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Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55
@El_Roberto
Standard 3 inch nails with the heads removed but I'm in the process of getting a Mu Metal projectile made. I looked at the v-switch and it not even close to what I'm using. The v-switch quenches and I have no reason to quench using an SCR because I have a quenching diode for that rated for 4000 amp pulses and has a 800v reverse voltage breakdown rating.
The capacitor doesn't detonate because it pulse through the SCR immediately shutting it off. It basically shorts out the SCR and by the time it is done discharging the SCR is off so it doesn't become negatively charged. I think you may be thinking that the capacitor is negatively charged but its not. It is charge normally and pulsed through the SCR in the opposite polarity as the current is flowing through it. If you copy and paste the link mentioned earlier it might help you understand it a little better. Warning though, using electrolytic capacitors may cause detonation if they aren't able to turn the SCR off. That is why a ceramic capacitor is used instead.
@rp181
555 times aren't fast enough believe it or not. That's why I'm not relying heavily on ICs to trigger. I'm tuning the coils so that they have fewer layers and become longer each stage. I don't plan on spending too much time on the tuning but I will make an effort to adjust the size each stage. Eventually I'm going to eliminate the need for finely tuned coils by switching over to a PIC controller. I already have the PIC but I've been too lazy to use because its not necessary until a few more stages are added. The operating frequency is very high and I will then be able to program custom delays in it.
Yes I've heard of using iron casing for the coils but it completely defeats the purpose of a multistage gun. Trying magnetizing and demagnetizing and iron casing in around 2 milliseconds, its simply not going to happen. I've played around with it a bit but its ruins a multistage gun trust me I've taken college courses in magnetodynamics and although it may improve flux density in a given area energy in guzzled trying to magnetize it and then on top of that fighting off eddy currents.
The ideas I'm generating are only improvements of old concepts. Too many people have been ridiculously sloppy in their approach and I intend on putting the science back into this design.
-------------
Understand that I'm not using a separate bank of capacitors for each stage. Doing so is inefficient and will introduce many problems as the power level increases and more stages are added. The only disadvantage is capacitor life is significantly reduced because they are basically being charged and discharge about 2 times a second.
Registered Member #1774
Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135
Oh I see what youre doing now, In my coilgun Im just going to tune the coils so that the pulse time is exactly right for the speed of the projectile etc and If I can ill use a V-Switch. If I understand correctly by doing what you are you will create a Back EMF which will cause suckback, but I believe that the V-switch does infact get the second capacitor negatively charged to counteract this, which is why I dont understand how it doesnt explode
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
A few nanoseconds are not going to be a problem. If it is that sensitive, It would be impossible to keep variables constant. Not saying you said your came up with the idea, But SCR;s have been turned of by reverse voltage before. Particularly old 3 phase power supplies. Good to see someone taking coilgun's seriously =)
EDIT: did somemore research, disregard the above =p
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
@El Roberto
The capacitor is not reverse charge it is charge to a polarity and then it is discharge into the scr anode and cathode where the positive side of the capacitor is connected to the cathode and the negative side is connected to the anode resulting in a reverse voltage being applied to the scr. If you look close at the URL i posted (you have to copy and paste it because it won't link for some reason) it clearly explains the concept and how it operates.
@rp181
I'm not sure exactly about what your referring to, to ignore but addressing the speed issue, it makes all the difference. A prime example is I was using 4069 logic gate as an inverter to trigger the scr shut off but I soon discovered even this high speed CMOS chip couldn't react fast enough to trigger. Keep in mind if the projectile is traveling at even 100 meters per second which is less than 5% effciency with a 2.5kJ bank then it will have already passed by the IR sensors before the voltage has reach the internal trip point for the logic gate. This is because voltage don't change instantaneously as we would like to think. It always takes time for the voltage to change under any circumstances; a key characteristic of analog circuitry. Now with the schmitt trigger I'm using the gate is either completely on or completely off and the voltage has a very specific point it has to reach in order for the output to go high or low. The properties of a schmitt trigger allow it to treat the voltage as though it is a digital signal that 'snaps' on and 'snaps' off.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
What is the bore of your cannon? To get 100 m/s your going to need a fairly thin projectile. At 2.5kJ, you may reach saturation. I shot a piece of 1/4" rod in a 1/2" bore gun with ~955J single stage, and the steel piece saturated. This was using a contact switch, no SCR.
You were talking about how it takes time for voltage to climb. For this, Ragnarok came up with an idea to switch a HV capacitor into the coil, to ramp up the voltage, and the main capacitor bank to keep it there.
