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Trying to build a coilgun, need help, suggestion and advice :)

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Rustycan

Tue Jun 21 2011, 04:03AM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

hi, peoples
i've build a (not actually) a coilgun, since it's small, not portable, and powerless - -"
(just a piece of coil-ed pipe connected to a charged capacitor, and "plup" the can piece flying out undesireablt :D)
so, i decided to build a better one...
it might be long due to my duty as a student, and intense exam, because i've just graduated from High School
and about to get in a College, btw Wish me luck :p

* Charging Circuit
first of all i will build a boost converter using 555 timer, they said it's rather inefficient compared to Mazilli's ZVS
but, since i don't understand it, i'll use boost converter, except any of you like to explain the ZVS for me :p
btw, this is how i'll build it, this is my Boost Converter schematic(pardon, i forgot to put a dot between ground and the battery's negative terminal, so they're connected), should i put a fuse there?
will it work, or any fault?
and, i don't have any NMOS, but ive found something has a label "MOSPEC TIP3055 GG 7J" on it,
is it the same as MOSFET ? this is the pic : MOSPEC(?)
i also found one K2746 MOSFET, can i use this instead of IRFP250??

* Cap Bank
last time i use a small 400V 150uF(those two holes was the results of my last one :) ), it's low capacitance make a very short pulse (correct me) so it lacks of power
then since this is my first HV project ill use these 4 200V 220uF caps scavenged from 2 Computer Power Suplies
this is the Capacitors

* Projectile
i'll use a 3.5cm blunt steel rod, because from what i've read, sharper tip means larger air gap, which causing inefficiency
and since this is still a low power coil gun, so i shouldn't worried on aerodynamics too much, right? The Projectile

* Coil and Tube
i've read somewhere, the Projectile length should be 2/3 of Coil length, so the coil will be 5.25cm long,
and i'll still play on barry's sim for inductance, or anyone has a suggestion?
for Tubing, i'll use a small indoor antenna, and chop it (just like my last -not actually- coilgun)
this is the pic for Antenna pipe

* Trigger and Timing
for triggering i'll use a relay, since i more understand it than SCR, it's cheaper, and the pulse can be timed
and for Pulse timing i'll use Brush, but not use it directly to deliver the cap bank's current O_O but to to turn
on and off the relay so less eddy current losses, this is the Schematic for timing

i'll simplify my questions here :

1. I wanna know how the Mazzili's ZVS Driver works, so i can build it, or anyone has a tutorial to build it?
2. Is 4 200V 220uF is enough?
3. If you can imagine my system, will it work? or i need to add something?
4. Could i use a single cap bank to fire multiple coil?
5. how to add a spoilers in this post??? T_T

sorry for my bad english :p and thanks before!

PSCG

Tue Jun 21 2011, 09:00AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136

This project reminds me my first coilgun i built when i was 15. 100 J stored energy (10 capacitors taken from TV sets - each 100uF/400V - charged at 450V with a simple 220/12VAC power transformer and a voltage doubler), barrel made from a radio antenna, coil taken from a 30A relay, projectile made from a 4mm diameter nail - 3cm length. Triggering was done with a 10A relay and a battery. Projectile - coil timing was done simply by the positioning of the nail (and a bit of luck.... ).

For the Mazzili driver there is the excellent Uzzor2k's 50W capacitor charger page (Google it and you will find it - i'm too noob and i can't make a link). It has everything you need to built such a driver (including the flyback transformer winding). But if you want to use a booster charger, Uzzors2k has also an excellent page (this charger i'll use for my 400 J BIG coilgun - i've heard very good results about it).

The caps you use store 17.6 Jouls. I do not think that it is enough to propel the projectile you use. My coilgun had a stored energy of 100 Jouls and was capable to punch through a soda can. So, if you want good results i suggest to increase the energy, at least to 50 Jouls. If you stick with your capacitors, you don't have to worry about the saturation of you projectile, sharp tips or aerodynamics.

The single cap - multiple coil topology is, from my opinion, impossible. The first coil stage will discharge to zero the cap bank, leaving no power for the next stages. I do not know any amateur coilgun design that use this topology, nor i can give you any other information about this topic.

Rustycan

Tue Jun 21 2011, 11:16AM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

hi, thanks for your reply :)

your first project was exactly the same with mine, the difference is i wasn't know anything about the voltage doubler that time, i just use a simple rectifier, and i use lesser caps too

and, yes, that's what i said i don't understand T_T it look too complex to me,
if you ever made one, please tell me how

i'll seek a better caps then. But, how much caps with what volt and capacitance would be good for my coilgun do you think? The projectile is a steel rod 3,5 cm long with ~3-4mm diameter, dunno how much its weight

I've used a coilgun sim, and put my coilgun data in it, then simulate it with the pulse timed, and after the pulse time ended, it shows there's still several volts left in the caps, so i just wondering if if posible to do what i asked.

