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Unread postAuthor: DYI » Tue Feb 08, 2011 11:55 am

Technician1002 wrote:...SCR... Maybe a coin shrinker instead.


:violent1: Come on Tech, surely you of all people know that making a coin shrinker with SCRs is like trying to build a hypersonic cannon with PVC pipes and a ball valve. Even for a U.S. dime (likely the easiest common coin to shrink, apart from the aluminum Yen coins), required currents are in the tens of kiloamps. I did the smallest one of these at 12kV (15kJ in the parallel bank configuration), and even then efficiency was fairly low due to the system inductance, and insufficient voltage:energy ratio.

Now, on to the coilgun topic. Firstly: WHY THE HELL DOES EVERYONE INSIST ON BUILDING RELUCTANCE COILGUNS?
They're a backward, dead-end, inefficient design. Pushing the projectile instead of pulling it removes the competition of propelling forces on the projectile, vastly widens the range of useable projectiles, and except in low velocity cases, improves efficiency.

As to this particular design, I think Ragnarok has already said almost everything that needs to be said about it. Spherical projectiles are just a bad idea. A suitable relay will not weld, as is evidenced by the one used on Larda's ETG for switching. With your current design, expect something less than 1J muzzle energy for the final build. Whether or not 1J is worth the trouble to you is a matter of personal preference.
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Unread postAuthor: Ragnarok » Tue Feb 08, 2011 12:44 pm

DYI wrote:Firstly: WHY THE HELL DOES EVERYONE INSIST ON BUILDING RELUCTANCE COILGUNS?

For the same reason I build pneumatics rather than hybrids. If I cared about the most powerful/"efficient" option, I'd be out there mixing solid propellants.

We all have different things we're looking for in this hobby. Power and efficiency may be way up your list, but that won't ring true for everybody.
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Unread postAuthor: Technician1002 » Tue Feb 08, 2011 1:31 pm

DYI, you have done your homework. I shot that out to see who was paying attention and knew their stuff. Well done. :D
Come on Tech, surely you of all people know that making a coin shrinker with SCRs is like trying to build a hypersonic cannon with PVC pipes and a ball valve.
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Unread postAuthor: jimmy101 » Tue Feb 08, 2011 4:26 pm

Instead of the relay, consider an SCR like this one here (scroll down to part #M25/16-1K):
SCR & Diode
1000v @ 16 amp (400 amp surge)
$4.50

The SCR replaces the relay and can be triggered by a couple volts at a couple dozen milliamps. SCR (or a solid state relay) has the advantage over a standard relay in that the contacts don't bounce. The high voltage diode is handy for a snubber so you don't have to worry about the coil+cap resonating, you get just a single half-wave across the coil.

You might want to take a look at this page for ways of triggering and an easy way to monitor the caps/coil voltage versus time during firing as well as being able to measure the muzzle velocity.

Partial circuit for the gun itself:
Image

Trigger circuit:
Image

The voltage monitor and chrono:
Image

Typical data stream (recorded on a laptop's MIC input) showing both the voltage versus time across the coil (first peak) and the two gates of the chrono:
Image
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Unread postAuthor: more_eggs » Tue Feb 08, 2011 5:00 pm

Haters gonna hate.

But thanks to those giving positive feedback and ideas.

Firstly: WHY THE HELL DOES EVERYONE INSIST ON BUILDING RELUCTANCE COILGUNS?


Why the hell not?
I dont give two shits if its not incredibly powerful, which I thought was understood by now.
Anybody can go out there and build a hybrid, or a pneumatic relatively easily, there is so much info out there on them its not funny.
For me I get more satisfaction from building something that it isnt so easy to come up with good plans/ideas etc there aren't any dedicated coil gun forums out there.
And how can you not see the cool side behind "hey check out his gun I made, it fires using electromagnetism"
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Unread postAuthor: Technician1002 » Tue Feb 08, 2011 9:25 pm

jimmy101 wrote:Instead of the relay, consider an SCR like this

The high voltage diode is handy for a snubber so you don't have to worry about the coil+cap resonating, you get just a single half-wave across the coil.

Partial circuit for the gun itself:
Image


That circuit does work, but an improvement can be made in regards to the snubbing diode.

The circuit works by dumping a high voltage into the coil. The coil current builds rapidly with a rise time limited by inductance. The LRC time constant can be used to find the discharge time. When the cap is fully discharged, the coil current is at maximum in a high Q circuit. At this point the diode begins to conduct effectively shorting the cap to prevent reverse polarity. Unfortunately the decay time of the coil is now an RL time constant and decay of the current is limited only by coil and diode resistance.

