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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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V3 Coil Carbine

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Author Post
ben5017
Wed Oct 09 2013, 12:30AM
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Joined: Thu Oct 14 2010, 04:23PM
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Posts: 154
12 Kohm between collector and emitter when gate voltage applied.

gate voltages used. 6 & 12 & 15 (when 6 didnt work)
do I need a resistor and/or diode between gate and emmitor?

IGBT: IXYS IGBT IXGN200N60A



(I wish they would tell you what the suffixes on the datasheet stand for)

By "on" I was looking for nearly no resistance (mohm) between collector and emitter. I had no trouble when driving the 422 off of a 555...

Saz: it does light up LED, I also tried my injector and nothing happened but a blown fuse after a second or so. there is to much resistance to get a magnetic field strong enough to move my bolt inside the injector.
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DerAlbi
Wed Oct 09 2013, 01:35AM
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Location: Dresden, Germany
Posts: 510
ben... you are using IGBTs. not Mosfets.
why do you expect anything even close resistve behavior!?

...well small signal behavior suggests 17.8mOhm with 10V @ Gate. but YOU cant measure that.
What sufix do you mean? The "A" is descibed in the datasheet first page..................
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Yandersen
Wed Oct 09 2013, 10:38AM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Funny, ben. The device you use rated 600V, 300A peak. The voltage drop is 2.5V. On digikey it costs $38.
Let's calculate the equivalent resistance of it when 300A conducted:
2.5V/300A = 8.3mOhm.
Now, the voltage rating of the switch must exceed the supply voltage 2 times. It means, that 100V rated device is good enough for you, ben.
So let's see what would be the MOSFET options. Here is the 100V rated MOSFET equivalent for $1:

Here is the 2 times better option for $4:

And 3 times better option for $7.4:


The MOSFETs linked are much smaller in size and have also lower driver's requirements then your IGBT. And unlike with the IGBT you can overload those higher then the pulse rating specifies given the pulse is shorter as MOSFETs do not latch. From the other hand, on low current the MOSFETS will show lower voltage drop while with IGBT you will always have 2.5V.
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Steve Conner
Wed Oct 09 2013, 10:53AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
An IGBT has a junction, so it doesn't have a Rds(on). It has an exponential voltage drop like an ordinary transistor, starting out about 0.5V for small currents and heading up logarithmically from there.

The lowest voltage rating of IGBT you can buy is something like 250V. Below 250V, MOSFETs work better.

Modern IGBTs are latchup free. One popular device is rated at 600V, 110A continuous, 300A pulsed at 25'c case temperature. In Tesla coil work we run it off a 320V DC bus and overload it to 450A or even 600 if you are feeling lucky. You would need a huge array of 600V MOSFETs to match this.
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ben5017
Wed Oct 09 2013, 03:16PM
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YAN: Don't use digi-key unless you have too. Kind of a rip off. FYI I got 20 of the IGBT's for 200$ on ebay.

from your math, I concluded my 12 Kohm is reasonable when only conducting the current of a single LED

2.5V/xA=12000ohm
x= .2 mA (for single LED)

(I could have sworn on my test setup when driving it off 555 & 422 that resistance was in mohm region with no load besides the multimeter??)

The reason I got higher voltage rated IGBT's (600v even though im powering my coils off of 24V) was to be able to withstand a larger voltage spike, allowing me to use more TVS diodes in series to kill the current as quickly as possible minimizing suck back.


STEVE: Can you explain further the significance of “Rds(on)” for a newbie and is my above reasoning correct?
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DerAlbi
Wed Oct 09 2013, 05:10PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
ben, you are completely off. sorry for that. your 200$ are spent compeltely wrong.
this is a nightmare in a 24V-System to have IGBTs.
You obviously are not interested in how things work. This is why you kind of deserve that desaster. Just using google and reading about IGBTs would have helped. Yes even for a starter - Yes its work, Yes it hard. Yes is important.
You seem to look at a datasheet and every charactierized value you dont understand seems not existant for you. so you ignore e.g. Vcesat instead of googling it and this brings you here.
You hold your multimeter somewhere in the circuits and getting some values. what for?

