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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Hey all. I have 5 350 uF (I measured most a bit above spec, and they're pretty old!) 450 vDC caps from Cornell Dubilier. All together, (with the above spec increase) I estimate it to be around 2000 uF total. As far as voltage ratings go, I should charge them around 420-430, instead of right at the spec, for a margin of error correct?
And, I have 614 fT of 14 gauge wire. It's not enameled, but it's PVC insulation. From what I understand, it's not ideal, but it'll work.
I don't know where to even begin in all honesty. How do I connect the capacitors together? They have screw terminals. ... I do have aluminum bars I could *try* to drill holes into, but the two screws on the caps are kinda close together to do that.
I looked around on Barry's website and his RLC simulator, but, where does one begin on this? I was looking at around 1 mH of inductance, but I don't know how wide or short or whatever to do.
As far as the actual switch goes, I have 3 of those SCR's in parallel.. I've heard mixed signals about paralleling SCR's for coilguns, some say it's dangerous, some say it's alright to do since the on time is so short. As for gate signalling, I'm just going to use a 9 v battery, resistor, and a push button switch to turn it on.
I also have an isotop diode I could use to prevent the negative voltage spike from killing my capacitors...
Should I parallel the two diodes in the package to increase the peak current ability? Or is the measurement in the datasheet with the two already paralleled?
Looking at the simulation, I should be in the ballpark of 500-600 amps or so of peak current. No paralleling, and my SCR's and the diodes would most likely be toast from the excessive current.
For external iron, I have a few iron core transformers I can tear apart to get the iron. Do I just place it on the barrel of the coilgun? The barrel I think I'm going to be using PVC piping (I think it's 3/4 inch wide? I'm at school typing this so I can't give an exact measurement atm)
For the projectile, will any iron object do? I read that recent thread about the saturation of iron projectiles... That got me wondering, maybe we ought to try something different that DOESN'T saturate. Is there any type of metal or material that will still become magnetized under such force? Aluminum/graphite is dimagnetic, wouldn't that mean it would work too? Or copper, since a magnetic field would be set up due to the current induced in it?
For the charging circuit, I'm probably going to wind my own transformer, and use a ZVS device (with a switch and multimeter) to be able to shut it off when the voltage gets too high.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
- those caps should be fine for a coilgun. yes, charge a little below the max voltage.
- try to find some magnet wire (even if it's a lot thinner, but no smaller than ~22 awg) thick insulation will add a lot of dead space in the coil as well as poor coupling to the projectile. you'll only need ~40ft at the most L of wire needed= (pi*coil length/wire radius)*the sum from x=1 to x=# of layers of [inner coil radius + wire radius*(2x-1)]
- bus bars are convenient for screw terminal caps. flatten out some lengths of copper tubing.
- for the simulators, i usually start by picking a projectile and using the inductor sim. I think it's best if the coil is the same length as the projectile: that gives you the length of the coil next, you want a nice fitting, thin walled barrel (or no barrel at all): that gives you the I.D. of the coil. for the outter diameter. ugh, i'd just wing it and try a few layers. or go the tedious way: time = 2*length of coil* sqrt[projectile mass/(efficiency * capacitance * voltage^2)] use the length, ID, and wire gauge to make coils of various number of layers and write down the inductance and resistance values. then use the RLC simulator with your capacitance, voltage, and try out each set of R and L until the peak time plotted equals the time you calculated with the above formula.
-try vishay's 70tps12 SCR in their sample program. 1200v 75A/1400A peak
-a diode in antiparallel with the coil is a good idea.
-the datasheet says 2 x 60A so i'd assume thats 60A per diode and you could parallel them for 120
-SCRs are pretty robust. just be careful with the gate input.
-external iron is placed on the outside of the coil, not the barrel. picture the field lines of your coil looping out and around to the other end. the external iron provides a shorter path for those lines to travel through.
- Iron will work fine for you. use a magnet and find what it sticks best to (besides another magnet) I'm currently working with solenoid plungers as they are already shaped and pointed, made of a good magnetic material (presumably silicon steel,) and are laminated.
-if winding your own transformer, you can make the output voltage (after being rectified) be equal to your desired voltage, so you wont have to worry about overcharging.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Forty wrote ...
- those caps should be fine for a coilgun. yes, charge a little below the max voltage.
- try to find some magnet wire (even if it's a lot thinner, but no smaller than ~22 awg) thick insulation will add a lot of dead space in the coil as well as poor coupling to the projectile. you'll only need ~40ft at the most L of wire needed= (pi*coil length/wire radius)*the sum from x=1 to x=# of layers of [inner coil radius + wire radius*(2x-1)]
- bus bars are convenient for screw terminal caps. flatten out some lengths of copper tubing.