From knowing the peak coil current and using ohm's law, a resistor in series with the snubber diode can be used to allow the coil to reverse voltage polarity up to the supply voltage to quickly decay the coil current with a current decay time that matches the coil current rise time. This dissipates the energy on the snubber diode resistor assy and greatly reduces the heat generated in the diode.

Another common option used in that type of circuit to save energy is to simply let it resonate for 1/2 cycle and recharge the cap saving the energy instead of dissipating it as heat. This involves placing the snubber diode in parallel with the SCR. The circuit is fired, the cap discharges, the coil current peaks and due to LC resonance, the voltage reverses polarity and the coil current decays. When the coil current and SCR current has decayed, the SCR recovers (switches off) and the reverse cap voltage then flows through the diode reversing the coil current for another 1/2 cycle. Again the coil current peaks as the cap is discharged, then the cap begins charging in the forward polarity while the coil current again drops. When the coil current (and snubber diode) reaches 0 current, the diode recovers and the system is almost fully recharged for the next shot. Energy lost to heat, projectile motion, and other losses will reduce the amount of energy recovered.

Proper consideration must then be given to the selection of the power supply (load will reverse polarity) and pulse cap along with the peak voltage and recovery speed of the SCR and diode. They will have to be fast recovery parts and the cap will need to be a non polar device.

The advantage is a double current pulse in the coil, and a quick snub of the coil current so the current is held to 0 for the projectile to exit the coil as both the SCR and diode have ceased conduction as the projectile reaches the center of the coil.

Sorry this is so long. Those who understand it will appreciate it.

The physical circuit mod is simply to place the diode across the SCR instead of snubbing the coil to ground with a shunt diode. The SCR trigger pulse will also need limiting so the SCR will recover instead of leaving the trigger current on for more than a 1/2 cycle.

I have some of these.. :D 600 Volt, 300 Amp. Fast recovery.
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Unread postAuthor: DYI » Wed Feb 09, 2011 11:53 am

For the same reason I build pneumatics rather than hybrids. If I cared about the most powerful/"efficient" option, I'd be out there mixing solid propellants.

We all have different things we're looking for in this hobby. Power and efficiency may be way up your list, but that won't ring true for everybody.


That's not an entirely valid argument - for what you're trying to do (my impression is ergonomic, fun, accurate, reasonably quiet) the pneumatics you build are an excellent choice, and a hybrid would probably be a step down. Are you saying that, assuming the ergonomics, fun, accuracy and noise levels stayed the same, you wouldn't want to switch to a different technology which allowed you to either increase muzzle energy or improve efficiency, for a similar build cost?



Why the hell not?
1. I dont give two shits if its not incredibly powerful, which I thought was understood by now.
2. Anybody can go out there and build a hybrid, or a pneumatic relatively easily, there is so much info out there on them its not funny.
For me I get more satisfaction from building something that it isnt so easy to come up with good plans/ideas etc there aren't any dedicated coil gun forums out there.
3. And how can you not see the cool side behind "hey check out his gun I made, it fires using electromagnetism"

(Numbered for ease of response)

1. I suggested a means to improve efficiency and range of projectile choice. Are you not interested in either of those?

2. And yet inductance coilguns are less frequently built than the reluctance variety, which are almost commonplace by now.

3. Hey, check out this gun I made - it fires using electricity. I've spent more time and money on this single (electric) launcher than on all my hybrids combined. It is roughly 1% as energetic as my last hybrid and yet, in my eyes, considerably more impressive. And you think the available information on hobbyist reluctance coilguns is sparse? Try building an ETG :wink:
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Unread postAuthor: jimmy101 » Wed Feb 09, 2011 5:40 pm

Technician1002 wrote:That circuit does work, but an improvement can be made in regards to the snubbing diode.

The circuit works by dumping a high voltage into the coil. The coil current builds rapidly with a rise time limited by inductance. The LRC time constant can be used to find the discharge time. When the cap is fully discharged, the coil current is at maximum in a high Q circuit. At this point the diode begins to conduct effectively shorting the cap to prevent reverse polarity. Unfortunately the decay time of the coil is now an RL time constant and decay of the current is limited only by coil and diode resistance.

