Wednesday, December 1, 2010

Getting down and dirty with shunt resistors

I'm firing the accelerator at ~400V. The red line is whatever. The blue line is the voltage across a 0.02 ohm resistor in line with the firing circuit. It looks like it's got about 20V across it, though my scope rails out at 10V. This works out to about 1000A or around half a megawatt at peak, given that the main coil is running at 400V.

Most importantly though we can see the coil is running at high power for about 1ms. That is a very long time. If the projectile is 2cm long that means we'll experience significant deceleration at just 20m/s. Perhaps we've not seen that yet because the second coil (which would be the one decelerating since the round is already moving >20m/s when it gets there) is firing well before the round and only using a piece of it's accelerating power.

Given that we don't want to make a new coil, the alternative is decreasing the capacitance. Halfing the capacitance will half the firing time. This, of course, goes hand-in-hand with increasing the voltage to pick up the additional power.


  1. Or, ya know, you could put in dump resistors :)

  2. yea, yea, V2. At $25/each for the SCRs I'm not in a super hurry to add another set of them.