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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
I just bought 2 caps for my coil gun today, costs $15 each they're 2500 uF each at 350Vdc and 400V max surge. does that mean its safe to charge it at 400 volts?
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
No, you can safely charge them to 350 vdc. They can tolerate 400v only briefly, such as a few msec during a transient, otherwise leakage current can cook them like popcorn.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Multiple capacitors connected together will always have the same amount of total stored energy, regardless of the connection scheme. You can trade off voltage and capacitance in various ways, but the total energy available is unchanged.
Cheers, Barry You don't become great by trying to be great. You become great by wanting to do something, and then doing it so hard that you become great in the process.
and that can be a very good thing, if you want to shorten the pulse, instead of 2 caps in parallel, you use 2 in series at twice the voltage. With the lower capacitance you get a shorter discharge time but the same energy.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
I have bank of 52x 3300µF 420V (483V surge) capacitors in series parallel to give me 10.73mF and 1680V (1932V surge)
I have charged this bank up to 1920V and discharged several times to various objects including highly inductive loads.
I didnt expect these to last longer than maybe 10-20 shots but somehow they are still going strong. Before putting this bank together each capacitor was tested with my standards (ESR/Capacitance reading and leakage current at 490V) Normally leakage should be tested at their nominal operating voltage..
According to datasheet these caps should leak max 4mA at their nameplate voltage of 420V. All caps i tested leaked less than 3mA at 490V
Out of curiosity, after 2 years of flawless operation i took the bank apart to see how caps are doing (retesting them with LCR meter and measuring their leakage). One cap of 52 tested was open circuit. Others were fine. ESR/Capacitance/leakage still clearly in spec.
So i ripped the failed capacitor open and found out that the failure was caused ba aging (these are used/scrapped inverter grade lytics). Metal foils from active element to positive terminal were badly corroded and broken off. From what i have seen that is pretty common failure mode when lytic runs out of electrolyte. There was no sign of arcing or electrical explosion so i believe that the cap failed when i did low energy testing at 1-5kJ
So i would say it is possible to charge lytics to their surge rating in this kinf of application. Or i have been just lucky.
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max surge rating on caps?