You would be much better with simple IRF1404 using active clamping (or let the avalance in the body didoe take care of). Did you calculate wow fast the current decays if you allow 80V? Was the value really that bad?
"the higher the better", right? Sounds cooler at least -.-

And PLEASE stop measuring resistance over a transistor. its a completely stupid value. it tells you exactly nothing.
what bring you to this idea? can you explain how an IGBT work, and why it would be usefull to measure resstance between C and E ??

When you finally show your ultra hyper cool thing on youtube or be the biggest guy in town for your friends, will you mention that you dont even know how the thing works? -.-
"somethihg switching in there somehow maybe a bit for some time"
I will laugh if you loose all your coolnes when finally someone asks why you didnt use a potentiometer with a verry fast motor if you just put in variable resistances in your design...

Sorry for that. And: Rsd(on) is verry well documented by google.
look what i have found on the first few links after googling for "rsd(on) mosfet"


really cool, issnt it? Takes much less time than waiting for a forum answer too.

heeey and look what i found when googling for "mosfet versus igbt".
First link:

And you know... if sometimes the first link issnt satisfiying, maybe reading it will help too. The things you dont understand will cross you at your search many times and then - when reading enough - you will get a picture of how things work. there are also good youtube videos.
But yes, all this takes time. Time you will not be admired by your buddys for beeing the cooles guy with the hyper ultra electrical cannon that spits out a iron projectile.

if i tell you now that Rsd(on) is the resistive value of a mosfet between Soruce and Drain when its fully on, so that adding extra gatevoltage will not dreacease Vds anymore, you will get the meaning of Vds, but you miss the bigger picture that would help you really to understand things.

Vcesat is the same thing for Transistors. Its the Vce-Voltage that is reached as soon as the basis-current is high enough so that Vce will not decrease anymore. Your IGBT has a 2.5V Vcesat. So it doesnt mater much, how much current you apply you will always measure the voltage around 2.5V. Sometimes less, (low current) somtimes more (high current)

End this is why measruring resistance is useless. you always have something around 2.5V. So you apply 15V at the gate, and measure resistance with 10mA. you get 250Ohm, you measure it with 10A, you get 0.25Ohm. you use 100A... and so on.
Its always the same Transsitor. so what physical meaning has this resistnace??? NO MEANING AT ALL.

The onyl thing is certain: by using IGBTs you waste 2-3V of your Battery-Voltage in your IGBT. This is a expected loss >10%. This will lower your effective Voltage at the coils -> you need lower inductance -> less turns -> less force -> less speed.

This is why IGBTs are high voltage devices. 2.5V taken from 400V dont matter much.

READ. start at wikipedia, then search for Application notes. Then start with wikipedia again when you encoutner something you dont understand. Forums arent that effective to read, because the content is often shitty.
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ben5017
Wed Oct 09 2013, 06:01PM
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Mostly fair enough. I am learning as I go, by the end of the project I will have a better grasp on how everything interrelates.

However I think "nightmare" is a little strong. I used the same IGBT's in my test rig with satisfactory results.

I will adjust my coil geometry to take into account the 2.5 voltage drop and still end up maxing out the pulse discharge of my lipo.
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Yandersen
Wed Oct 09 2013, 07:53PM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
Why don't you simply buy some MOSFETs instead? Can't spend extra $20 for more suitable devices? If your current is limited by the coils' resistance, then optimizing the speed of current decay by 600V is so nonsense...
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DerAlbi
Wed Oct 09 2013, 08:32PM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
Well learning by making mistakes is ok so far. Even yan makes mistkes *troll*

If the results are ok for you then its fine. However keep in mind that there are other posibilities that dont require that much amount of money to achieve the same result OR spend the same amount and get even more "satisfactory" results. .... i must confess that even my charger costs more.
Have fun so far

Just a tip: if you spend your LiPo a big cap, maybe the voltage will not break down so much due to internal resistance so you can compensate some effects...
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ben5017
Wed Oct 09 2013, 10:24PM
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YAN: I probably will. Coil Inductance plays a larger role than resistance in limiting the current true?

any tips/ things to look out for when paralleling mosfets?

