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Small Coilgun Project with Trigger Circuit

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LAN_403

Backyard Skunkworks

Mon Jan 28 2008, 06:20AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Mouser has those TYN640RGs for $2.12, they seem to be the cheapest for about everything...

How would I go about getting an SCR to control the 50v side? Its not like a relay where one coil with 12 volts can be used to control a 50v switch, right?Would I need to have the timing circuits powered by the main caps? That sort of screws up one stage being abel to start the timer on the next...
BTW Thanks for all the help so far man.

Barry

Mon Jan 28 2008, 05:06PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

I'm not sure if you're still considering relays so just let me mention ... A relay can work very well for the first stage, with its very low on-resistance giving you every bit of energy possible. And arcing on closure is not as big a problem as you might think, since a high-inductance circuit takes time for the current to build up. As long as the contact bounce has settled quickly enough, little arcing will occur.

But relays have timing problems in subsequent stages. That is, the mechanical speed of the relay might be the same order of magnitude as the velocity of the projectile. This could mean firing the relay well in advance of the projectile reaching the coil. All told, it adds timing problems which are easily avoided with solid state switches like SCRs.

The latest schematic of yours that I saw shows an open-loop design with pre-set timing for each coil. I built a 3-stage fixed-timing coilgun something like that, and was pretty disappointed. The results were quite variable, since the acceleration is extremely dependent on the exact position of the projectile in relation to each coil. Tiny variations in starting position, voltage, timing and speeds are amplified in each stage and the open-loop design just can't compensate for them. Sometimes it would even shoot backwards. Imagine my surprise. Also I had other problems (like way too much inductance) so I try to forget that I ever built my open-loop Mark 1 design.

Cheers, Barry
If at first you don't succeed, call it version 1.0

Shaun

Mon Jan 28 2008, 06:19PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

And you could get 10 of those SCRs for $1.78 each

gotta love bulk pricing!

The way you connect the SCRs is you connect its cathode and anode in series with the capacitor and coil, so cathode is connected to negative end. To fire, connect +3v through a limiting resistor (50ohm or so for these SCRs) to the gate relative to the cathode. If I mixed something up there (I tend to do that), check barry's site for schematics.

Backyard Skunkworks

Tue Jan 29 2008, 02:27AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

The god of coilguns has visited! Thanks for helping with my project, Barry!

I took Shaun's idea about the SCRs and re-worked the schematic again, now all the timing stages begin counting at the same time so I'll just compensate by tuning each stage off the inital launch time.

So does this latest schematic version look like it should work?

Aside from confirming the circuit as working (atleast in theory) the last thing to settle here is just inductance, after reading Barry's site and playing with the simulator the idea of progressivly longer, lower inductance coils in the later stages seemed like a good one to me. I'm thinking of using large 12 gauge wire for the coils and having the first one with higher inductance, the second one a bit longer and less inductive, the third even more so and the fourth one maybe just one long layer of 12 gauge wire.

Does that seem good? I'm limited to 50 volts by the fact no one trusts a 16 year old evil genius with high voltage and current so I gotta go for high currents alone to discharge the later caps fast.

Also SCRs arn't uber sensitive to static like FETs are, correct?

Shaun

Tue Jan 29 2008, 03:49AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

You need to have a return path for the +3V trigger voltage. The cathode of the SCR needs to be connected to 0V for them to trigger. Also, 10kohm seems like alot for these devices. I would use 100 ohm at most.

Your whole timing system is weird; since its based on an RC constant the voltage on each SCR gate will slowly rise as the caps charge; this could result in the SCRs partially firing at too low a voltage and destroying themselves. Most multi-stage guns based on variable timing alone still used timer circuits or a microcontroller.

It may be a good idea to put all your capacitors together in a bank and use a single stage. You'll have more than enough power to punch through paper, and the SCRs can easily take it.

Please don't think I'm implying that a multi-stage gun is beyond your skill level or anything, because thats not what I think. I only say it because multi-stage guns are usually used because just the one stage can no longer handle the massive amount of energy, so it is split up among multiple coils. You are nowhere near that much energy, or the ~1.2 Tesla saturation point of an iron projectile. There's really no reason for a four stage gun; its what you might call needless complication.

