Barry's Coilguns 

Mark 4

Bleeder ResistorAn electrolytic capacitor can store considerable charge for weeks and months. How do we ensure this does not endanger people? Every highpower capacitor should have a bleeder resistor to remove a dangerous charge. Bleeder ResistorR3 is the safety bleeder resistor. It ensures the capacitor is drained during overnight storage to improve safety. We will examine longterm and shortterm discharge rates to determine the appropriate resistance. Longterm Discharge:If the charging time is 5 sec, then the bleed time should be at least 100x longer and preferably 1000x longer, or 83 minutes (e.g. 5,000 sec). Assume this is 3x the RC time constant, which then becomes a desired t = RC = 1600 seconds or longer. If t = RC, then R3 = t/C = (1600 sec) / (0.012 F) = 133 Kohm Now check the power used in the bleeder resistor. P = V^{2}/R = (200 v)^{2} / (133 Kohm) = 0.3 watts Therefore a conservative rating would be 1watt resistor or larger. Some experimentation found that a 150 Kohm 1watt carbon composition resistor showed no noticeable warming. However, a 47 Kohm 1watt resistor became quite warm. Shortterm DischargeThe operator needs some time with a reasonably stable capacitor voltage between charging it up and firing the coilgun. Let’s assume we want to lose < 5% of the charge in five minutes. This permits plenty of time between charge and discharge. For short periods at full charge, the decay is closely approximated by a linear rate.
Based on the calculations for shortterm (500 Kohm) and longterm (133 Kohm) values, and some experimentation, the coilgun used a 150 Kohm 1watt resistor. The smaller value was chosen because a little faster decay was useful when the capacitor was charged a few volts beyond a target value. If it received, say, 52v instead of 50v, then we need to wait only a short time for it to return to the desired voltage. 



Last Update 20080614
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