Coilgun

[x88x]

Yes! Well, at least somebody got it.

Along those lines, the Arc Reactor has always bothered me...it's just too convenient. (Yeah, yeah, I know...)

So, I got the charging circuit better assembled now...and ran into a few problems. First off, I accidentally blew out an I/O pin on my uC before I thought to use a diode instead of just hooking it straight in.... ..yeah, that's fixed now...feel free to ridicule. The second thing I ran into was rather unexpected...

Once I had a good switching rate (10 5Hz atm), I realized that the charge rate is waaaaaay faster than I was expecting from previous tests (100V in 1.9s!), so I needed to start playing with higher voltages to continue testing the circuit. So, I pulled out one of my 3uF 450V caps and let it run. ...now, the thing is, I neglected to check the limits of the transistor I was using before I did that, so I was rather perplexed when it stopped charging at 100V. ...and then ceased to work at all.. I checked the datasheet again, and it's actually 60V, not 100. So no wonder it burned out.

So, into the parts bin to find a higher rated transistor! Hurray for plasma TV parts!

Here's a shot of the circuit as it is now. Yes, I am being semi-safe and using fairly hefty jumpers and a 0.01Ohm 10W resistor to drain the caps when I'm done with them. Another benefit of the tiny capacitance of the cap I'm using is that it drains really fast by itself, so even when I had it up to 100V, by the time I had shut off the power and reached over for the jumper, it was down to ~35V.

[x88x]

Hmm, so, I think I'm missing something..

I have a nice little 600V 45A IGBT, and I was thinking that I should just be able to drop it in where I had the NPN transistor...but that's not working... It's not that it needs a higher voltage across the Base-Emitter lines to switch, because I tested it with the 2xAA's, and it switched just fine (I'm running this circuit on 5V atm). IDK, like I said, I'm guessing I'm just missing something about using IGBTs, since, well, I've never used one before. Anyways, it's late, I'm tired, I'm gonna go to bed now.

If anyone's interested:
the transistor I had been using:
http://docs.google.com/viewer?a=v&q=...uZqBOaOQ3I-4vA
the IGBT I want to use:
http://docs.google.com/viewer?a=v&q=...rYZ-CWjdQSppgg

[x88x]

Heeheeheehee!!! I had a great moment just now where I finally worked out the last kink in my charging circuit and it suddenly worked and worked AWESOME!

Ok, let's see, what did I do today...

Ah, right, IGBTs. ..yeah, #&%@ IGBTs. Ok, so maybe not; they'll be great for switching the acceleration coils...not so much for a charging circuit though. The one I was trying to use has a switching frequency of something like 1/7-1/8Hz....yeah...not so much with the working...ness.... -_^ Needless to say, when I figured that out I was pretty pissed that I had wasted so much time trying to get it to work.. On the other hand, now I know how IGBTs work, so it wasn't a complete loss.

Instead, I pulled out one of the transistors I had bought with switching the coils in mind, but knowing what I do now, there's no way on earth these transistors could handle the current levels that's gonna require. These transistors are 1500V max, 8A max (not both at the same time, but that shouldn't be a problem..I think they top out at 700V when at 8A.. datasheet if anyone's interested), so they should work marvelously for any capacitor bank I anticipate charging any time in the near future.

And indeed, it worked out great! ...once I figured out that the first one I was using was DOA..

...ok, so maybe it might have also had something to do with me hooking it up wrong and possibly frying it...but I'm gonna go with DOA. ..yeah..

So, long story short, I finally had a circuit that wouldn't blow up at random intervals, so I decided to see what this thing could do.

....and promptly ran into a 100V ceiling

Now, after switching out different caps, waiting a long time for them to charge, that not fixing the problem, and being on the verge of swapping out inductors to try and find the problem, I started thinking.. Yes, yes, I know, shut up. When I changed the switching frequency from ~3Hz using a manual switch to 5Hz (I miscalculated in a previous post: 100ms on, 100ms off == 5Hz, not 10Hz) using the uC, I noticed a quite large improvement in charging times. So, since I basically just pulled 5Hz out of nowhere because 100 is a nice round number, I decided to start experimenting with that instead. Now, instead of trying different frequencies at random, I decided to go back to that instructible that that I linked in my first post, and see what frequency he used. After checking his circuit diagram:

(again, mirrored to save Instructible's bandwidth)

...and spending some time with our great lord and master the almighty Wikipedia may it live forever, to figure out how to figure out the frequency..

