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Charging problems (too fast)

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yoyoweb

Sat Mar 05 2011, 01:42PM

Registered Member #3744 Joined: Sat Mar 05 2011, 12:40PM
Location:
Posts: 5

Hello everybody ! i apologise if my english is not very good.

For my coilgun, i try to chage the caps bank (4x470ÂµF = 1880ÂµF and 450) as fast as possible !

For this i use a 9.6 V Ni-Cd battery who power an ocsillator, then a transformer and a voltage multiplier and here is my problem :

I have too much voltage ! the caps bank raise to 450V in around 8s but they are only charged at 50%

I think it must seems like this :
http://img846.imageshack.us/i/elev4.png/
And if i reduce the output voltage around 500 V, it take 45 to 60 s to charge the capacitors (really too long for me).
So, how can i reduce the charging time by charging near full capacity ? maybe by raising the current ?

Thanks and sorry if this question was already post, it didn't find it.

[Edit: Fixed link]

Marko

Sat Mar 05 2011, 02:06PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi,

just curious, what kind of oscillator are you using?

If it's a royer driving an ATX PSU transformer in reverse or something similar, you can use a comparator with hysteresis to turn it's gate drive on and off (via series PNP transistor) by feedback from the output.

Marko

yoyoweb

Sat Mar 05 2011, 02:17PM

Registered Member #3744 Joined: Sat Mar 05 2011, 12:40PM
Location:
Posts: 5

Here is my circuit !!

[http://img225.imageshack.us/i/grapho.png/

Marko

Sat Mar 05 2011, 02:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi,

you could still apply the same for your circuit... just use a LM311 or similar to compare divided output voltage to a reference and pull the base of your bjt down when it reaches a limit. Add some positive feedback to keep the system stable.

Marko

yoyoweb

Sat Mar 05 2011, 02:55PM

Registered Member #3744 Joined: Sat Mar 05 2011, 12:40PM
Location:
Posts: 5

Thanks for your answer ! can you explain me how does it work please ? what will be the effects ?

genious 7

Sat Mar 05 2011, 09:07PM

Registered Member #2887 Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107

He means having an operational amplifier (Such as LM311) to compare your actual voltage with a reference voltage. When the actual cap voltage reaches your reference voltage, it turns off the supply. When it discharges below the reference voltage, the supply turns on again.

Oviously inputing 450 V into any operational amplifier will kill it, so you will need a voltage divider (made of resistors). Don't forget to scale down your reference voltage accordingly. You could have the reference voltage from a zener.

Hysterisis means, lets say, the supply turns off as soon as it passes 450 V. It should not turn on again when it goes below 450V, but instead, something lower, like (Guessing a number), 400V. This avoids repetitive on off cycles.

To Illustrate the concept, see this

Bear in mind this is just to illustrate the concept, as it has several practical and technical flaws. Do not replicate.
If you need help for a good design, I could try, altough more experienced members will have a better solution.

yoyoweb

Sun Mar 06 2011, 11:48AM

Registered Member #3744 Joined: Sat Mar 05 2011, 12:40PM
Location:
Posts: 5

Ok i understood ! i may just remove the voltage doubler and add a 12 V battery with more current (and another npn transistor) instead of my litte Ni-Cd 9.6 V.
What do you think about it ?

genious 7

Mon Mar 07 2011, 03:15AM

Registered Member #2887 Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107

Remember you would still need to rectify it to be able to charge the cap.

The 12v battery should work, but I haven't done any math about it. Maybe you would need to change the resistors values or wattages as well. Anyways, the move to the 12V battery will help.

Is portability (batteries) a requirement? There might be easier ways to make it work if it isn't.

yoyoweb

Mon Mar 07 2011, 12:24PM

Registered Member #3744 Joined: Sat Mar 05 2011, 12:40PM
Location:
Posts: 5

Yes it must be portable!
I have 2 questions :
Is it better toincrease the voltage with a voltage doubleur or put a battery with more voltage ?
And if i put 2 caps in parallel + 2 in parallel the all in sÃ©rie, it skould be 470 uF and 800 v. So in that way it should be easier to charge isn't it ?

genious 7

Mon Mar 07 2011, 10:49PM

Registered Member #2887 Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107

Is it better toincrease the voltage with a voltage doubleur or put a battery with more voltage ?

As the application is charging capacitors, I'd say they are just two different ways to do it. Eliminating the multiplier would certainly make the circuit simpler, although you would need to recalculate resistors and transistors.

And if i put 2 caps in parallel + 2 in parallel the all in sÃ©rie, it skould be 470 uF and 800 v. So in that way it should be easier to charge isn't it ?

Well, due to the greater voltage tolerance, it is certainly more difficult to over volt, a more resistant solution. Nevertheless, you will need to recalculate the coil gun's coil parameters. You should really check Barry's Coil Gun Page. I personally haven't built any, so don't take my comments as finals.

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