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El_Roberto

Wed Oct 22 2008, 03:07AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Im making a 600J coilgun for my electronics project at school, it will be using two 5000uf 350v capacitors. I have just soldered together 5 1000v 6amp doides in parralell and i am wonering if this will be enough protection for the capacitors and the SCR. I also want to know if the SCR i have bought will be powerful enough, The coil will be about 50 turns and the capacitors will be in series. Here is the spec sheet for the SCR http://www.datasheetcatalog.org/datasheet/irf/irktf72.pdf It is rated the 08 version rated at 800v 71amps continuos. I looked at barry's calculator for current but no matter what i do i cant get it to install on my pc (very annoying because I would really like to use this). I have looked on google and i found one 1200j coilgun that only uses four of the diodes i choose but every other lower powered one ive seen uses massive 100amp diodes. Any help would be appreciated.

[Edit: URL fixed]

guitardude012

Wed Oct 22 2008, 06:15AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Welcome to the board =)

You didn't mention enough parameters for me to determine. I need to know wire gauge, inner diameter of the coil, number of layers for the coil. The link isn't working for me.... edit: nvm I figured out that there was an extra space before the filename. As crazy as it may sound, you will probably be exceeding the 2200A current limit of that SCR(max peak, one cycle, non repetitive surge for 8.3ms).

I use a 70 amp diode (stud) for my gun that I currently fire at 477J. I have blown many smaller 1000v 3amp diodes with <200J . You will want an ultrafast recovery diode like: 70HFLR100S05 if you plan on being seriously underdamped. If you are planning on firing the caps in parallel a lot you will probably want to choose working voltage no higher than 300V and 1000v diodes are probably overkill.

Sorry for the brief response, pretty tired =) I can help more tomorrow if you want.

El_Roberto

Wed Oct 22 2008, 10:45AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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Posts: 135

Cool thats a start, im looking at another scr rated at 1700v 850amp continuous, im unsure on the peak current rating just waiting for the seller to get back to me but i think its in the 8000amp range.
i will most likey be firing the caps in series for 700 volts. This will hopefully be the first coilgun of many so i want to start off without a bang!
my plan for a second coilgun is ten stages each at about 60j. I am thinking of trying to get some high rating igbt's so i can use an optical sensor to turn off the
first coil and fire the next coil when the projectile is exactly at the centre of the first (ie the coils will have a 3mm gap in the middle for the optical sensor). What i hope to do with this to greatly increase efficiency is to
use the back emf to charge the the caps im using now, with could then be used to recharge the firing caps. any thoughts? I might try to see if i can get some good higher power diodes, if i cant i might have to make my second
idea my first project (that way ill only need 250v diodes)

/end ramble :p

rp181

Wed Oct 22 2008, 11:57AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

that scr is fine. The surge rating of an SCR goes up exponentially as time deacreses, and with those caps your looking at a maximum time of 2ms (1ms will be reasonable)

Where are you buying those SCR (800A)? those can get expensive fast.

aonomus

Wed Oct 22 2008, 12:30PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

The LCR and induction sims on Barrys website are Java applets, you'll need the Java Runtime Environment to get them to work.

Depending on the LCR simulation, you can typically up-rate the SCR's pulse current up to 5-10x the continuous, but as rp181 said it goes up exponentially as time decreases so figure out the peak current and the pulse length. I pushed 1kA through a 80A SCR, peak current lasted almost 2 mS.

As for the diodes to protect against reverse-bias, I used RURG8060s (800v 60A I believe) but there isn't a need to go to that extreme, I just had them sitting around (I believe I bought them off of ... years ago). Design your coil right and you'll have little-no reverse bias so you can use something lighter.

El_Roberto

Wed Oct 22 2008, 06:57PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Cool thanks for your replies, ill try the scr i got if it doesnt work i have a supplier that can get me pretty much anything i want (his words) he is selling 6 of those scr's for NZ$150 (about US$90)
at the moment the caps should just be emptying into the coil without stopping so the back emf will be minimised (accourding to my teacher). But do you think those diodes will hold? i need a yes/no awnser because i dont particularly want to experience a diode blowing up next to me.
Also what guage wire would be reccomended im thinking 12awg as that is what ive seen allot of.

guitardude012

Wed Oct 22 2008, 07:12PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

12 awg sounds good. What projectile diameter are you planning for?

I still think we need more info to determine if the diodes will hold. We don't have enough information to approximate resistance and inductance yet. Like aonomus said, it depends on your coil and the amount of reverse bias.

El_Roberto

Wed Oct 22 2008, 07:15PM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Sorry i meant to say before, i will most likely be firing 9mm, but it will be a plastic barrell so it would probably be about 12mm - 14mm thick total, the coil will most likely be 5 lots of ten windings on top of each other

Edit: sorry to ask so many questions at once but i cant find the information on google, what is the peak current for an igbt? is it 3x the rated continuous? and can igbt's be run in parralell like scr's? because i found an electronics shop that sells 800v 12amp igbts that could be useful for my second idea as the are only $3 each.

aonomus

Wed Oct 22 2008, 07:48PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

SCR's are fairly forgiving for current, however they 'latch on' (ie: once you trigger, the SCR will stay forward conducting until there is no more current flow above the threshold). IGBT's on the other hand are not so forgiving for current, however you can shape the pulse since you can actually turn off the IGBT. With a 12A IGBT you would be lucky for it not to explode and damage anything around imo...

rp181

Wed Oct 22 2008, 08:52PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I think they are usually twice the rating for IGBT's. A word of caution: Make sure you look at the data sheet for the SCR's well. The peak current could be 10us or 1 us, and to reach the peak safley, you need a specific waveform.

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