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
@rp181
The bore of my gun is roughly 6.35mm and the projectile is about 6mm. The 100m/s calculation is based off the weight of the projectile which is .10kg (10g). If you use KE=(.5)mv^2 (conservation of energy) then plug in your efficiency rating which is the capacitor energy multiplied by efficiency then solve for 'v' you will get around 223m/s at 10% efficiency.
I'm not talking about coil voltage I'm talking about the voltage changes inside of a logic gate. The idea to use a HV capacitor is nice but inductance still applies. All the HV capacitor does is put high voltage through the coil putting the inductive forces on the HV cap instead of the regular capacitor bank. Although it cuts back on wasted energy in your main bank due to induction you end up sacrificing precious time.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
blackgrunge wrote ...
@rp181
The bore of my gun is roughly 6.35mm and the projectile is about 6mm. The 100m/s calculation is based off the weight of the projectile which is .10kg (10g). If you use KE=(.5)mv^2 (conservation of energy) then plug in your efficiency rating which is the capacitor energy multiplied by efficiency then solve for 'v' you will get around 223m/s at 10% efficiency.
I'm not talking about coil voltage I'm talking about the voltage changes inside of a logic gate. The idea to use a HV capacitor is nice but inductance still applies. All the HV capacitor does is put high voltage through the coil putting the inductive forces on the HV cap instead of the regular capacitor bank. Although it cuts back on wasted energy in your main bank due to induction you end up sacrificing precious time.
Finding velocity from bank size and desired efficiency is quite optimistic at best. Like rp181 said, you may be better off going with a bigger projectile. I found a 3/8ths (8mm) projectile works quite well at around 500J, and probably a lot higher. 6mm may just not cut it, go for a bigger projectile and less velocity, unless you want a dozen stages...
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
@Backyard Skunkworks
How is using the conservation of energy and compensating for losses optimistic. If the coilgun has a 10% efficiency then you use 10% of the capacitor energy as KE. Its the exact way you would calculate your efficiency given the velocity. A larger projectile just means that when it becomes saturated the maximum force that can be applied is larger than a smaller projectile. While this true it also creates more recoil, lower velocities and shortens the range. My projectile is 6mm but its also 3 inches long so it still has the mass necessary to not look like a pea shooter. A realistically the bore of the gun has little to do with the effectiveness of the gun. Its all about mass. I could have a 3mm bore but have a projectile that's a foot long, it really wouldn't matter as long as it still has the appropriate mass.
Everything I'm saying is backed by hard calculations and calculus models. I have some great equations and reference material if anyone would like to take a look at the math behind this concept just grasp a little bit better what's happening every time one of these are fired up.
------
There's something I would like to point out. It seems as though people believe that when the projectile becomes saturated its a bad thing. Its actually a good thing up to a certain point. For a projectile to become magnetized it takes energy. So when you have saturated your projectile it just means that you are no longer dumping energy into it. Just because it is saturated doesn't mean its at its maximum velocity it just means that the maximum force it can experience has been reached. It doesn't, however, stop it from gaining velocity it actually will continue to accelerate until the applied force equal the force of air resistance. So when you talk about saturation keep in mind that just because wikipedia says its a limiting factor it only limits the maximum velocity. And when you have an impulse of just a few milliseconds its highly unlikely that you will see the effects of saturation. And if you are totally convinced that you have maxed your velocity then sit down with a grinder and round some edges and smooth some surfaces and I guarantee that you might be able to squeeze a few extra meters/second out of it.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Its better to have energy being used to accelerate it, rather then keeping its velocity constant. Also, with long projectiles, it is effectively dead weight. The magnetic field will only extend so far, so mass is not the only key point. Theres also issues of eddy currents working against you in such long projectiles.
Ive done some testing with lots of materials ranging from iron to hymu-80, and saturation does affect it.
Also, Do you plan on changing SCR's? those look too small for your planned eventual energy.
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
@rp181
Now what you said I have to point out is simply not true. I've taken college level magnetodynamics courses and to say the magnetic field will only extend so far is wrong. Take a look at how a solenoid produces an em field and then we'll talk more about it but a longer projectile isn't bad if you have longer coils to match. And it also cut down on eddy currents becuase they flow in an axial pattern not down the length. Think about it currents only flow in a closed circuit, if length caused more eddy currents that'd mean they would traveling down it projectile and then back up it. Please do a bit more research before you make blanket statements like that. Also, if you read closely I didn't day saturation doesn't matter I said that its effects ate unlikely to be seen at the velocities most people play with and if you are then consider reducing the bore size and lengthening the projectile, the exact reason why my projectile is shaped the way it is.
The SCRs are 2N5207. I'm getting new ones but I'm going to use these ones until they start failing which may be right away or the next time I step up the voltage.
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6 Month Gauss Cannon Project