I figured out that his circuit has a switching frequency of about... 3.1kHz. ..yeah, I was waaaay off.

So, since I wasn't quite sure what would happen if I put decimals in the Arduino 'delay' function, and I couldn't be bothered to find out, I tried 1 instead. Rather, now I have it generating a square wave 1ms on, 1ms off, working out to only 500Hz, but still way faster than I had been doing. Ok, typing that made me curious, and as it turns out, you can put decimals in the 'delay' function. So, I have it set to 0.15 now, which results in a 3kHz frequency.

..what? Oh, right, the point of that. ..it's late...

Right, so with this new crazy fast switching frequency (and even the 500Hz I had when I started writing this post), the charge rate is INSANE! Seriously, with the little 3uF cap it takes 2.5s to get to 450V!

If it can do that now, just imagine how fast it'll charge stuff once I upgrade that inductor. *cue evil laugh here*

So, the circuit as it is now:

You like my TT blast shield? Finally, that stupid V9 was useful for something.

..ok, that's not fair...it has been being a wonderful bedside laptop stand...

And, on that note, I'm gonna head to bed. Hopefully tomorrow later today I'll be able to actually get up at a reasonable hour.

[mDust]

Haha! The excited tone (that I inferred) of the posts to the trial and error methodology to the evil laughter...I love it! Keep up the excellent work and you'll be a mad scientist in no time...

[x88x]

Heheh, thanks. Yeah, that definitely is an excited tone you're inferring. ..also, all of these posts have been finished around 2-3am, so I wasn't exactly in a well-rested state of mind...which probably contributed to the, shall we say, unusual..tone.

[Oneslowz28]

That charging rate is insane! I can not wait to see the work on the bank and then see some projectiles hurling through the air at near warp speeds....!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[x88x]

Ok, some updates on this. I went ahead and added some good contacts on the inductor I'm using right now, so it's not a pain to actually swap in and out of the circuit.

So, that makes things easier for now, but as I mentioned before, I'm going to be replacing it. On that note, I unwound the giant inductor I pulled off the TV...for now it's empty, but I have 100' of 24AWG magnet wire on order that I'll be rewinding it with.

I also threw together a quick testing circuit to test the discharge into a coil. Good news, it definitely worked, so no worries there.

Now to work on building a good cap bank and making a switching method for coil discharge. Right now I loose a lot of charge through the simple little wire switch.


On another note, OneSlowz28, crenn, and I were talking on Saturday, and crenn suggested a reason why the transistor overloaded. It made sense then, but I just thought about it again, and I think you were wrong. ..or I could be wrong again, idk.

What I'm thinking is the problem now is this:

When the transistor is conducting, the path of least resistance follows:
power supply -> inductor -> conducted through transistor -> power supply

Then, when the transistor is non-conducting, the circuit cuts to:
power supply -> inductor -> diode -> capacitor -> power supply

In this state, there is the constant possibility of the current passing through the transistor instead of the PSU and inductor, if the resistance of the transistor becomes less than the resistance of the PSU and inductor. Normally this will never be the case, however the 60V limit on the transistor determines the max voltage that can be kept from conducting between the collector and the emitter (iirc). So when the voltage in the capacitor passes 60V, the circuit breaks down into two parts.
Capacitor circuit:
capacitor -> burned out transistor -> diode -> capacitor
PSU circuit:
PSU -> inductor -> burned out transistor -> PSU

Like I said, I could definitely be wrong again, but this explanation makes the most sense to me so far, considering that the diode has never fried.

[crenn]

Well, I've been looking into it with the circuit simulator that Oneslowz28 linked us to. I think I'm partially right... but I don't know why yet!

