Coil Gun test firing.

 

Maybe I didn't shoot very far, nor punch any holes in tin cans.. but still pretty good for first attempt I think!

Using the scavenged Back EMF really added some kick to the firing! It may be a strange use of a relay, but its a new trick in my toolbox. :) But first lets get some Hero shots of the coilgun in all its glory.

Still brings a smile to my face. I'm glad the failed symet spinner capacitors found a new home. The rings were kept from the old design, and added a nice "raygun" look to the little cannon. I suspended the coilgun in the middle on the rigid solid core wires, hoping for a recoil effect as it shoots (not enough power yet for that). The barrel is a brass tube, so it wont mess up with the magnetics of it all, and the outer ring is + and the inner ring - of the capacitor bank.

The Capacitors were angled forward a bit, and the rear ring was bent back some, to give it a bit more of a tripod look, as well as some more interesting angles to look at. And of course it HAD to appeal to my sense of aesthetics... like a Corvette, if the coilgun LOOKS like its shooting even when it isn't, well then it MUST be cool!

 

Here is the new fangled power booster, fresh from the Back EMF experiments. Reading back, you could see how it is truly a booster, in that I put 7v in, and the back EMF it generates trickle charges up the capacitor bank... to 23volts!  The white sleeved leads are the EMF, the unmarked are the power. The only real drawback is the whine the clacking contactors do on it as it cycles.. at about a 400 hz whine. (one I recognize from my Air Force days)

After trying to get a 555 timer to monopulse to make a quick fire of the capacitors through the coil, to looking into using a camera flash trigger to do the job, I instead went old school: relay logic (!!), as something I could wrap my head around, and had past experience with, as well as something I could build right then with parts at hand.

So here it is. 2 double contactor relays, and 2 pushbuttons. The function is straightforward, if a little interconnected: the left relay is the arming relay, the right one is the fire/disarm relay. The top left contacts act as a holding circuit, that keep that relay on once I press the button. The bottom left contact is the Enable, sending power to be available at relay #2. Relay2 wont energize until I complete the ground with the other pushbutton, the fire button. When the relay energizes, two things happen: the bottom right contact switches the contacts, moving the connection of the capacitors from the power source to the coil for as long as the relay is on. That wont be very long, however, as the top right contact breaks the ground to Relay #1... which you may remember, was supplying power to relay2 so long as IT was on. So, what happens is the firing relay turns on-OFF really fast, but long enough for the bullet to be fired.

 

Here's the wiring schematic, for anyone whose interested. This is a simple way to have a pulse of capacitor dump that is only a fraction of a fraction of a second. Maybe 100ms or so. A quick on-OFF is important to shoot something with a coil, as if the power stays flowing too long, the coil will just be a magnet, sucking the bullet to the center of the coil, and keeping it there. The Capacitor bank I had held enough power that the discharge time would have been too long for them to just dump their charge, so I had to engineer a rapid switching pulse... thingy.

I could adjust the "on time" by simply adding a capacitor across relay2's coil taps.. when relay1 kills its power, the capacitor will leave it on a little bit longer... but just like this seemed to work the best. 

 

 

 

Back EMF demonstration (and a peek at a coilgun)

Picture is worth 1000 words, as they say:

So, going counter clockwise from the blue lit power supply: it is producing 7 volts, shown on the gauge at top. the relay at left takes that 7V- the minimum to energize it, and mechanically oscillates on and off very rapidly due to how it is wired. The schottky diodes, wired reverse to the input polarity, deliver the Back EMF spike power to the bottom power rail of the breadboard, where it charges a 630v capacitor. A meter on that capacitor, shows its level of charge.

48 volts! So 7v in, 48 out.

Here's the wiring 'under the hood' of the car relay. The white marked wires are the Back EMF, the unmarked are the power and grnd leads. Notice that the power in doesnt go right to the coil, but to the Common contactor tap. The + side of the coil is jumpered to the NC contact.. meaning that the relay is energized as long as its OFF... see the trick? on then off then on then off then on then off... as fast as this little relay contactor can physically switch.. whicih turns out to be pretty darn fast. The Schottky diodes are wired in reverse, because the back EMF pulse is reversed polarity as well.. we dont want the power supply to add its power to the meter at the end! The giant diodes were 100v schottky ones.. because I didnt know how high voltage the EMF was going to be, and schottky diodes drop less voltage across them than normal diodes do. (.2 instead of .7 V drop)

 

This just amazes me. Why don't we use this power?! 7 piddly volts- less than a full amp of supply out of that little hobby power supply chip, and 50 volts pouring into the capacitor!! Sure its pulses, but even the way I've set it up in the video is so rapidly pulsing it might as well be steady.

This gets me thinking! See, I was building a mini coilgun that would shoot paperclip clippings, and it shoots pretty well with a 9volt powering it. but after some failed experiments to build a 555 chip voltage double to try to boost the power, THIS idea came along!

In fact, while this video is uploading, I'm going to go kitbash it right now!

Hmm didn't work for some reason. Maybe he 650v capacitor didn't have enough power capacity to fully energize the gun barrel coil I set up. Or something. bummer. Good news though: it DOES make an effective voltage tripler!! Those giant 10v 4700 Caps I tried to make my first symet out of were the original battery bank for the coilgun, Arranged in a neat circular frame that the coilgun could be suspended in the middle of in a sort of "raygun" configuration that I liked.

See? Isn't that awesome looking?! I don't bother making something unless it makes the inner child in me giggle with joy a bit.

 Together their capacity was 14100uf.. which.. I don't know, may equal 1.41 Full Farad. But in my initial build, all I could pump into those Capacitors was the maximum voltage my little hobby power supply could provide: 10V. this was the upper limit of the pulse I could send through the coilgun... meaning the 1.4" clip of paperclip only shot about 3 feet straight up.. pretty good for no moving parts and all, but not really a GUN. My goal for this coilgun is to have it punch a hole in a piece of paper in front of it... here's hoping!

Oh (lost track) so anyway, this Back EMF rig I have didn't work as a power bank, so, I tried it on my coilgun power bank seen here, to see if the EMF pulses could pump higher voltages into them. It DID. Sure the buildup was slow, so I guess the EMF spikes were infinitesimal (high voltage)spikes, but nevertheless, the volt gauge I had clipped onto them showed the charge rising, and rising, and rising, the meter slowing down its charge around 23V (remember the starting voltage was 7). I didn't have the patience to let it climb up to 50, to see if it could actually reach that charge, and anyway, the caps were only 10v, so if I pushed them too far, they go POOF- so I've heard.

 

So, if I can start with 7v and end up with 25v+ in a capacitor bank, then the back EMF rig is something I have a real use for; a way to double, triple, even septuple voltage, without any complicated electronics nor timing circuits.. nor reall any other parts at all! And thats cool enough to keep.