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Registered Member #7829
Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16
Given the cost of high power switching devices, what is the feasibility of not using IGBTs, SCRs, etc. and instead using contacts on the inside of the barrel? The projectile would complete the circuit of each stage as it passes down the barrel and the placement of the contacts could be tuned to achieve maximum performance. Obviously, this design would require some sort of initial kick to get going.
I think Ash Small brought this up in Saz's project thread. What're your thoughts?
Registered Member #8497
Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74
What you're referring to is spark-gap triggering. If you're utilizing decent high power (>1kJ) capacitor banks, you can afford the losses from the arcing.
This method has been implemented very successfully by one of the members here who almost broke the sound barrier.
However - the consequences are: 1. Terminal contact erosion caused by arc'ing. 2. Very noisy.
Registered Member #7829
Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16
My current design (on paper) has 3 optically triggered IGBTs at much lower voltage, but I thought I might be able to eliminate some part costs by utilizing spark gaps.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I wouldn't want to try it
To get a good mechanical contact, which you'll want for a high current, you'll end up with a load of friction slowing down the projectile almost to a standstill. Switching devices are generally cheap anyway, and have very low conduction losses.. so stick with them! My first coilgun used a mechanical switch, and even that's probably better than including the projectile as part of the switching action. Coilguns are very inefficient (in terms of joules in the capacitor converted to joules in the projectile, 1/2mv^2) that anything that impedes the projectile will just make it worse.
Registered Member #7829
Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16
Yeah, I realize the spark gap was just a hopeful attempt to save money and keep performance at the same time (unrealistic). I think I'll take all of your advice and keep to the switching elements especially since this is my first build. Thanks everyone
Registered Member #4034
Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138
My first coilgun did this, some odd 800volts ish (long time ago) and a spark gap trigured by a relay (single stage) so timing wasn't a issue. Worked really well. The relay never really wore either.
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Something worth experimenting with?
