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New to Coilguns - Coilgun Buildlog (Charger apparently fixed)

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Artikbot

Mon Sept 27 2010, 10:08AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Hi there,

I'm new in coilguns and in REAL electromagnetism as well (done years of theory in high school) and I've finally decided to build my coilgun prototype.

So far it features about 20 capacitors rated at 330V and 120uF each, connected in parallel. I'm using a camera circuit to charge then -here is where my problem kicks in- it can't lcharge more than 4 capacitors at a time without kinda... destroying the 1.5v battery :P (nah, it just drains so much power the battery is left dry). And even with 4 caps, it takes about 3 minutes to charge to its maximum.

As it's sad a nail can't go through a 25mm styrofoam piece, I'm willing to develop a new charger in order to charge those 20 capacitors and have some REAL power on there.

So far I've figured how to build the transformer stage:

A transistor able to handle 330V connected to the primary and the secondary stage of the transformer which would serve as an oscillator, and two diodes on the secondary stage to make the positive and negative terminals. Ofc a small capacitor to stabilize that ripply current.

That would be enough to fill up the capacitors... but when do I know where to stop charging? AFAIK capacitors only let current through when fully loaded, so just adding a LED with the proper resistor at the transformer's secondary after capacitor bank would work?
I've also seen lots of people adding a small voltmeter to the coilgun, to view the charging of the capacitors I suppose... Would a regular cheapish voltmeter work?

Okay, having asked that avmount of stuff, ima say the coil specs if more info is needed:

230mm aluminium barrel with 1mm thick walls. 8mm outer diameter.
40mm solenoid with about 400-410 turns in 8 layers, 20AWG wire.

What do you guys think about it? If my ideas are correct... should I wind the transformer myself or just buy a pre wound transformer that fits my needs? I have like 2m of spare magnet wire from the coil.

Thanks a lot!

Adam Munich

Mon Sept 27 2010, 10:47AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Capacitors will let current flow regardless of their state of charge. To let the juice into the coil, you're going to want a big SCR.

Simply hook the trigger through a pushbutton switch and resistor.

2 m of wire won't be long enough to wind the secondary. If you want this to be portable, I recommend the zvs circuit that was used on that portable 1.25kj coil gun.

To know when the caps are fully charged, use a 0-10M trimmer and an led. Fully charge the caps (measure their voltage. anything above 370V is OK). Then hook the led and trimmer between them. MAKE SURE THE TRIMMER IS MAXED OUT. Then slowly turn the trimmer (lower it's resistance). When the led begins to light, there's your full charge "tester".

I would use a plastic barrel too. I think the aluminum may rob some energy.

GhostNull

Mon Sept 27 2010, 11:26AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

This thread here covers practically all your questions:

Basic Info:

: Read this through, it has heaps of simple easy to understand information on coil guns (you da man Barry) ,
Then this site:

for more advanced info on coil guns. Also search around the forum you'll find almost everything you need to know.

Capacitor bank:
You will probably need more power. Assuming all your capacitors are exactly their rated capacitance and charged to their rated voltage you will get aprox. 130.6 joules (

). So don't expect anything near what Saz43 got. What you need are some bigger "Computer Grade" capacitors. This is the name for a type of capacitor that is designed for high power energy. Hopefully this one:

will work for you.

Chargers:
You can not over charge a capacitor if that is what you are thinking. That would be against the laws of science. However, you can over volt them, where you charge them to a higher voltage than they are rated and that will make em go poof, pop or if you're luck BOOM.
If you want to make a proper coil gun then you will need something better than a camera circuit
A Boost or a Buck-Boost converter are should suit you. They are much of efficient, portable and much higher power than Camera circuits.
Here are a few:
Uzzor's:

Coilgun systems':

rwilsford07's :

As for the zvs circuit:

I wouldn't recommend it for what you are doing. While it is yet more powerful, it is more complex, expensive and would be unnecessary for a a cap bank of your current size.

Charge indication:
Any voltage meter with the required voltage rating should work, including a DMM but if all you want is a simple coil gun with some charge indication then like grenadier said a LED in parallel with your capacitor should do.

Barrel and Coil:
This would be a better explanation than anything I could write up in my current time:
Coil: Simple:

Advanced:

Barrel:

Artikbot

Mon Sept 27 2010, 09:21PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Thanks to both, Grenadier and GhostNull. Really helpful :)

I made some schematics with an idea I have rounding my head for a step up charger. It's not exactly like the one in Instructables, and it has something from the ZVS one.

