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Alex Smith

Wed Nov 10 2010, 06:03PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Hi there!

I'm building a coilgun for my A-Level physics project and think my plans are ok but would appreciate if you folk could check it over and make sure I'm not about to blow a hand off or anything stupid ;) The experiment requires me to change some variables in order to try and optimise launch velocity, so i am choosing to vary voltage, starting distance of projectile and length/mass or projectile.

So I have 20x 9,000 uF 50V Sprague capacitors (previously used in ampifers) and im planning to use 2 of them in parallel (would you recommend more or less??). For health and safety reasons I cant use more than 50V.
I will add a bleeder resistor of course as I can see peeps here are really fond of that ;)

I also have a SCR (IXYS MCD95-16IO1B Datasheet: http://www.wattbits.com/media/datasheets/MCC95.pdf) and plan to use this to switch on the circuit. I know its overkill but its also reusable for Mk.II whenever that comes along!

For the coil i have 2.0mm magnet wire and for charging the capacitors i have a 50V power supply. I will be firing the snuggest nail i can find which fits in the launch tube (which i have also yet to find)

So my questions are.. :)

1) looking at the sims below, the current hits negative 100A and i take it this will have and adverse effect on the caps? could is use the diode of my SCR/diode combo to prevent this or would that slow down the pulse too much? Or i could add a resistor - but that slows the pulse down to 12ms from 7ms and decreases the peak current by 50%

2) what length coil should I use - read somewhere that 110-120% length of projectile is a good idea? But i am varying the length of the projectile...

3) im slightly confused about the SCR datasheet - what are the currents required to trigger the scr? I think it is 150mA @2.5V! but are these minimum values or recommended values etc.

4) do i need to cover/protect the capacitors?? Also is anything else in this experiment dangerous other than the nail being fired or the possibility of electric shock?

5) my scr/diode rattles a bit. is this normal?

Barrys sims are attached below - they are a bit rough when its comes to the coils as it isnt built yet, but hopefully it is enough to get the gist of it.

Any help is greatly appreciated!
Cheers Alex

P.S. sorry for the spelling errors, ive found about 10 already but im really tired right now, what with black ops and everything...

GhostNull

Thu Nov 11 2010, 09:23AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Look at the 4hv wiki page on coil guns:

1) That will probably put too much stress on your SCR. Use a flyback diode or modify your coil; 2mm wire is much more than enough.
2) Yes, is your projectile going to be shorter than 21mm? look here:

If you are varying the length of the projectile and that effects the results then that is part of your results...
3) That should be the minium recommended gate trigger current. The maximum input is given by Gate Power, Pg
4) Yes, you should eliminate all possible dangers to the best of your ability.
5) What do you mean by rattles?

Other Issues:
Pulse is a bit long, 7ms might be too long.

Read through those two links and other pages on the second one. You should be able to find all the answers to all your questions.

Best of luck =D
-Ken

Alex Smith

Thu Nov 11 2010, 11:33AM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Hi!
1) Right. Off to buy a diode....
2,3,4) Thanks ;)
5) Err yeah it makes a small metallic knocking sound if you shake it?
Cheers!
Alex

EDIT: After careful examination, I think the clinking is coming from the screw mounts. Its very well built so unlikely to have broken in transit as it was also very well packaged.

EDIT 2: Using one capacitor reduces the peak current by 50A (to 200A) and shortens the pulse to 5ms? Im going to try and build the coil now so I can optimise it.

ElectronicBob

Thu Nov 11 2010, 01:11PM

Registered Member #2973 Joined: Tue Jul 06 2010, 01:13PM
Location: Leeds UK
Posts: 9

You are probably right about the rattle....it sounds like the SCR/diode you have is similar to ones I use at work. A bit of rattle seems to be normal...

klugesmith

Thu Nov 11 2010, 08:57PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715

Alex Smith wrote ...
1) looking at the sims below, the current hits negative 100A and i take it this will have and adverse effect on the caps? could is use the diode of my SCR/diode combo to prevent this or would that slow down the pulse too much? Or i could add a resistor - but that slows the pulse down to 12ms from 7ms and decreases the peak current by 50%

Negative current is never a problem for capacitors, it just means they are charging back toward a positive voltage. Of course if you use an SCR switch, it will simply turn off when the current reaches zero, so the actual pulse will be just the first half-cycle of the simulation.

The problem is with negative -voltage- on the capacitors, and when the current pulse reaches zero you have already reached maximum negative voltage. Barry's RLC simulator has a box you can check to display the voltage waveform as well as the current waveform.

As for adding a series resistor to increase damping: that can be done more efficiently by just winding the coil with thinner wire. Can get the same inductance in smaller volume, with stronger field at the axis (for a given current). Remember, it isn't amperes that count, it's amperes times the number of turns.

Alex Smith

Thu Nov 11 2010, 10:13PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Thanks! That clarifys quite a lot!
If only I had taken electronics.... :/

Alex Smith

Thu Nov 18 2010, 05:57PM

Registered Member #3258 Joined: Wed Sept 29 2010, 06:25PM
Location: Ascot, UK
Posts: 25

Just want to say a big thank you guys. Info really helped! Had my practical day today and got very consistent results and managed over 20m/s!! Needless to say my teacher was impressed - after he stopped being hysterical about a couple of gigantic capacitors being in the lab ;)

Now for Mk. II...

Navrit Bal

Thu Nov 18 2010, 11:38PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Congratulations Alex, did it end up critically dampened or not?

Neuuubeh

Sat Nov 20 2010, 09:27AM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

Why do you guys always strive to have a critically dampened circle? Guess its much much easier on the capacitors and does simplify the circuit (no high powered diodes or similar needed), but then, you are losing so much power?

What do I know tho, Im still waiting for my components to get to me, been a while since ive messed with such things :D

Navrit Bal

Sat Nov 20 2010, 10:05AM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I asked because thats what his simulation might not have accounted for additional resistance etc,... so could have quite easily been critically dampened

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