Forty

Tue Jun 21 2011, 05:47PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

1. This will give you a nice description of the zvs circuit as well as a pretty picture showing its layout. In my opinion, and from what i've read of other peoples' experience, the zvs is a lot easier to build and to get working than the boost converter. point to point construction is easiest (as opposed to a circuit board) and can be made quite compact as well. start by sticking the two mosfets together side by side on a small heatsink (w/ insulating pads), hold a zener diode up against a 10k resistor and twist their leads together on both sides, repeat with another zener and resistor. connect the anode side of a zener/resistor to the source of each mosfet. the rest of the circuit just kinda falls into place. your first one will look messy but then you'll have the hang of it.
as for getting the components:
i imagine you've also taken apart a crt tv or monitor before, which will have your needed
-flyback core and probably the
-power resistors (the 470 ohm 3W ones in the schematic. they dont have to be that exact value but they do have to be equal and of high wattage)
-vishay and fairchild will sample mosfets to you (50-200v, greater than 15A, lowest rds on value available, avalanche rated... or just go with the irfp260n or irfp250 as in the schematic)
-texas instruments will sample the zeners to you.
-the inductor is easy enough to make but if not, coilcraft will sample you one.
the capacitor on the primary side can easily be salvaged. look for the plastic box caps or the coated mylar caps, rated anywhere from 0.5uF @200v on up (psu supplies will have several)

cap bank: the 200v ones might work. and more are easily found in lots of stuff. capacitors from camera flashes are a nice start for a coilgun. look around at any thrift/second hand/ junk stores in your area for cheap cameras with flashes. the best things to look for are the independent flash units that mount on top of cameras and old polaroid cameras where the front flips up. 300-600uf @350v capacitors can be had with this shopping agenda, and for a low price. you could also try the disposable camera route and go to your local film developers and ask them for any used camera shells they might have.

projectile: your's should work. don't worry about aerodynamics yet.

opinions seem to vary on the ratio of coil to projectile length. I've only used 1 to 1 and ~1.5 to 1 (which is your ratio)
just about any ratio (within reason) should work so don't worry about it too much. you can always make more projectiles of different lengths to compare. use the thickest enameled wire you can find. (16awg to 20awg if youre buying it)

barrel: conducting barrels aren't good to use as they will rob some of the energy because of induced eddy currents. if youre only doing one stage for now, don't even use a barrel. take a piece of your projectile material, put a couple layers of paper on it, and form your coil around that. the coil itself will be the barrel and you'll have very high coupling. since i usually use lots of liquid super glue on the coil to keep it together and sturdy, i've just adapted the paper spacing between the form and the coil into the barrel by saturating the paper with super glue. even using a pen tube that is larger ID than your projectile would probably be better than the conductive barrel.

for triggering, sample the 70tps12 from vishay. SCRs are just a diode that doesnt turn on until you send a signal to an extra pin. using relays will eventually just turn the contacts into a plasma deposited layer of metal on the inside of the plastic case.

Rustycan

Tue Jun 21 2011, 09:46PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

thanks for your reply

wow, that's a complete explanation, thank you very much!
and the components aren't as much as i thought too.
you said that the power resistor don't have to be exact, will a 5W 0.5ohm works?
I'll buy 2 mosfets then, but for inductors, they only shown henries there if i read correctly, how many turns on toroid it should be?

I was trying to make a photoflash coilgun before, but it's hard to find a disposible cams in my city, and it sounds better to build a booster charger :D so, i'm trying to make this one now

I use the 1-1.5 one
and 2-3 layers of 14awg (still finding out)

your paper advice sounds good, i'll try it then

but the Scr won't turned off until no currents flowing through it, right?
So, how can i time the pulse with that?

Thanks before

Forty

Tue Jun 21 2011, 11:22PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

0.5 ohms is probably too low. but i've used anything from 200 to 500 and they all seem to work the same.
the toroids from the psu's you took apart will work. take off the thinner gauge wires from one of the multiturn toroids and use the thickest that's left. or strip one bare and do anywhere from 20 to 50 turns on it with beefy wire (thats for the zvs inductor) for the boost converter, going off of the one from anothercoilgunsite.com, about 90 turns of 22awg on ferrite should do it.

only do the paper/superglue ceramic thing if you really dont care about getting superglue on your fingers. i personally don't feel that i have accomplished anything in a day unless my fingers are covered in superglue, cuts, and burns.

correct, the scr will stay on until the current hits ~0. you adjust the timing by varying the voltage, capacitance, coil inductance, and initial position of the projectile. or just keep the first 3 variables fixed and adjust the projectile position.

Rustycan

Wed Jun 22 2011, 02:43AM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

aw, should i try to use it to know what'll happens?
Btw, what's this resistor do ? What'll happen if i don't put any resistor there?
(stupid question, sorry, i'm just curious)

burns??? From what did you get burns?

@.@ sounds harder.. But, still doable,
but how much voltage requirede to make an arc? I'll use a voltage below that so i can use relays instead of SCRs

i'll shop for the mosfets and power resistor today, and build the zvs by afternoon :)

thanks

Forty

Wed Jun 22 2011, 05:01PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

burns from soldering stuff.
the relays will arc when closing and opening if there's a lot of current flowing through it.
as far as the resistor, it's just to help protect the mosfets.

Rustycan

Mon Jun 27 2011, 04:14PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

can i have another mosfet to substitute that irfp250?

Forty

Mon Jun 27 2011, 04:40PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

for the zvs, yes but you'll need two of them, as they need to be the same.
for the boost converter, yea sure, just find one with similar voltage, current rating, and rds on resistance

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