated with salt, the water would have too high of a resistance. The water would become the load.

Navrit Bal

Fri Oct 29 2010, 08:22PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Yeah, I just checked, I thought it was a lot better a conductor than it actually is

I think Aluminium might be my best option for the V wedge, as a compromise between price and electrical conductivity, mostly price though...

Navrit Bal

Sun Oct 31 2010, 09:10PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Since I need a voltage tripler for charging the capacitors (330V), from a 110V transformer, I was looking at this voltage tripler

.
Would it be suitable ? I have 30 2.2uF 630V, 8 3.3uF 400V and 8 3.3uF 630V capacitors. I ask because on the diagram it labels the capacitors as 10uF.

If this isn't suitable, what other options would you suggest? Preferably cheap and easy xD

Also, wouldn't the best coil be a relatively short, thick wire (~3mm for example) ? I'm saying this because it gives low resistance, compared to thin wire and inductance. Although I do know that a lot of people keep saying trial and error seems like a good way, I would prefer something more scientific and explainable than that

I was thinking of these

Barry

Mon Nov 01 2010, 01:43PM

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

Navrit Bal wrote ...

Since I need a voltage tripler for charging the capacitors (330V), from a 110V transformer, I was looking at this voltage tripler

Another option worth considering is a voltage-doubler transformer and full-wave diode bridge rectifier. It's simple to build and reliable and rugged. It offers the quickest charging rate of all approaches. I can't help with your proposed tripler schematic, I hope others will review it and lend advice.

The idea is that 2:1 transformers are relatively common and may be found in surplus stores. They're also known as control transformers. Although designed for voltage step-down, they can be run backwards by connecting 110v to the "secondary" winding. They run fine albeit at lower than rated current.

So a control transformer wired for 220v AC output can be rectified to 370v DC. Why the jump in voltage? Because "AC" is measured in rms, and the peak voltage of a sinusoid is 1.7 times higher than the rms voltage.

I know 370vdc is higher than your intended 330vdc. Do not ever charge your caps higher than their rated voltage. So how to avoid overcharging? Insert a series resistor to slow down the rate of charge, so you can watch the voltmeter and stop charging when it reaches the desired voltage. Better yet, get a Variac so you can dial in any charge voltage you want. But Variacs cost usually $40-60 so maybe it can wait til later.

Cheers, Barry
Happy Halloween

Navrit Bal

Mon Nov 01 2010, 09:46PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I know its a bit late, but Happy Halloween anyway.

Would you say that my best option is probably to order from

I asked, and the total is $63.45 which is ~Â£40 +/- 2 which I think is pretty good, although I won't be able to have the fun of making it

. It therefore seems that this will be the best, most reliable and cheapest option.

Navrit Bal

Tue Nov 02 2010, 07:22PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

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.

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