Starting with the basics... I cannot afford those capacitors. I rely completely on my funds, and having passed almost a whole year since I last got paid on my job (working only on holidays, this one I got some PC hardware instead).
I said 20 because it's a nice number and the infrastructure needed to charge them is not really massive (~2200-2400uF @330V). But I'll add that I can get up to 50, maybe more if I do some "carving" on my local photo stores :D
Would 4000uF @330V be enough for some WOW power? Ofc not like saz43's ultraoverpowered coilgun, just to do some sniping from let's say 50m far and be able to do some damage on a soda can (I'm enough if I leave a mark on it, even if it's just a slight bump).
The projectiles are, basing on barrel size, about 40mm long and 4mm diameter. Basing on iron's density, that's a 14-gram projectile, maybe it's too heavy? I can perforate it and leave it around 5-gram'ish, much better for both me and my coil (more acceleration, better top speed, therefore more straight projectile path, and less recoil).

Regarding to the barrel issue... I'll try to find a carbon fiber/fiberglass tube with 8mm outer diameter to change it for my aluminium tube (as I read eddy currents can be pretty strong there as it's not slotted).

No more delays, here it goes my schem. Don't take into account diodes or capacitors' polarities, I'm no good at putting them in the right direction. Just get the idea of the circuit, and correct me if something is going to blow up/burn out or something. Also I don't know the values for the rectifier capacitor and for the LED resistors. But the resistors I'll figure out with Ohm law.

Also I don't know which part nÂºs to pick up, advise on that would be great :)

Edit: Oh crap on my circuit the solenoid&SCR's positions are wrong. They're attached to the cap bank instead of the rectifier cap :)
Edit2: I was searching for some SCRs, but i found as maximum 400V at 6A. Can I add some of them (let's say... 10) in parallel? Or will they go BOOM?

Thanks a lot!

Side note: No, I'm not trying to overcharge them or something. I'm not that dumb :) I just want to charge them to their maximum rated capacity and voltage. In this case, 120uF 330V each. I've seen so many big AC capacitors explode near me, trust me. I don't want one of those electrolytic devils explode in my face.

GhostNull

Mon Sept 27 2010, 11:22PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

I can't under stand your schematic

The your transistor doesn't make sense, You've name the base an emitter and the emitter a collector!? The emitter and collector are electrically common?! Cap batt? I have know idea where the AC to drive the transformer is coming from.

How is this supposed to work!?

Although there are a few things that remotely make sense.
D2 and D3 are there to rectify the AC from the Secondary I assume, the should be the other way around unless the capacitor has it's negative on the top and positive on the bottom. A Bridge rectifier would be better but it a single diode setup to half wave rectify will work as good.
With the SCR you have a switch there instead =S but that is in the right place. The solenoid and SCR should be in parallel to the Capacitor bank with the SCR Anode on the Capacitor Positive. You will also need something to trigger your SCR.

I really don't see this thing working :-/ I think you either need to do some studying up on your electronics:

or copy someone else's design.

Artikbot wrote ...

Edit: Oh crap on my circuit the solenoid&SCR's positions are wrong. They're attached to the cap bank instead of the rectifier cap :)
Edit2: I was searching for some SCRs, but i found as maximum 400V at 6A. Can I add some of them (let's say... 10) in parallel? Or will they go BOOM?

Um, rectifier cap?
For the SCR that 6A 400v probably will go boom unless it's one of those with crazy surge capability.
Find the aprox. specs for your coil with this:

Than plug the numbers into this:

Look at the peak current, then look for an SCR with an Itsm greater than that.
These two sims are also good for getting an approximation to design your coil/solenoid. You want a pulse time around 7mS.

Artikbot wrote ...

Side note: No, I'm not trying to overcharge them or something. I'm not that dumb :) I just want to charge them to their maximum rated capacity and voltage. In this case, 120uF 330V each. I've seen so many big AC capacitors explode near me, trust me. I don't want one of those electrolytic devils explode in my face.

I suggest using a zener diode with a resistor to limit the current. I assume you can use R=V^2/P to find out what resistance you'll need.

I think all you need to do is crunch down and get some reading done, then you'll be fine.

Artikbot

Mon Sept 27 2010, 11:41PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Yeah well, I did a serious re-look on my circuit and I found it has more mistakes than good things. I'll redo it and post tomorrow.