ALBI: Its all good. I just dont make the same mistake twice. + its not like i wont use the igbts for something ;) ,especially for the good deal I got. Mistakes are inevitable in my first serious electronics project. just hoping this will be the biggest one of the project.. (fingers crossed)
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DerAlbi
Wed Oct 09 2013, 11:02PM
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Location: Dresden, Germany
Posts: 510
Current waveform is... fuck it. Not worth asking the question. its answered many times already. its all out there.
google: "inductor resistance current"
second link:
That was hard. i know.

Paralleling mosfets is not as complex as paralleling IGBTS. But really.. are you kidding me, that you wait for an answer here?
-> Google. "paralleling mosfets"
First hit:
Including causes of imbalance and solutions.

Boy... are you living in china? i think yahoo is capable of searching the internet too.
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ben5017
Thu Oct 10 2013, 12:11AM
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YOU ARE NOT FORCED TO READ/RESPOND TO MY POSTS
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DerAlbi
Thu Oct 10 2013, 12:39AM
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 510
no, but how else to show you that you are better off with stopping to ignore all the answers to your questions. They are there. you are not doing anything fancy. its done before. many times. you struggle with basics. this is explained everywhere.
its mostly about using the keywords or alternative words of a question.
Its the most important thing to learn.. how to learn. trust me.
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ben5017
Thu Oct 10 2013, 01:11AM
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I told you this was my first serious electronics project I AM LEARNING THE BASICS. And I never claimed this project was "fancy". The project goals were to make something with my new cnc router, learn to program Arduino, and learn basic electronic design. By the end of the build , whenever that is I will have accomplished all 3

if you do not like my build YOU ARE NOT FORCED TO READ/RESPOND TO MY POSTS
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DerAlbi
Thu Oct 10 2013, 02:22AM
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how old are you?
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Yandersen
Thu Oct 10 2013, 10:33AM
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Joined: Fri Sep 28 2012, 04:54PM
Location: Canada
Posts: 340
He is an American, Albi...
Easy, Benny. Learning can be done individually by using google and it is more efficient, fast and reliable then asking on forums. And learning cnc before understanding the basics of electronics is... seems to be the same nonsence as switching off coils at 600V having 24V for switch on. It is like building the cage of the gun having no guts solidly optimised... And so on.

As for MOSFETs, the devices you should have used instead of IGBT, the paralleling is as simple as putting the devices as close to each other as possible connecting the pins in parallel. But there are plenty of good 100V MOSFETs available which will perform perfectly alone (see my links again, or you can ask me to search digikey for you for the devices with required parameters).

"Coil Inductance plays a larger role than resistance in limiting the current true?"

Not in your design - you limit the current by coil's resistance, right? The speed of current rise is inversely proportional to inductance and proportional to applied voltage, which is the source voltage minus the voltage drop on the switch and minus the coil's resistance multiplied by current. So the higher the current, the slower it rises. When coils shut off, the speed of current fall is proportional to the zener voltage plus resistance*current - so resistance plays controversal here, see? As result, with 24V on input and 24V on zeners you will have current decaying faster then it was rising. That is why 600V is nonsence and 100V MOSFETs is the top devices for your application, even though 60V ones would be awesome too.

And BTW, as MOSFETs can handle higher pulse currents, you can achieve higher speed, but you need a cap in parallel to battery for that. Shorting the projectile and coils 2-3 times would let you achieve that as well as higher efficiency.
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Steve Conner
Thu Oct 10 2013, 11:11AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
First of all...
site rules wrote ...

