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Registered Member #55
Joined: Thu Feb 09 2006, 04:56AM
Location:
Posts: 149
Hi Mero So the V-switch you implemented is the type devloped by Evgeny Vasiljev? I have looked at his website in the past and am now intreged to try this topology. I always figured I would get the same if not better control with IGBTs, but your results are very good and are tempting me to have a go at it.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
basically yes. It is the E.V. v-switch except that i would a small coil around the launch tube instead of using a ferrite stick placed near the coil. Used zeners to clamp spikes and a toroidal GDT for the work SCR, since they are high sided (Cap+ , SCRS , LOAD, ground) so the gates float a 300V. The key to the success is to wire the sensing coil in the right way (otherwise both scrs will trigger at start, and the current will not turn off) and chosing appropriate damping resistor following 2 simple guidelines: 1) avoid that the induced voltage go higher than your scr rating , in my case 800V( for worst case use Ipeak * Rdamp + V@peak, need a rcl simulator), or the SCR will latch back on.
2) Calculate the proper resistor value to have a not-too fast or not-too slow damping.
calculate the time that takes for the projectile from turn off to reach the coil center, subtracting the scr turn off time (approx 40us) obtaining the time toff. Calculate the current in the coil after turn off with an LR model with this formula I= Ioff* e^(-toff*R/L) and make sure it falls in the 1-10A range.
3) Size the second damper resistor (the one on the small cap bank) 3-4 times the main damper resistor
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Quick coilgun project: Single stage SCR V-switch coilgun pistol