Some symbols are'nt normativized, so it's normal that you don't understand half of the things there.

As I said, I'll repost this tomorrow with a new schem. It's too late now and I'm sleepy :)

Edit: Answering your questions...

God's sake. I messed up with the transistor. The collector is supposed to be connected directly to the power supply. The emitter is connected alone to the primary (yep, forget about any other thing connected to the primary, should go into the collector instead)

Cap batt is a wrong branded Cap Bank (LOL).

D2 and D3 yep, are for that purpose. But as I said, they might be wong oriented. They are... They should go all the way around. Single diode? Hm. I've always used two because I don't really trust on single setups. And as it doesn't interfere... I'll keep going that way unless you say they actually do some bad to the circuit :)

The SCR is incorrectly drawn... I forgot about the gate, should go into the power with a small mechanical switch + resistor in between. I'm using a 40A sustained 600V SCR instead. It can hold up to 400A for 20ms -more than enough for my cap bank-.
Also incorrectly placed, I forgot to put it in parallel with the bank, thanks for correcting me :)

With rectifier cap I mean the cap that makes the signal less ripply and therefore more capacitor friendly. I'll use a polyester cap for that purpose.

I used those sims and I found out a 125A Itsm for my coil. Waaaay under SCR's peak specs.

Zener diodes... To limit it? Okay. Suggesting any specific part?

So far I've found the NE555 timer, the SCRs, the cap for the rectifier, the diodes, the potentiometers... basically everything but the transformer. I'll need help on that one :<

Hope with this explanations you understand better what I mean

All I know about electronics is because I love to experiment with stuff and I need to learn to do my projects... Maybe everything is a little messy (maybe A HELL LOT of messy) but in that chaos, I have a bit of order. Just need to clean my head a bit to make this work.

Thanks a lot!

GhostNull

Tue Sept 28 2010, 01:55AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Sorry to doubt you

But still that schem is pretty lazy -.-
Hopefully this will all make sense with a better schematic. So i'm not going to say so much until I see your new schematic. Although, I still think a boost converter would be easier.

Moving away from the charger now, Concerning D2 and D3 There is nothing wrong with having two diodes except for the voltage drop. I can't think of any real negative impact on the circuit it might have.

With the rectifier cap, I think smoothing capacitor is the more common term but let's not get hung up on that. They would be completely unnecessary since you are charging capacitor. Using a capacitor to charge a capacitor. There is nothing wrong with a ripply input voltage as long as it does not go over the capacitors rated voltage.

Zener diodes:
"I just want to charge them to their maximum rated capacity and voltage."

: So what I'm suggesting is you use a zener diode so if voltage across the capacitor exceeds the voltage rating the zener will beak down and stunt the extra voltage but it probably won't be easy finding a 330v zener with a descent power rating.

Also the inductor sim will give you the length of wire required to theoretically wind that coil. Just make sure you got plenty more than that.

That's all I have to say for now.

Cheers
-Ken

ScotchTapeLord

Tue Sept 28 2010, 02:54AM

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

GhostNull wrote ...

So what I'm suggesting is you use a zener diode so if voltage across the capacitor exceeds the voltage rating the zener will beak down and stunt the extra voltage but it probably won't be easy finding a 330v zener with a descent power rating.

Use an MOV or TVS for this type of application.

GhostNull

Tue Sept 28 2010, 04:36AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

MOV or TVS diode would not be well suited for this more repetitive sustained over voltage encountered in this situation. IMO

Artikbot

Tue Sept 28 2010, 07:27AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Aaaaham. Oki things are pretty clearer now :)

I wouldn't call it "lazy", I'd rather call it... "done in CAD class in order to not fall asleep" :P

About the smoothing cap yep. I agree. It's kinda redundant. Capacitor to charge capacitors seems... oh well a bit LOLish. What was I thinking? :P

According to the Zener, so if I place a reverse biased Zener in parallel with the circuit, it should act as a breakdown just in csae voltage goes above capacitor's rated voltage? Wow, never thought about that used for zeners (well in fact I never used zeners >.<)

I found a (oh crap) 390V Zener, rated @5W. It would be pretty excessive, considering that my capacitors have a maximum rated voltage of 350V in abnormal situations... I think they'll blow up if I drive a greater voltage across them. Is there any way to connect a 200 and a 130V together? I guess there isn't...

Okay, ima redraw the schem and upload on a sec ;)

And once more, thanks a lot for your helpful posts :)

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