Simple R L C circuit Critically Damped Response

andy93 · Jan 18, 2013

I wonder if anyone can help with my magnet launcher experiments? I've obtained 56 1000uF 450V caps which I plan to wire in parallel to make one 56,000uF 450v which I will discharge into a coil (by smacking two copper plates together with a hammer) .I guess I need to design the coil so that the resistance and inductance is such as to cause a critically damped response? How do I do this , first ignoring the effect on inductance caused by the projectile, then what effect would an iron or neodymium magnet entering the coil have on the calculations. Is it OK to take the caps a little over their rated value, just for a few seconds? Thanks in advance for your help...

chuckey · Jan 18, 2013

Old fahioned caps used with valve equipments were often rated for different "surge" and "working" voltages. Caps now a days are much better in terms of leakage but as the manufacturing processes are much tighter, I would suspect that they are running at 90% of their breakdown voltage. Try them out individually with a 100K current limiting resistor and make up your own mind. By the way when these capacitors fail, they go off like a bomb, blowing out the seal or rubber bung, throwing out a lot of ali foil and chemicals. So point them away from yourself when testing them (or using them).
The effect of the projectile would be to kill the Q of the inductance and its inductance. I was under the impression that the projectile had to be high conductance as it was the induced magnetic field in it that caused the movement*. And copper will only reduce the inductance of a coil by 20%, not to 3%?
Frank
* Look at any AC induction motor, they have a magnetic rotor to capture the field but with copper or ali bars to provide the high current for the repulsive forces.

andy93 · Jan 19, 2013

Thanks for that ... regarding the copper projectile, if it were a ring or cylinder preferably with an insulated iron core , I could see that working ,worth investigating , but wouldn't solid copper 'short out' the effect?

chuckey said:
Old fahioned caps used with valve equipments were often rated for different "surge" and "working" voltages. Caps now a days are much better in terms of leakage but as the manufacturing processes are much tighter, I would suspect that they are running at 90% of their breakdown voltage. Try them out individually with a 100K current limiting resistor and make up your own mind. By the way when these capacitors fail, they go off like a bomb, blowing out the seal or rubber bung, throwing out a lot of ali foil and chemicals. So point them away from yourself when testing them (or using them).
The effect of the projectile would be to kill the Q of the inductance and its inductance. I was under the impression that the projectile had to be high conductance as it was the induced magnetic field in it that caused the movement*. And copper will only reduce the inductance of a coil by 20%, not to 3%?
Frank
* Look at any AC induction motor, they have a magnetic rotor to capture the field but with copper or ali bars to provide the high current for the repulsive forces.

Simple R L C circuit Critically Damped Response

andy93

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chuckey

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andy93

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Simple R L C circuit Critically Damped Response

andy93

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