- for the simulators, i usually start by picking a projectile and using the inductor sim. I think it's best if the coil is the same length as the projectile: that gives you the length of the coil next, you want a nice fitting, thin walled barrel (or no barrel at all): that gives you the I.D. of the coil. for the outter diameter. ugh, i'd just wing it and try a few layers. or go the tedious way: time = 2*length of coil* sqrt[projectile mass/(efficiency * capacitance * voltage^2)] use the length, ID, and wire gauge to make coils of various number of layers and write down the inductance and resistance values. then use the RLC simulator with your capacitance, voltage, and try out each set of R and L until the peak time plotted equals the time you calculated with the above formula.
-try vishay's 70tps12 SCR in their sample program. 1200v 75A/1400A peak
-a diode in antiparallel with the coil is a good idea.
-the datasheet says 2 x 60A so i'd assume thats 60A per diode and you could parallel them for 120
-SCRs are pretty robust. just be careful with the gate input.
-external iron is placed on the outside of the coil, not the barrel. picture the field lines of your coil looping out and around to the other end. the external iron provides a shorter path for those lines to travel through.
- Iron will work fine for you. use a magnet and find what it sticks best to (besides another magnet) I'm currently working with solenoid plungers as they are already shaped and pointed, made of a good magnetic material (presumably silicon steel,) and are laminated.
-if winding your own transformer, you can make the output voltage (after being rectified) be equal to your desired voltage, so you wont have to worry about overcharging.
As far as the SCR goes, I asked for it via the sample program; I hope they have some in stock.
With the antiparalllel diode, I was concerned about PEAK current, and was wondering if they were describing the peak current of ONE diode in the datasheet, or both diodes in parallel.
As for the busbars, I changed my mind, and I'm going to use these aluminum bars I have leftover from an old project. They should work fine.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
oh. whoops. I'm fairly certain that the peak current is for one of the diodes for two reasons: the rating above it for continuous current indicates that it is for a single diode, and, i don't see why they would make two individual diodes into a package and then put the rating for a given combination of them into the datasheet.
edit: so in parallel the peak current should be approx. double the rating
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Forty wrote ...
oh. whoops. I'm fairly certain that the peak current is for one of the diodes for two reasons: the rating above it for continuous current indicates that it is for a single diode, and, i don't see why they would make two individual diodes into a package and then put the rating for a given combination of them into the datasheet.
Then I should assume that the peak current rating is doubled when paralleled?
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
Having done this myself because of anothercoilgunsite.com (I think), I would definitely recommend using superglue (perhaps cyanoacrylate not I that I used that type) on every layer that you wind. Since I used a relatively thick wire and fudged up the initial layer winding repeatedly I got loads of work hardening on the inner layer so that definitely helped. The reason behind this is so that you don't have to use an inner barrel which increases the magnetic coupling between the projectile and the coil, which is of course great. I have to agree with using magnet wire, enamelled copper just to be clear, its not exactly expensive, depending on your budget of course.
I think everyone has been saying that if you do put the SCRs in parallel then make sure you add what is apparently called a current sharing resistor (of equal values) with the gates of each of the SCRs so that they share current equally.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
yes, superglue is amazingly useful for the coils. make sure you get the liquid stuff, not the gel. (superglue=cyanoacrylate) for your first coil, it might be a good idea to put taps in after each layer (after the first couple). just take a small (2-3 in.) loop of the wire, hold both ends of the loops together on the coil (the end thats coming off the coil, and the end after the loop you made) and twist the loop from the end down to the coil. to quote uzzor "it'll make sense when you do it." then just keep winding the coil with the twisted bit left out. this will leave you with a bunch of taps so that you can experiment with the number of layers without having to make a several coils. just be careful where the ends of the wires are when you fire it.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Parallel those SCRs! I've done it time and time again with no problem, upwards of 600A. The people who say that it won't work are those who haven't tried it, those that have tried it know how wonderfully it works. As for current rating, just add them up and multiply by .8 for a safety margin.
As for the anti-parallel diodes, I haven't tried paralleling them so I don't know.
At the energy level you're using, don't worry about saturation. There are people experimenting with different materials (such as ferrite) but you have to have really strong magnetic fields before saturation becomes a problem.
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I would agree, make sure you add taps since you haven't wound the coil yet, I wish I had now :D also about the liquid superglue, absolutely make sure that it doesn't drip inside the coil though... I think when I was considering putting about 50 diodes in anti-parallel with the coil, I was told that they almost definitely wouldn't share current equally, and would be destroyed, besides the diode I put in anti-parallel can handle 600A peak and only cost £5, so no great loss there!
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Working on a coilgun, have questions...