Part I: Behavior and Conduct
A. Be respectful. This includes everyone: other members of the forum, moderators and administrators, other people on the internet, and any other person, living or dead. Making false statements that are damaging to another person's reputation is not allowed. Insults, threats, and harassment are prohibited!


site rules wrote ...

Part II: Posting
B. Do your own research. If you intend to ask a question, please make sure it isn't already addressed in the archives, or on the wiki. Also, be sure to perform at least a cursory Google search. Remember, nobody wants to do your homework for you. Show some effort, and the community will repay you in kind.


Now onto the science... Coilguns are pretty inefficient so it hardly matters how you drive them in practice. But in theory, the most efficient way is probably to drive from a supply of several hundred volts and use IGBTs in an energy recovery scheme such as the diagonal half-bridge. You minimise suck-back by turning the drive off early and allowing the coil current to decay through the energy recovery circuit. If the coil resistance was negligible, you would shut the drive off when the projectile was about halfway in.

Try Barry's coilgun simulator to get a feel for the way things interact.
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ben5017
Thu Oct 10 2013, 01:23PM
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"He is an American, Albi..." And proud of it (albeit a little less lately.., say the last 5 yrs or so...)

I Forgot you Eastern Europeans are so superior to us westerners HAHAHA

-26 yrs old, I don't know why that matters but ill play along
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ben5017
Thu Oct 10 2013, 06:10PM
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"He is an American, Albi..." And proud of it (albeit a little less lately.., say the last 5 yrs or so...)

I Forgot you Eastern Europeans are so superior to us westerners HAHAHA

-26 yrs old, I don't know why that matters but ill play along
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ben5017
Wed Sep 16 2015, 04:39PM
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Worked on the V3 for the first time in months and finally have some performance data to share

Changed battery to 12s 3.7 AH lipo

performance data:
115 ft/s
32g slug
19j KE

drum mag is still work in progress
I am reading the encoder with the Arduino, but have not written the control code yet.

more updates to come when I find time to work on her some more.
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DerAlbi
Wed Sep 16 2015, 06:09PM
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Location: Dresden, Germany
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Cool! Do you have an oscilloscope?
1) I would be interested in the battery voltage waveform during shot (AC coupled is ok)
2) how many working stages?
3) Fresh charged LiPo means 12*4.2V, right? ~50V That should give some pretty high current. Do you know how much (estimated)
4) how do you stop the projectile?
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ben5017
Thu Sep 17 2015, 03:42AM
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Here is a bit more data on the test firing.

Note: data point 5 on the charts are ESTIMATED values for a 5th stage.

I am encouraged by the linear nature of the performance data to consider squeezing a 5th stage on the V3.
Although it will be a considerable amount more work on a project that never ends.

ALBI:

-No scope. sorry. any recommendations?
-4 working stages, with no injector. I havnt had time to adjust the pulse length in the software so it blows its fuse with the higher volt batteries.

The coils used are the same ones I made and tuned for a 6s pack (82 ft/s).

Hopefully i can squeeze some more power out of her by winding new ones in the future

Maby a new goal of 25J muzzle energy could be possible...
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DerAlbi
Thu Sep 17 2015, 07:35AM
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Recommendations for an Oscilloscope? Uhhm. Rigol DS1000Z-Series. but it costs 400$. (if that kind of money is too much, please tell me)
Its interesting how your kinetic energy rises linear while in my design the velocity rises linear - i dont know hoewever how long that trend will hold in my design due to current limitations. Your graph should break down too due to the same reason soon. The faster the projectrile get, the higher the acceleration power. So with the increasingly higher current the LiPos should draw a natural boundary somewhere.
5J per stage is impressive. I mean.. maybe not, because you cheat by having a heavy projectile... still double of my design with way lower voltages and much lower costs.