This is the code for the circuit I set up:

$ 1 4.9999999999999996E-6 0.028650479686019012 43 5.0 50
t 336 320 384 320 0 1 -0.9084951952828586 0.15636575820549847 1000.0
R 256 256 256 240 0 2 3100.0 2.5 2.5 0.0 0.5
v 192 384 192 320 0 0 40.0 5.0 0.0 0.0 0.5
l 272 144 384 144 0 0.148 0.09018997524958472
d 384 144 464 144 1 1.1
w 192 192 192 144 0
w 192 144 272 144 0
w 384 336 384 384 0
w 384 384 192 384 0
w 256 320 336 320 0
w 464 384 384 384 0
r 576 144 576 384 0 1.0
s 464 144 576 144 0 1 false
w 576 384 464 384 0
s 256 320 256 256 0 0 false
s 192 320 192 192 0 0 false
c 464 208 464 320 2 0.0033 0.046955623951132175
w 464 208 464 144 0
w 464 320 464 384 0
w 272 112 384 112 0
r 272 80 384 80 0 1.0
w 384 304 384 144 0
o 16 64 0 35 0.15625 0.4 0 -1
o 18 64 0 35 5.0 0.4 1 -1

And the link to the simulator:
http://www.falstad.com/circuit/

Interesting things I noted: If you place just a wire where the inductor is, the circuit still works... but the voltage on the capacitor, when you turn it on, is pretty much instantly at the 'stable' final level, which means it's drawing a lot of current at that moment. Changing the inductor to a resistor has the interesting result of the capacitor not getting much higher than a value around 4.5v.

Another interesting I noticed that it seems you can do the circuit without the transistor, it slightly slows down the charging process, but not by much and it's probably safer to have it in to help regulate the amount of current drawn.

The other intesting thing was I couldn't get higher than 8.43v. There are things I still don't quite understand, but I don't think I'm fully wrong yet. I'm curious to what you think. The simulation currently is semi supporting my theory, although probably on 'ideal' conditions.

[SXRguyinMA]

that circuit simulator is awesome! now if only there was a way to interface that with arduino

[x88x]

Ok, I took a look at the simulator and it's doing things correctly. When you have the wire or resistor there, it goes immediately to almost the voltage of the power supply. That's because you're basically just hooking the cap in parallel with the power supply. The resistor will block more voltage than the wire (you know, it being a resistor and all ), so that's why it goes to a lower voltage.

The reason you're maxing out at 8.43V, and why it takes so long for it to get there, is because you set the cap to 3.3mF, not 3.3uF. 3.3mF is massive. Also, 134mH is pretty small. I think the one I'm working with is around 900mH, if my estimates of the number of coil windings is correct....actually, it's topping out at 8.14V now with 3.3uF and 900mH...hmmm IDK, here's the simulator code for my circuit with the small testing cap. IDK why the simulator is capping it there, but I promise it works irl.

$ 1 4.9999999999999996E-6 0.7703437568215379 43 5.0 50
t 240 320 288 320 0 1 -4.225879159308766 0.15544009054704944 1000.0
R 160 256 160 240 0 2 3000.0 5.0 5.0 0.0 0.5
v 96 384 96 320 0 0 40.0 5.0 0.0 0.0 0.5
l 176 144 288 144 0 0.9 5.010000069016896E-11
d 288 144 368 144 1 1.1
w 96 192 96 144 0
w 96 144 176 144 0
w 288 336 288 384 0
w 288 384 96 384 0
w 368 384 288 384 0
r 480 144 480 384 0 260000.0
s 368 144 480 144 0 1 false
w 480 384 368 384 0
s 160 320 160 256 0 0 false
s 96 320 96 192 0 0 false
c 368 208 368 320 2 3.0E-6 7.917887980984668
w 368 208 368 144 0
w 368 320 368 384 0
w 176 48 288 48 0
r 192 80 304 80 0 1.0
w 288 304 288 144 0
d 160 320 240 320 1 0.805904783
o 15 64 0 35 20.0 0.05 0 -1