Barry's Coilguns 

Mark 4

CapacitorThere are several important factors in choosing an energy storage capacitor for a coilgun. VoltageThe PE (potential energy) depends greatly on voltage. Note that PE is proportional to voltagesquared, so it is a great advantage to use higher voltages. PriceCosts are favorable in the range 100 – 300 wvdc (working volts of direct current). We recommend comparing capacitors on the basis of “cost per joule” when considering them for purchase. TypeWe can choose between standard powersupply filter capacitors, and pulserated type. The pulserated will have lower ESR (equivalent series resistance), and larger terminals. However, the standard aluminum electrolytic capacitors have lowenough resistance to achieve an underdamped circuit, and are much more commonly available. CapacitanceTo estimate the desired capacitance for the LC tuned circuit, we plug in approximate values of the time period and probable inductance. Note that a tuned LC circuit has time constant Solving this for C yields: We expect low inductance, in the range 1 – 100 uH, and time t in the range 1  4 ms. Minimum C is at low t, high L: This is a very broad range, so we chose the middle value of capacitance of about 10,000 uF. 



Last Update 20080614
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