How long are your Coils and your projectile?
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ben5017
Thu Sep 17 2015, 12:28PM
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Thanks for the tip on the scope, I will give that one a look. I don't mind paying for a nice one.

Attached is a picture of the slug. they are modified solenoid cores. I ordered 50 of them (from Germany no less)
Only modification made was to cut of the back portion off.

Slugs are 68mm Lg.

Coil geometry:
12 ga. magnet wire, 75mm Lg.

Stage 1: 5 layers
Stage 2: 4 layers
Stage 3: 3 layers
Stage 4: 3 layers



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ben5017
Fri Sep 18 2015, 01:58AM
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Some photos of my slug catcher. made from the 1.17mm stainless steel shell of a broken meat grinder.

Inside is lined with 1/2" thick rubber.

I had to start using a 3/4" thick mdf wear-plate on inside once i noticed the stainless was still denting even with the rubber.












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ben5017
Fri Sep 18 2015, 02:03AM
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Since im in the posting mood, Here are some more recent photos of the V3











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DerAlbi
Fri Sep 18 2015, 04:28AM
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Location: Dresden, Germany
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Quite some build how heavy?
Your energy density (Joules per acceleration length) is also interesting. You get 19.3J / (4*6.8cm)=0.71J/cm while in my design i achieve 1.43J/cm. So all my optimizations make it only twice as compact.
Not bad considering the coil geometry and beeing battery driven and so on.. nice!

Are those fuses for the coils? What is for coil behind the magazine or?
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ben5017
Fri Sep 18 2015, 02:03PM
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I cant remember the exact weight, will weigh it next time i get a chance to take it off its stand. (breadboard is attached to the stand for development so the V3 has not been off it for months) It was pretty heavy before upgrading the batteries (+400ish grams) & possibly adding a 5th stage... However the weight is well balanced & the front grip give additional stability, so its not to awkward to hold at all.

You are correct, the fuses (60 amp) are for the coils. Trying to protect from catastrophic failure.. After many f-ups in the development of the V3, they proved their worth already (have a box full of burnt ones... the 60 amp fuses will get your attention when they go...).

The coil behind the drum mag is the "injector" it is a homemade spring return solenoid that pushes the slug out of the drum.
I am planing on redesigning it to be much more compact, allowing me to move the drum towards the rear and should give me enough room to add a 5th stage.
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DerAlbi
Sat Sep 19 2015, 03:24AM
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Do you have any schematic on how you switch and what did you actually decide for? I hope you use mosfets at this voltage

The fuses surely blow spectacular if they do. They are not made for the high voltage.. if they melt they can sustain a spark that dissipates quite a lot of bang.
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ben5017
Thu Sep 24 2015, 01:14PM
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Even after the success of recent upgrades, I am still not satisfied with the power of the V3. I am debating more radical changes to increase performance.

I have two options that would keep overall dimensions of the V3 the same but increase performance.

#1) stay with 12s (50 volts) and add 2 additional stages (cheaper/safer route)
#2) increase to 18s (75 volts) and add 1 additional stage (would need to buy different mosfets & tvs diodes and an additional 6s lipo)

Interested in hearing feedback from my fellow coil gunners on what option i should take. I believe #2 will give me biggest boost in KE, But not sure it will be worth extra expense/weight

Additional changes that will be made

Change 1) gear motor/encoder drum mag is replaced with a direct drive stepper motor. this configuration is more compact and should be easier to program and integrate on the arduino.

Change 2) Injector is removed and replaced by a full stage coil (one of the additional stages mentioned above) directly in front of the drum mag. this coil will be pulse length controlled by the arduino to pull the projectile out of the drum and will act as the first accelerator stage.

-Albi no schematic to post right now sorry. 100v 420 amp mosfets were used. driven by 12V 1.2A gate drivers tc422 series (mouser# off top of my head). Gate driver gets 5 volt signal directly from arduino mega.

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