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Charging a capacitor? Answered

Recently I've had an idea for a project that requires a relatively low voltage (5 to 30 volts) capacitor bank. However, that capacitor bank needs to be charged. While I do know you can easily just hook it up to power, I find that too simple, and lacking of safety, and knowing when it is fully charged. So, could anyone please enlighten me on how i can charge my cap. bank relatively slowly, with, say, an LED that lights up when charged (to avoid them blowing up in my face), and a kill switch/button that discharges the capacitor bank in the event of an overcharge. If anyone could provide an explanation or circuit diagram that would be great, thanks. BTW, the capacitors I was planning on using are http://www.goldmine-elec-products.com/prodinfo.asp?number=G17623 or http://www.goldmine-elec-products.com/prodinfo.asp?number=G16573.

2 Replies

orksecurityBest Answer (author)2010-09-14

To charge slowly, charge in series with an appropriate size resistor. If you know the capacitance and the characteristics of the power source, you can calculate the RC time constant and determine exactly how long it will take to charge to a given voltage.

To discharge slowly, discharge through a resistor. Again, the RC time constant will tell you the relationship between resistance and discharge rate. Resistance and voltage will tell you how much power is being dissipated, which will tell you what size (wattage) the resistor needs to be.

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Jack A Lopez (author)2010-09-14

Those memory backup capacitors tend to have large series resistance. I forget how large, but it might be something like hundreds of ohms for a single capacitor. It's not a big deal for memory backup applications where the current delivered to and from the capacitor is very small, but you won't be able to get them to deliver large currents the way a typical electrolytic capacitor can.

But don't take my word for it.  You should actually get a few of these memory backup capacitors and measure their natural series resistance yourself.

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