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LAN_403

Mycologist

Sat Oct 24 2009, 02:41PM

Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22

Hi there,

I've been working on a 1280J capacitor bank that I can plug my coilgun or railgun into, and have had some success with a 200V 64mF configuration. However, I found that 64mF was way to much capacitance for my coil at that low a voltage, and I got really awful suckback because the pulse time was so long. That and the railgun needed a bit more voltage, so I decided to rewire it as an 800V 4mF bank. The problem I have now however, is that it's a lot harder to get 800V from 240VAC than 200V; to get the 200V before I was just using a bridge rectifier and a potential divider. It seemed obvious that I either needed a transformer, or a cw multiplier, and the multiplier option seemed a lot easier than wiring a transformer, so I went ahead and put one together with some 1kV rectifier diodes, and some 1kV 10nF ceramic caps. It was at this point that I realised that it would work none too well with the mains frequency being ~60Hz and all...
After doing some calculation, I figured that I'd need 4 380uF 1kV capacitors to make a quadrupler for 60Hz input; not exactly the commonest items to come by! So I thought a transformer may be a better idea, though I have little idea of how to go about winding one. I've got an old crt that I was going to take apart to make a flyback arc generator for some fun, and I could use that for the core I guess, but I'm unsure if it would work well at 60Hz or not...
So if anyone has any advice regarding increasing mains frequency, or transformer winding, it would be greatly appreciated!

Matt

Wolfram

Sat Oct 24 2009, 03:13PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

With a voltage doubler connected directly to mains, you should get 2 * sqrt(2) * 230V, or around 650V. This is not too far from the 800V rating of the bank. To get closer to 800V, a good idea would be to use a 230V-48V or so transformer, and connect it up as an autotransformer to get around 280V AC out. When put through a doubler this should yield just under 800V, not much safety margin.

A lightbulb in series with the mains input of the circuit would limit the peak current on turn-on, which can otherwise be very high. It would also serve as a nice charging indicator.

Anders M.

Mycologist

Sat Oct 24 2009, 03:20PM

Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22

Wow, speedy reply! :D Thanks Anders, I'll give that voltage doubler a try, I assume you're talking about the 'Charge-Pump Voltage Doubler' on the wiki right?

Proud Mary

Sat Oct 24 2009, 03:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Why not series the capacitors in your voltage multiplier, Matt? That's what I'd do. But take the greatest care if you are not using an isolation transformer.

1256405922 543 FT78322 Mains Voltage Doubler

There you go!

Mycologist

Sat Oct 24 2009, 07:29PM

Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22

Thanks for the schematic Harry! Haven't tried it yet though, as I've just got back from making the Voltage Doubler as per the wiki, and as per Anders' prediction, it outputs exactly 650VDC! Now to try it attached to the actual capacitor bank rather than just across a load resistor...
Also, I don't quite understand some of the points in your schematic Harry; I can see that there are now four diodes so you can have either the 300 or the 600 volt output, but why all the small (I assume ceramic) caps and chokes?

Proud Mary

Sat Oct 24 2009, 08:59PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The chokes and caps in the mains input line (50Hz if live you in Britain!

) form a standard noise filter. These should be caps specified for direct connection across the mains.

The capacitors in parallel with the diodes are there to protect them from surges.

The very cheap 1N4007 diode would be a good alternative for the diodes.

The design should also incude a fuse.

If you want more current, increase the size of the capacitors.

Mycologist

Sun Oct 25 2009, 09:17PM

Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22

Ok, so the small caps and chokes are basically just to regulate it a little, I'll probably add them sooner or later then, seeing that trying to charge the bank earlier today with just the raw design (two caps, two diodes through a fused plug and socket protector) yielded a popped load resistor after about 0.1s, after only reaching 35V. I was considering just letting it draw without a load resistor in series, but would that blow the fuse?

Proud Mary

Tue Oct 27 2009, 11:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Mycologist wrote ...

Ok, so the small caps and chokes are basically just to regulate it a little, I'll probably add them sooner or later then, seeing that trying to charge the bank earlier today with just the raw design (two caps, two diodes through a fused plug and socket protector) yielded a popped load resistor after about 0.1s, after only reaching 35V. I was considering just letting it draw without a load resistor in series, but would that blow the fuse?

You'll have to learn some very basic Mathematics for Electronics, won't you, or you will ever be dependent on others to work out the solution to even the simplest of circuits. There are great stacks of books on basic electronics, and I must now refer you to these.

Mycologist

Thu Oct 29 2009, 09:29PM

Registered Member #2413 Joined: Sat Oct 03 2009, 08:27PM
Location: Essex, UK
Posts: 22

Hmm, you do have a point there Harry! Hopefully I'll be a little more proficient when I'm at uni studying this kind of thing, rather than just messing around in my spare time... So yeah, I had a more detailed look at it, and while I still don't quite understand the noise filter (I'll do some more reading

), I went ahead and risked the massive current draw by charging it without a load resistor, and everything was ok! Charged to 640V almost instantly, with no real problems, well other than I've gone and wrecked my rails by zapping too much tin foil on them...
So yeah, thanks guys! I'll get and do some more reading and chipping molten crud off my rails!

Zeus

Thu Oct 29 2009, 10:21PM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

Have you considered placing a 1N4007 in series with 80Î¼F photo flash caps like this
-l>--l l--l> with the positive of the cap on the left,
-<l--l l--<l and the positive of the cap on the right.
Then connect said caps in parrelel. ( Sorry about the ASCII schematic I,m on my
phone)

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