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Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
I am in the process of designing a quarter shrinker. I have 2 energy discharge capacitors available for use. I am trying to decide which one to use. The specs are as follows:
Capacitor #1:
Maker: Aerovox Capacitance: 100 uF Maximum Voltage: 22 KV Maximum voltage reversal: 20% Maximum energy: 24 KJ
Capacitor #2:
Maker: Maxwell Capacitance: 100 uF Maximum Voltage: 10 KV Maximum voltage reversal: 80% Maximum energy available: 5 KJ
At first glance I would pick the Aerovox because of its maximum available energy of 24 KJ, even though I would probably never use all of that. However the Maxwell will tolerate an 80% voltage reversal whereas the Aerovox will only tolerate 20%. So for maximum life I could actually charge the Maxwell close to its maximum of 10 KV without violating its 80% voltage reversal spec. I could only charge the Aerovox to about 4.5 KV without violating its 20% voltage reversal spec. Thus, in this scenario, the Maxwell would deliver more energy without a lifespan reduction.
I am guessing that the Aerovox can probably take a lot more punishment than its 20% spec would indicate since I will only be firing this thing maybe a few hundred times over its life.
Does anyone that has built a quarter shrinker have any data suggesting the best choice?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Sounds like your voltage reversal estimates are too conservative. If the charged voltage is X, the only way a voltage of -X can develop is if the LCR circuit is lossless (R=0). Not only would that do no work, it would oscillate forever after you close the switch.
I'm going to guess that in low voltage tests, where the wire doesn't break and the quarter doesn't shrink, there's enough damping that the reversed voltage magnitude never exceeds X/2 or X/3. Try it with both of your capacitors, using an ordinary oscilloscope and an initial voltage on the order of 10 V or 100 V. How much does the observed reversal percentage change when you don't have a coin in the middle of the coil? It's a linear system, eh, so the levels will simply scale up with initial voltage, until the wire gets significantly hot or things physically move (resulting in more damping, not less). When actually shrinking a coin, and splintering the work coil, the voltage might not reverse at all. What do the websites of Bert Hickman, or Hackaday, etc. , or Signification here say about the sign & magnitude of residual voltage after a shot?
Finally, based on what I remember from datasheets and online-accessible articles, those voltage reversal numbers (for device lifetime of N shots) are based on 100% charge. The increase in N for charge voltages less than 100% are dramatic, outweighing the extreme-reversal penalty. A 20% reversal spec would allow 80% reversal if the initial charge level were 25% of max. Don't take my word for it -- I put no money into your capacitor purchase -- but do some Internet searching.
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Which capacitor for quarter shrinker?