If you still want the multiple stages, I strongly suggest basing it on some sort of feedback. With dial-in coil timing, as Barry said, it doesn't work well because each shot is different.

Barry

Tue Jan 29 2008, 06:55AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

As he correctly suggested, you need a return path for the coil + capacitor portion. Let me redraw your schematic to show where it should go. This is just another view of one of your stages, exactly what you've drawn it but with parts rearranged a little:

I've shown the + and - return path for the trigger, so that both can be connected into your remaining circuitry somewhere. Now another little issue comes up ... the charging circuit for the capacitor also needs a + and - connection. Since the capacitor "-" is not connected to the trigger "-" then the circuit can get really messy as you keep these two connections separated from each other. Wouldn't it be a nice simplification if all the "-" wires were tied together and became common? Then the charging circuit and the trigger circuit could share a lot of wiring.

Let me draw this again to illustrate one way to achieve common negative power supply wiring:

It provides the same function but now you have a common place to measure all voltages from and the wiring is a lot easier.

Now how about that trigger? Keep in mind that SCRs require a significant gate current to turn on, in the case of TYN640 the datasheet suggests 3.5 mA to 35 mA. This puts a limit on the maximum value for a series resistor to the gate terminal. If you use a 3v power pack then the resistor cannot be larger than R = (3v)/(3.5mA) = 850 ohms or there's not enough current to turn on the SCR. For sure the schematic you drew with values of 10K and 100K won't be able to turn on the SCRs. For reliable operation you'll probably end up using a series gate resistor around 100 ohms.

I gotta go now, so chew on this for awhile and see what you come up with. Hope it helps! Also, you'll find things get a lot easier with all your capacitors in parallel for one big single stage.
Barry

Backyard Skunkworks

Tue Jan 29 2008, 11:05PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Ok I think I got itâ€¦ Finally.

Final? Schematic:

Iâ€™ll have the timing circuit switch a big oleâ€™ 2N3055 which will complete the ground path for the SCR trigger, avoiding the problem of slowly ramping up the voltage to it.

Did I get it wired right?

If I did I guess that just leaves my question about trying lengthening, lower inductance coils in later stages.

And the whole goal of this project is to try and make it difficultâ€¦ I already shot a nail though a piece of paper using a photoflash cap bank of 1,200uF @ 330V and a coil made of 22 ga. wire wrapped around a drinking straw, triggered by a weed trimmer pistol grip.

Shaun

Wed Jan 30 2008, 04:28AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

This schematic actually looks okay, although you dont need a huge 2n3055 for the SCRs.

Only suggestion I can make here that I havent made already is use higher energy in your banks! The SCRs can take so much more energy! Since you are limited to 50V for safety, why not just, you know, make banks of 3 of those in series by 3 in parallel...So maybe the way you understood it you were only allowed to use 50V CAPACITORS, which you have. Nobody needs to know the exact charge voltage of the bank...If they do just take a voltage reading across one capacitor...reading 50V, and life is good.

That's what I would do; I mean come on, who's gonna know...

(and P.S if you stick with current capacitors, you in no way need the double SCR on coil #1.)

Backyard Skunkworks

Wed Jan 30 2008, 05:42AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Well I figure it can't hurt to WAAAAAAY overbuild things, I've got the 2N3055s laying around. This coilgun is gonna be modular too, I'm not soldering anything so I can always modifiy/use components for something else later. And the problem with higher voltage is I gotta demonstrate the thing in march and I can't believe I'd get away with it. Besides it's kinda nice to only be dealing with 50VDC, when I was experimenting with the 330VDC 1,200uF bank (65.34 heart stopping joules) I was wearing leather gloves over latex gloves and rubber sleaves, with it sitting on a rubber mat using insulated plyers to handle everything. Its gonna be awful nice to just have the same voltage as a phone

So I'm guessing the lowering inductance/increasing length for later coils should work, making it time to start construction?

I'll get some vids of this thing on youtube when everythings done, maybe even make a site about it.

Steve Conner

Wed Jan 30 2008, 10:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you think overbuilding things never hurts, take a ride on a 300lb cast-iron skateboard. Overbuilding can have similar consequences in electronics.

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