From knowing the peak coil current and using ohm's law, a resistor in series with the snubber diode can be used to allow the coil to reverse voltage polarity up to the supply voltage to quickly decay the coil current with a current decay time that matches the coil current rise time. This dissipates the energy on the snubber diode resistor assy and greatly reduces the heat generated in the diode.

Another common option used in that type of circuit to save energy is to simply let it resonate for 1/2 cycle and recharge the cap saving the energy instead of dissipating it as heat. This involves placing the snubber diode in parallel with the SCR. The circuit is fired, the cap discharges, the coil current peaks and due to LC resonance, the voltage reverses polarity and the coil current decays. When the coil current and SCR current has decayed, the SCR recovers (switches off) and the reverse cap voltage then flows through the diode reversing the coil current for another 1/2 cycle. Again the coil current peaks as the cap is discharged, then the cap begins charging in the forward polarity while the coil current again drops. When the coil current (and snubber diode) reaches 0 current, the diode recovers and the system is almost fully recharged for the next shot. Energy lost to heat, projectile motion, and other losses will reduce the amount of energy recovered.

Proper consideration must then be given to the selection of the power supply (load will reverse polarity) and pulse cap along with the peak voltage and recovery speed of the SCR and diode. They will have to be fast recovery parts and the cap will need to be a non polar device.

The advantage is a double current pulse in the coil, and a quick snub of the coil current so the current is held to 0 for the projectile to exit the coil as both the SCR and diode have ceased conduction as the projectile reaches the center of the coil.

Sorry this is so long. Those who understand it will appreciate it.

The physical circuit mod is simply to place the diode across the SCR instead of snubbing the coil to ground with a shunt diode. The SCR trigger pulse will also need limiting so the SCR will recover instead of leaving the trigger current on for more than a 1/2 cycle.

Yes, the trigger circuit already does that. RC of the trigger is in the low microsecond range, much less than the RLC of the launcher.

As to the snubber, you don't want the circuit to resonate. Yes, it'll partially recharge the cap but it also means the magnetic field persists for much longer than you want, resulting in the ammo being sucked back towards the coil. Once the ammo is beyond the center point of the coil you want the field to be at zero and to stay there.

You want the field to build up exactly once, you don't want it to resonate. Like I said, you want a half wave on both the Voltage and current plots.

Barry's RLC simulator (http://www.coilgun.info/mark2/rlcsim.htm) is excellent for estimating the RLC characteristics. It is easy to get the circuit to resonate but you don't want it to do that. The snubber diode will pretty effectively keep the current through the coil from ever reversing.
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Unread postAuthor: Technician1002 » Wed Feb 09, 2011 8:18 pm

With the diode shunting the coil, the coil has a long current decay curve.. The diode starts to conduct while the coil current is at maximum. Did you miss that in the original design shown?

Coil current decay is a function of the collapse of the magnetic field inducing a current in the coil (maintaining current) which decays as a function of coil resistance and diode voltage drop.

High speed fuel injectors and other high speed electromagnetic devices use other means other than a shunt diode to ensure rapid dissipation of the current in the coil.

Options include a zener diode, series resistance, the shunt cap and diode (used in ignition circuits to generate the rapid voltage spike and TV flyback circuits for rapid retrace times). A simple shunt diode is the slow recovery option. The shunt diode is often used on relays as it reduces the voltage spike of relay current shut off by permitting the relay current to flow long after the applied voltage is removed. The slow drop in current provides low induced voltages. A fast drop in current requires a high voltage.

Re study the energy in a LC resonant circuit to understand the voltage current relationship in both the inductor and capacitor. Shorting the inductor at the zero voltage point with a diode results in shorting the inductor at the peak current point. Current will continue to flow until dissipated by resistance.

A shunt diode LR constant is much longer than a 1/2 cycle of LC constant.
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Unread postAuthor: Ragnarok » Wed Feb 09, 2011 10:21 pm

DYI wrote:Are you saying that, assuming the ergonomics, fun, accuracy and noise levels stayed the same, you wouldn't want to switch to a different technology which allowed you to either increase muzzle energy or improve efficiency, for a similar build cost?

No. But that's assuming I would find it as fun, which I don't think I would.

It's about the character of each. As I've said before, I'm a steam locomotive enthusiast - while it's undeniable that steam rail traction is technically inferior to its modern replacements in every way*, I still find it far more interesting and "fun".

*Well, almost every way.
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Unread postAuthor: Lockednloaded » Mon Feb 14, 2011 3:36 am

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