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6 Month Gauss Cannon Project

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blackgrunge

Thu Jan 01 2009, 03:32AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55

@El_Roberto

Wow that's ridiculous $400 dollars, what are they thinking? I guess I won't complain.

@rp181

Junior in high school at an international bacheloriate school. That just mean my courses are all college level during junior and senior year. Plus I've taken courses at the local community college to help with the physics course I'm in.

blackgrunge

Fri Jan 02 2009, 06:20PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55

The Calvary has arrived!!

A week later and $67 out of pocket and the new capacitors have arrived. For one I can say how disappointed I am with UPS Ground and their holiday shipping schedule but I really can't complain now that I have them. Rated 400v @ 3900uf, these 8 capacitors weigh in at a little less than 2.5kJ. Being just the first step in upgrading to the 5kJ model I hope that these bad boys will do some serious damage.

In the mean time I'm searching for SCRs that can handle these pulses since 2N5207s don't stand up too well to a 1500amp discharge. Anyone got any suggestions for part numbers or places where I can buy some more robust SCRs, everywhere I look I can't find to seem any decently priced SCRs.

(Sorry for double post)

rp181

Fri Jan 02 2009, 06:24PM

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

Check out chtechnology SCR's. I posted a thread earlier on popping noises from my capacitor bank, and it turns out the aluminum terminals where oxidising. I was suggested Oxguard. You might want to take a look at that. and the thread i posted ( named popping noises from capacitor bank).

big5824

Fri Jan 02 2009, 07:54PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

i got a 6000A pulse scr from ebay for 15 pounds a while ago, take a look there

rp181

Fri Jan 02 2009, 08:00PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Beware of quality also, Ive seen Some small studs (like yours) rated higher than 3/4in stud SCR's.

guitardude012

Fri Jan 02 2009, 08:53PM

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

Blackgrunge, take a look at the TVR SCRs in TO-220 package. You will have to parallelize them to handle your levels of energy, but they should work in your CG.

Turn off time will be critical and the 50RIA80 for example are rated at 110us(typical). Run the numbers at even 50m/s and I think you will find this will be an issue. However, a SCR such as TVR's S8055R has a max turnoff time of 35us.

Myke

Fri Jan 02 2009, 09:33PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Remember that the current that goes through SCRs in parallel isn't shared perfectly.

guitardude012

Sat Jan 03 2009, 02:10AM

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

It helps that they have a positive thermal coefficient(though at these speeds I'm not sure how much that really matters). The important thing is to plan for a little margin of error and to drive the gates very strongly(within spec. sheet limits of course)

Mine have lasted over 200 shots and my setup is almost exactly the same as yours.

edit: Which wire gauge and IR sensors are you using? I currently use lasers and have found that photo diodes are a must. I doubt the rise/fall time of photocells and phototransistors are going to work at the speeds you want(without extending the barrel)

You will find that it is necessary to fire coils before the projectile gets there to establish the field. You don't want to waste a precious 1 ms fighting the RLC circuit to get the field established after the projectile has actually reached the coil. This of course has a negative effect on efficiency but I believe it is important if you really want to hit 100m/s.

blackgrunge

Sat Jan 03 2009, 03:19AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55

I deal with the rise and fall time of the photo diode by making my inverting hex trigger extremely sensitive. I have the IC being pull low by a 300k resistor, so the photo diode has to get above 300k ohms to pull it high. I could further increase the sensitivity of the circuit by making the resistor even larger but its sufficient. Its a great way of battling the response time.

rp181

Sat Jan 03 2009, 03:42AM

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

It would help if your photogate was easy to move up and down your barrel.
What is your barrel going to be? You are trying to maximize effeciancy, so a really strong material would be good to keep dead space down. Maybe you should look into carbon fiber tubes (gets pretty expensive though. one 1ft length for a 3/8in bore from mcmaster-carr is 25$, and that isn't even smooth on the inside).
Decide what SCR you are going to get?

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