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Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
Saz43, What was the current & voltage rating was your relay? This guy here: has used relays many times apparently with out problems. It may be that your voltage rating was lower (mains over here is 240v and in some countries it is 400v). From my research, relays are okay, if they have sufficient rating but welding can a definite problem if relays are used beyond their ratings and welding is better than your semiconductor exploding and sending shrapnel every where XD. I think relays are an okay, low cost choice for smaller coilguns.
Touching wires is a definite no no for anything more than an single week-end coilgun. Arcing and contact resistance lead to unacceptable energy loss and inefficiency.
After looking at the RLC simulation you might be worried about finding a semiconductor that can handle the current but remember you need to look at the surge rating of the semiconductor since you are putting it under a current surge. Logically current rating will go up the shorter the surge. To estimate the surge capability i believe the approximation is given by:
I_peak = Given_I_Peak * sqrt(f/given_f)
where: I_peak = the peak current at your frequency, in Amps (non repetitive) Given_I_Peak = the peak current rating (non-repetitive) is your semiconductor, in Amps f = your equivalent frequency, in Hz given_f = the frequency at which Given_I_Peak is rated, in Hz (usually 60Hz)
If your peak current is well below the current rating given by this equation then you should be fine, otherwise you will need to consider a higher rated semiconductor. Here is a good discussion on the topic
If you look around you might be able to find the thread where someone is giving away SCRs
Registered Member #2961
Joined: Sun Jul 04 2010, 01:21PM
Location:
Posts: 2
Hey, I have made two coilguns before, and have always used photoflash circuits to charge my capacitor bank. I saw that GhostNull posted some links for methods to charge capacitors, but the maximum foltage for the boost converter is 500; I'm upgrading to a 900v bank. I currently have no method of charging this bank. Does anyone have any ideas? I want to have something portable to charge these capacitors.
Also, I have used a relay before, but the immense energy in the current welded the relay a few times. The relay I used is an automotive relay with a rating of 40 amps. That was the highest I could find at Radio Shack. Does anyone have any suggestions? Any help would be much appreciated.
Registered Member #2648
Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291
I don't know so much about buck/boost converters but I know that you should be able to design 1 that can charge at 900v.
How big is your bank?
If it is enough to weld your 40amp relay then you might want to consider solid state switching. You won't find any solid state switchers at radio shack that can handle the current. Look on ebay, there a many (relatively) cheap SCRs that can handle the current, esp. if you live in the US.
Registered Member #2961
Joined: Sun Jul 04 2010, 01:21PM
Location:
Posts: 2
the bank i'm planning on making is going to be using (4) 450v 2400uf capacitors. I'm putting them into pairs in series, then put the two banks in parallel. I'm not absolutely limited to using a boost converter. I know they are more portable, that's all. I'm open to other methods of charging.
What src ratings would you suggest if i make a coilgun with a bank like above?
One other thing... I know i can make a simple coilgun, but i know slightly more advanced ones utilizes diodes to prevent back emf and scr for something... What are the SCR's for? i found some here. http://www.anothercoilgunsite.com/nf-portable-construction.htm And how do i position the diodes?
I really appreciate your patience and response. =)
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New Coilgunner, need advice and tips!
Sorry, didn't mean to give the impression that I had.
