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ZakWolf

Sun Oct 16 2011, 08:45PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

I have 2 450v 1800uf caps and was wanting to make a coil gun with them.

If I put them in series I get 850v(safe charge voltage) at 1800uf = 650.25 Joules

In parallel I have 400v at 3600uf = 288 Joules

My question is which is better, I think that at 850v at 1800uf would be a quicker pulse, isn't that what you would want?

Is uzzors2k's cap charger able to regulate that voltage?

Lastly I would need an SCR capable of switching that, anyone know of a good site to get one and what would the ratings need to be?

thanks

Legit_bacon

Sun Oct 16 2011, 09:09PM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Wrong! When caps are parralled they half there capacitance. I've been tricked by this before.
You would indeed get a quicker pulse yes, this is better. But finding means of switching will be trickier, aswell as charging it.

As for regulating voltage with a zvs I believe that it can via a voltage comparator. But then again I'm fairly new to all this and could be wrong, (am yet to build one :s )

As for the switch I have no idea about a good source, I believe I have a scr lying around that would be capable of switching that, but got it from a surplus store, and I'm in newzealand, so not much help there.
Plenty of people here would know though.

Good luck!

Turkey9

Sun Oct 16 2011, 09:09PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

When you connect capacitors in series, the capacitance does not add. The relationship is(1/Ctotal) = (1/C1) + (1/C2). That means that you will actually have 900uF @ 850V for 325J. Total energy stored in capacitors cannot change no matter how you connect them together.

Edit** Too slow on the reply! But it still isn't quite true that the capacitance in half. That is only true with identical capacitors.

How long of a pulse you want is dependent on your coil. It seems that this may be one of your first coilguns so I would go with them in parallel as it is much safer and will require a smaller SCR.

The SCR you will need is also dependent on your coil. You need to figure out the dimensions of your coil before you choose an SCR. Once that happens, Vishay and ST are good companies to sample from.

ZakWolf

Mon Oct 17 2011, 01:49AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608

I will go parallel thanks for clarifying.

I dont know the resistance of my coil or the inductance, im gonna take a gamble and by some SCR's soon and hope they can hold up.

I was recommended

I would put two in parallel and use a resistor to make sure the switch at the same or close to time.

Will these hold up?

Or is there something else but a little more robust ?

Legit_bacon

Mon Oct 17 2011, 01:59AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

It's easy to find the resistance and the inductance of you coil. Barry has a simulator that will tell you these, just stick in your coil dimensions, wire used etc and it will tell you what the resistance and inductance is.

Or if you are a school student your school will have a LCR meter (inductance, resistance, capacitance). You can simply hook this up to you coil and it'll tell you what you need to know. Then head over to Barry's site and find the appropriate simulators to tell you you peak current flow, aswell as you revers voltage. This will help you determine the needed ratings for you scr, projectile and revers protection diode.

Edit: as for those scr's I'm currently on an iPad and it's refusing to open the link, so some else will have to look at it for you.

ZakWolf

Mon Oct 17 2011, 02:12AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

I will try the sim.

I just dont have any dimensions yet.

enter this part number in mouser VS-70TPS12PBF

EDIT: APPROXIMATION !!

inductance .45uh

resistance .39 ohms

turns 18awg 376

Legit_bacon

Mon Oct 17 2011, 02:27AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

That SCR looks fine, although I could be wrong wait for the results from the sim, it pep ends on the ESR of you caps among other things.

ZakWolf

Mon Oct 17 2011, 02:42AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

what do you mean wait for the results form the sim?

So do i only need one?

thanks for for help

EDIT: I used Barry's RLC Simulation and crudely input the results and got a peak current of 1040 amps.

EDIT EDIT: quote form Barry "Always use a coil protection diode to dissipate stored energy. This will save your switches and SCR's. Not to mention your bacon." What would be a good rating for this diode considering my spec?

Legit_bacon

Mon Oct 17 2011, 04:11AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Ok judging by the results of the sim that SCR will do fine. As for the protection diode I believe around 200v at a couple hundred amps, altought there is another sim on Barry's site for this. Thank god for Barry!!!

ZakWolf

Mon Oct 17 2011, 04:50AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

so i only need one!?! :D

I just finished my transformer, it puts out 556 volts, perfect...

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